Ship Construction

Question 1

Explain the meaning and purpose of EACH of the following ship terms:
a) Hawse pipe; (4)
b) Chain stopper; (4)
c) Fairlead; (4)
d) Bollard. (4)

Answer 1

a.) an iron or steel pipe in the stern or bow of the ship through which the anchor chain passes from the cable lifter, through the forecastle deck and the ship’s side. Purpose - to provide an easy lead for the cable from the windlass to the anchors,

b.) The chain stopper holds the anchor while the ship is underway and guides the chain during anchor manoeuvring. During anchoring, the chain stopper
withstands the forces from the anchor, so the anchor winch is protected. fitted on the deck between the cable lifter and the hawse pipe, which automatically prevents the anchor chain running out. When letting go of the anchor the
stopper bar of the chain stopper is swung out of the way its main purpose is take load from anchor and transmit it into ships hull.

c.) Fairleads guide the
mooring lines/cables through the ships side and into the winch or
windlass assembly. enables the line to be passed through the bulwark and allows for a change in direction without snagging or fouling.

d.) A rectangular base welded to the deck of the ship, upon which two vertical parts are welded. Bollards are used to secure the mooring lines.

Question 2

a) Describe, with the aid of a sketch, a freeing-port. (8)

b) Explain how freeing-ports assist in maintaining the stability of a ship. (8)

Answer 2

a) A freeing port is an open hole cut into a bulkward that allows rapid draining of green seas and collected rain water from the weather deck. Its sealed by a hinge plate that will open when a weight of water acts on the inside surface. These can be to the hull side plating, such as on Ro-Ro ferries in order to drain the water from internal car decks. if water was left on the main deck and kept continuously accumulating the mass of the vessel would increase, which would increase displacement, raise the centre of gravity and decrease GM and righting lever.

sketch freeing port (check notes)

b) The freeing port maintains stability by quickly allowing large volumes of water to drain away which would otherwise be able to gather on the main ship deck. The effect of this water would increase the mass of the vessel which would increase displacement and draft. Having this water collecting on the upper deck would raise the centre of gravity therefore decreasing metacentric height (GM), consequently the righting lever (GZ) would decrease and this will have a negative impact on the ships stability. Free surface effect could further decrease stability due to the large volume of water accumulating on deck, which could cause a permanent list.

Question 3

a) Describe how the thrust of the propeller is transmitted to the hull of the ship. (4)

b) Describe, with the aid of a sketch, how the surfaces, within the thrust block, transmitting the thrust are separated by the lubricating oil. (8)

c) Explain the importance of the oil temperatures within the thrust block. (4)

Answer 3

a) thrust is created axially by the ships propeller, when the propeller spins axially through the water acting as a hydrofoil that cuts through the water.
suction is created on the back of the propeller blades and pressure on the front part of the blades. high pressure side of propeller generates thrust force that propels the vessel through the water. thrust force is passed down the propeller shaft until it hits the thrust collar on the thrust block. The thrust of the propeller is transmitted axially through a shaft to a heavily reinforced point on the ship’s hull, where a thrust block will then transmit thrust force from the propeller shaft to the ships hull.

draw propeller diagram. (check notes)

b.) Hydrodynamic fluid film wedges separate the thrust pads and thrust collar. The trust pad transmits trust to the lower half of the casing. The lower half of the casing is connected to the ships Hull. The ship moves when the trust is transferred to the ship’s Hull. Oil from the top cover cascades over the kidney pads, bearings and fall back to the sump. The top cover acts as a pad stop. The thrust shaft transmits the thrust onto the thrust collar.

draw Mitchell thrust bock

c.) Maintaining this oil wedge is essential to the performance of the block, it the wedge was to fail the Kidney pads would contact the collar cause sever damage. The oil temperature is critical to maintaining this pressure, to hot the pressure will be low, to cold the pressure will be high.

Question 4

a) Describe the actions that the EOOW should take on finding that the
temperature of the thrust block is rising above normal acceptable range. (6)

b) Explain why the thrust block temperature is critical. (10

Answer 4

a.) firstly contact the bridge and chief engineer to ask to reduce the engine load when its safe to do so.

if thrust block bearing temperature exceeds 75degrees Celsius you should stop the engine when safe to do so to reduce damage dealt to the components.

monitor the lube oil and water level for thrust bearing and fill if necessary.

investigate the surrounding pipes and equipment if the thrust block has jacket pumps incorporated into the system then use them to increase cooling to thrust block.

b.) thrust bearing is very highly loaded and relies on hydrodynamic lubrication there is no forced pressure lubrication. Hydrodynamic lubrication is where lube oil is taken from a lube oil reservoir and the lube oil forms a barrier between running surfaces its formed by motion of moving parts and self generated pressure. For this, hydrodynamic lubrication to be effective temperature needs to maintained in order to maintain barrier between the running surfaces. This barrier will be affected if the temperature is increased because this will reduce viscosity and increase flow rate meaning the barrier will collapse and the running surfaces will come into contact and create friction and cause and wear tear which will damage the thrust block. This will also occur if the temperature decreases because then viscosity increases and flow rate reduces. Additionally thrust block is made from white metal. this metal loses its tensile strength at 120degrees and melts at 180degrees. therefore thrust block temperature must be monitored and controlled by controlling lube oil temperature through the use of LO cooler or using water cooling or there will be severe damage caused.

Similar to sleeve bearings, the performance of fluid film thrust bearings substantially deteriorates as operating temperatures rise. Oil viscosity in the bearing clearance drops drastically due to viscous heating in the tight film gaps. As a result, the oil film thins and is less able to cool the bearing. These factors, in turn, increase the likelihood temperatures will be high enough to soften bearing material and even result in bearing wipe, smearing of the bearing material along the contact surface.
In thrust block, the two basic ways to cool the unit and cut effective running temperature is to increase oil-flow rate and reduce bearing friction.
Some friction within a bearing comes from the viscosity of the lubricant itself. Lowering lubricant viscosity can cut viscous friction and corresponding power loss. But any benefit is usually negated by a drop in oil-film thickness and a higher oil-film shear rate. Still, less-viscous lubricants can improve oil-cooler efficiency and lower friction.
Increasing the volume of oil passing over each pad segment is the most-effective way to lower temperature. incorporate higher oil volume by increasing the taper over a segment, pressurizing oil feed in oil-distributing grooves, and minimizing the bearing outside diameter to keep down net surface velocity. Finally, engineers can directly cool the oil feed or the bearing housing from the outside. Minimizing carryover of hot oil from the trailing edge of one segment into the next segment’s leading edge can be helpful. External cooling fins on the bearing housing lower overall bearing temperature, as does blowing a stream of cooling air at the housing.

Question 5

what are thrust block used for?

Answer 5

Thrust Blocks are used to transmit the ahead and astern thrust from the propeller into vessel motion.
The axial thrust from propeller is transmitted along the rotating shaft into the stationary
thrust block which in turn is strongly connected to the ships hull using hydraulic holding
down bolts.
This bearing uses a form of hydrodynamic lubrication using the motion of the shaft
collar and multiple stationary plates to create an oil wedge.

Question 6

in the event that stern tube bearing temperatures are high what are your actions?

and what are your actions If stern tube temperature does not decrease or rise above 85°C?

Answer 6

In case that high temperatures occur in the stern tube bearing:

Reduce shaft revolutions immediately to Dead Slow. In case protection system is only set up to give an alarm or manual Slow Down, it is of high importance that duty officer immediately reduce rpm on the ME telegraph;

Keep rudder position at mid-ship position as far as possible;

Monitor stern tube bearing temperatures rise, if temperature is stabilizing keep RPM and monitor that temperature is gradually going down.

In case temperature is continuously decreasing, continue with Dead Slow RPM until temperature is stabilized below sea water temperature + 30 ºC;

At above stages never stop the Main Engine, as this could result in the tail shaft being bent due to spot heating of the propeller shaft.

If stern tube temperature does not decrease or rises above 85°C with above procedures, then:

Stop the Main Engine;

Engage the turning gear immediately and start turning of shaft to avoid spot heating of the propeller shaft;

Monitor cooling down of stern Tube;

Turning gear must not be stopped during this process.

Obviously vessel must liaise with the superintendent to coordinate on further actions in case of reduction in shaft revolutions due to abnormal conditions in the stern tube system and above checks are to be initiated subject to safe navigational conditions. If high temperatures have occurred, check the filter in the oil system for impurities from the bearings

Question 7

what alarms are provided for stern tubes?

Answer 7

On some vessels additional alarms and checks are available in order to ensure stern tube safety and proper functioning.

they are;

Temperature rise max. 5 ºC/min ( m/e Slow Down)

ΔT Max differential temp. between SW and S/T temp. ( m/e Slow Down)

Increased monitoring of stern tube bearing temperatures, stern tube seal drains and LO water content during the entire low draft operation.

In general, temperature alarms for stern tube bearings are recommended to be set at:

High Alarm setting 62 ºC
High High Alarm and Slow Down 65 ºC
Other settings may have been applied originally and should only be changed in agreement with the superintendent.

Question 8

how is propeller immersion stopped?

Answer 8

Under normal circumstances in order to avoid propeller immersion issues, the minimum draft aft must be:

Draft required for min. 100% propeller immersion (as per Trim & Stability book) + 0.6 meters.

During navigation in stormy conditions, a ship can think about postponing or eliminating trim optimization altogether, bringing the ship to an even keel instead, or adjusting the trim by the stern as necessary depending on the severity of the weather. If the propulsion shaft system is experiencing an abnormally high amount of vibration, you may want to consider increasing the aft draft in order to reduce the level of vibration.
When the propeller is only partially submerged during operation, this can result in an excessively eccentric force on the propeller and, as a consequence, a downward bending moment on the shaft. Because of this, there is a possibility that the aft bearing will experience increased localized loads (edge loading), as well as surface pressure, as a consequence of the increased relative slope and lower bearing contact area.

In exceptional cases it may not be possible to achieve 100% propeller immersion + 0.6m, for example:

Vessel going in/out of dry-dock

Phasing in/out of a certain trade

Low cargo load

Vessel trading in areas with limiting factor e.g. minimum water depth and/or port restrictions on maximum vessel draft.

In such cases vessel superintendent is to be informed to ensure that appropriate measures are planned, and following risk mitigation measures are put in place:

All options to increase propeller immersion to greater than or min. 100% must be considered, and cargo planner may be contacted if any concerns with ballast intake and/or stress & stability limits.

At propeller immersions between 87% to 100%, the maximum load on main engine should not exceed ME power corresponding to “Half Ahead”.

It must be ensured that all stern tube and intermediate bearing temperature alarms are checked and slow down functions (Manual or Automatic) are tested.

Vessels equipped with ‘Manual Slow Down’ require immediate attention during a high temperature alarm.

Question 9

how does propeller immersion occur?

what is meant by immersion of propeller

and what are the effects of propeller immersion

Answer 9

vessel propeller immersion issues occur due to uneven load of cargo, lack of cargo or impossibility of ballasting/de-ballasting the vessel due to shear forces or bending moments. This is a very serious issue as propeller immersion less than 100% will result in loss of vessel performance, main engine over speeding and stress or damage to vessel machinery.

The immersion of propeller is defined as the ratio of the distance between free surface and propeller blade tip to propeller diameter.

If propeller is not completely immersed, it will result in:

excessive eccentric thrust
increased downward bending moment at the aft end of propeller shaft, leading to higher edge loading of stern tube bearing.
breakage of oil film and ineffective hydrodynamic lubrication in the aft stern tube bearing.
increased shaft system vibrations
increased cavitation of propeller

When propeller and shaft lines are operated outside the design criteria there is a risk of:

Stern tube seal leakage
Increased wear of stern tube bearing
Fatigue failure and subsequent damage of stern tube bearings.
Wear and damage to shaft line bearings
Cavitation and wear of propeller

Question 10

how is stern tube cooling achieved?

Answer 10

Vessel crew must ensure efficient stern tube cooling by always keeping the cooling water tank around the stern tube filled with fresh water. As mentioned above LO water content should be checked regularly due the entire low draft operation, as in case of stern tubes with white metal bearings, water in the lubricating oil can cause severe damage with considerable repair expense and time loss. On the other hand, Wartsila Railko stern tube bearings can work with a limited amount of sea water in the lubrication oil without damage to the bearings.

Question 11

what is the purpose of the stern tube?

Answer 11

The stern tube is a hollow tube which accommodates the bearings, the seal boxes and the propeller shaft. The stern tube is filled with oil, grease or water and forms a barrier between the water outside and the engine room inside the vessel.

Question 12

explain the operation of the OWS

Answer 12

The bilge separator operates automatically and discharges water overboard or back to the bilge water holding tank depending on the oil content of the discharged liquid and separated oil to the waste oil drain tank. Bilge water is drawn from the bilge main by the attached pump and into the bilge separator where it passes, usually through a two-stage separation process. The separator uses the difference in density and surface tension between oil and water in usually two stages that are housed separately or in the same compartment. The separator is initially filled with clean water before admitting bilge water. The pump supplies the oil water mixture to the first stage where most of the oil is retained. Oil droplets are attracted to the coalescer surface or gravity plates, forming into increasingly larger drops until they float. The coalescer has a very large open pore surface area and a very low pressure loss and is stable against suspended matter found in bilge water, hence these particles have no detrimental effect on the coalescer. This means that the coalescer will still continue to operate effectively even with considerable fouling.

Following separation in the first stage, the water, now with a very low oil content, is passed into the second stage chamber, which contains, usually, a second coalescer filter to separate out any remaining oil particles, leaving water that may now be discharged overboard.
A conductive oil/air sensing probe at the top of the first stage (HEC) chamber constantly monitors the oil level in the separator, the length of the probe’s electrode determining the operating range. When oil (or air) is detected, the valve to the oil drain tank opens and the valve to the second stage chamber closes and the oil is discharged to the oil drain tank. The supply pump remains
running during the oil discharge. When most of the oil has been displaced, the oil sensing probe is again immersed in water and activates the control system to resume the separating operation.
The separator works automatically and will operate as long as there is water in the bilge water holding tank. Heating may be applied to improve separation, but the heater will only operate when the separator is full of liquid. The separator is fitted with sampling valves which allow oil samples to be drawn and enable the oil/water interface level to be determined. The Oil Content Discharge monitor samples the bilge water as it passes out of the separator. Should the oil content exceed maximum 15ppm, the three-way valve changes the output flow from the overboard discharge to discharge to the bilge water holding tank. An audible alarm sounds to warn the operator of the alarm condition. The 15ppm device setting can be adjusted from 1ppm up to the maximum 15ppm, but cannot be set higher. The monitor sensing element may be, normally flushed through with fresh water when in operation by moving the supply lever from the SAMPLING to the FLUSHING position.

This action automatically operates the three-way valve on the discharge line and returns the water to the bilge holding tank. Nowadays, the monitor contains a memory card recording the monitor readings for a period of 18 months, after which the data is automatically overwritten. The card is not to be removed from the instrument as it records the following information:
Time;
Date;
Oil content greater than 15 ppm;
Separator status

The oil content monitor must be checked each month and must be flushed through in order to remove any debris which could influence the reading. The maximum flow capacity should not be exceeded, as excess flow will prevent effective separation. The bilge pump suction strainer should be kept clean in order to avoid large solid particles entering the separator, as these will have a detrimental effect on the separation process. It is important to notice that the oily water separator is designed to separate oil from water, not water from oil. Therefore, if the bilge water supply to the separator contains excessive amounts of oil it will render the equipment inoperable and result in unnecessary maintenance. Same, if the separator uses flocculation chemicals, great care must be taken when handling the treatment chemicals, as these substances are caustic and can cause chemical burning on contact with skin and will cause severe damage to eyes. The appropriate protective clothing, including eye protection, must be worn when handling the chemicals.
When operating the oily bilge water separator and the overboard oil monitoring
system, the date, quantity and location of the discharge overboard is to be recorded in the Oil Record Book. All pumping operations and discharges are also to be in accordance with the latest MARPOL Regulations, Annex I, Regulations 9, 10, 11 and 16. The date, operational code and item number needs to be written in appropriate columns and the required particulars should be recorded chronologically in the blank space.For discharges overboard, the ship’s position at the start and end of the discharge should be entered. Each completed operation shall be signed for and dated by the officer or officers in charge of the operation and each completed page must be countersigned by the master of the vessel.

Question 13

what mooring equipment is used on ship?

Answer 13

Winches with various arrangements of barrels are the usual mooring equipment used on board ships. The winch barrel or drum is used for hauling in or letting out the wires or ropes which will fasten the ship to the shore. The warp end is used when moving the ship using ropes or wires fastened to bollards ashore and wrapped around the warp end of the winch. Modern mooring winches are arranged as automatic self-tensioning units. The flow of the tides or changes in draught due to cargo operations may result in tensioning or slackening of mooring wires. To avoid constant attention to the mooring wires the automatic self tensioning arrangement provides for paying out (releasing) or recovering wire when a pre-set tension is not present.

Question 14

what anchor handling equipment is used on ships

Answer 14

The windlass is the usual anchor handling device where one machine may be used to handle both anchors. A more recent development, particularly on larger vessels, is the split windlass where one machine is used for each anchor. The rotating units consist of a cable lifter with shaped snugs to grip the anchor cable, a mooring drum for paying out or letting go of mooring wires and a warp end for warping duties. Each of these units may be separately engaged or disengaged by means of a dog clutch, although the warp end is often driven in association with the mooring drum. A spur gear assembly transmits the motor drive to the shaft where the various dog clutches enable the power take-off. Separate band brakes are fitted to hold the cable lifter and the mooring drum when the power is switched off. The cable lifter unit, is mounted so as to raise and lower the cable from the spurling pipe, which is at the top and centre of the chain or cable locker. Anchor capstans are used in some installations where the cable lifter rotates about a vertical axis. Only the cable lifter unit is located on deck, the driving machinery being on the deck below. A warping end or barrel may be driven by the same unit and is positioned near the cable lifter.

Question 15

what is a stabilising system used for?

Answer 15

There are two basic stabilising systems used on ships—the fin and the tank. A stabilising system is fitted to a ship in order to reduce the rolling motion. This is achieved by providing an opposite force to that attempting to roll the ship.

Question 16

what is a bow thruster?

Answer 16

The bow thruster is a propulsion device fitted to certain types of ships to improve manoeuvrability. The thrust unit consists of a propeller mounted in an athwartships tunnel and provided with some auxiliary drive such as an electric or hydraulic motor. During operation water is forced through the tunnel to push the ship sideways either to port or starboard as required. The unit is normally bridge controlled and is most effective when the vessel is stationary. A servo motor located in the gear housing enables the propeller blade pitch to be altered, to provide water flow in either direction. With this arrangement any non-reversing prime mover, like a single-speed electric motor, may be used. The prime mover need not be stopped during manoeuvring operations since the blades can be placed at zero pitch when no thrust is desired. The drive is obtained through a flexible drive shaft, couplings and bevel gears. Special seals prevent any sea water leakage into the unit. The complete assembly includes part of the athwartships tunnel through which water is directed to provide the thrust.

Question 17

define the term LOA

Answer 17

Length overall (LOA)

The distance from the extreme fore part of the ship to a similar point aft and is the greatest length of the ship. This length is important when docking.

Question 18

define the term LBP

Answer 18

Length between perpendiculars (LBP)

The forward perpendicular is the point at which the summer load waterline crosses the stem. The aft perpendicular is the after side of the rudder post or the centre of the rudder stock if there is no rudder post. The distance between these two points is known as the length between perpendiculars, and is used for some ship calculations.

Question 19

define the term breadth

Answer 19

Breadth

The greatest breadth of the ship, measured to the outside of the shell plating.

Question 20

define the term BMld

Answer 20

Breadth moulded (BMld)

The greatest breadth of the ship, measured to the inside of the inside strakes of shell plating.

Question 21

define the term bulwark

Answer 21

Bulwark

Vertical plating that extends upwards and is fitted around the perimeter of the main deck or weather deck.

Question 22

define the term coaming?

Answer 22

Coaming

Vertical side of the hatch extending from the main deck and forming a structure for the hatch lid to sit upon.

Question 23

define the term DExt

Answer 23

Depth extreme (DExt)

The depth of the ship measured from the underside of the keel to the top of the deck beam at the side of the uppermost continuous deck amidships.

Question 24

define the term DMld

Answer 24

Depth moulded (DMld)

The depth measured from the top of the keel.

Question 25

define the term double bottom

Answer 25

Double bottom Name given to the area that includes the outer hull, girders and stiffeners inside the vessel and a layer of plating to form in effect a double skin. This should not be mixed up with the new form of ‘twin hulled’ tankers. This is due to the double bottom being used as a space for liquid storage and the twin hull arrangement being an empty space.

Question 26

define the term dExt

Answer 26

Draught extreme (dExt) The distance from the bottom of the keel to the waterline. The load draught is the maximum draught to which a vessel may be loaded. This will vary depending upon the service and type of water. (See Chapter 10 for more details.)

Question 27

define the dMld

Answer 27

Draught moulded (dMld) The draught measured from the top of the keel to the waterline.

Question 28

define the term freeboard

Answer 28

Freeboard The distance from the waterline to the top of the deck plating at the side of the deck amidships.

Question 29

define the term camber or round of beam?

Answer 29

Camber or round of beam The transverse curvature of the deck from the centreline down to the sides. This camber is used on exposed decks to drive water to the sides of the ship. Other decks are often cambered. Most modern ships have decks which are flat transversely over the width of the hatch or centre tanks and slope down towards the side of the ship.

Question 30

define the term rise of floor

Answer 30

Rise of floor The bottom shell of a ship is sometimes sloped up from the keel to the bilge to facilitate drainage. This rise of floor is small, 150 mm being usual.

Question 31

define the term sheer

Answer 31

Sheer The curvature of the deck in a fore and aft direction, rising from midships to a maximum at the ends. The sheer forward is usually twice that aft. Sheer on exposed decks makes a ship more seaworthy by raising the deck at the fore and after ends further from the water and by reducing the volume of water coming on the deck.

Question 32

define the term bilge radius

Answer 32

Bilge radius The radius of the arc connecting the side of the ship to the bottom at the midship portion of the ship.

Question 33

define the term bilge keel

Answer 33

Bilge keel A section of plating fixed to the outside of the hull running for the length of the ship protruding at right angles to the bilge radius.

Question 34

what is tumble home?

Answer 34

Tumble home In some ships the midship side shell in the region of the upper deck is curved slightly towards the centre line, thus reducing the width of the upper deck and decks above. Such tumble home improves the appearance of the ship.

Question 35

what is displacement?

Answer 35

Displacement This is a measurement of the mass of the ship and everything it contains when the measurement is taken. The term comes from the amount of water that a ship will ‘displace’ when it is fully floating. Please note that there will be a difference between a ship floating in fresh water and the same ship, loaded exactly the same, floating in salt water. This is due to the difference in density between the salt water and fresh water. Displacement can be calculated as the underwater volume times by the density of the water that the vessel is floating in, times by the value of gravity.

Question 36

define the term lightweight

Answer 36

Lightweight This is a measure of the mass of the empty ship, without stores, fuel, water, crew or their effects. The hull and machinery and all the fixtures and fittings are also included in this measurement.

Question 37

define the term deadweight

Answer 37

Deadweight The deadweight is a measure of the mass that a ship is carrying at a given time. It is the sum of the weight of cargo, fuel, water, stores and people that a ship has on board when the measurement is taken. The deadweight is therefore the difference between the displacement and the lightweight: Displacement = Lightweight + Deadweight It is usual to categorise a vessel by reference to its deadweight. Thus a 10 000 tonne ship is one which is capable of carrying a deadweight of 10 000 tonne.

Question 38

what is uppermost continuous deck or bulkhead deck?

Answer 38

Uppermost continuous deck or bulkhead deck This is one of the most important features of a ship as it makes a watertight seal with the vertical watertight bulkheads. With cargo ships, this deck could also be the same as the freeboard deck. This deck should be provided with the means to close all openings that could be accessed by the sea, thus making a sealed watertight box with the watertight bulkheads. The uppermost continuous deck or bulkhead deck is also taken as the ‘strength’ deck when calculating the girder strength of the vessel

Question 38

what is registered tonnage?

Answer 38

Registered tonnage It is necessary to have an official measurement for ships and in the past the value of the gross registered tonnes (grt) has been used. However there was never a universally agreed standard definition of grt. IMO’s International Convention on Tonnage Measurement of Ships entered into force in July 1982 and as a consequence the two measurements of gross tonnage (gt) and net tonnage (nt) have been agreed upon and are now in universal use for all ships. However, they are not straight forward mathematical calculations. IMO describe ‘gross tonnage’ as a function of the volume of all the internal spaces within the ship. These include the volume of appendages but not volumes that are open to the sea.

Question 39

what is ballast water used for

Answer 39

Ballast water Ballast water has been used since the introduction of steel hulls in order to stabilise the vessel especially when it has little or no cargo. When ballast water treatment system was developed so was the ballast water management convention, which requires every ship to manage their ballast water and sediments to a predetermined standard in accordance with the ballast water management plan. In addition to this all ships are required to carry a ballast water record book.

Question 40

explain what is meant by condition of assignment load line survey?

Answer 40

These are the conditions which must be met before free board is assigned to a ship and load line certificate is issued following a load line survey. Free boards are computed assuming ship to be a completely enclosed and water tight / weather tight envelop. The convention then goes onto recognize the practical need for opening in the ship and prescribes means of protection and closure of such openings. These are called condition of assignment since the assignment of computed free board is conditional upon the prescribed means of protection and closure of openings such as hatchways, doorways, ventilation, air pipes, scuppers, etc. Following are the conditions which must be met before assigning the load line. 1.Enough structural strength should be possessed. 2.Enough reserve buoyancy should be possessed. 3.Safety and protection of crew. 4.Prevent entry of water through hull. Ships to be surveyed annually to ensure that they fulfill the condition of assignment. Most of the condition of assignment are concerned with the water tight integrity of the ship. Hull construction should meet the highest standard laid down by the classification society. This ensures protection against flooding of the ship. The superstructure and bulkheads must be strengthened sufficiently. Some of the condition of assignment which contribute towards water tight integrity are: 1.Hatchways 2.Machinery space openings 3.Details of opening in free board 4.Details of opening in superstructure deck 5.Ventilators 6.Cargo ports 7.Air pipes 8.Scuppers 9.Side scuttles 10.Inlet and discharges All the above parameters ensures water tight integrity and protection against flooding of compartment. If above are not water tight then during rough weather water can enter into the areas below main deck causing to reduce the free board. So, condition of assignment very much contributes towards water integrity of the ship. Also if green sea effect is not reduced and water is being accumulated on the deck, it can cause free board to reduce and add free surface effect. In rough weather if any longitudinal or transverse girder give way it can cause structural failure and water can enter area below main deck.

Question 41

what force act on a ships structure?

Answer 41

A number of forces act on a ship’s structure; some are static forces while others are dynamic. The static forces are set up due to the differences in weight and support along the length of the ship, while the dynamic forces are created by the force of the wind pushing on the ship, as well as the water interacting with the ship, by the passage of waves along the ship and by the moving propulsion parts.

Question 42

explain the overall design concept of a ship

Answer 42

A ship may be regarded as a non-uniform beam, carrying both uniformly distributed and non-uniformly distributed loads and having varying degrees of support along its length. Some of the load will be distributed evenly over a section of the ship while some will be more concentrated. The overall strength of the beam is referred to as the ‘girder strength’ and the overall bending moment envelope curves are used to calculate the required girder strength for all circumstances that the vessel is likely to encounter. Then the internal structures and the hull plating are sized and arranged to meet the minimum girder strength required for the design and duty of the vessel. Values for the still water bending moments, under all conditions of operation, are calculated. The results of all the bending moments give the overall bending moment for the hull girder, and this divided by the maximum design stress allowed by the classification society, will give the strength modulus required for the hull girder steel sections. The components designed to resist the buckling of the girder and contributing to the ultimate hull strength are the: *size, number and strength of longitudinal beams *thickness and strength of the shell plating *bilge keels *quality of welding *number and strength of the transverse sections. The ultimate hull girder strength for tankers, bulk carriers and now container ships is determined using a system known as the iterative-incremental method, which is where the hull girder is divided into a set of transverse elements. The forces are calculated in one element and the resultant strain (for any one given condition) is used to modify the stress calculations on the next connecting transverse element. This is repeated until the final strength profile for the hull girder has been calculated. For the hull girder, to keep the ‘box section’ in shape there needs to be a combination of beams running along the length of the box and a series of shapes designed to maintain the square shape of the box. The way that these two are combined will determine the overall design of the vessel.

Question 43

what are the different system of framing used for a ship

Answer 43

There are two different systems of framing, called longitudinal framing and transverse framing. As the two names suggest, longitudinal framing is where the main load carrying sections of steel run parallel to the sides of the vessel. Transverse framing on the other hand, has the framing arranged at right angles to the sides of the ship. These frames are then supported by girders, or stringers, running in parallel with the sides of the vessel. Not only can the overall vessel be of one or the other or indeed a combination of both, but so can smaller parts of the overall vessel, with respect to hatch openings for example.

Question 44

with the aid of a cross section of a ship drawing explain how and why the transverse structure is designed and with reference to transverse bending?

Answer 44

see figure 2.7 The transverse structure may be subjected to different types of loading, such as the weight of the ship’s structure, machinery, fuel, water and cargo due to the water pressure causing longitudinal bending. The decks must be designed to support the weight of accommodation, winches and cargo, while exposed decks may also have to withstand a tremendous weight of water that might be taken on board during heavy weather. The deck plating is connected to beams which transmit the loads to the longitudinal girders and to the side frames. In the area of heavy local loads such as cranes and windlasses and so on, additional stiffening will be required. The shell plating and frames form pillars which support the additional weights that are situated on the deck. Tank tops are required to be strong enough to keep the cargo in place or resist the upthrust exerted by the liquid in the tanks. In the machinery space other factors must be taken into account. Fluctuating forces transmitted from the reciprocating machinery through to the structure need to be accommodated. Modern resilient mountings reduce the magnitude of the forces and the strength of the fixings means that the machinery is well supported to prevent any excess movement. Under the position of the engine additional girders are fitted in the double bottom and the thickness of the tank top increased to ensure that the main propulsion remains fixed despite the additional stresses caused by rough weather acting upon the vessel. Special consideration must be given to the thrust block, the propeller shaft and the propeller. Thrust to push the ship along is generated by the propeller and must be carefully transmitted to the hull of the vessel. This is a difficult process as the propeller shaft is relatively small in diameter when compared with the area of the hull. The thrust block is first in line to take the force from the propeller shaft. The important issue is for the thrust block to be connected to as large an area of the hull as possible. This will transmit the force generated by the thrust to the hull evenly. The weight of the propeller shaft is supported by intermediate bearings which in turn must be supported by the vessel’s hull structure. The arrangement of the stern of the vessel needs to counter the forces transmitted through the stern tube bearing. These forces will mostly be the weight of the propeller which will be acting at the end of the shaft. Any out of balance forces will have a significant effect as will any vibration caused by cavitation. A considerable force is exerted on the bottom and side shell by the water surrounding the ship and the double bottom floors and side frames are designed to withstand these forces, while the shell plating must be thick enough to prevent buckling as it spans the distance between the floors and frames. Since water pressure increases with the depth of immersion, the load on the bottom shell plating will be greater than the load on the side shell, therefore the bottom shell must be thicker than the side shell to withstand the increased force. When the ship passes through waves, these forces are of a pulsating nature and may vary considerably in high waves, while in bad weather conditions the shell plating above the waterline will receive severe hammering.

Question 45

with the aid of a sketch explain what is meant by racking and how its prevented

Answer 45

see figure 2.8 When a ship rolls there is a tendency for the ship to distort transversely due to the fluctuating forces. Due to the fluctuating forces the vessel structure distorts transversely and this causes deformation, the deck moves laterally relative to the bottom structure and shell on one side moves vertically relative to the other side, this action is known as racking and is reduced or prevented by the beam knee and tank side bracket connections, together with the transverse bulkheads, the latter having the greatest effect. Transverse bulkheads divide the ship from side to side and are habitually used to create watertight compartments on the vessel. Additionally, they stiffen the structure of the hull, preventing deformation and racking stresses The stress mainly affects the corners of the ship, i.e., on the tank side brackets and the beam knees, which must be made strong enough to resist it. Transverse bulkheads, frames and web frames provide very great strength to resist racking.

Question 46

what are deck air vents? explain their purpose

Answer 46

Deck air vents are devices that allow the passage of air in and out of the tanks onboard vessels, such as cargo holds, ballast tanks, fuel tanks, and fresh water tanks. They are essential for maintaining the pressure balance, the quality of the cargo or fluid, and the safety of the vessel and crew. The Purpose of Deck Air Vents Deck air vents are strategically placed openings on the deck of a vessel, each serving a specific purpose for various onboard tanks. Their primary functions include: Preventing Overpressure: One of the most critical roles of deck air vents is to prevent the overpressure of tanks. When a tank is loaded or unloaded, it undergoes changes in volume due to temperature fluctuations and the addition or removal of liquid cargo. Without proper venting, pressure imbalances can develop within the tank, leading to structural damage or even catastrophic failure. Deck air vents provide a controlled release of excess pressure to ensure the tank’s integrity. Minimizing Vacuum Conditions: During the discharge of liquid cargo, especially in tanks like ballast or cargo tanks, a vacuum can form as the liquid is pumped out. This vacuum can potentially collapse the tank structure if not relieved. Deck air vents allow air to enter the tank, equalizing the pressure and preventing collapse. Reducing Gas Build-up: Certain tanks, like fuel oil or sewage tanks, may produce gases or vapours that need to be vented to prevent the build-up of hazardous conditions or explosions. Deck air vents allow these gases to dissipate safely into the atmosphere. Maintaining Tank Integrity: Proper ventilation helps to reduce the corrosion of tank internals caused by moisture accumulation. It also minimizes the risk of contamination, ensuring the quality and safety of stored liquids.

Question 47

what are some common issues that occur on deck air vents and how to fix them?

Answer 47

deck air vents common issues and rectifcations : Blockages: If you suspect a blockage, inspect the vent for debris or obstructions. Remove any foreign materials and clean the vent thoroughly. Leakage: If you notice leaks, inspect the sealing gaskets and connections. Replace any damaged components and ensure a tight seal. Inoperative Valves: If pressure relief or vacuum-breaking valves fail to operate, consult the manufacturer’s guidelines for maintenance or replacement instructions.

Question 48

what kind of maintenance is carried out on deck air vents

Answer 48

To ensure the effectiveness of deck air vents, regular maintenance and inspections are essential. Maintenance tasks: Clean and Clear Vents: Deck air vents should be cleaned periodically to remove any dirt, dust, salt, rust, or debris that may accumulate inside them. This can be done by using compressed air, water jets, brushes, or solvents. Cleaning should be done more frequently for cargo hold vents that handle dusty or dirty cargoes Functional Valves: If equipped with pressure/vacuum relief valves, make sure they are in good working condition. Replace any malfunctioning valves promptly. Leak Checks: Inspect for leaks around the vent openings. Leaking vents can compromise the tank’s integrity and should be repaired immediately. Deck air vents that are damaged, corroded, leaking, blocked, or malfunctioning should be repaired as soon as possible to restore their normal operation and prevent further deterioration. This can be done by replacing worn-out parts, welding cracks, sealing leaks, clearing obstructions, or adjusting settings. Repairing should be done by qualified personnel following the manufacturer’s instructions and safety precautions. Corrosion Prevention: Apply appropriate anti-corrosion coatings to vent openings and surrounding areas to protect against corrosion. Deck air vents that have moving parts, such as valves, springs, hinges, or flaps, should be lubricated regularly to ensure smooth operation and prevent seizing or jamming. This can be done by using grease, oil, or spray lubricants. Lubricating should be done more frequently for deck air vents that are exposed to salt water spray or humid conditions. Operational Testing: Deck air vents should be tested periodically to check their performance and functionality. Testing should be done more frequently for deck air vents that handle hazardous cargoes or fluids. Periodically test the pressure relief and vacuum-breaking functions of the vents to ensure they are functioning as designed.

Question 49

why is it it important that deck air vents operate properly

Answer 49

Deck air vents are important for ensuring the safety and efficiency of the vessel and its cargo. They help to: Prevent damage to the cargo: By ventilating the cargo holds properly, deck air vents prevent moisture condensation, heating, gas emission, odour generation, or tainting that can affect the quality and integrity of the cargo. Prevent damage to the vessel: By maintaining the pressure balance in the tanks, deck air vents prevent overpressure or vacuum that can cause structural damage to the tank walls or hull deformation. Prevent fire or explosion: By removing hazardous gases from the cargo holds or fuel tanks, deck air vents prevent fire or explosion that can endanger the vessel and crew. Prevent asphyxiation or poisoning: By removing hazardous gases from the cargo holds or fresh water tanks, deck air vents prevent asphyxiation or poisoning of the crew or other personnel who may enter the tanks. Prevent contamination: By preventing seawater, fuel, dust, insects, or bacteria from entering the tanks through the vent pipes, deck air vents prevent contamination of the ballast water, fuel, or fresh water. On the other hand, deck air vents also pose some risks if they are not operated or maintained properly. Some common risks include: Cargo damage: If deck air vents are not opened or closed at the right time or frequency, they may cause cargo damage due to excessive or insufficient ventilation. For example, if deck air vents are opened too frequently or for too long, they may cause cargo sweat or ship sweat by introducing moist air into the cargo holds. If deck air vents are closed too early or for too long, they may cause cargo heating or gas accumulation by trapping warm air or gas inside the cargo holds. Vessel damage: If deck air vents are not opened or closed properly during ballasting or de-ballasting operations, they may cause vessel damage due to overpressure or vacuum in the ballast tanks. For example, if deck air vents are not opened sufficiently during deballasting, they may cause vacuum in the ballast tanks that can suck in seawater through the vent pipes. If deck air vents are not closed sufficiently during ballasting, they may cause overpressure in the ballast tanks that can blow out seawater through the vent pipes. Fire or explosion: If deck air vents are not closed properly when handling hazardous cargoes or fluids, they may cause fire or explosion due to ignition of flammable gases. For example, if deck air vents are not closed tightly when loading or unloading coal, they may allow oxygen to enter the cargo holds and ignite the coal dust. If deck air vents are not closed securely when re-fuelling or transferring fuel, they may allow fuel vapour to escape and ignite by sparks or static electricity. Asphyxiation or poisoning: If deck air vents are not opened properly when entering confined spaces, they may cause asphyxiation or poisoning due to lack of oxygen or presence of toxic gases. For example, if deck air vents are not opened sufficiently before entering a cargo hold that contains carbon dioxide, carbon monoxide, methane, or hydrogen, they may cause asphyxiation or poisoning of the personnel who enter the hold. If deck air vents are not opened adequately before entering a fresh water tank that contains bacteria, they may cause poisoning of the personnel who enter the tank. Contamination: If deck air vents are not fitted with proper filters, screens, covers, or caps, they may cause contamination of the tanks due to ingress of foreign substances. For example, if deck air vents are not fitted with filters that can remove dust particles from the air, they may cause contamination of the fresh water tanks by introducing dust into the water. If deck air vents are not fitted with screens that can prevent insects from entering the vent pipes, they may cause contamination of the fresh water tanks by introducing insects into the water.

Question 50

with the aid of sketches explain how a ship enters dry dock

Answer 50

see figure 2.9 and 2.10 Dry-docking A ship usually enters dry dock with a slight trim aft. This means that as the water is pumped out, the after end touches the blocks first. As more water is pumped out an upthrust is exerted by the blocks on the after end, causing the ship to change trim until the whole keel from forward to aft rests on the centre blocks. At the instant before this occurs the upthrust aft is at maximum. If the design of the ship results in this thrust being excessive, it may be necessary to strengthen the after blocks and the after end of the ship. Such a problem arises if it is necessary to dock a ship when fully loaded or when trimming severely by the stern. As the pumping continues the load on the keel blocks is increased until the whole weight of the ship is taken by the blocks in the dry dock. The ship structure must be strong enough to withstand this unevenly distributed load. The ‘docking’ plan, carefully worked out before the ship arrives, ensures that the blocks are all placed in the correct position. The strength of the hull is carefully considered during the design of the ship and there could be up to three different docking arrangements specified for each vessel. Different systems can be used on subsequent dry docks so that the spaces not examined at the last docking can be covered during the current one. It will also be obvious to students that the hull cannot be prepared and painted in the area in contact with the blocks. In most ships the normal arrangement of keel and centre girder, together with the transverse floors, is quite sufficient for the purpose. If a duct keel is fitted, however, care must be taken to ensure that the width of the duct does not exceed the width of the keel blocks (Figure 2.9). The keel structure of a longitudinally stressed vessel such as an oil tanker, bulker or container ship is strengthened by fitting docking brackets and tying the centre girder to the adjacent longitudinal frames at intervals of about 1.5 m. Bilge blocks or shores could be fitted to support the sides of the ship. The arrangements of the bilge blocks vary from dock to dock. In some cases they are fitted after the water is pumped out of the dock, while other dry docks may have blocks which can be slid into place while the water is still in the dock. The latter arrangement is preferable since the sides are completely supported. At the ends of the ship, where the curvature of the shell does not permit blocks to be fitted, bilge shores are used. The structure at the bilge must prevent these shores and blocks buckling the shell.As soon as the after end touches the blocks, shores are inserted between the stern and the dock side to centralise the ship in the dock and to prevent the ship slipping off the blocks. When the ship grounds along its whole length additional shores could be fitted on both sides, holding the ship in position and preventing tipping. These shores are known as breast shores and have some slight effect in preventing the side shell bulging. They should preferably be placed with respect to transverse bulkheads or side frames as these offer more resistance to buckling than the side placed do on their own (Figure 2.10). On the larger vessels the side supports are not necessary as the ship sits safely on the blocks situated on the floor of the dock. When undocking the vessel, care must be taken to ensure that all the ballast, fresh water and fuel oil tanks are in the same condition as they were when the vessel came into the dry dock. It is usual to start filling the dock and then, when the ship’s side valves are just covered, stop the filling to check for any leaks.

Question 51

what is pounding?

Answer 51

Pounding When a ship meets heavy weather and commences heaving and pitching, the rise of the fore end of the ship occasionally synchronises with the trough of a wave. The fore end then emerges from the water and re-enters with a tremendous slamming effect, known as pounding. While the event does not occur with great regularity, it may nevertheless cause damage to the bottom of the ship at the forward end. The designers must ensure that the shell plating is stiffened to prevent buckling due to the forces involved in the pounding. Pounding also affects the aft end section of the vessel but the effects are not nearly as great. Nevertheless, provision must be made in the design of the hull to counteract the effects of pounding at the aft end.

Question 52

what is panting?

Answer 52

Panting As waves pass along the length of a ship the various parts of the vessel are subjected to varying depths of water which causes fluctuations in water pressure. This tends to create an in-and-out movement of the shell plating. The knock-on effect of this is found to be greatest at the ends of the ship, particularly at the fore end, where the shell is relatively flat. Such movements are termed panting, and, if unrestricted, panting could eventually lead to fatigue of the material and must therefore be prevented as much as possible. This is achieved by the structure at the ends of the ship being stiffened to prevent any undue movement of the shell plating.

Question 53

when are stresses and bending moments calculated?

Answer 53

Highly sophisticated computer tracking systems monitor the movement of containers as they are transported around the world. Therefore, ships will have their loading and discharging sequencing calculated before the vessel reaches port and stresses and bending moments will be calculated while the vessel is moving between ports.

Question 54

with the aid of sketches explain what effect does wave bending/dynamic loading have on the bending moment?

Answer 54

see figure 2.5 and 2.6 When a ship passes through waves, alterations in the distribution of buoyancy cause alterations in the bending moment. The greatest differences occur when a ship passes through waves whose lengths from crest to crest are equal to the length of the ship thereby placing the greatest bending moment on the hull. When the wave crest is amidships (Figure 2.5), the buoyancy amidships is increased while at the ends it is reduced. This tends to cause the ship to hog. A few seconds later the wave trough lies amidships. The buoyancy amidships is reduced while at the ends it is increased, causing the vessel to sag (Figure 2.6). The effect of these waves is to cause fluctuations in stress, or, in extreme cases, complete reversals of stress every few seconds. The ship is designed to withstand this cycle of stressing without causing undue damage. However, if any part of the structure has already been damaged or is corroded, then the hogging and sagging will make the weakened structure worse.

Question 55

with the aid of a sketch describe how a vessel withstands longitudinal bending?

Answer 55

see figure 2.4 The structure resisting longitudinal bending is made up of all the continuous longitudinal material. The features farthest from the axis of bending (the neutral axis) are the most important (Figure 2.4). These features are the: *keel *bottom shell plating *centre girder *side girders *tank top *tank margin *side shell *sheer strake *stringer plate *deck plating alongside hatches *and in the case of oil tankers, any longitudinal bulkheads. Buckling and/or deformation may occur at a point in the structure that is the greatest distance from the neutral axis which will become a high stress point, such as the top of a sheer strake, such points are avoided as far as possible, since they may result in the plate cracking. The greatest stresses set up in the ship as a whole are due to the distribution of loads along the ship, causing longitudinal bending. The efficiency of the ship structure in withstanding longitudinal bending depends to a large extent on its girder strength and the ability of the transverse structure to prevent the buckling of the shell plating and decks. Some ships are required extra structure in order to withstand stresses like In many oil tankers the structure is improved by joining the sheerstrake and stringer plate to form a rounded gunwale.

Question 56

with the aid of sketches explain how load is distributed for a ship with reference to still water bending/static loading explain how hogging and sagging occurs?

Answer 56

Still water bending – static loading If we consider a loaded ship lying in still water, then the upthrust at any 1 m length of the ship depends upon the immersed cross-sectional area of the ship at that point. If the values of upthrust at different positions along the length of the ship are plotted on a base representing the ship’s length, a buoyancy curve is formed (Figure 2.1).This curve increases from zero to a maximum value in the midship portion, then decreases back down to zero. The area of this curve represents the total up-thrust exerted by the water on the ship. The total weight of a ship consists of a number of independent weights concentrated over short lengths of the ship. These include; cargo, machinery, accommodation, cargo handling gear, poop and forecastle sections of the hull construction, and a number of items which form continuous material over the length of the ship, such as decks, shell and tank top. A curve of weights is shown in Figure 2.1. The difference between the weight and buoyancy at any point is the load at that point. In some cases the load is in excess of weight over buoyancy and in other cases there is an excess of buoyancy over weight. A load diagram formed by these differences is shown in the figure. Since the total weight must be equal to the total buoyancy (assuming that the vessel is still floating), the area of the load diagram above the base line must be equal to the area below the base line. Due to the unequal loading, however, shearing forces and bending moments are set up in the ship with the maximum bending moment occurring around the midship section. The load distribution will determine the direction in which the bending moment will act, and this in turn will create the state of hogging or sagging. Class terminology for the condition of hogging and sagging in the bending moment calculations is to go negative for sagging and positive for hogging. If, for example, the buoyancy amidships exceeds the weight, the ship will hog, and this may be likened to a beam supported at the centre and loaded at the ends. As with a simply supported beam, when a ship hogs, the deck structure is in tension while the bottom plating is in compression (Figure 2.2). If the weight amidships exceeds the buoyancy, the ship will sag, which is equivalent to a beam supported at its ends and loaded at the centre. When a ship sags, the bottom shell is in tension while the deck is in compression (Figure 2.3). Students will be able to appreciate that when a hull is continuously changing between hogging and sagging, as in a rough sea, considerable cyclical stresses happen in the deck and the bottom shell plating.Changes in bending moments also occur in a ship due to different loading conditions. This is particularly true in the case of cargoes such as iron ore which are heavy compared with the volume they occupy. When these types of cargo are loaded into a ship, especially if it is on the spot market or performing the role of a tramp ship, care must be taken to ensure a suitable distribution throughout the ship. The even distribution of stresses is calculated by using the on-board loading computer. In the past these calculations have proved difficult especially if the ship has a machinery space and deep tanks/cargo holds amidships. These older vessels would also have had only a very basic method of calculating the bending moment. There would however be a tendency in such ships, when loading heavy cargoes, to leave the deep tank empty. This results in an excess of buoyancy by way of the deep tank. This action must be considered carefully as there could also be an excess of buoyancy by way of the engine room, since the machinery (especially if large two stroke engines are fitted) might be light when compared with the volume it occupies. The International Association of Classification Societies (IACS) sets out guidance for the loading sequences in its rules for the ‘Strength of Ships’. Careful consideration should also be given to the sequencing of loading and using bunker fuel as well as the filling or emptying of the ballast tanks. A ship loaded carelessly, might hog considerably, creating unusually high stresses in the deck and bottom shell. This may be very dangerous and could lead to the vessel breaking in two if loaded using an incorrect sequence. If the owners intend for the ships to be regularly loaded in this manner, additional hull strength must be provided to ensure the safe operation of the vessel. In cases where there is a long transmission shaft between the main engine and the propeller, excess hogging or sagging could also lead to excessive bending of this shaft and the engineering staff would continually be checking for any overheating of the main shaft bearings.

Question 57

what is minimum freeboard?

Answer 57

The minimum freeboard is based on providing the vessel with a volume of reserve buoyancy which cannot be loaded with cargo and therefore may be regarded as making the ship safe and ensuring that the ship proceeds to sea in a stable condition.

Question 58

define freeboard?

Answer 58

Freeboard is the distance from the waterline to the top of the deck plating at the side of the freeboard deck amidships.

Question 59

define free board deck

Answer 59

The freeboard deck is the uppermost continuous deck (also known as the ’bulkhead deck’) that has the necessary equipment to close all openings to the outside weather.

Question 59

the exact level of ‘reserve buoyancy’ required depends upon several factors: What are these factors?

Answer 59

*conditions of service of the ship *type of vessel *stability of the vessel in still water *degree of subdivision after suffering ‘prescribed damage’ *safety of the ship’s staff when out on deck *ability of the vessel to protect the weather deck from taking on water *fixtures and fittings used to allow any ‘shipped’ water to be removed.

Question 60

what is reserve buoyancy?

Answer 60

Reserve buoyancy is the volume equivalent of freeboard. Its the difference between the volume of a hull below the designed waterline and the volume of the hull below the lowest opening incapable of being made watertight. Reserve buoyancy is a very crucial yardstick for the seaworthiness of a vessel. The extent of the reserve buoyancy determines the safety limit or margin before the vessel can sink. The greater the reserve buoyancy, the safer the vessel is regarding sinkage, and vice-versa. In deep sea ships, for example, sufficient reserve buoyancy must be provided to enable the vessel to rise up again when shipping the heavy seas that could be encountered in the oceans of the world, small vessels on the inland waterways will not encounter such conditions and therefore are allowed to sail with a different level of ‘reserve buoyancy’.

Question 61

with reference to load line regulation why is bow height required?

Answer 61

One point to consider is the likelihood of water coming onto the fore deck. This is largely a function of the distance of the fore end of the deck from the waterline and for this reason a minimum bow height is stipulated. The bow height required depends upon the length of the ship and the block coefficient and may be measured to the forecastle deck if the forecastle is 7% or more of the ship’s length. Should the bow height be less than the minimum then either the freeboard is increased or the deck raised by increasing the sheer or fitting a forecastle.

Question 62

function of forecastle?

Answer 62

the forward part of the upper deck of a ship, a superstructure at or immediately aft of the bow of a vessel, used as a shelter for stores, machinery, The function of the forecastle has been brought into sharp focus recently as some bulk carriers suffered weather damage to the forward hatch coaming due to the lack of protection offered by the inadequate forecastle.

Question 63

how is freeboard calculated?

Answer 63

Vessels conforming to the Load Line Rules are assigned a freeboard according to a table of values, and this is termed the tabular freeboard. Its like the starting point used to calculate the actual freeboard, there are two set of tables split into TYPE A ship and TYPE B ship. Type A ships are designed to carry only liquid cargoes and hence have a high integrity of exposed deck, together with excellent subdivision of the cargo space. The hatches are small openings and are oil/water tight, and heavy seas are unlikely to cause flooding of the cargo space or the accommodation. As a result, these vessels are allowed to load to a comparatively deep draught than the type B ships.While these ships have a high standard of watertight deck, they have a comparatively small volume of reserve buoyancy and may therefore be less safe if damaged. It is necessary, therefore, in all such vessels over 150 m in length, to investigate the effect of damaging the underwater part of the cargo space and, in longer ships, the engine room. Under such conditions the vessel must remain afloat without excessive heel and have positive stability. Type B ships cover the remaining types of vessels and are assumed to be fitted with steel hatch covers. In older ships having wood covers the freeboard is increased.Should the hatch covers in Type B ships be sealed with efficient securing arrangements, then their improved water tight integrity is rewarded by a reduction in freeboard of up to 60% of the difference between the Type A and Type B tabular freeboards. If, in addition, the vessel satisfies the remaining conditions for a Type A ship (e.g. flooding of cargo spaces and engine room), 100% of the difference is allowed and the vessel may be regarded as a Type A ship. The tabular freeboards for Types A and B ships are given in the Rules for lengths of ship varying between 24 m and 365 m. each table used has various values dependant upon the type of ship and its length and is based on a standard vessel having a block coefficient of 0.68, length ÷ depth of 15 and a standard sheer curve. Once a value is chosen the freeboard, is then calculated from the tabular freeboard and Corrections are then made to this value for any variation from the standard, together with deductions for the reserve buoyancy provided by weather tight superstructures on the freeboard deck.

Question 64

where are freeboard markings located? what effect does timber have on reserve buoyancy?

Answer 64

The freeboard markings (Figure 10.1) are cut into the shell plating with the centre of the circle at midships. Special provision is made in the Rules for vessels carrying timber as a deck cargo. The timber increases the reserve buoyancy and hence the vessels are allowed to float at deeper draughts. An additional set of freeboard markings is cut in aft of midships with the normal letters prefixed by L (lumber).

Question 65

how stresses occur on a ship?

Answer 65

The modern ship is made up steel plating, section and builds up girders so connected as to provide adequate strength in all parts to withstand the forces acting on the ship under all condition of service. The forces acting on a ship may be static or dynamic. The static forces are due to the difference in the weight and buoyancy, which occur through out the ship. The dynamic forces are cause by the motion of the ship at sea and the action of the wind and wave. These forces create:  1.Longitudinal stress 2.Transverse stress 3.Local stress The greatest stress set in the ship as wholes are due to the distribution of load along the ship, causing longitudinal bending.

Question 66

what is hogging and what is sagging?

Answer 66

Hogging If the buoyancy amidships exceed the weight due to loading or when the wave crest is amidships, the ship will Hog, as a beam supported at mid length and loaded at the end. Sagging If the weight amidships exceed the buoyancy or when the wave trough amidships the ship will sag, as a beam supported at a ends and loaded at mid length.

Question 67

what is transverse stress

Answer 67

Occur when transverse section of amidships is subjected to static pressure due to the surrounding water as well as  internal loading due to the weight of the  structure, cargo, etc. The parts of the structure, which resist transverse, are 1.Transverse bulkhead. 2.Floor in the double bottom. 3.Bracket between deck beam and side frame, together with bracket between side frame and tank top plating, or margin plate . 4.The pillars in hole and tween deck.

Question 68

how do local stresses occur

Answer 68

created by: 1.Heavy concentrated load like boiler, engine etc. 2.Dead cargo such as timber 3.Hull vibration 4.Ship resting on block on a dry dock (Static Stress)

Question 69

describe with the aid of a sketch how dynamic forces occur?

Answer 69

Dynamic Forces The dynamic forces arise from the motion of the ship itself. Due to the action of the waves on the ship the following motions occur 1.Surging: The forward and aft linear motion (along x) of a ship is called surging. 2.Heaving: The vertical up and down linear motion (along y) of a ship is called heaving. 3.Swaying: The side to side linear motion (along z) of a ship is called swaying. 4.Rolling: The rotational motion of a ship about longitudinal axis is called rolling. 5.Yawing: The rotational motion of a ship about vertical axis is called yawing. 6.Pitching: The rotational motion of a ship about transverse axis is called pitching. When the ship motions are large particularly in pitching and heaving, considerable dynamic forces are created in the structure.

Question 70

what is panting?

Answer 70

Panting As wave passes along the ship they cause fluctuation in water pressure which tends to create in and out movement of the shell plating. This is particularly the case at the fore end. The rules of the classification societies required extra stiffening, at the end of the ship, in the form of beams, brackets, stringer plate, etc. in order to reduce the possibility of damage. This in and out movement is called panting.

Question 71

what is slamming or pounding?

Answer 71

Slamming or Pounding In heavy weather when the ship is heaving and pitching, the fore end emerges from the water and reenters with a slamming effect which is called  pounding. Extra stiffening require at the fore end to reduce the possibility of damage.

Question 72

what is apparent slip?

Answer 72

Apparent Slip Since the propeller work in water, the ship speed Velocity will normally be less than the theoretical speed. The difference between the two speeds is known as Apparent slip and is usually express as a ratio or percentage of the theoretical speed.

Question 73

what is real slip?

Answer 73

Real slip or true slip This is the difference between the theoretical speed and the speed of advance, express as a ratio or percentage of the theoretical speed. The real slip is always positive and it dependant of current.

Question 74

what is meant by load line?

Answer 74

The load line is a term given to a mark located amidships on both sides of a ship to show the limiting draught to which the vessel may be loaded. This limiting draught is obtained by measuring from the uppermost continuous weather tight deck (normally the freeboard deck) down to the load line mark amidships. This distance is called the freeboard of the ship.

Question 75

what is a pilgrim nut?

Answer 75

Pilgrim nut The pilgrim nut provides a predetermined grip between the propeller and its shaft. The propeller boss is fitted with a S.G cast iron internally tapered sleeve, which is secured (fixed firmly in position) into the boss. This sleeve is bedded to the shaft cone before mounting in the boss so that better fit is achieved which, combined with the pilgrim nut push up, ensure a good friction grip. No key is required. The pilgrim nut is a threaded hydraulic jack, which screwed on to the tailshaft. A steel ring receives thrust from a hydraulically pressurised nitrile rubber tyre. This thrust is applied to the propeller to force it onto the taper sleeve. Propeller removal is achieved by reversing the pilgrim nut and using a withdrawal plate, which is fastened to the propeller boss by studs. When the tyre is pressurised the propeller is drawn off the taper.

Question 76

what pressure tests are done for cargo tanks?

Answer 76

1.A structural test by testing with water to a height of 2.45 meters above the tank crown, or 2.A leak test consisting of a soapy solution test while the tank is subjected to an air pressure of 0.14 bar. It is recommended that the air pressure is initially raised to 0.21 bar and then lowered to the above test pressure before inspection is carried out.

Question 77

Difference between Stiff ship and Tender ship?

Answer 77

Stiff Ship 1.Greater GM due to high density cargo on bottom 2.Ship rolls very fast 3.Very  uncomfortable Tender Ship 1.Small GM (but not negative ) due to loading on top 2.Ship rolls very slowly 3.Uncomfortable but better than stiff ship

Question 78

What are water tight door requirements ?

Answer 78

1.The door may be either vertical or horizontal sliding 2.The means of closing the door must be positive i.e.. They must not rely on gravity or a dropping weight  3.They capable for operating with a list of 15 degree and to be capable of being quickly closed form an accessible position above the bulkhead deck 4.It must be operated from the vicinity of the door in addition to a point above the bulk head deck  5.If no power is available in hydraulically operated system, the door may be closed and opened by manual operated pump 6.Must have an index at the operating position showing whether the door is opened or closed.

Question 79

describe the theory of rudder for a ship?

Answer 79

Theory of Rudder on Ships The rudder is used to steer the ship. The turning action is largely dependent on the area of the rudder. The required area of the rudder varies with different type of vessels since desired maneuvering ability differs considerably and the general ship design may imposed restriction. In practice the rudder area is usually relative to the area of the immersed metal plane. The ratio of the depth to width of a rudder  is known as the aspect ratio and its value is generally 2. High aspect ratio is used in large vessels, where depth is not a constraint. Higher aspect ratio reduces the astern torque considerably.  The force on the rudder depend on: 1.Area of the rudder 2.The form of rudder 3.The speed of the ship 4.The angle of helm Rudder may be hinged on the pintles and gudgeon, or the may turn about an axle which passes down through the rudder. The weight of rudder may be taken by bearing pintles, or by a bearing at the rudder head (rudder carrier), or by a combination of both.

Question 80

what are the different types of rudder?

Answer 80

Balanced rudder When 20% to 37% of the area is forward of the turning axis there is no torque on the rudder stock at certain angles. At some angle of rudder, it is balanced. i.e., torque is zero, to keep rudder at that angle. Axis of rotation lies between 0.2 L and 0.37 L. Semi-balanced rudder A rudder with a small part of its area, less than 20%, forward of the turning axis. At no angle rudder is balanced. Axis of rotation lies less than 0.2 L. Unbalanced rudder A rudder with all of its area aft of the turning axis. At no angle rudder is balanced. Axis of rotation is the leading edge.

Question 81

describe the the construction of a rudder?

Answer 81

Modern rudders are of stream lined form and are fabricated from steel plate, the plate size being stiffen by internal webs. Where the rudder is fully fabricated, one side plate is prepared and the vertical and horizontal stiffening webs are welded to this plate. The other plate often called the closing plate is then welded to the internal webs from the exterior only. This may be achieved by welding, flap bars to the webs prior to fitting the closing plate, and then slot welding the plate. The upper face is formed into a usually horizontal flat palm, which acts as the coupling point for the rudder stock. A lifting hole is provided in the rudder to enable a vertical inline lift of a rudder when it is being fitted or removed. This lifting hole takes the form of a short piece of tube welded through the rudder with doubling at the side and closing plate. A drain hole is provided at the bottom of the rudder to check for water entry when the ship is examined in dry dock.  To prevent internal corrosion the interior surfaces are suitably coated, and in some cases the rudder may be filled with inert plastic foam. The rudder is tested when complete under a head of water 2.45 M above the top of the rudder.

Question 82

Why is Rudder Angle Limited to 35 Degrees ?

Answer 82

Beyond 35 degree rudder efficiency is reduced due to formation of eddies on the back of rudder as the flow is no longer streamlined. This is called stalled condition. The manoeuvrability does not increase beyond 35 degree, but rudder torque increases and ship’s turning circle increases.

Question 83

Why Steering Test Rudder angle 35 degree to 30 degree ?

Answer 83

So that the point at which it is reached can be exactly judged as it crosses 30 degree. As hunting gear puts pump stroke to zero, the rudder movement slows down progressively as it approaches 35 degree.

Question 84

Why is Astern Turning Moment much less than Ahead ?

Answer 84

The propeller thrust adds to the force on the rudder when going ahead, but in astern that thrust is lost. The pivoting point (point about which ship turns) shifts aft to 1/3 rd the length from aft. This reduces turning moment greatly.

Question 85

What is the Pivoting Point for Ships ?

Answer 85

The ship turns about a point called pivoting point. This is situated about 1/3 rd to 1/6 th of the ship length from forward, depending on the ship design.

Question 86

Why Rudder is situated Aft of the Ship ?

Answer 86

To make use of propeller wash for thrust. The pivoting point of ship is 1/6 to 1/3 rd of length of ship from bow, the greater the perpendicular distance between point of action of force and pivoting point, the better rudder movement. Better protected at astern from damage. Drag is reduced if rudder is situated aft.

Question 87

Why Full Astern Power is usually Less than Full Ahead Power ?

Answer 87

Propeller blade section is designed for maximum efficiency in ahead. In astern direction, angle of attack is high on back of blade. Propeller will absorb very little available power, severe eddying occurs on face. Therefore, efficiency is very low. Hence, if 80% of full ahead power is available for astern, then boosting it to 100% will have minimal return in thrust from propeller.

Question 88

function of duct keel?

Answer 88

Duct Keel An internal passage of water tight construction (two longitudinal girders spaced not more than 2.0 m apart) running same distance along the length of the ship, often from the forepeak to the forward machinery space bulkhead. To carry the pipe work, and an entrance is at forward machinery space via a watertight manhole.

Question 89

function of bulbous bow?

Answer 89

Bulbous bow A bulb shaped under water bow which is designed to reduce wave making resistance and any pitching motion of the ship.

Question 90

function of camber?

Answer 90

Camber Curvature given to a deck transversely. It is measured by the difference between the heights of the deck at side and centre. The camber amidships is frequently one fiftieth of the breadth of the ship.

Question 91

what is carving note?

Answer 91

Carving note Form completed by the owner of the ship under construction. Gives details of tonnage, name, port of registry, etc.

Question 92

what is cofferdam

Answer 92

Cofferdam  Narrow void space between two bulkheads or floors that prevents leakage between the adjoining compartment.

Question 93

what are coffin plates

Answer 93

Coffin plates They are used to connect stern frames to the flat plate keel. The stern frame is extended forward far enough, two or three frame spaces, to provide a good connection with a flat plate keel. The aft most plate of the keel, coffin plate is dished around the extension.

Question 94

what is CRP?

Answer 94

CRP Contra – rotating propeller. A propulsion arrangement with two propellers rotating in opposite direction on the same shaft.

Question 95

define freeboard?

Answer 95

Freeboard Vertical distance from the load water line to the top of the freeboard deck. Freeboard has considerable influence on sea worthness of the ship. The greater the freeboard larger is the above water volume of ship. This provided reserve buoyancy assisting the ship to remain afloat in the event of damage.

Question 96

what are the types of keel?

Answer 96

Types of keel Bar keel, Duct keel, Flat plate keel.

Question 97

what is a freeing port?

Answer 97

Freeing port   An opening in the lower portion of a bulwark, which allows deck water to drain overboard. Some freeing ports have hinged gates that allow water to drain overboard but that swing shut to prevent seawater flowing in board.

Question 98

define dead light

Answer 98

Dead light A hinged steel cover, which is part of a port or scuttle. It is used to cover the glass in heavy weather.

Question 99

define dead rise

Answer 99

Dead rise Athwartship rise of the bottom from the keel to the bilge. Also known as ‘ Rise of floor ‘.

Question 100

what is a devils claw

Answer 100

Devil’s claw   A stretching screw with two heavy hooks or claws. It is used to secure the anchor in the hawse pipe.

Question 101

define displacement

Answer 101

Displacement A ship floating freely displaces a mass of water equal to its own mass and this mass is known as the ship’s displacement.

Question 102

define light displacement

Answer 102

Light displacement It is the displacement of the ship complete & ready for sea but no crew, passengers, baggage, stores, fuel, water, cargo on board. Boilers, if any, are filled with water to working level.

Question 103

define load displacement

Answer 103

Load displacement   It is at the maximum permissible draught and is made up of the light displacement and the dead weight.

Question 104

what is a docking plug

Answer 104

Docking plug A brass screw fitted in the garboard strake of the shell plating at the bottom of each compartment to drain the water, which remains in the ballast tanks while the vessel is in dry dock.

Question 105

define flare

Answer 105

Flare The spreading out of the hull form from the centre vertical plane usually in the fore body above waterline.

Question 106

define floodable length

Answer 106

Floodable length The length of the ship that may be flooded without sinking below her safety or margin line.

Question 107

what are floors

Answer 107

Floors They are transverse vertical plates those run across the bottom of the ship from the centre girder to the bilge. Watertight or oil tight floors are used to divide the double bottom space into suitable tanks.

Question 108

what is a flush deck ship

Answer 108

Flush deck ship A ship constructed with an upper deck extending through out her entire length without a break or a super structure such as forecastle, bridge or poop.

Question 109

define free surface

Answer 109

Free surface Liquid in a partially filled tank that tends to remain horizontal as the vessel heels or rolls

Question 110

what are deep tanks?

Answer 110

Deep tanks Tanks extending from the bottom or inner bottom up to or higher than the lowest deck. They are often fitted with hatches so that they also may be used for dry cargo in lieu of fuel oil, ballast water of liquid cargo.

Question 111

define garboard strake

Answer 111

Garboard strake The strake of the bottom shell plating adjacent to the keel plate.

Question 112

define intercostals

Answer 112

Intercostals These are plates, angles, etc., fitted down between others or cut to allow other parts to pass through them. Side girders, parallel to the centre girder & fitted between the floors, are intercostals. Vessels of up to 20 meters in breadth must have one intercostal side girder on each side. Vessels of greater are to have two such girders on each side.  

Question 113

define Kort nozzle

Answer 113

Kort nozzle A shroud or duct fitted around a propeller in order to increase thrust at low speeds. It is often fitted to tugs and trawlers.

Question 114

define length over all

Answer 114

Length overall The extreme length of a ship measured from the foremost point of the stem to the aftermost part of the stern.

Question 115

define length between perpendiculars

Answer 115

Length between perpendiculars The length of a ship between the forward and after perpendiculars. The forward perpendicular is a vertical line at the intersection of the fore side of the stem and the summer load waterline. The after perpendicular is a vertical line at the intersection of the summer load waterline and the after side of the rudder post or stern post, or the centreline of the rudder stock if there is no rudder post or stern post.

Question 116

define gross tonnage

Answer 116

Gross tonnage This is the total of the under deck tonnage and the tonnage of the following spaces. 1.Any tween deck space between the second and upper decks. 2.Any enclosed spaces above the upper deck. 3.Any excess of hatchways over 0.5 % of the gross tonnage. 4.At the ship owner’s option and with the surveyor’s approval, any engine light and air space on or above the upper deck.

Question 117

define exempted spaces

Answer 117

Exempted space These are spaces, which are not measured for the gross tonnage calculation. 1.Wheel house, chart room, radio room, navigation aid room. 2.Spaces for machinery & condenser, stability tanks 3.Safety equipment and battery spaces. 4.Gallery, washing & sanitary. 5.Sky lights, domes & trunks.

Question 118

define net tonnage

Answer 118

Net tonnage This is the tonnage value obtained by deducting from the gross tonnage the total value of the deducted spaces. The net tonnage is considered to represent the earning capacity of the ship.

Question 119

What is bow thruster ?

Answer 119

Bow thrusters are manoeuvring devices which are fitted in an athwartship tunnel near to the bow and aft of the collision bulkhead. It gives additional manoeuvring ability.

Question 120

What is bulwark ?

Answer 120

 A barrier fitted at the deck edge to protect passenger and crew to avoid the loss of items overboard while the ship rolls excessively.

Question 121

Why double bottom or DB tanks are fitted on ships ?

Answer 121

fitted to prevent foundering (flooding) in the event of hull damage. To control the stability by ballasting To provide buoyancy To store F.W and F.O

Question 122

Why wing tanks are fitted?

Answer 122

To control the ship stability (healing) To carry cargo To store F.W and F.O

Question 123

What is half beams ?

Answer 123

Transverse beams which are cut at hatch side coamings are termed half beams.

Question 124

What is water tight bulkhead ?

Answer 124

Water tight bulkheads are important element of transverse strength, particularly against racking stress. They divide the ship into subdivision. They also give protection against fire

Question 125

What is collision bulkhead ?

Answer 125

Collision bulkhead is a forepeak watertight bulkhead to protect foundering and against racking stress. It is fitted not less than 5% or not more than 7% of the ship length aft and stern at load water line. It must extend to upper deck. Stiffeners may be spaced 600 mm apart. Its water tightness can be tested by filling the fore peak tank to the level of water line or hose test along the boundary. No water leak through other sides.

Question 126

What is after peak bulkhead ?

Answer 126

One bulkhead at each end of the machinery space To enclose the stern tube in a watertight compartment. This bulkhead need only extend to first deck above load water line, if it forms a watertight flat. Plating in after peak bulkhead must be doubled or thickened around the stern tube to resist vibration.

Question 127

How do you check water tight door’s tightness ?

Answer 127

Watertight hatch cover and watertight doors’ tightness can be check by chalk method or hose methods.

Question 128

What is chalk method ?

Answer 128

Apply chalk to watertight flat sealing continuously. Close the door tightly, then open Check the watertight door sealing. The chalk must have continuously around the watertight sealing, it has water tightness.

Question 129

What is hose method ?

Answer 129

Close the watertight door or watertight hatch cover tightly. Hosing with water jet with a pressure of 2 bar and directed to the sealing edges away from 1.5 m. There must be no water leak through the other side. That door or hatch are good in order for watertight.

Question 130

Dead weight and light weight ?

Answer 130

Dead weight is the weight of cargo, fuel, water, store etc. that a ship can carry. Light weight  is ready to sea going but no fuel, no crew, no cargo. Displacement   = Dead weight +Light weight

Question 131

What is windlass ?

Answer 131

It is a machine used for hoisting or lowering anchor.

Question 132

What is stern tube ?

Answer 132

It is a watertight tube enclosing and supporting the propeller shaft. It consists of a cast iron or cast steel cylinder fitted with bearing surface up on which the propeller shaft (enclose in a sleeve) rotates.

Question 133

What is hatch coaming ?

Answer 133

The vertical plating bounding a hatch for the purpose of stiffening the edges of the opening and resisting entry of water to ship hull.

Question 134

What are girders?

Answer 134

The continuous stiffening members which run fore and aft in a ship to support the deck.

Question 135

What are stealer strake, stringer, stringer plate ?

Answer 135

Stealer strake: A single wide plate which replace two narrow plates in adjacent strake of a ship. Stringer: A horizontal stiffener fitted along the ships’ side or a longitudinal bulkhead, in order to provide strength and rigidity. Stringer plate: The outboard strake of plating on any deck.

Question 136

with the aid of a sketch explain the stability of a ship during grounding? when will a ship capsize?

Answer 136

Stability of Ships During Grounding If a ship runs aground in such a manner that the bottom offers little restraint to heeling as in the figure below, the reaction of the bottom may produce a heeling moment. As the ship touches ground during its forward motion, part of the energy due to the forward motion may be absorbed in lifting the ship, in which case a reaction, R, between the bottom of the ship and the ground would develop. This reaction may be increased later as the tide ebbs. Under these conditions, the force of buoyancy would be less than the weight of the ship, since the ship would be supported by the combination of buoyancy and the reaction of the bottom. The ship would heel until the moment of buoyancy about the point of contact with the bottom became equal to the moment of ship’s weight about the same point, when (W-R) X a equals W x b. There is only a remote possibility of a stranded ship capsizing as a result of ebbing tide. For this to occur the ship will have to be grounded on a bottom such that there is no restraint to heeling in one or both directions until a very large angle is reached, as for example, on a peak which is considerably higher than the surrounding bottom. In a case as in the figure, the heel would increase as the tide ebbs. The position of the ship will always be such that the moment of buoyancy equals the moment of weight i.e. W x b will be equal to (W-R) x a. Capsizing Before the ship could capsize, reaction R must be reduced to zero. Since W x b = (W-R) x a, when R=0, a and b would be equal, which means that G would be directly above B1. This is the same as the ship heeling to its maximum angle of heel for positive stability. It can therefore be said that the stranded ship will not capsize (in the absence of external forces) until it reaches the angle of heel equivalent to its range of positive stability when afloat.

Question 137

describe the Stability of a Ship During Dry docking

Answer 137

Stability of Ships During Dry docking When a vessel enters dry-dock, it has a trim and hence the keel makes an angle with the dock bottom/keel blocks. The dock bottom may be slightly sloped to help draining of the water when the dock is emptied. However the keel block top may be horizontal. As the water level falls due to pumping after the vessel has entered the dock, the keel of the ship touches the keel blocks. Vessels are usually trimmed by the stern and if so, the stern end of the ship touches the keel blocks first. When a ship has just touched the keel block, the position is as shown in the sketch below. When ship’s stern just touches keel block, the force of buoyancy would be less than the weight of the ship, since the ship would be supported by the combination of buoyancy and the reaction of the bottom (P). This scenario is same as removing a mass from the ship, where ship’s stern touches keel block. The result is shifting centre of gravity (G) upwards. This reduces metacentric height (GM). So it is evident that there is a reduction in metacentric height during docking, when ship just touches keel block. This reduction must be taken into consideration to ensure that a positive GM is there during the entire dry docking period.

Question 138

what is grounding?

Answer 138

Grounding Grounding means the vessel touching sea bed due to lack of depth of water or presence of protrusions from the sea bed. If the sea bed is soft, grounding may not cause any serious damage to the hull. However if hull touches any hard objects like rock, the shell plate may get dented deeply or even gashed. If the shell plate is cut open due to the grounding, sea water will get into the space thus opened to sea. This can result in ship taking dangerous list or even sinking unless proper action is taken immediately.

Question 139

what checks are made during grounding of ships?

Answer 139

Checks During Grounding of Ships 1.In order to detect any ingress of water into the vessel after grounding, it is necessary to take soundings of all tanks at regular intervals. 2.Any increase in the tank level (or decrease if the grounding is by way of deep tanks having fluid to a level higher than water line) indicates that the tank boundary has suffered damages. 3.If a DB (double bottom) tank has been damaged and water ingresses into the tank, it is essential to ensure that the manhole covers of the concerned tank are closed tight. 4.Then the water ingress will stop as soon as the tank is full and the situation may be under control.

Question 140

what are some of the Checks made when in Dry Dock

Answer 140

1.After the propeller has been satisfactorily fitted and the hydraulic nut properly tightened check that the bores on the propeller hub and on the hydraulic nut used for the hydraulic hose connectors have been sealed with gaskets and screwed plugs and that the plug heads on the propeller hub are locked by punch marks. 2.Check that the locking of the propeller nut has been done as per design. 3.Check that the propeller cap has been properly fitted with gasket and secured. Ensure that the void space in the cap has been filled with grease or tallow, as per Shipyard practice. The recesses on the cap for the securing bolts have to be filled with cement to provide a smooth streamlined surface. 4.After the aft seal housing and its liner have been properly secured, ensure that the bolt heads are locked either by the washers or by stainless steel wire locking two bolt heads at a time. The same is to be checked for the forward  seal housing also. 5.A concentricity check of the aft seal liner with the propeller shaft by dial gauge and a concentricity check of the aft seal housing with the liner by wedge gauge have to be carried out, as recommended by the seal Maker. 6.After oil has been filled in the stern tube, a tightness check of the oil seal rings in the forward and aft seal housings have to be carried out over a period of about 12 hours. Since the permanent oil piping for the stern tube lube oil system will not have been installed at the time of launching, a temporary plastic hose is usually rigged and filled up to the oil level normally maintained in the top header tank to provide the necessary pressure head. 7.Finally the wear-down gauge readings (top & bottom) at the aft seal have to be taken and recorded. Since there might be some eccentricity between the seal liner and shaft, to avoid error because of this it is necessary that ‘wear-down’ measurements are taken with the liner in the same position as when the reading was taken first time. For this reason it is necessary to identify the position by a mark on the propeller shaft flange in ‘top’ position. 8.Before fitting the wear-down gauge, ensure that no copper washer is left in the hole and also that the hold bottom is clean. After fitting the gauge and tightening it in the normal manner, a mark should be made on the seal housing in line with the mark on the gauge to ensure that the gauge is tightened to the same extent as at the first time in the building dock. 9.Check that the rope guard is properly welded to the stern boss and that the rope cutter knives are fitted as per supplier’s instructions. 10.If the condition of the propeller blades is not in ‘as manufactured’ condition, it is normal for the Shipyard to have the propeller polished and coated with varnish so that it is maintained in a clean condition thereafter. 11.The clearances in the rudder stock guide bush and pintle bush and rudder jumping clearance have to be checked. 12.If an Impressed Current Cathodic Protection system has been provided, it is necessary to ensure that the epoxy shield around the anodes has been correctly done. An insulation test has to be carried out for the anodes as per Makers instructions. 13.Besides the ICCP system, zinc anodes are provided at the stern area and on the rudder blade, in sea chests and in the tunnel for the bow thruster, if provided. Their condition should be checked and if any anode found wasted, they should be renewed. It is not advisable to fit anodes on the stern-tube boss immediately ford. of the propeller because of the turbulence they will create in the water flow to the propeller. All such anodes are masked before the final coats of anti-fouling are applied and it is important to ensure that the masking is removed before launching. 14.An inspection of all sea chests has to be made before the grids are closed. The vent and drain holes should be clear and all outfitting inside done as per drawings. The zinc anodes and MGPS anodes, if fitted, should be clean and without any paint. 15.If the ship has a BIS notation, the grids will be of the hinged type. Check that the grids swing easily and that the material of hinge pins, and securing bolts or studs/nuts are of stainless steel. After the grids are secured the locking of the bolts has to be confirmed. If locking wire is used it should also be of stainless steel.

Question 141

What is Paint DFT (Dry Film Thickness) Check?

Answer 141

Just prior to flooding the Shipyard QC dept. together with the Paint supplier’s inspector will carry out a ‘dft’ (Dry Film Thickness) check of the exterior hull, over flat bottom and vertical bottom area covered by anti-fouling paint. A ‘roughness check’ is also carried out at the same time. Ship’s officers may participate in these checks.

Question 142

What is In Water Survey (IWS)?

Answer 142

The CLASS will accept an In water Survey in lieu of the intermediate docking between Special Surveys required in a five year period on ships where an *IWS notation is assigned. The In water Survey is to provide the information normally obtained from the Docking Survey. Special consideration shall be given to ascertaining rudder bearing clearances and stern bush clearances based on the operating history, on board testing and stern L.O. analysis. The In water Survey is to be carried out at an agreed geographical location, with the ship at a suitable draught in sheltered waters and with weak tidal streams and currents. The in water visibility is to be good and the hull below the waterline is to be clean.

Question 143

What are some In Water Survey (IWS) Requirements?

Answer 143

Prior to commencing the In water Survey, the equipment and procedures for both observing and reporting the survey are to be agreed between the Owners, Surveyor and diving firm. In water Survey is to be carried out by a qualified diver of a firm approved by the Class. If the In water Survey reveals damage or deterioration that requires early attention, the Surveyor may require that the ship be dry-docked in order that a fuller survey can be undertaken and the necessary work carried out. Where a vessel has an *IWS notation, the conditions of the high resistant paint is to be confirmed at each dry-docking in order that the *IWS notation can be maintained. Where an *IWS (In water Survey) notation is to be assigned, plans and information covering the following items are to be submitted: Details showing how rudder pintle and bush clearances are to be measured and how the security of the pintles in their sockets is to be verified with the vessel afloat. Details showing how stern bush clearances are to be measured with the vessel afloat. Details of high resistant paint, for information only. Where an *IWS notation is to be assigned, means are to be provided for ascertaining the rudder pintle and bush clearances and for verifying the security of the pintles in their sockets with the vessel afloat. Where an *IWS (In water Survey) notation is to be assigned, means are to be provided for ascertaining the clearance in the stern bush with the vessel afloat.

Question 144

What are some Inspections which must be made during Docking Survey?

Answer 144

The shell plating is to be examined for excessive corrosion, deterioration due to chafing or contact with the ground and for undue unfairness or buckling. Special attention is to be given to the connection between the bilge strakes and the bilge keels. The clearances in the rudder bearings are to be measured. Where applicable, pressure testing of the rudder may be required if deemed necessary. The sea connections and overboard discharge valves and cocks and their attachments to the hull are to be examined.  The propeller, stern bush and sea connection fastenings and the gratings at the sea inlets are to be examined. The clearance in the stern bush or the efficiency of the oil glands is to be ascertained. When chain cables are ranged, the anchors and cables are to be examined by the Surveyor.

Question 145

List the Preparations made before Dry-docking?

Answer 145

Prepare Repair Specifications in advance for budgeting and inviting quotations Pipe Lines Structural steel renewal, for which plate thickness gauging may be required. Overhauling of Mooring Equipment. Special servicing requirements Overhauling and testing of Cargo Gear ( cargo pumps if the vessel is a tanker) Prepare Spare Parts Plan and carry out Hold/tank cleaning/gas freeing before arrival Check and confirm availability of special tools, instruments and manuals on board. Confirm availability of drawings like Docking Plan, GA etc. on board.

Question 146

List Activities carried out In Dry dock?

Answer 146

Bottom Inspection/corrective action Bottom painting Anchor cable ranging and painting. Cleaning and painting of chain locker. Echo-sounder and speed log checks MGPS and ICCP checks Sea chests/filters cleaning/painting Storm valves overhauling Rudder pintle and jumping clearances checking Rudder pressure testing/internal painting if required Poker gauge reading (Propeller drop) Stern seal leak check Propeller polishing/repairs if required. Tail shaft survey, if required. Bottom plugs removal/re-fitting as required

Question 147

What are some Safety considerations which must be made in Dry Dock

Answer 147

Warning notices Fire line under pressure/ fire fighting appliances kept ready Means of communications established Usage of galley and WCs restricted as required. Hot work permissions/precautions Earthing of vessel

Question 148

What are the Checks before/during Flooding in Dry Docking

Answer 148

Sea connections, sea chests checking Bottom plugs fitting/leak test by vacuum Any other hull opening Removal of masking of anodes Securing of rudder fittings, propeller nut/cone fitting, rope guard and net-cutter fitting Ensuring ballast/weight distribution same as when docked After commencement of flooding, when the water level has risen enough to cover all shipside opening, flooding needs to be stopped and all ship side connections checked for any leakage. After confirming that all are satisfactory, flooding is resumed and the vessel floated.

Question 149

What is meant by Dry Docking ?

Answer 149

Dry docking of a vessel refers to taking the vessel into a dry dock and placing her on a series of blocks (known as ‘keel blocks’ as they are placed under the keel of the vessel) arranged on the bottom of the dry-dock. The keel blocks are fabricated out of steel and have wooden tops so that the ship’s hull plating will not come into hard contact with steel, which may result in damage to the plate. Also the wooden tops are shaped as wedges so that the height of the block can be adjusted by pushing two wedges placed with their tapers in the opposite direction, into each other or pulling them apart.

Question 150

What are Keel Blocks ?

Answer 150

This facility of adjusting the height is necessary because the ship’s bottom plate may not be exactly in a straight line (even a few millimetres of difference can result in overloading the hull structure and deforming the same and all classification rules allow certain tolerance in the straightness of the keel) and blocks of the same height can result in some portion of the hull not touching the blocks below that. This will in turn load the adjacent blocks which are touching the hull.

Question 151

What is Docking Plan ?

Answer 151

The number of keel blocks, their spacing and exact location are decided taking into consideration the ship’s hull structure viz. the frame spacing, location of additional brackets (known as docking brackets) and stiffeners provided specially for the purpose of strengthening the hull at the locations where the hull is resting on the blocks and also distributing the weight evenly on the blocks. The shipyard which builds the vessel prepares a drawing called ‘docking plan’ which gives the exact locations where the blocks should support the hull when the vessel is dry-docked. The repair yard undertaking the repairs/docking will need this drawing in advance so that they can arrange the blocks accordingly before flooding the dock prior to docking of the vessel.

Question 152

What are the Ballast Requirements for Dry Docking ?

Answer 152

The docking plan also specifies the number of blocks and the minimum load carrying capacity of the blocks. As the entire weight of the ship in the docking condition has to be borne by the keel blocks, it is also necessary to limit the weight of the ship while she is being docked. Thus the extra weight of ballast, a minimum of which is required to be on board to provide stability when the vessel is afloat/sailing, has to be reduced to the maximum just before the vessel enters the dry-dock. Since the weight of ballast required for the vessel to stay upright in afloat condition may be more than the weight acceptable by the keel block, it may be necessary to take the vessel into the dock with that much ballast and then de-ballast the vessel when she is inside the dry-dock but is still afloat. In such a case, the vessel will be taken into the dock and then de-ballasted while the dock is being emptied after closing the dock gate.

Question 153

What is Dock Sill Height?

Answer 153

There is one more reason for reducing the docking draft of the vessel and that is the ‘sill height’ of the dry dock. Although the depth of water in the channel may be sufficient to accept vessels of a certain draft, the sill height at the entrance of the dock may be insufficient to allow this draft. Due to this reason, the vessel is docked when the depth of water near the dock entrance is maximum, which condition exists during the high tide only. Thus docking and un-docking dates have to be planned after taking into consideration the tidal variations at the place where the dry-dock is situated.

Question 154

How are Dry Docking Intervals decided?

Answer 154

Every vessel needs to be taken into a dry-dock once in a while in order to examine the condition of the under-water hull as well as some under-water fittings. All Classification Societies have rules covering this requirement. The docking intervals are decided after taking into consideration many facts like the facilities provided on board for ‘condition monitoring’ as well as for inspecting the hull in the afloat condition and ascertaining the state of the hull and other items under the water line. These intervals may later be changed depending upon the age of the vessel, the condition of the hull and under-water fittings during a survey etc.

Question 155

What are Important Classification Society Rules on Dry Docking ?

Answer 155

A minimum of two Docking Surveys are to be held in each five-year Special Survey period and the maximum interval between successive docking. Surveys is not to exceed three years. One of the two Docking Surveys required in each five year period is to coincide with the Special Survey. Consideration may be given in exceptional circumstances to an extension of this interval not exceeding three months beyond the due date. The class may accept an In-water Survey (IWS) in lieu of the intermediate docking between Special Surveys. Special Surveys are different from routine surveys which may be carried out annually. The scope of inspection is more during the Special Surveys and the timing of the Special Surveys also is covered by the class rules.

Question 156

What are Special Surveys – Class Rule Requirements

Answer 156

All ships to be subjected to Special Surveys in accordance with the requirements. These Surveys become due at five-yearly intervals, the first one five years from the date of build or date of Special Survey for Classification as recorded in the Register Book, and thereafter five years from the date recorded for the previous Special Survey. Consideration may be given to any exceptional circumstances justifying an extension of the hull classification to a maximum of three months beyond the fifth year. If an extension is agreed the next period of hull classification will start from the due date of the Special Survey before the extension was granted. In this context ‘exceptional circumstances’ means unavailability of dry-docking facilities, repair facilities, essential materials, equipment or spare parts or delays incurred by action taken to avoid severe weather conditions.

Question 157

What are Docking Surveys – Class Rule Requirements?

Answer 157

At Docking Surveys or In-water Surveys the Surveyor is to examine the ship and machinery, so far as necessary and practicable, in order to be satisfied as to the general condition. For oil tankers (including ore/oil and ore/bulk/oil ships), chemical tankers and bulk carriers over 15 years of age the intermediate docking between Special Surveys is to be held in dry-dock. Further, this survey is to be held as part of the Intermediate Survey. Where a ship is in dry-dock or on a slipway it is to be placed on blocks of sufficient height, and proper staging is to be erected as may be necessary, for the examination of the shell including bottom and bow plating, keel, stern, stern frame and rudder. The rudder is to be lifted for examination of the pintles if considered necessary by the Surveyor. Vibration is defined as the motion of a particle or a body by which it is displaced from its stable equilibrium. Vibration can be ‘in and out’ or ‘to and fro’ or ‘oscillation’ of a mass. As far as marine vessels are concerned, vibration can be broadly classified into two categories.

Question 158

What are Hull Vibrations

Answer 158

Hull vibrations include vibration of main hull, sub structures (such as deck house, uptakes, masts, deck, bulkheads, etc.) and local structures (such as panels, plated and minor structure members).

Question 159

what are machinery vibrations?

Answer 159

Machinery Vibrations Machinery vibrations are caused by ship’s propeller, shafting, thrust block, main engine and other machinery. THE WORST THING IS THAT HULL AND MACHINERY VIBRATION INTERACTS EACH OTHER. THAT MEANS ONE CAN CAUSE EXCITATION FOR THE OTHER.

Question 160

what are the Effects of Vibrations for Ships

Answer 160

Vibrations are unwanted on board ships and it must be minimized. Vibrations may lead to: Damages to hull structure Machinery failure or malfunction Personal discomfort Loosening of joints, nuts, bolts, etc. Fatigue fractures Fluctuation of stresses, etc.

Question 161

with the aid of a pendulum sketch explain how vibrations occur?

Answer 161

How Vibrations Occurs There are 3 positions shown in the figure. The pendulum is at stable equilibrium when it is at position ‘b’. When pendulum is hit, it oscillates between ‘a’ and ‘c’, passing through ‘b’. So it can be concluded that: Vibration starts on a rigid body because of an external force or excitation on the body. Then the body is displaced from its stable equilibrium. Now the body tends to return to its equilibrium position under the action of a restoring force, which can be gravitational force or elastic forces. The body keeps moving back and forth crossing its position of equilibrium due to inertia. Finally amplitude of vibration reduces gradually or vibration dampens and die out and body regains its stable equilibrium, if no more excitation is given to it.

Question 162

What are the Elements for Vibration There are 4 elements for vibration.

Answer 162

1.Mass [m]: Rigid body executes vibration 2.Spring [k]: Elasticity of body 3.Damper [c]: Means of energy dissipation 4.Excitation [F(t)]: System receives energy from excitation CHARACTERISTICS OF A VIBRATION CAN BE CHANGED WHEN ANY OF THESE ELEMENTS ARE CHANGED.

Question 163

what is Degree of Freedom (DOF) of Vibrating Mass

Answer 163

The degree of freedom or DOF is the number of independent co-ordinates needed to represent the motion or vibration of a body.

Question 164

Dry docking is one of the best opportunity to carry out inspection of , shell plating, fore end and aft end of ship, opening in shell plating, rudder, propeller and stern tube, describe the checks made for each of these.

Answer 164

Shell Plating Shell plating is inspected before cleaning, after cleaning and after painting. Following checks are carried out: Check the condition of sea growth and shell plating Check the amount of marine growth before cleaning to evaluate the effectiveness of anti-fouling  paint Check welds at the joint, after cleaning Check condition of anodes. If anodes are fully corroded or consumed, that means either quality or quantity of the anodes used were insufficient and inadequate. If anodes are not at all corroded, that indicates improper fitting. Normally 75% of the anodes will be consumed. Check the welded area for any cracks, corrosion or deformations Check for buckling or corrosion of plates Cracks are most common near welded areas. The possibility of cracks is more at the forward end (due to waves) and aft end (due to main engine and propeller induced vibrations) Check for any twisting, damage or cracks on the bilge keel Check damages to the bulbous bow Fore End of Ship Check bulbous bow for indentation, bulging or cracks Check for chain marking and impressions on it Check for bow thruster tunnel and grating condition If ship is more than 5 years old, surveyor may insist on gauging thickness of some portions on hull Aft End of Ship Aft end requires very close examination since it is subjected to frequent vibrations slamming (impact loading) Presence of different types of materials (for propeller, rudder, hull) may accelerate galvanic corrosion. Check the Impressed Current Cathodic Protection (ICCP) and sacrificial anode conditions Check the stern frame for cracks, bend, erosion, buckling, etc. Opening in Shell Plating It consist of high and low sea chest, emergency sea chest, bow thruster tunnel, overboard openings, etc. Inspect corrosion and condition of sea chest plating Check sea chest grating Inspect MGPS (Marine Growth Prevention System) Check for steam injection and vent valves in sea chest Gauging of sea chest plating for old ships Check overboard valve connections to sea chest Boiler blow down valves require more attention since it is subjected to high temperature, pressure, corrosion and erosion Overboard valves to be tested for leakage Any repair carried out on shell to be inspected thoroughly Rudder When ship enters dry dock and pumping out water, check water whether water is coming out from rudder or not. If yes, then rudder is breached. Open the top and bottom plug and check any water inside. Pressure test rudder at a water head of  2.46 meters. If the rudder is badly pitted or ship is older, surveyor may insist on thickness gauging of the rudder plate. Check the sacrificial anode condition on the rudder. Check the cement on coupling bolts for rudder and rudder stock. Remove the cement and check condition of the palm nut. Check the rudder pintle clearance. Check the rudder jumping clearance. Check the rudder drop. Carry out hammer test by surveyor’s hammer, by tapping on the rudder to evaluate the plate condition. Check the actual position or true position of the rudder, compared to rudder angle indicator and see whether any difference is there by bending or deformations Carry out a visual inspection. Propeller and Stern Tube Check oil leakage from stern tube. Cut off the rope guard and check any fouling by fishing nets or ropes. Propeller removed and inspected after alternate dry docks. If stern tube is oil cooled, remove propeller every 5 years. Inspect blade surfaces for erosion, pitting, blade surfaces, corrosion and bending. Take measurements of propeller and compare with last records. Take propeller drop. Check stern tube condition for leakages. For CPP (Controllable Pitch Propeller), check for blade movements and zero pitch settings with respect to wheel house. Check hydraulic oil leakages. Check securing arrangements for blades and carrier. Inspect propeller for cavitation damages.

Question 165

What are the Conditions of assignment?

Answer 165

These are the conditions which must be met before free board is. assigned to a ship and load line certificate is issued following a load line survey. Most of the condition of assignment are concerned with the water tight integrity of the ship. Hull construction should meet the highest standard laid down by the classification society. This ensures protection against flooding of the ship. The superstructure and bulkheads must be strengthened sufficiently. Standards of stability are given in the Rules for both small and large angles of heel. Details of the information required to be carried on a ship are stated, together with typical calculations. All the information is based on an inclining experiment carried out on the completed ship in the presence of a Flag surveyor. It is essential that all openings in the weather deck are weathertight. Hatch coamings, hatch covers, ventilator coamings, air pipes and doors must be strong enough to resist the pounding from the sea and standards of strength are given in the Flag/Classification society rules. The Rules also specify the height of coamings, air pipes and door sills above the weather deck, those at the fore end being higher than the remainder. It is also important to remove the water from the deck quickly when a heavy sea is shipped. With completely open decks, the reserve buoyancy is sufficient to lift the ship and remove the water easily. When bulwarks are fitted, however, they tend to hold back the water and this may prove dangerous. For this reason openings known as freeing ports are cut in the bulwarks, the area of the freeing ports depending upon the length of the bulwark, and again the area required for releasing the water from the deck is set out in the load line regulations. If the freeing ports are wide, grids must be fitted to prevent crew being washed overboard. In addition, scuppers are fitted to remove the surplus water from the deck. The scuppers on the weather deck are led overboard while those on intermediate decks may be led to the bilges or, if automatic non-return valves are fitted, may be led overboard. Type A ships, with their smaller freeboard, are more likely to have water on the deck and it is a condition of assignment that open rails be fitted instead of bulwarks. On the older ships with the accommodation midships, a longitudinal gangway was fitted to allow passage between the after end and midships and between the forward end and midships, without setting foot on the weather deck. In larger ships it is necessary to fit shelters along the gangway. Alternatively, access may be provided by an underdeck passage, but while convenient for bulk carriers and container ships would prove dangerous in oil tankers.

Question 166

how is net tonnage or registered tonnage obtained?

Answer 166

The net tonnage or register tonnage is obtained by deducting from the gross tonnage, the tonnage of spaces which are required for the safe working of the ship: *master’s accommodation *crew accommodation and an allowance for provision stores *wheelhouse, chartroom, radio room and navigation aids room *chain locker, steering gear space, anchor gear and capstan space *space for safety equipment and batteries *workshops and storerooms for pump men, electricians, carpenter, boatswains and the lamp room *auxiliary/emergency diesel engine and/or auxiliary boiler space if outside the engine room *pump room if outside the engine room *in sailing ships, the storage space required for the sails, with an upper limit of 2% of the gross tonnage *water ballast spaces if used only for that purpose (the total deduction for water ballast, including double bottom spaces, may not exceed 19% of the gross tonnage), and *propelling power allowance – this forms the largest deduction and is calculated as follows: If the machinery space tonnage is between 13% and 20% of the gross tonnage, the propelling power allowance is 32% of the gross tonnage. If the machinery space tonnage is less than 13% of the gross tonnage, the propelling power allowance is a proportion of 32% of the gross tonnage.

Question 167

what is net and gross tonnage used for?

Answer 167

Net tonnage is used to determine canal dues, light dues, some pilotage dues, some harbour dues and national ‘tonnage taxes’. Gross tonnage is used to determine manning levels, safety requirements such as fire appliances, some pilotage and harbour dues, towing charges and graving dock costs. In 2010 the IMO’s International Code on Intact Stability, 2008 (2008 IS Code) came into force. This provides all the requirements for ship’s designers and operators in one document.

Question 168

What is Probabilistic method of calculating damage stability

Answer 168

In an effort to improve the ‘survivability’ of ships in the event of collision, the IMO requires designers to initially assess the internal structure of their design based upon the assumptions of damage occurring for a given type of ship, cargo carried, length, width and draught. (Called the deterministic method)However, for passenger ships and dry-cargo ships over 80 m long designers must consider the probable survivability of the vessel in the event of different damage scenarios. The results of the survivability assessments will then require a modification to the internal structure to improve the ability of the vessel to survive the damage.

Question 169

What is Alternative tonnage?

Answer 169

The owner may, if he wishes, have assigned to any ship reduced gross tonnage and net tonnages calculated by the method given above. This is an alternative to the normal tonnages and is penalised by a reduction in the maximum draught. A tonnage mark must be cut in each side of the ship at a distance below the second deck depending upon the ratio of the tonnage length to the depth of the second deck. If the ship floats at a draught at or below the apex of the triangle, then the reduced tonnages may be used. If, however, the tonnage mark is submerged, then the normal tonnages must be used. The principle behind the modified and alternative tonnages is that reduced tonnages were previously assigned if a tonnage hatch were fitted. This hatch seriously impaired the safety of the ship. Thus, by omitting the hatch the ship is more seaworthy and no tonnage penalty is incurred. The tonnage mark suitable for alternative tonnage is shown in Figure 10.1. The distance W is × the moulded draught to the tonnage mark.

Question 170

What is Modified tonnage?

Answer 170

Many ships are designed to run in service at a loaded draught which is much less than that allowed by the Load Line Rules. If the freeboard of a vessel is greater than that which would be assigned taking the second deck as the freeboard deck, then reduced gross and net tonnages used to be allowed. In this case the tonnage of the space between the upper deck and the second deck is not added to the underdeck tonnage and is therefore not included in the gross tonnage or net tonnage, both of which are consequently considerably reduced. As an indication that this modified tonnage has been allocated to the ship, a tonnage mark must be cut in each side of the ship in line with the deepest load line and 540 mm aft of the centre of the disc. If any cargo is carried in the 'tween decks, it is classed as deck cargo and added to the tonnage.

Question 171

what are classification societies and how is a ship classified and why is this important?

Answer 171

A classification society is an organisation whose function is to ensure that a ship is built to match its rules of construction and that the standard of construction is maintained. classification societies are independent of the insurance companies. A ship is classified according to the standard of construction and equipment. The cost of insurance of both ship and cargo depends to a great extent upon this classification, and it is therefore to the advantage of the shipowner to have a high class ship. There are a number of large societies, each being responsible for the classification of the majority of ships built in at least one country, although in most cases it is left to the shipowner to choose the society. Each of these societies has its own rules which may be used to determine the design, composition and therefore the ultimate strength of the structural members. A classification society has the power to require owners to alter the structure of an existing ship if they consider that the structure is weak.

Question 172

What are annual and special surveys?

Answer 172

To ensure that the ship remains worthy of its classification, annual and special surveys are carried out by the surveyors. The special surveys are carried out at intervals of 4–5 years. In an annual survey the ship is examined externally and, if considered necessary, internally. All parts liable to corrosion and chafing are examined, together with the hatchways, closing devices and ventilators to ensure that the standards required for the Load Line Regulations are maintained. The steering gear, windlass, anchors and cables are inspected. A more thorough examination is required at the special surveys. The shell plating, stern frame and rudder are inspected, the rudder being lifted if considered necessary. The holds, peaks, deep tanks and double bottom tanks are cleared, examined and the tanks tested. The bilges, limbers and tank top are inspected, part of the tank top ceiling being removed to examine the plating. With respect to any corroded parts, the thickness of the plating must be determined by ultrasonic testing.

Question 173

What is IWS?

Answer 173

The in-water survey (IWS) is used to check those parts of a ship which are usually surveyed in dry dock. It includes visual examination of the hull, rudder, propeller, sea inlets and so on, and the measurement of wear down of rudder bearings and stern bush. There are several requirements to be met before IWS is allowed. High resolution colour photographs are taken of all parts which are likely to be inspected, before the ship is launched. The rudder and stern frame are designed for easy access to bearings. The ship must be less than 10 years old, have a high resistance coating on the underwater hull and be fitted with an impressed current cathodic protection system. At the time of the inspection the hull is cleaned by one of the many brush systems available. The water must be clear and the draught less than 10 m. The inspection may be made by an underwater closed-circuit television camera. The camera may be hand-held by a diver or carried by a hydraulically propelled camera vehicle, remotely controlled from a surface monitoring station. The use of remotely operated unmanned vehicles is transforming this area of ship inspection/maintenance.

Question 174

explain what is meant by discontinuities?

Answer 174

Survey of Ships – Discontinuities If there is an abrupt change in section in any type of stressed structure, particularly high stresses occur at the discontinuity. Should the structure be subject to fluctuating loads, the likelihood of failure at this point is greatly increased .A ship is a structure in which discontinuities are impossible to avoid. It is also subject to fluctuations or even reversals of stress when passing through waves. The ship must be designed to reduce such discontinuities to a minimum, while great care must be taken in the design of structural detail with respect to any remaining changes in section. The most highly stressed part of the ship structure is usually that within 40%–50% of its length amidships. Within this region every effort must be made to maintain a continuous flow of material. Difficulties occur at hatch corners (Figure 10.4). Square corners must be avoided and the corners should be radiused or elliptical. With radiused corners the plating at the corners must be thicker than the remaining deck plating. Elliptical corners are more efficient in reducing the corner stress and no increase in thickness is required.Similarly openings for doors, windows, access hatches, ladderways and so on in all parts of the ship must have rounded corners with the free edges dressed smooth. If a bridge structure is fitted over more than 15% of the ship’s length, the bridge side plating must be tapered or curved down to the level of the upper deck. The sheerstrake is increased in thickness by 50% and the upper deck stringer plate by 25% at the ends of the bridge. Four 'tween deck frames are carried through the upper deck into the bridge space at each end of the bridge to ensure that the ends are securely tied to the remaining structure.In bulk carriers, where a large proportion of the deck area is cut away to form hatches, the hatch coamings should preferably be continuous and tapered down to the deck level at the ends of the ship.Longitudinal framing must be continued as far as possible into the ends of the ship and scarphed gradually into a transverse framing system (Figure 10.5). This problem is overcome in oil tankers and bulk carriers by carrying the deck and upper side longitudinals through to the collision bulkhead, with transverse framing in the fore deep tank up to the tank top level and in the fore peak up to the upper deck. Similarly at the after end the side and deck longitudinals are carried aft as far as they will conveniently go. A particular difficulty arises with the longitudinal bulkheads which must be tapered off into the forward deep tank and engine room. In the larger vessels it is often possible to carry the bulkheads through the engine room, the space at the sides being used for auxiliary spaces, stores and workshops. A similar problem occurs in container ships, when it is essential to taper the torsion box and similar longitudinal stiffening gradually into the engine room and the fore end. The fine lines of these ships cause complications which are not found in the fuller vessels. In these faster vessels the importance of tying the main hull structure efficiently into the engine room structure cannot be sufficiently emphasised.

Question 175

What are Load line surveys?

Answer 175

To ensure that the vessel is maintained at the same standard of safety as when it was built, annual load line surveys are made by the Assigning Authority. An inspection is made of all those items which affect the freeboard of the ship and are included in the Conditions of Assignment. A note is made about of any alterations to the ship which could affect the assigned freeboard. The main areas of the ship that are checked relate to the: *visibility of the freeboard marks on both sides of the vessel *accuracy of the freeboard marks *quality of the stability information available to the officers *condition of any openings that need to be watertight *ability of the vessel to allow water to drain from the decks *structure is of sufficient strength – no significant corrosion or damage which would impact on the design strength of the vessel *suitability of the crew accommodation.

Question 176

explain the constructional arrangements of the stem?

Answer 176

Stem The stem is formed by a solid bar which runs from the keel to the loaded waterline at the very front of the vessel’s bow. The shell plating is stopped about 10 mm from the fore edge of the bar in order to protect the plate edges. At the bottom, the foremost keel plate is wrapped round the bar and is known as a coffin plate, due to its shape. A similar form of construction is used at the top of the stem. The ‘stem’ bar is solid and rounded which improves the appearance considerably, particularly where the keel and side plates overlap. Above the stem bar the stem is formed by plating which is strengthened by a welded stiffener on the centreline, the plating being thicker than the normal shell plating near the waterline but reduced in thickness towards the top. The plate stem is supported at intervals of about 1.5 m by horizontal plates known as breast hooks, which extend from the stem to the adjacent transverse frame. The breast hooks are welded to the stem plate and shell plating and are flanged on their free edge.Modern stems are raked at 15°–25° to the vertical, with a large curve at the bottom, running into the line of the keel. Above the waterline some stems curve forward of the normal rake line to form a clipper bow. The flare of the bow increases the water plane area the deeper it is immersed in the water due to the pitching motion of the ship. This increase in area in turn increases the ship’s buoyancy, thus helping to resist the pitching, and the additional forces must be taken into account when calculating the maximum bending moment caused by wave action. The downside of this arrangement is the added possibility of damage due to pounding.

Question 177

with the aid of sketches explain the fore end constructional arrangements provided to resist panting?

Answer 177

The structure of the ship is strengthened to resist the effects of panting from 15% of the ship’s length from forward to the stem and aft of the after peak bulkhead. In the fore peak, side or panting stringers are fitted to the shell at intervals of 2 m below the lowest deck (Figure 7.2). No edge stiffening is required as long as the stringer is connected to the shell, a welded connection being used in modern ships. Depending upon the size of the vessel, and the class rules, panting stringers may be omitted if the hull thickness in increased. The side stringers meet at the fore end, while in many ships a horizontal stringer is fitted to the collision bulkhead in line with each shell stringer. This forms a ring round the fore peak tank and supports the bulkhead stiffeners. Channel beams are fitted at alternate frames in line with the stringers, and connected to the frames by brackets. The intermediate frames are bracketed to the stringer. The free edge of the bulkhead stringer may be stiffened by one of the beams. In fine ships it is common practice to plate over the beams, lightening holes being punched in the plate. The tank top is not carried into the peak, but solid floors are fitted at each frame. These floors are slightly thicker than those in the double bottom space and are flanged on their free edge.An interesting development is the X-bow (see Figure 7.3) design that was originally designed by the Norwegian shipbuilders ‘Ulstein’ for use on offshore vessels. The design gives the vessel an increasing underwater volume as the vessel pitches, similar to the flare in the bow of a conventional ship. This reduces the motion of the vessel due to wave action. The design is now finding its way on to passenger ships where the comfort of the passengers, and crew, is a prime concern. The same consideration can be extended to the comfort of the crew working on conventional ships that are operating in rough seas for long periods. In the traditional arrangement, side frames are spaced 610 mm apart and, being so well supported, are much smaller than the normal hold frames. The deck beams are supported by vertical angle pillars on alternate frames, which are connected to the panting beams and lapped onto the solid floors. A partial wash-plate is usually fitted to reduce the movement of the water in the tank. Intercostal plates are fitted for 2 or 3 frame spaces in line with the centre girder. The lower part of the peak is usually filled with cement to ensure efficient drainage of the space.Between the collision bulkhead and 15% length from forward, the main frames, together with their attachment to the margin, are increased in strength by 20%. In addition, the spacing of the frames from the collision bulkhead to 20% of the length from forward must be 700 mm. Light side stringers are fitted in the panting area in line with those in the peak. These stringers consist of intercostal plates connected to the shell and to a continuous face angle running along the toes of the frames. These stringers may be dispensed with if the shell plating is increased in thickness by 15%. This proves uneconomical when considering the weight but reduces the obstructions to cargo stowage in the hold. The peak is usually used as a tank and therefore such obstructions are of no importance. The collision bulkhead is stiffened by vertical bulb plates spaced about 600 mm apart inside the peak. It is usual to fit horizontal plating because of the excessive taper on the plates which would occur with vertical plating. Figure 7.4 shows the construction of a collision bulkhead.The structure in the after peak is similar in principle to that in the fore peak, although the stringers and beams may be fitted 2.5 m apart. The floors should extend above the stern tube or the frames above the tube must be stiffened by flanged tie plates to reduce the possibility of vibration. It is recommended that the bow of an ice breaking vessel is constructed without a bulbous bow. However, a vessel with limited ice breaking capacity should be constructed with added strength as detailed in the new ‘polar code’ that has recently been developed by the International Maritime Organization (IMO).

Question 178

explain the fore end constructional arrangements fitted to resist pounding?

Answer 178

Arrangements to Resist Pounding The structure is strengthened to resist the effects of pounding from the collision bulkhead to 25% of the ship’s length from forward. The flat bottom shell plating adjacent to the keel on each side of the ship is increased in thickness by between 15% and 30% depending upon the length of the ship, larger ships having smaller increases.In addition to increasing the plating, the unsupported panels of plating are reduced in size. In transversely framed ships the frame spacing in this region is 700 mm compared with 750–900 mm amidships. Longitudinal girders are fitted 2.2 m apart, extending vertically from the shell to the tank top, while intermediate half-height girders are fitted to the shell, reducing the unsupported width to 1.1 m. Solid floors are fitted at every frame space and are attached to the bottom shell by continuous welding.If the bottom shell of a ship is longitudinally framed, the spacing of the longitudinals is reduced to 700 mm and they are continued as far forward as practicable to the collision bulkhead. The transverse floors may be fitted at alternate frames with this arrangement and the full-height side girders may be fitted 2.1 m apart.

Question 179

with the aid of a sketch explain the purpose of a bulbous bow?

Answer 179

Bulbous Bow One of the most important tasks facing designers of ships is to optimise hull performance. There are two aspects to this activity: the first is to reduce hull resistance and the second is to reduce the effect of ‘wave drag’. The act of pushing the vessel through the water will create a bow wave. If a sphere is immersed just below the surface of the water and at the bow of the ship the wave from the sphere interferes with the normal bow wave created by the vessel and results in a smaller bow wave. Thus the force required to produce the bow wave is reduced. At the same time, however, the wetted surface area of the ship is increased, causing a slight increase in the frictional resistance. In slow ships the effect of a bulbous bow could be an increase in the total resistance, but in faster ships, where the wave making resistance forms a large proportion of the total resistance, the latter is reduced by fitting a bulbous bow. A bulbous bow also increases the buoyancy forward and hence reduces the pitching of the ship to some small degree. Optimising the design of the bulbous bow is very important. On modern ships the operating range might also be increased and therefore the design of the bow might not be as straight forward as first thought by the designers. Although the actual design of a bulbous bow will be optimised to an actual vessel, a typical construction of the bulbous bow is shown in Figure 7.6. The stem plating is formed by steel plates supported by a centreline web and horizontal diaphragm plates 1 m apart. The outer bulb plating is thicker than the normal shell plating, partly because of high water pressures and partly due to the possible damage by anchors and cables. It is often found that due to the reduced width at the waterline caused by the bulb, horizontal stringers in the fore peak prove uneconomical and complete perforated flats are fitted. The framing of the bulb could be joined to the general frame of the fore peak using diaphragm plates.

Question 180

explain the anchor and cable arrangements?

Answer 180

A typical arrangement for raising, lowering and stowing the anchors of a ship is shown in Figure 7.1. The anchor is attached to a heavy chain cable which is led through the hawse pipe over the windlass and down through a chain pipe or spurling pipe into the chain locker. The hawse pipes may be constructed of mild steel tubes with castings at the deck and shell, or cast in one complete unit for each side of the ship. There must be ample clearance for the anchor stock to prevent jamming and they must be strong enough to withstand the hammering which they receive from the cable and the anchor. The shell plating is increased in thickness with respect to each hawse pipe and adjacent plate edges are fitted with mouldings to prevent damage. A chafing piece is fitted to the top of each hawse pipe, while a sliding cover is arranged to guard the opening. The cable stopper is a casting with a hinged lever, which may be used to lock the cable in any desired position and thus relieve the load from the windlass either when the anchor is out or when it is stowed. The drums of the windlass are shaped to suit the cable and are known as cable lifters. The cable lifters are arranged over the spurling pipes to ensure a direct lead for the cables into the lockers. The windlass may be either steam or electric in common with the other deck auxiliaries. Warping ends are fitted to assist in handling the mooring ropes. The windlass must rest on solid supports with pillars and runners with respect to the holding down bolts, a 75 mm teak bed being fitted directly beneath the windlass. The chain pipes are of mild steel, bell mouthed at the bottom. The bells may be of cast iron, well rounded to avoid chafing. The pipes are fitted as near as possible to the centre of the chain locker for ease of stowage. The chain locker may be fitted between the upper and second decks, below the second deck or in the forecastle. It must be of sufficient volume to allow adequate headroom when the anchors are in the stowed position. The locker is usually situated forward of the collision bulkhead, using this bulkhead as the after locker bulkhead. The locker is not normally carried out to the ship side. The stiffeners are preferably fitted outside the locker to prevent damage from the chains. If the locker is fitted in the forecastle, the bulkheads may be used to support the windlass. A centreline division is fitted to separate the two chains and is carried above the stowed level of the chain but is not taken up to the deck. It is stiffened by means of solid half-round bars while the top edge is protected by a split pipe. Foot holds are cut in to allow access from one side to the other. A hinged door is fitted in the forward bulkhead, giving access to the locker from the store space. Many lockers are fitted with false floors to allow drainage of water and mud, which is cleared by a drain plug in the forward bulkhead, leading into a drain hat from where it is discharged by means of a hand pump. The end of the cable must be connected to the deck or bulkhead in the chain locker. With this method, use is made of the existing stiffeners fitted to the fore side of the collision bulkhead. Two similar sections are fitted horizontally back to back, riveted to the bulkhead and welded to the adjacent stiffeners. A space is allowed between the horizontal bars to allow the end link of the cable to slide in and be secured by a bolt.

Question 181

describe what a passenger ship is some of the rules pertaining to this vessel and draw the general arrangement sketch for a passenger ship

Answer 181

Passenger ship is defined as ‘a vessel that is designed to carry more than 12 passengers on an international voyage’. These vessels must comply with the relevant International Maritime Organization (IMO) regulations included in the Safety Of Life At Sea (SOLAS) and the Load Line Conventions for passenger ships. Cargo ships can still carry up to 12 passengers without being re-classed as a passenger ship. Passenger ships, in general, range from small river ferries to large ocean-going vessels. Cruise ships can now carry over 8000 passengers and are designed to provide maximum comfort for all guests on board. These ships include in their services large dining rooms, luxury restaurants, theatres, cinemas, swimming pools with water slides, gymnasia, open deck spaces and shops. They are being designed increasingly to provide the majority of rooms with a balcony and a ‘sea view’. Ferries are also being designed after listening to customers’ needs. For example, ferries carrying large numbers of trucks might provide a specific place where the drivers can rest and have a meal in a restaurant that has been designed with them in mind. Roll-on/roll-off (Ro-Ro) ferries are now ‘double decked’ and arranged so that both decks can be loaded at the same time. This is important as the time in port for any vessel is non-revenue-earning and therefore needs to be reduced to a minimum. Minimum standards for crew accommodation are now required under the International Labour Organization’s (ILO) Maritime Labour Convention 2006 (MLC 2006).A sign of the times is that most people look to manage risk as they go about their daily work. The IMO is no different and during the 1990s and into the early twentieth century they worked on developing a risk-based approach to the operation and construction of ships. SOLAS Chapter II-1. This section of SOLAS relates to the structure, subdivision and stability, machinery and electrical installations for passenger and cargo ships. This part of SOLAS is continually being updated in light of experience and to improve the survivability of ships in the event of damage. In 2006 came the plans to improve the safety of passenger ships based around the Safe Return to Port (SRtP) concept for these ships. SRtP centres around the notion that ‘your ship is your best lifeboat’, and therefore if the ship can be constructed with maximum ‘survivability’, then there will be less of a need to resort to the much smaller and more vulnerable ‘lifeboats’ in the event of an accident. A further development for the very large passenger ship is the ‘diesel electric’ (power station) concept. This is where the main engines are large diesel alternators producing high voltage electricity that is used to either power the ship or run large electrical loads servicing the passengers, such as the air conditioning compressors, ventilation fans or galley and laundry equipment. Cruise ships that can carry more than 8000 people are now being built, and in the event of a major disaster, those people may need evacuating from the vessel. SOLAS sets out the rules for calculating the minimum number and maximum capacity for the lifeboat requirements for each vessel (see Figure 1.1).SOLAS regulations set out the general requirements for lifeboats and state that no lifeboat shall be approved with a passenger capacity of more than 150 persons. However, on a vessel carrying more than 8000 people, larger lifeboats have been developed that require special permission from the flag state to enable them to be used as they are outside of the current regulations

Question 182

what are cargo liners?

Answer 182

Cargo liners are vessels designed to carry a variety of cargoes between specific ports and, as stated, the modern configuration of this type of vessel is the container ship and most of the non-liquid cargoes and some liquid cargoes are now carried around the world by this type of ship.

Question 183

with the aid of a sketch describe the general arrangement layout of a container ship?

Answer 183

Figure 1.2 shows the layout of a modern container ship, the size of which can be measured by the number of containers it can carry. As with many ships, at the extreme forward end is a tank known as the fore peak which may be used to carry water ballast or fresh water. Above this tank is an area called the chain locker and also a storage space. At the after end is a tank known as the after peak which generally encloses the stern tube in a watertight compartment. At the bottom of the vessel and between the two peak bulkheads is a continuous tank top forming a double bottom space which is further subdivided into smaller tanks suitable for carrying oil fuel, fresh water and water ballast. The machinery space consists of heavy equipment and as such will place considerable ‘local’ stresses on the structure of the ship. If placed at the aft end, as with Figure 1.3, the machinery will exert a maximum bending moment on the ship’s hull. If placed in a more central position, as shown in Figure 1.2, then the ‘light ship’ bending moment and hull stress will be reduced. The latest diesel electric propulsion systems enable the designer to place the main diesels in the best possible place for the benefit of the hull thereby maximising the cargo carrying capacity. The reason for this is that the positions of the engines are not determined by the propeller shafting and gearboxes. The connection to the inboard electric motor or podded drive will be via electric cables and not by mechanical equipment, such as power transmission shafts and gearboxes. Currently, in 2022, most ships use some form of oil as their major energy source. The general term for this fuel is bunkers, which is a legacy from the early days of shipping when coal was the main source of fuel and the coal was loaded into coal bunkers. The oil fuel bunkers are taken on board when required and loaded into designated tanks called bunker tanks. Due to the possibility of the presence of water, the fuel is transferred into settling tanks where any water in the mix will settle to the bottom of the tank. The water can then be drained off prior to the fuel being further treated on board with filters and centrifugal purifiers before being used in the engines. In order to help the stability of a ship, it is generally a good idea to have the bunker tanks as low in the ship as practical, and therefore the tanks used as bunker tanks are usually the double bottom tanks. It is also advantageous to have the bunkers arranged as close to the machinery space as possible. When ships have the engine room sighted around the centre part of the vessel, there is a long path for the propeller shaft to take before it exits the hull via the stern tube bearing. Between the aft engine room bulkhead and the after peak bulkhead is a watertight shaft tunnel enclosing the shaft and allowing access to the intermediate shaft and bearings directly from the engine room. An exit in the form of a vertical trunk is arranged at the after end of the tunnel in case of emergency. The arrangement of the machinery space on many modern ships is further aft and therefore the propeller shafting and intermediate bearings are sighted in the machinery space. A walkway and guard rail are placed close to the rotating shaft so that the watch keeping engineer can safely inspect the drive train for correct operation at any time. On container ships, the hatches will be designed to carry the weight of loaded containers. Here, several tiers of containers will be carried on top of the hatch covers. The calculations relating to the strength of the hatch coamings will also need to consider the maximum forces set up by the weight of the containers carried on top of the hatches, as well as the forces due to the pitching and rolling of the vessel.

Question 184

with the aid of a sketch describe the general arrangement of a cargo ship

Answer 184

The general arrangement is shown in Figure 1.4. To assist the safe stowage of cargo the cargo space is divided into lower holds and compartments between the decks, or ’tween decks. Many ships have three decks, thus forming upper and lower ’tween decks. This system allows different cargoes to be carried in different compartments and reduces the possibility of the cargo getting crushed. Access to the cargo compartments is provided by means of large hatchways which are all closed by hydraulically operated steel hatch covers Some of these ships will be refrigerated, giving it the ability to carry fruit and other ‘perishable goods’ such as fish and sometimes meat. However, the general arrangement of the reefer ship is very similar to the example of the general cargo vessel shown in Figure 1.4.The difference would be that the holds and hatch covers will be insulated and additional refrigeration equipment will be required to reduce the temperature in the hold and to keep the temperature low for the duration of the voyage. For example, a hold full of bananas will need to be loaded as quickly as possible, and then as soon as the hatch covers are closed, the hold temperature will need to be reduced to 4°C +/− 0.1 °C within 48hrs of loading the cargo.The cargo of bananas is carried at this temperature for the duration of the voyage. To achieve this the machinery must be working to the peak of its ability. Therefore, for this to happen, the ship’s staff must ensure that all the equipment’s maintenance is up to date. Suitable cargo handling equipment is provided in the form of hydraulic or electrically powered cranes. Heavy lift equipment may be fitted and is usually situated next to one or more hatches. A forecastle is fitted to reduce the amount of water shipped forward and to provide adequate working space for handling ropes and cables.The forecastle also acts very effectively to protect the forward hatches from heavy weather damage. Hatch covers are a prime area to guard against a breach of watertight integrity. The hatch covers are well designed, but the forecastle provides a very good first line of defence and deflects the full force away from the hatches. However, the work of these vessels is now being taken over by bulk carriers and smaller container vessels. Figure 1.4 shows the layout of a typical cargo ship. The significant advantage of these vessels is ‘flexibility’ so a large variety of cargo can be carried. The space immediately forward of the machinery space may be subdivided into lower ’tween decks and hold/deep tank, thus improving the ability to even out the stress on the hull and/or give different options for carrying the different cargo and/or liquids such as fuel or water or dangerous goods. To contribute to the flexibility of these vessels they are fitted with cargo handling equipment, such as cranes. General cargo ships have hatches which will allow cargoes such as timber, cars, locomotives and crates of machinery to be loaded. A cargo tramp of about 10 000 tonne deadweight may have five hatches, each 10 m long and 7 m wide, although one hatch, usually to No. 2 hold, is often increased in length. Large hatches also allow easy handling of cargoes.

Question 185

with the aid of a sketch describe the general arrangement of a RO-RO ship?

Answer 185

Roll-On/Roll-Off Vessels These vessels are designed with flat decks and have moveable watertight divisions to enable vehicles and tractor-trailer units to be driven into and off the vessel under their own power. Having such a large continuous deck means that any appreciable accumulation of water will have a magnified adverse effect on the vessel’s stability. This is due to the ‘free surface effect’ inherent in such a body of water. The Ro-Ro vessel is particularly susceptible to this feature, and all the staff should be well aware of the dangers of the ship becoming unstable if it is not operated correctly. A ramp is fitted at one or at both ends of the ship allowing direct access for cars, trucks and buses which remain on board in their laden state. These ramps lead to large outer doors which have in the past been a source of leakage due to damage and/or incorrect operation. Any ingress of water will come straight onto the Ro-Ro decks leading to the possible problem with free surface effect mentioned earlier. Containers may be loaded 2 or 3 high by means of fork lift trucks. Lifts and inter-deck ramps are used to transfer vehicles between decks. Most modern vessels have stern ramps that are angled to allow vehicles to be loaded from a straight quay (Figure 1.6). This would circumvent the need for a special ’link span’ arrangement or blind end berth. Some specialist Ro-Ro vessels have very large car carrying capacity and are used to move considerable numbers of new cars around the world. Loading these weights (containers) will quickly alter the trim of the vessel and move it away from the vertical position. This will have the effect of not being able to drop subsequent containers vertically into position. Therefore, there must be a means of keeping the ship upright. This is achieved by pumping water from one side of the vessel to the other. The movement of the water weight counterbalances the added weight of the containers (or cars) and is the method used to achieve the requirement of keeping the vessel vertical. The name of the system is called the 'heeling arrangements' and more details can be seen here. Where the vessel has a combined container/Ro-Ro capacity the term Lo-Lo is sometimes used. This refers to the lift-on lift-off feature of the containerised cargo. Accommodation for crew and passengers includes restaurants and fast food outlets as well as specific areas for long distance lorry drivers where they can complete ‘official’ rest periods that satisfies the ‘drivers’ hours’ regulations. Again these vessels tend to work as liner vessels and ferries. The ports of Dover and Calais, either side of the English Channel, are very good examples of ports that are highly specialised in handling the Ro-Ro ferry operation. During the 1970s and 1980s designers started to produce roll-on roll-off car ferries and cargo ships. The problem was that the main ro-ro deck was continuous from bow to stern and had large doors at either end. In general, seafarers realised that only a very shallow depth of water across that main deck would be sufficient to capsize the vessel. Following a major disaster, where that actually happened, the rules were changed and watertight divisions were included in the design of these vessels to reduce the ‘free surface effect’ in the event of any water making its way onto the main deck.

Question 186

with the aid of a sketch describe the general arrangement of an oil tanker

Answer 186

Tankers are used to carry oil in bulk, and they can be divided into two different basic types. One type carries unrefined ‘crude’ oil while the other type, known as a product carrier, carries different types of ‘refined’ oils such as lubricating oil, naphtha, petrol or diesel oil.The crude oil carriers are termed very large crude carriers (VLCC; 200 000–300 000 gt) or ultra large crude carriers (ULCC; 560 000+ gt). If the crude oil is unrefined, then it will contain all the different types of oil all in one. This makes the crude oil quite volatile, and in the past there have been some significant explosions due to the mixture of hydrocarbons in a cargo tank. Modern tankers are now fitted with fixed inert gas systems (all tankers over 8000 gt as of 1 January 2016). This means that when the oil is pumped from a tank during a discharge, the space above is filled with a gas that will not support a fire or explosion. Conversely as the oil is loaded the inert gas is released. The machinery space and accommodation on oil tankers is situated aft. This means that the designers can provide an unbroken cargo space forward of these features. The cargo tanks are subdivided by longitudinal and transverse bulkheads, and the tanks are separated from the machinery space by an empty compartment known as a cofferdam. A pump room may also be provided at the after end of the cargo space just forward of the engine room and may form part of the cofferdam (Figure 1.3). It is then possible to have the cargo pumps situated in the pump room and the prime mover (diesel engines, steam turbines or electric motors) situated in the machinery space. A gas tight seal is maintained around any rotating drive shaft penetrating the bulkhead between the machinery space and the pump room.In the older vessels a double bottom was required only with respect to the machinery space and may have been used for the carriage of oil fuel and fresh water. Modern vessels must now have a ‘double hull’ covering the length of the ship. A forecastle is sometimes required and is used as a storage space, although on larger tankers this area is usually a continuation of the deck rather than a step change in the line of the ship. As the accommodation and navigation bridges are provided at the after end, the deck space may be left unbroken by superstructure and all the services and living arrangements, including catering equipment and facilities, are concentrated in one area.In the smaller tankers much of the deck space is taken up by pipes and hatches. Therefore it is usual to provide a longitudinal platform or pathway to allow easy access to the forecastle and bow sections. The walkway is situated above the pipes and is known as the ‘flying bridge’. On the VLCCs and ULCCs there is sufficient space to walk easily around the pipework. However the distance is so great that sometimes bicycles are provided for the crew. An alternative arrangement is for the ship to have a pump allocated to each cargo tank. Known as ‘deepwell’ pumps they have the prime mover sited on-deck and the pump at the bottom of each tank driven by a long drive shaft. Another feature required by modern tankers is the ability to moor up to single buoy moorings (SBMs). These are typically arrangements where the output from an oil production field is fed along a pipe line, resting on the sea/river bed and leading to a mooring buoy that could be several miles away. Readers can see that this single point, situated on the surface of the water, would warrant a special arrangement for securing the vessel. The Oil Companies International Marine Forum (OCIMF) produces guidelines for the design strength of systems for mooring to SBMs. The method is to use the anchor chain; however, the angle of the pull on the securing equipment will have moved from the sea/river floor to the surface. Therefore, the weight on the equipment will be at a much shallower angle.

Question 187

draw a midship section of an oil tanker showing longitudinal arrangement and mid section showing web strengthening?

Question 188

what is an important feature for chemical carriers?

Answer 188

A considerable variety of chemical cargoes are required to be carried in bulk. Many of these cargoes are highly corrosive and incompatible with each other while others require close control of temperature and pressure. Special chemical carriers have been designed and built, in which safety and avoidance of contamination are of prime importance. To avoid corrosion of the structure, stainless steel is used extensively for the tanks, while in some cases coatings of zinc silicate or polyurethane are acceptable. Protection for the tanks is provided by double bottom tanks and wing compartments which are usually about one-fifth of the midship beam from the ship side (Figure 1.11, lower drawing)

Question 189

with the aid of sketch describe the general arrangement of bulk carriers

Answer 189

Bulk carriers are vessels built to carry such cargoes as ore, coal, grain and sugar in large quantities. They are designed for ease of loading and discharging with the machinery space aft, allowing continuous, unbroken cargo space forward of the accommodation. They are single deck vessels having long, wide hatches, closed by steel covers. The double bottom runs from stem to stern.In ships designed for heavy cargoes such as iron ore the double bottom is very deep and longitudinal bulkheads are fitted to restrict the cargo space (Figure 1.9). This system also raises the centre of gravity of the ore, resulting in a more comfortable ship. The double bottom and the wing compartments may be used as ballast tanks for the return voyage. Some vessels, however, were designed to carry an alternative cargo of oil in these tanks. With lighter cargoes such as grain, the restriction of the cargo spaces is not necessary although deep hopper sides may be fitted to facilitate the discharge of cargo, either by suction or by grabs. The spaces at the sides of the hatches are plated in as shown in Figure 1.10 to give self-trimming properties. In the past many bulk carriers had a tunnel is fitted below the deck from the midship superstructure to the accommodation at the after end. The remainder of the wing space was used for water ballast. Some bulk carriers are built with alternate long and short compartments. Thus if a heavy cargo such as iron ore is carried, it is loaded into the short holds.A cargo such as bauxite would be carried in the long holds, while a light cargo such as grain or timber would occupy the whole hold space.The double bottom is continuous in the cargo space, and it is raised at the sides to form hopper sides which improve the rate of discharge of cargo. Wide hatches are fitted for ease of loading, while in some ships small wing tanks are fitted to give self-trimming properties. In bulk carriers, if it is anticipated that cargo will be regularly discharged by grabs or by forklift trucks, it will be necessary to fit either additional protection to the ceiling of the tank or heavier flush plating. Bulk carriers have long, wide hatches to allow the cargo to fill the extremities of the compartment without requiring trimming manually.

Question 190

what is the red ensign yacht code?

Answer 190

The only departure from this requirement is in the super yacht sector, where the UK’s Maritime Coastguard Agency (MCA) has developed the 13–36 Passenger Yacht Code. This code is for use by the Red Ensign Group to register large passenger yachts that carry up to 36 passengers. These vessels find it very difficult to comply with the full requirements of the IMO regulations for passenger ships, which led the UK administration to develop the regulations for this sector. Red Ensign Yacht Code and comes in two sections, where part A is the Large Yacht Code and part B is the Passenger Yacht Code. The rules cover information about the: *strength and standard of design of a yacht’s structure *water and weather tight integrity *requirements for the machinery and electrical installation *steering arrangements *bilge pumping requirements *stability and freeboard *lifesaving equipment required *fire resistance and Firefighting equipment *radio communication *navigation equipment As well as things such as the use of helicopters and tenders/survival craft that are specific to yachts. The regulations also cover the requirements for the qualifications of the officers and crew that operate the yachts.

Question 191

what is SOLAS regulation II-1/3–10?

Answer 191

The MSC (maritime safety committee), at its 87th session in May 2010, adopted a new SOLAS regulation II-1/3–10 this contains ship construction standards for bulk carriers and oil tankers (resolution MSC.290(87)).These rules require that vessels have a design life of at least 25 years and that all the structural and manufacturing rules are consistent with this requirement.

Question 192

with reference to tanker what is the special survey?

Answer 192

Tankers five years old or more are subject to a ‘special’ survey. This survey will cover all the items in the ‘annual’ survey but will also examine all cargo tanks, ballast tanks, double bottom tanks, pump rooms, any pipe tunnels, cofferdams and void spaces. The aim is to ensure that the vessel is structurally sound for the next five years. The survey inspection will be backed up by hull thickness data and surveyors will look for corrosion, damage, fractures, deformation or set and any other structural deterioration. The vessel will need to be dry-docked and all the crude oil washing and ballasting systems need to be in good working order. The tank coatings also need to be in good condition. The first special survey will require a spot check of the double hull but subsequent surveys will require a more rigorous inspection. In addition to these inspections most respectable charters will insist on conducting a ‘tanker vetting’ process to ensure that the vessel is in a satisfactory condition to complete its charter and that it conforms to all the necessary requirements. Tanker vetting will cover the condition of the vessel as well as inspect all the necessary records and documents to ensure that the tanker has been operated and maintained to the level required by the flag administration, by the classification society and by the insurers.

Question 193

with reference to bulk carriers and tankers what is the harmonised Common Structural Rules (CSR)?

Answer 193

The harmonised Common Structural Rules (CSR) for Bulk Carriers and Tankers were introduced on 1 July 2015. This set of rules, developed by the International Association of Classification Societies (IACS), replaced the rules set independently for bulk carriers and for double hull oil tankers. The rules are updated regularly in light of new information. IACs are careful to explain that these rules cover self-propelled bulk carriers and double-hulled tankers, which are able to sail to any part of the world in almost any weather condition. The only restriction is sailing in icy conditions, which requires special arrangements. The first and common part is arranged in 13 chapters and covers the minimum requirements for the strength of the hull, such as expected wave loads, hull girder strength as well as minimum buckling and fatigue characteristics. A design life of 25 years is assumed and forms the base for the size of the scantlings and so on. The second part includes information specifically about the construction of the two different types of vessels.

Question 194

explain the construction and purpose of the double bottom system on ships?

Answer 194

Ocean-going ships (with the exception of tankers, which now have to be double hulled) and most coastal vessels are fitted with a double bottom system of construction, which extends from the fore peak bulkhead almost to the after peak bulkhead. The double bottom consists of the outer shell and an inner skin or tank top between 1 m and 1.5 m above the keel. This provides a form of protection in the event of damage to the bottom shell, and it also provides protection to the environment from any oil or contaminants that may be in the bilges at the time of a breach of the hull. However, the International Convention for the Prevention of Pollution from Ships (MARPOL) specifies whether, and how, fuel and lubrication oil is permitted to be stored in ‘double bottom’ tanks. The tank top, being continuous, contributes to the hull girder strength. From 1997 additional support was required for the platforms around the cargo and machinery areas. These added strength items are called ‘solid floors’ and are thicker than the normal double bottom plating. Additional strength may also be required for high powered engines or gearboxes and thrust blocks bolted directly to the bottom plating will need to be fixed to plating of at least 19 mm thickness. Additional brackets or high strength material might have to be used to withstand the high ‘local’ loading that will need to be transmitted to support the vessel during this time. The double bottom space contains a considerable amount of scantlings and is therefore unsuitable for carrying much cargo. Where the regulations allow, double bottom tanks may be used for the carriage of oil fuel, fresh water and water ballast. They are subdivided longitudinally and transversely to reduce any free surface effect. Double bottom tanks can be filled or emptied with the different liquids that are required to be carried, and they can also be used to correct the heel of a ship or to change the trim. Access to these tanks is arranged in the form of manholes with watertight covers (Figure 4.1) and care must be taken when entering these tanks as they are dangerous spaces and could have an oxygen-depleted or poisonous atmosphere. Under hatchways, where the tank top is most liable to damage, the plating or protection must be increased to the tank’s ceiling, and the plating is at least 10% thicker in the engine room. In the lower part generally considered to be the bilges, the tank top may be either continued straight out to the shell, or knuckled down to the shell by means of a tank margin plate set at an angle of about 45° to the tank top and meeting the shell almost at right angles. It has the added advantage, however, of forming a bilge space into which water may drain and has, in the past, proven to be popular. If no margin plate is fitted it is necessary to fit drain hats or wells in the after end of the tank top in each compartment so that the bilges can be pumped dry.

Question 195

explain the construction and purpose of a continuous centre girder and side girders

Answer 195

The hull girder strength can be enhanced with the inclusion of a continuous centre girder and/or side girders, these extending longitudinally from the fore peak to after peak bulkhead. The centre girder is usually watertight except at the extreme fore and after ends where the ship is narrow, although there are some designs of ship where the centre girder does not form a tank boundary and is therefore not watertight. Centre girders must also provide sufficient strength to withstand the docking loads and additional ‘docking brackets’ may need to be included in the design. A pipe tunnel may be substituted for a centre girder as long as the construction of the tunnel is of sufficient strength. If the ship’s designer wishes to include this feature, a full set of detailed plans must be submitted for approval.Additional longitudinal side girders are fitted (at a maximum of 5 m apart) depending upon the breadth of the ship but these are neither continuous nor watertight, having large manholes or lightening holes in them. Special consideration must be given to providing side girders under the machinery space and/or the thrust block seating.

Question 196

sketch a typical watertight floor and state the function of watertight floors?

Answer 196

The tanks are divided transversely by watertight floors, which in most ocean-going ships are required to be stiffened vertically, to withstand the liquid pressure. Figure 4.2 shows a typical watertight floor

Question 197

describe the construction of a solid floor

Answer 197

In ships less than 120 m in length the bottom shell and tank top are supported at intervals of not more than 3 m by transverse plates known as solid floors. The name slightly belies the structure since large lightening holes are cut in them. In addition, small air release and drain holes are also cut at the top and bottom, respectively. These holes are most important since it is essential to have adequate access and ventilation to all parts of the double bottom. There have been many cases of personnel entering tanks which have been inadequately ventilated, with resultant gassing or suffocation. These tanks must still be regarded as enclosed spaces. The solid floor is usually fitted as a continuous plate extending from the centre girder to the margin plate. The side girder is therefore broken on each side of the floor plate and is referred to as being intercostal. Solid floors are required at every frame space in the machinery room, in the forward quarter length and elsewhere where heavy loads are experienced, such as under bulkheads and boiler bearings.

Question 198

with the aid of a sketch describe the construction of a bracket floor?

Answer 198

The shell and tank top between the widely spaced solid floors are stiffened by bulb angles or similar sections running across the ship and attached at the centreline and the margin to large flanged brackets. Additional support is given to these stiffeners by the side girder and by intermediate struts which are fitted to reduce the span. Such a structure was known as a bracket floor and is still referred to by some classification society rules for the construction of a ship’s hull (Figure 4.3).

Question 199

how is buckling caused?

Answer 199

Buckling is caused by distortion due to the welding of the floors and frames, together with the bending of the ship, needs to be guarded against and designers are now required to specify longitudinal stiffening in the double bottom for all ships over 120 m long.

Question 200

describe the construction of longitudinal frames

Answer 200

Longitudinal frames are fitted to the bottom shell and under the tank top, at intervals of about 760 mm. They are supported by the solid floors, the spacing of these floors may be increased to 3.7 m. Intermediate struts are fitted so that the unsupported span of the longitudinals does not exceed 2.5 m. Brackets are again required at the margin plate and centre girder, the latter being necessary when docking. The longitudinals are then arranged to line up with any additional longitudinal girders which are required for machinery or thrust block support.

Question 201

explain the construction and purpose of duct keel

Answer 201

Some ships might still be fitted with a tunnel or tunnels which is a convenient method of routing any pipework that is required to supply services to the forward part of the vessel. These are known as duct keels (see Figure 4.5) and as long as they are of equal strength, they can be used in place of the centre girder. The pipe tunnel extends from within the engine room along the length of the vessel to the forward holds. This arrangement then allows the pipes to be carried beneath the hold spaces and are thus protected against cargo damage. Access into the duct is arranged from the engine room. The pipes can then be inspected and repaired at any time independent of the weather (within working constraints) and cargo operations. At the same time it is possible to carry oil and water pipes in the duct, preventing contamination which could occur if the pipes passed through tanks. Duct keels are particularly important in insulated ships, allowing access to the pipes without disturbing the insulation. Ducts are not required aft since the pipes may be carried through the shaft tunnel. The duct keel is formed by two longitudinal girders up to 1.83 m apart. This distance must not be exceeded as the girders must be supported by the keel blocks when docking. The structure on each side of the girders is the normal double bottom arrangement. The keel and the tank top centre strake must be strengthened either by supporting members in the duct or by increasing the thickness of the plates considerably. It is vital that the duct space is treated as an enclosed space and great care must be taken before any person enters the area.

Question 202

with the aid of a sketch state the function of web frames

Answer 202

Web frames may be fitted in the machinery space and connected to strong beams or pillars in an attempt to reduce vibration (Figure 4.12). These web frames are about 600 mm deep and are stiffened on their free edge. It is usual to fit two or three web frames on each side of the ship, a smaller web being fitted in the 'tween decks. The exact scantling requirements will be determined by the strength calculations completed by the designer.

Question 203

with the aid of sketches explain the constructional arrangements of a ships side framing?

Answer 203

Side Framing The side shell is supported by frames which run vertically from the tank margin to the upper deck. These frames, which are spaced about 760 mm apart, are in the form of bulb angles. The lengths of frames are usually broken at the decks, allowing smaller sections to be used in the 'tween deck spaces where the load and span are reduced. The hold frames are of large section (300 mm bulb angle). They are connected at the tank margin to flanged tank side brackets (Figure 4.7). To prevent the free edge of the brackets buckling, a gusset plate is fitted, connecting the flange of the brackets to the tank top. A hole is cut in each bracket to allow the passage of bilge pipes. In insulated ships the tank top may be extended to form the gusset plate and the tank side bracket fitted below the level of the tank top (Figure 4.8). This increases the cargo capacity and facilitates the fitting of the insulation. Since the portion of the bracket above the tank top level is dispensed with, the effective span of the frame is increased, causing an increase in the size of the frame. The tops of the hold frames terminate below the lowest deck and are connected to the deck by beam knees (Figure 4.9) which may be flanged on their free edge to give added stiffness and strength. The bottoms of the 'tween deck frames are usually welded directly to the deck, the deck plating at the side being knuckled up to improve drainage. At the top, the 'tween deck frames are stopped slightly short of the upper deck and connected by beam knees (Figure 4.10). In some cases, the 'tween deck frames must be carried through the second deck and it is necessary to fit a collar round each frame to ensure that the deck is watertight. Figure 4.11 shows a typical collar arrangement, the collar being in two pieces, welded right round the edges.

Question 204

What does the external hull of a ship consist of?

Answer 204

The external hull of a ship consists of bottom plates making up the bottom of the shell. Plates making up the side shell and the decks are formed into longitudinal strips of plating known as strakes. The strakes themselves are constructed of a number of plates joined end to end and large, wide plates are used to reduce the welding required.

Question 205

Explain the importance and construction of side Shell Plating

Answer 205

The bottom and side shell plating of a ship also forms a major part of the longitudinal strength of the vessel. The most important part of the shell plating is that on the bottom of the ship, since this is the greatest distance from the neutral axis. As it is subjected to the highest forces, it is slightly thicker than the side shell plating. The keel plate is about 30% thicker than the remainder of the bottom shell plating, since it is subject to additional wear and tear when dry-docking. The strake adjacent to the keel on each side of the ship is known as the garboard strake which is the same thickness as the remainder of the bottom shell plating. The uppermost line of plating in the side shell is known as the sheerstrake which is 10–20% thicker than the remaining side shell plating. The thickness of the shell plating depends mainly on the length of the ship, varying between about 10 mm at 60 m and 20 mm at 150 m. The depth of the ship, the maximum draught and the frame spacing are, however, also taken into account. If the depth is increased, it is possible to reduce the thickness of the plating. In ships fitted with long bridges which extend to the sides of the ship, the depth with respect to the bridge is increased, resulting in thinner shell plating. Great care must be taken at the ends of such superstructures to ensure that the bridge side plating is tapered gradually to the level of the upper deck, while the thicker shell plating forward and aft of the bridge must be taken past the ends of the bridge to form an efficient scarph. If the draught of the ship is increased, then the shell plating must also be increased. Thus a ship whose freeboard is measured from the upper deck has thicker shell plating than a similar ship whose freeboard is measured from the second deck. If the frame spacing is increased the shell plating is required to be increased. The maximum bending moment of a ship occurs at or near amidships. Therefore the shell plating must be of sufficient strength to ensure its contribution to the hull girder strength, and it is reasonable to build the ship stronger amidships than at the ends. The main shell plating has its thickness maintained for 40% of its length amidships and tapered gradually to a minimum thickness at the ends of the ship. While the longitudinal strength of shell plating is of prime importance, it is equally important to ensure that its other functions are not overlooked. It is essential that the shell plating is watertight and, at the same time, capable of withstanding the static and dynamic loads created by the water. The shell plating, together with the frames and double bottom floors, resist the water pressure, while the plating must be thick enough to prevent undue distortion between the frames and floors. If it is anticipated that the vessel will regularly travel through ice, the shell plating in the region of the waterline forward is increased in thickness and small intermediate frames are fitted to reduce the widths of the panels of plating. The bottom shell plating forward is increased in thickness to reduce the effects of pounding. The shell plating and side frames act as pillars supporting the loads from the decks above and must be able to withstand the weight of the cargo. In most cases the strength of the panel which is required to withstand the water pressure is more than sufficient to support the cargo, but where the internal loading is particularly high, such as with respect to a deep tank, the frames must be increased in strength.

Question 206

Draw a midship section of a welded ship

Question 207

With the aid of a sketch describe the function of deep tank hatches

Answer 207

Deep tank hatches have two functions to fulfil (Figure 5.10). They must be watertight or oil-tight and thus capable of withstanding a head of liquid, and they must be large enough to allow normal cargoes to be loaded and discharged if the deep tank is required to act as a dry cargo hold. Such hatches may be 3 m or 4 m square. Because of the possible liquid pressure, the covers must be stiffened, while some suitable packing must be fitted in the coamings to ensure water tightness, together with some means of securing the cover. The covers may be hinged or arranged to slide.

Question 208

Explain the construction and purpose of Hatches

Answer 208

Large hatches must be fitted in the decks of dry cargo ships to facilitate loading and discharging of cargo. It is usual to provide one hatch per hold or 'tween deck, although in ships having large holds two hatches are sometimes arranged. The length and width of hatches depend largely upon the size of the ship and the type of cargo likely to be carried. The hatches are framed by means of hatch coamings which are vertical webs forming deep stiffeners. The heights of the coamings are governed by the Load Line Rules. On weather decks they must be at least 600 mm in height at the fore end and either 450 mm or 600 mm aft depending upon the draught of the ship. Inside superstructures and on lower decks no particular height of coaming is specified. It is necessary, however, for safety considerations, to fit some form of rail around any deck opening to a height of 800 mm. It is usual, therefore, at the weather deck, to extend the coaming to a height of 800 mm. In the superstructures and on lower decks portable stanchions are provided, the rail being in the form of a wire rope. These rails are only erected when the hatch is opened. The weather deck hatch coamings must be 11 mm thick and must be stiffened by a moulding at the top edge. Where the height of the coaming is 600 mm or more, a horizontal bulb angle or bulb plate is fitted to stiffen the coaming which has additional support in the form of stays fitted at intervals of 3 m. Hatch covers are made weathertight by means of tarpaulins which are wedged tight at the sides and ends, at least two tarpaulins being fitted on weather deck hatches. Modern ships are fitted with steel hatch covers (Figure 5.7). There are many types available, from small pontoons supported by portable beams to the larger self-supporting type, the latter being the most popular. The covers are arranged in 4 to 6 sections extending right across the hatch and having rollers which rest on a runway. They are opened by rolling them to the end of the hatch where they tip automatically into the vertical position. The separate sections are joined by means of wire rope, allowing opening or closing to be a continuous action, a winch being used for the purpose. Many other systems are available, some with electric or hydraulic motors driving sprocket wheels, some in which the whole cover wraps round a powered drum, while others have hydraulic cylinders built into the covers. In the latter arrangement pairs of covers are hinged together, the pairs being linked to provide continuity. Each pair of covers has one or two hydraulic rams which turn the hinge through 180°. The rams are actuated by an external power source, with a control panel on the side of the hatch coaming. The covers interlock at their ends and are fitted with packing to ensure that when the covers are wedged down, watertight cover is provided. Such covers do not require tarpaulins. At the hatch sides the covers are held down by cleats which may be manual or hydraulically operated. Composite hatch covers have just been approved and fitted to the first bulk carrier during 2015. One significant problem has been that the existing rules were set around steel hatches. Therefore, none of the elements of the rule book fitted to the composite structure; this is however a logical development as the material is strong and light. Hatch covers are very important and need to have the strength to withstand a pounding from the waves that could come over the side of the ship during heavy weather. ‘Tween deck covers can also constructed of composite material, which makes them lightweight and easier to handle.

Question 209

With the aid of sketches explain the construction of Beams and Deck Girders

Answer 209

The decks may be supported either by transverse beams in conjunction with longitudinal girders or by longitudinal beams in conjunction with transverse girders. The transverse beams are carried across the ship and bracketed to the side frames by means of beam knees. A continuous longitudinal girder is fitted on each side of the ship alongside the hatches. The beams are bracketed or lugged to the girders, thus reducing their span. With respect to the hatches, the beams are broken to allow open hatch space, and are joined at their inboard ends to either the girder or the hatch side coaming. A similar arrangement is necessary in way of the machinery casings. These broken beams are known as half beams and are usually shaped as bulb plates. There are several forms of girder in use, two of which are shown in Figure 5.2.If the girder is required to form part of the hatch coaming, the flanged girder (Figure 5.2a) is most useful since it is easy to produce and does not require the addition of a moulding to prevent chafing of ropes. Symmetrical girders such as in Figure 5.2(b) are more efficient but cannot form part of a hatch side coaming. Such girders must be fitted outboard of the hatch sides. The girders are bracketed to the transverse bulkheads and are supported at the hatch corners either by pillars or by hatch end girders extending right across the ship. Tubular pillars are most often used in cargo spaces since they give utmost economy of material and, at the same time, reduce cargo damage. In deep tanks, where hollow pillars should not be used, and in machinery spaces, either built pillars or broad flanged beams prove popular. Most modern ships are fitted with longitudinal beams which extend, as far as practicable, along the whole length of the ship outside the line of the hatches. They are bracketed to the transverse bulkheads and are supported by transverse girders which are carried right across the ship, or, with respect to the hatches and machinery casings, from the side of the ship to the hatch or casing. The increase in continuous longitudinal material leads to a reduction in deck thickness. The portion of deck between the hatches may be supported either by longitudinal or transverse beams, neither having any effect on the longitudinal strength of the ship. At points where concentrated loads are anticipated it is necessary to fit additional deck stiffening. Additional support is required with respect to winches, windlasses, deck cranes and capstans (Figure 5.3). The deck machinery is bolted to seatings which may be riveted or welded to the deck. The seatings are extended to distribute the load. With respect to the seatings, the beams are increased in strength by fitting reverse bars which extend to the adjacent girders. Solid pillars are fitted under the seatings to reduce vibration.

Question 210

Describe the construction of Deck Plating

Answer 210

The deck plating of a ship carries a large proportion of the stresses due to longitudinal bending, the upper deck carrying greater loads than the second deck. The continuous plating alongside the hatches must be thick enough to withstand the loads. The plating between the hatches has little effect on the longitudinal strength. The thickness of plating depends largely upon the length of the ship and the width of deck alongside the hatchways. In narrow ships, or in vessels having wide hatches, the thickness of plating is increased. At the ends of the ship, where the bending moments are reduced, the thickness of plating may be gradually reduced in the same way as the shell plating. A minimum cross-sectional area of material alongside hatches must be maintained. Thus if part of the deck is cut away for a stairway or similar opening, compensation must be made in the form of either doubling plates or increased local plate thickness. The deck forms a cover over the cargo, accommodation and machinery space and must therefore be watertight. The weather deck, and usually the second deck, are cambered to enable water to run down to the sides of the ship and hence overboard through the scuppers. The outboard deck strake is known as the stringer plate and at the weather deck is usually thicker than the remaining deck plating. It may be connected to the sheerstrake by means of a continuous stringer angle or gunwale bar. Exposed steel decks above accommodation must be sheathed with wood which acts as heat and sound insulation. As an alternative the deck may be covered with a suitable composition. The deck must be adequately protected against corrosion between the steel and the wood or composition. The deck covering is stopped short of the sides of the deck to form a waterway to aid drainage.

Question 211

Explain the importance and construction of the bulwark with the aid of a sketch

Answer 211

It is necessary on exposed decks to fit some arrangement to prevent personnel falling or being washed overboard. Many ships are fitted with open rails for this purpose while others are fitted with solid plates known as bulwarks at least 1 m high. These bulwarks are much thinner than the normal shell plating and are not regarded as longitudinal strength members. The upper edge is stiffened by a ‘hooked angle', that is, the plate is fitted inside the flange. This covers the free edge of the plate and results in a neater arrangement. Substantial stays must be fitted from the bulwark to the deck at intervals of 1.83 m or less. The lower edge of the bulwark in riveted ships was fixed to the top edge of the sheerstrake. In welded ships, however, there must be no direct connection between the bulwark and the sheerstrake, especially amidships, since the high stresses would then be transmitted to the bulwark causing cracks to appear. These cracks could then pass through the sheerstrake. Large openings, known as freeing ports, must be cut in the bottom of the bulwark to allow the water to flow off deck when a heavy sea is shipped. Failure to clear the water could cause the ship to capsize. Rails or grids are fitted to restrict the opening to 230 mm in depth, while many ships are fitted with hinged doors on the outboard side of the freeing port (see Figure 5.1), acting as rather inefficient non-return valves. It is essential that there should be no means of bolting the door in the closed position.

Question 212

What is the function of bulkheads?

Answer 212

There are three basic types of bulkheads used in ships; watertight bulkheads, tank bulkheads and non-watertight bulkheads. These bulkheads may be fitted longitudinally or transversely. Their function is to keep enough buoyancy in the ship in the event of a rupture in the outer hull. As we know from Titanic, if enough watertight compartments are breeched, then the ship will sink. However, as we also know from the Titanic, a watertight compartment not only has to be watertight, but it must also extend high enough in the vessel that it will not be breached, in order to maintain enough buoyancy to keep the vessel afloat in the event of a rupture of the hull.

Question 213

Explain with the aid of a sketch the construction and purpose of Watertight Bulkheads

Answer 213

The transverse watertight bulkheads of a ship have several functions to perform. They divide the ship into watertight compartments and thus restrict the volume of water which may enter the ship if the shell plating is damaged. The watertight compartments also serve to separate different types of cargo and to divide tanks and machinery spaces from the cargo spaces. In the event of fire, bulkheads significantly reduce the rate of spread of the fire. Controlling of the spread of fire, however, also depends upon the ‘fire potential’ on each side of the bulkhead, that is, the likelihood of the material near the bulkhead being ignited. SOLAS Chapter 2 requires that vessels are constructed to significantly reduce the spread of fire. When a vessel is designed, this is achieved by dividing the vessel into different sections by using insulated bulkheads made of steel, or other material that is equivalent to steel. These are known as class ‘A’ or ‘B’ fire divisions. Fire divisions made up of bulkheads and approved non-combustible material are further subdivided into ‘A-60, A-30, A-15, A-0, B-15 and B-0. The number after the class standard A or B relates to the number of minutes that the bulkhead + insulation, is required to restrict the rise in temperature on the opposite side to a fire. For example, A-60 relates to the A standard for 60 minutes. There is a class ‘C’ division, but this is only to be made of non-combustible material and is not subject to the rigorous testing of the class A and B divisions. The class A division standard means that the temperature on the opposite side to the fire must be restricted to a rise of not more than 140°C, and a peak temperature of 180°C above the initial temperature. The length of time for this restriction will relate to the number of minutes specified in its classification. The transverse strength of the ship is also increased by the bulkheads which have much the same effect as the ends of a box. They prevent undue distortion of the side shell and reduce racking considerably. Longitudinal deck girders and deck longitudinals are supported at the bulkheads which therefore act as pillars, while at the same time they tie together the deck and tank top and hence reduce vertical deflection when the compartments are full of cargo. This whole structure contributes to the ultimate strength of the hull girder. Thus it appears that the shipbuilder has a very complicated structure to design. In practice, however, it is found that a bulkhead required to withstand a load of water in the event of flooding will readily perform the remaining functions. The number of bulkheads in a ship depends upon the length of the ship and the position of the machinery space. Each ship must have a collision bulkhead and from 2010 this should be at least 0.05L or 10 m (whichever is the least) from the forward perpendicular and no greater than 0.08L or 0.05L + 3 m (whichever is greater). The bulkhead must be continuous up to the uppermost continuous deck (also called the ’bulkhead deck’). Special considerations are made for Ro-Ro vessels that have a bow door. The stern tube must be enclosed in a watertight compartment formed by the stern frame and the after peak bulkhead which may terminate at the first watertight deck above the waterline. A bulkhead must be fitted at each end of the machinery space although, if the engines are aft, the after peak may form the after boundary of the space. In certain ships this may result in the saving of one bulkhead. In ships more than 90 m in length, additional bulkheads are required, with the actual number depending upon the length. Thus, a ship 140 m long will require a total of 7 bulkheads if the machinery is amidships or 6 bulkheads if the machinery is aft, while a ship 180 m in length will require 9 or 8 bulkheads, respectively. These bulkheads must extend to the freeboard deck and should preferably be equally spaced in the ship. The bulkheads are fitted in separate sections between the tank top and the lowest deck, and in the 'tween decks. In ships employing diesel electric propulsion systems the actual engines might not be in line across the vessel. Therefore, to keep the propulsion systems separate, giving improved redundancy, additional watertight transverse and longitudinal bulkheads would be required. Watertight bulkheads are formed by plates which are attached to the shell, deck and tank top by welding (Figure 6.2). Since water pressure increases with the head of water, and the bulkhead is to be designed to withstand such a force, it may be expected that the plating on the lower part of the bulkhead is thicker than that at the top. The bulkheads are supported by vertical stiffeners spaced 760 mm apart. Any variation in this spacing results in variations in size of stiffeners and thickness of plating. The ends of the stiffeners are usually bracketed to the tank top and deck although in some cases the brackets are omitted, resulting in heavier stiffeners. The stiffeners are in the form of either bulb plates or toe welded angles. It is of interest to note that since a welded bulkhead is less liable to leak under load, or alternatively it may deflect further without leakage, the strength of the stiffeners may be reduced by 15%. It may be necessary to increase the strength of a stiffener which is attached to a longitudinal deck girder in order to carry the pillar load.The May 2010 revision of IACS rules requires that bulkheads are tested for water tightness by the application of a hydrostatic test. If this is not possible, then hydro-pneumatic testing can be used. This is where a tank is part filled with water and then pressurised with the use of compressed air. If neither of these methods are feasible, then they can be tested using water pressure of 200 kN/m2 from a hose with a nozzle of at least 12 mm diameter applied from a distance of no more than 1.5 metres.The hose test is carried out from the side on which the stiffeners are attached. It is essential that the structure should be maintained in a watertight condition. If it is found necessary to penetrate the bulkhead, precautions must be taken to ensure that the bulkhead remains watertight. If the after engine room bulkhead is penetrated by the main shaft, which passes through a watertight gland, and by an opening leading to the shaft tunnel, then this opening must be fitted with a sliding watertight door. When pipes or electric cables pass through a bulkhead, the integrity of the bulkhead must be maintained.

Question 214

What is the probabilistic method?

Answer 214

This new method of calculating the ‘survivability’ of a vessel, in the event of damage, is called the ‘probabilistic’ method of stability calculations.. The calculations work out the probability of a vessel surviving if the hull is holed in a specific position. The internal design of the vessel is then examined and changed, if necessary, to improve the ‘survivability’.

Question 215

Describe the construction of Watertight Doors

Answer 215

A watertight door will be fitted to any access opening in a watertight bulkhead. Such openings must be cut only where necessary for the safe working of the ship and are kept as small as possible, 1.4 m high and 0.75 m wide being usual. The doors may be mild steel, cast steel or cast iron, and could be either vertical or horizontal sliding, the choice being usually related to the position of any fittings on the bulkhead. Watertight doors should be strong enough to withstand the pressure of water that it could be subjected to. The method of construction would be a matter for the designer but on ships where the door is to be closed in operation at sea the door must be of the sliding type. The means of closing the doors must be positive, that is, they must not rely on gravity or a dropping weight. Watertight doors for all ships can be tested before fitting by a hydraulic pressure equivalent to a head of water from the door to the bulkhead deck. All such doors are hose tested after fitting. Hinged watertight doors may be fitted to watertight bulkheads in passenger ships, above decks which are 2.2 m or more above the load waterline. Similar doors are fitted in cargo ships to weather deck openings which are required to be watertight. The doors are secured by clips which may be fitted to the door or to the frame. The clips are forced against brass wedges. The hinges must be fitted with gunmetal pins. Some suitable packing is fitted round the door to ensure that it is watertight.

Question 216

Revise the operation of A horizontal sliding door shown in Figure 6.5. bulkheads and deep tanks chapter reed ship construction book

Question 217

Revise operation of The older type of vertical sliding doors (Figure 6.4) bulkheads and deep tanks chapter reed ship construction book

Question 218

What are Non-watertight Bulkheads?

Answer 218

Any bulkhead which does not form part of a tank or part of the watertight subdivision of the ship may be non-watertight. Many of these bulkheads are fitted in a ship, forming engine casings and partitions in accommodation.

Question 219

Describe the purpose and construction of Corrugated Bulkheads?

Answer 219

A corrugated plate is stronger than a flat plate if subjected to a bending moment or pillar load along the corrugations. This principle may be used in bulkhead construction, when the corrugations may be used to dispense with the stiffeners, resulting in a considerable saving in weight. The troughs are vertical on transverse bulkheads but must be horizontal on continuous longitudinal bulkheads which form part of the longitudinal hull girder strength of the ship. The bulkhead must be provided with adequate support to avoid stress concentrations. They should be supported by floors or girders with the stiffening members providing additional support if necessary.A load acting across the corrugations will tend to cause the bulkheads to fold in concertina fashion. It is usual, therefore, on transverse bulkheads to fit a stiffened flat plate at the shell, thus increasing the transverse strength. This method also simplifies the fitting of the bulkhead to the shell which may prove difficult where the curvature of the shell is considerable. Horizontal diaphragm plates are fitted to prevent collapse of the troughs. These bulkheads form very smooth surfaces which, in oil tanks, allows improved drainage and ease of cleaning. A vertical stiffener is usually necessary if the bulkhead is required to support a deck girder.

Question 220

A partially full deep tank carrying oil or water will have a free surface and will therefore be subjected to dynamic forces. In the case of an oil fuel or fresh water bunker tank they will also be subjected to different levels of forces during the voyage. This results in reduced stability, while at the same time the momentum of the liquid moving across the tank may cause damage to the structure. How is this reduced?

Answer 220

To reduce this surging it is necessary to fit divisions or deep swashes to minimise the dynamic stresses caused by this arrangement. These divisions may be intact, in which case they must be as strong as the boundary bulkheads, or perforated, when the stiffeners may be considerably reduced. The perforations must be between 5% and 10% of the area of the bulkhead. Any smaller area would allow a build-up of pressure on one side, for which the bulkhead is not designed, while a greater area would not reduce surging to any marked extent. Sparring must be fitted to the cargo side of a bulkhead which is a partition between a bunker and a hold. If a settling tank is heated and is adjacent to a compartment which may carry coal or cargo, the structure outside the tank must be insulated.

Question 221

Explain the construction and purpose of Deep Tanks?

Answer 221

It might be necessary, in ships with machinery amidships, to arrange a deep tank forward of the machinery space to provide sufficient ballast capacity in order to help trim the vessel correctly. This deep tank might also be designed to allow dry cargo to be carried and some ships may carry vegetable oil or oil fuel as cargo; however no flammable liquid can be carried in a deep tank forward of the collision bulkhead.Deep tanks may also be provided for the carriage of oil fuel to be used as a ship’s bunker fuel. The structure in these tanks is designed to withstand a head of water up to the top of the overflow pipe, the tanks being tested to this head or to a height of 2.44 m above the top of the tank, whichever is greater. It follows, therefore, that the strength of the structure must be much superior to that required for dry cargo holds. Where this is intended the designers must make this clear in their plans.If a ship is damaged with respect to a hold, the end bulkheads are required to withstand the load of water without serious leakage. Permanent deflection of the bulkhead may be accepted under these conditions and a high stress may be allowed. There must be no permanent deflection of a tank bulkhead, however, and the allowable stress in the stiffeners must therefore be much smaller. The stiffener spacing on the transverse bulkheads is usually about 600 mm and the stiffeners are much heavier than those on hold bulkheads. If, however, a horizontal girder is fitted on the bulkhead, the size of the stiffeners may be considerably reduced. The ends of the stiffeners are bracketed, the toe of the bottom bracket being supported by a solid floor plate. The thickness of bulkhead plating is greater than required for hold bulkheads, with a minimum thickness of 7.5 mm. The arrangement of the structure depends upon the use to which the tank will be put.

Question 222

What are Controllable Pitch Propellers?

Answer 222

A controllable pitch (c.p.) propeller is one that always rotates in the same direction but the pitch of the blades may be altered by remote control. The blades are separately mounted onto bearing rings in the propeller hub. A valve rod is fitted within the hollow main shaft and this actuates a servo-motor cylinder. Longitudinal movement of this cylinder transmits a load through a crank pin and sliding shoe to rotate the propeller blade (Figure 11.3).The propeller pitch is controlled directly from the bridge and hence closer and quicker control of the ship speed is obtained. This is of particular importance when manoeuvring in confined waters when the ship speed may be changed, and indeed reversed, at constant engine speed. Because full power may be developed astern, the stopping time and distance may be considerably reduced. The initial cost of a c.p. propeller is considerably greater than that of a fixed pitch installation. On the other hand a simpler non-reversing main engine may be used or a reversing gear box is not required, and since the engine speed may be maintained at all times the c.p. installation lends itself to the fitting of shaft-driven auxiliaries such as a shaft alternator. The efficiency of a c. p. propeller is less than that of a fixed pitch propeller for optimum conditions, due to the larger diameter of boss required, but at different speeds the c.p. propeller has the advantage. The cost of repair and maintenance is high compared with a fixed pitch propeller although it might be possible to repair or replace a single blade of the c.p. arrangement.

Question 222

What is meant by a ships natural rolling period and what effect does a large or small GM have ?

Answer 222

When a ship is heeled by an external force, and the force is suddenly removed, the vessel will roll to port and starboard with a rolling period which is almost constant. This is known as the ship’s natural rolling period. The amplitude of the roll will depend upon the applied heeling moment and the stability of the ship. For angles of heel up to about 15° the rolling period does not vary with the angle of roll. The angle reduces slightly at the end of each swing and will eventually dampen out completely. This dampening is caused by the frictional resistance between the hull and the water, which causes a mass of entrained water to move with the ship. Large metacentric height will produce a small period of roll, although the movement of the ship may be decidedly uncomfortable and possibly dangerous. A small metacentric height will produce a long period of roll and smooth movement of the ship. The resistance to heel, however, will be small and consequently large amplitudes of roll may be experienced.

Question 222

What are Contra-Rotating Propellers?

Answer 222

This system consists of two propellers in line, but turning in opposite directions. The after propeller is driven by a normal solid shaft. The forward propeller is driven by a short hollow shaft which encloses the solid shaft. The forward propeller is usually larger and has a different number of blades from the after propeller to reduce the possibility of vibration due to blade interference. Research has shown that the system may increase the propulsion efficiency by 10%–12% by cancelling out the rotational losses imparted to the stream of water passing through the propeller disc. Contra-rotating propellers are extremely costly and are suitable only for highly loaded propellers and large single screw tankers, particularly when the draught is limited. The increased surface area of the combined system reduces the possibility of cavitation but the longitudinal displacement of the propellers is very critical.

Question 223

With reference to tankers and bulk carriers is it possible to change the radius of gyration?

Answer 223

In tankers and bulk carriers vessels it is possible to change the radius of gyration. If the cargo is concentrated in the centre compartments, with the wing tanks empty, the value of the radius of gyration is small, producing a small period of roll. If, however, the cargo is concentrated in the wing compartments, the radius of gyration increases, producing a slow rolling period. The value of the radius of gyration will therefore vary with the disposition of the cargo.

Question 224

Explain with the aid of sketches one type of Vertical Axis Propellers

Answer 224

The Voith–Schneider propeller is typical of a vertical axis propeller and consists of a series of vertical blades set into a horizontal rotor which rotates about a vertical axis. The rotor is flush with the bottom of the ship and the blades project down as shown in Figure 11.4. The blades are linked to a control point P by cranked control rods (Figure 11.5). When P is in the centre of the disc, the blades rotate without producing a thrust. When P is moved away from the centre in any direction, the blades turn in the rotor out of line with the blade orbit and a thrust is produced. The direction and magnitude of the thrust depends upon the position of P. Since P can move in any direction within its inner circle, the ship may be driven in any direction and at varying speeds. Thus the Voith–Schneider propeller may propel and manoeuvre a ship without the use of a rudder. The efficiency of a Voith–Schneider propeller is relatively low but it has the advantage of high manoeuvrability and is useful in harbour craft and ferries. Two or more installations may be fitted and in special vessels (e.g. firefloats) can move the ship sideways or rotate it in its own length. Replacement of damaged blades is simple although they are fairly susceptible to damage. A tubular guard is usually fitted to protect the blades. The propeller may be driven by a vertical axis motor seated on the top or by a diesel engine with a horizontal shaft converted into vertical drive by a bevel gear unit.

Question 225

What are Tunnel Thrusters (Bow and Stern Thrusters) ?

Answer 225

Many ships are fitted with bow thrust units to improve their manoeuvrability (Figure 11.6). They are an obvious feature in ships working within, or constantly in and out of harbour where close control is obtained without the use of tugs. They have also proved to be of considerable benefit to larger vessels such as oil tankers and bulk carriers, where the tug requirement has been reduced. Several types of tunnel thrusters are available, each having its own advantages and disadvantages. In all cases the necessity to penetrate the hull forward causes an increase in ship resistance and hence in fuel costs, although the increase is small and with the podded drives the tunnel thrusters at the stern of the vessel are not required. A popular arrangement is to have a cylindrical duct passing through the ship from side to side, in which is fitted an impeller that can produce a thrust to port or to starboard. The complete duct must lie below the waterline at all draughts, the impeller acting best when subject to a reasonable head of water and thus reducing the possibility of cavitation. The impeller may be of fixed pitch with a variable-speed motor which is reversible or has reverse gearing. Alternatively a controllable pitch impeller may be used, having a constant-speed drive. Power may be provided by an electric motor, a diesel engine or a hydraulic motor. Some vessels are fitted with Voith–Schneider propellers within the ducts to produce the transverse thrust. As an alternative the water may be drawn from below the ship and projected port or starboard through a horizontal duct which may lie above or below the waterline. A unidirectional horizontal impeller is fitted in a vertical duct below the waterline. The lower end of the duct is open to the sea, while the upper end leads into the horizontal duct which has outlets in the side shell port and starboard. Within this duct two hydraulically operated vertical vanes are fitted to each side. Water is drawn from the bottom of the ship into the horizontal duct. By varying the position of the vanes the water jet is deflected either port or starboard, producing a thrust and creating a reaction which pushes the bow in the opposite direction. This system has an advantage that by turning all vanes 45° either forward or aft an additional thrust forward or aft can be produced. A forward thrust would act as a retarding force while an aft thrust would increase the speed of the ship. These actions may be extremely useful in handling a ship in congested harbours. The efficiency of propeller thruster falls off rapidly as the ship speed increases. The rudder thrust, on the other hand, increases in proportion to the square of the ship speed, being relatively ineffective at low speeds. The water jet unit appears to maintain its efficiency at all speeds, although neither type of thrust unit would normally be used at speed.

Question 226

What are hinged fins? Explain their purpose with the aid of a sketch

Answer 226

Hinged fins are used when there is a restriction on the width of ship, which may be allocated, such as in a container ship (Figure 11.13). The equipment is controlled by means of two gyroscopes, one measuring the angle roll and the other the velocity of roll. The movements of the gyroscopes actuate relays which control the angle and direction through which the fins are turned. It should be noted that no movement of stabiliser can take place until there is an initial roll of the ship and that the fins require a forward movement of the ship to produce a righting moment.

Question 227

Explain What are Ship Vibrations, how they occur and how they can be reduced?

Answer 227

Ship vibration is the periodic movement of the structure and may occur vertically, horizontally or torsionally. There are several sources of ship vibration, any of which could cause discomfort to personnel, damage to fittings and instruments and structural failure. If the frequency of the main or auxiliary machinery at any given speed coincides with the natural frequency of the hull structure, then vibration may occur. In such circumstances it is usually easier to alter the source of the vibration by changing the engine speed or fitting dampers, than to change the structure. The natural frequency of the structure depends upon the length, mass distribution and second moment of area of the structural material. For any given mass distribution a considerable change in structural material would be required to cause any practical variation in natural frequency. There is a possibility of altering the natural frequency of the hull by redistributing the cargo. If the cargo is concentrated at the nodes, the natural frequency will be increased. If the cargo is concentrated at the anti-nodes, the natural frequency and the deflection will be reduced. Such changes in cargo distribution may only be possible in vessels such as oil tankers or bulk carriers in the ballast condition. Similarly vibration may occur in a machinery space due to unbalanced forces from the main or auxiliary machinery or as a result of uneven power distribution in the main engine. This vibration may be transmitted through the main structure to the superstructure, causing extreme discomfort to the personnel. The variation in blade loading due to the wake may create vibration of the after end, which may be reduced by changing the number of blades. The turbulence of the water caused by the shape of the after end is also a source of vibration which may be severe. It is possible to design the after end of the ship to reduce this turbulence resulting in a smoother flow of water into the propeller disc. Severe vibration of the after end of some ships is caused by insufficient propeller tip clearance. As the blade tip passes the top of the aperture it attempts to compress the water. This creates a force on the blade which causes bending of the blade and increased torque in the shaft. The periodic nature of this force, that is, revs × number of blades, produces the vibration of the stern. Classification societies recommend minimum tip clearances to reduce propeller-induced vibrations to reasonable levels. Should the tip clearance be constant, for example, with a propeller nozzle, then this problem does not occur. If an existing ship suffers an unacceptable level of vibration from this source, it may be necessary to crop the blade tips, reducing the propeller efficiency, or to fit a propeller of smaller diameter. A damaged propeller blade will create out-of-balance moments due to the unequal weight distribution and the uneven loading on the blades. Little may be done to relieve the resultant vibration except to repair or replace the propeller. Wave induced vibrations may occur in ships due to pitching, heaving, slamming or the passage of waves along the ship. In smaller vessels pitching and slamming are the main sources but are soon dampened. In ships over about 300 m in length, however, hull vibration has been experienced in relatively mild sea states due to the waves. In some cases the vibration has been caused by the periodic increase and decrease in buoyancy with regular waves much shorter than the ship, while in other cases, with non-uniform waves, the internal energy of the wave is considered to be the source. Such vibrations are dampened by a combination of the hull structure, the cargo, the water friction and the generation of waves by the ship.

Question 228

What are Active fin stabilisers? Explain their purpose with the aid of sketches

Answer 228

These are Two fins which extend from the ship side at about bilge level. They are turned in opposite directions as the ship rolls. The forward motion of the ship creates a force on each fin and hence produces a moment opposing the roll. When the fin is turned down, the water exerts an upward force. When the fin is turned up, the water exerts a downward force (Figure 11.11). The fins are usually rectangular, having aerofoil cross-section (Figure 11.12) and turn through about 20°. Many are fitted with tail fins which turn relative to the main fin through a further 10°. The fins are turned by means of an electric motor driving a variable delivery pump, delivering oil under pressure to the fin tilting gear. The oil actuates rams coupled through a lever to the fin shaft. Most fins are retractable, either sliding into fin boxes transversely or hinged into the ship.

Question 229

Explain the purpose and working principle of vessel’s active fin stabilizers?

Answer 229

The ship’s stabilizers are active underwater fins that are designed to reduce the amount of rolling that the ship experiences during an ocean voyage, where rolling is brought on by the effect that sea action has on the hull of the vessel. This becomes especially apparent when the frequency of the waves draws closer to the ship’s natural frequency. During a roll, actively reducing the amount of roll that occurs is accomplished by tilting the extended fin. The effect that the ship’s forward motion has on the surface of the fin produces a lifting moment that acts against the roll of the ship. Some vessels are outfitted with two retractable tilting fin units, one on each side of the vessel. The tilting of these is accomplished by hydraulic units that are powered by electro-hydraulic pumps. When the fins are not in use, they are stored in fin boxes that are located in the hull. These fin boxes have been specifically designed to become a part of the hull and the structure of the ship, which helps to reduce the amount of drag that the hull experiences. By folding the fin through an angle of 90 degrees, a hydraulic ram mechanism enables the fin to be rigged in and out (housed and extended). Fins produce a hydrodynamic lift effect as a result of the flow of water over the hydrofoil shaped fins. One fin is in the nose up position, which exerts an upward thrust, and the other fin is in the nose down position, which exerts a downward thrust. Because the up and down thrust is produced by the flow of water over the fin, the ship must be moving through the water at a speed that is at least sufficient to maintain the flow. If the ship is traveling at a speed of less than 12 knots, the fin stabilizers will not be able to do their job effectively. The fins have a design similar to that of a symmetrical hydrofoil, and inside the housing for the fin shaft there is a rotating shaft that is supported by two bearings. Additionally, the housing for the fin shaft is mounted in two bearings, which together make it possible for the fin to rotate into and out of the fin box. The required housing for connecting the fin to the hull is provided by the steel casing, which also provides support for the bearing structure of the fin. Example of stabilizers machinery To enable the fin to be tilted in the desired direction, a hydraulic tilting cylinder has been coupled to the fin shaft. The trailing edge of the fin is movable, so it moves along with the rest of the fin and helps to increase the hydrodynamic lift effect. The tilting shaft has lip seals installed in it to prevent water from the sea from getting into the machinery and also to prevent oil from leaking out into the water. A sliding block is rotated by the rigging lever through the use of a mechanical linkage, which is moved by a rigging cylinder. When everything is completely rigged out, this linkage will put the fin in a position where it cannot move. After the fin has been completely rigged in, a latch is used to secure the sliding block in its position. The stabilizers’ operation is monitored and controlled to ensure that cavitation does not occur, which helps to ensure that erosion issues are avoided. The vessels are equipped with a gravity header tank that is located above the water line in order to supply the machinery with lubricating oil. The header tank can be broken down into two distinct parts. The first section supplies LO for the mechanism that controls the fins, and the second section supplies LO for the rigging system. In the event that the seal fails to keep water out of the housing, the presence of the pressure caused by the positive head of oil will prevent seawater from entering. The electro-hydraulic power unit can be found atop the fin housing box in its designated location. Each unit has a main variable delivery piston pump, a tandem vane pump, and an oil cooler. These three components make up the unit. The tandem vane pump is used for controlling the pressure, rigging the fins, and replenishing the pump; the electrical motor that drives this pump is the same as the one that drives the main pump. The oil that is fed to the tilting circuit by the variable delivery pump is of the axial piston type and is controlled by servo. The pump is responsible for delivering oil to the various chambers of the tilting cylinder. The chamber that is assigned to receive oil pressure is determined by the required attitude of the fin at that particular time and, as a result, the position of the pump stroke control unit. An emergency pump that is powered by electricity and has manual controls is made available. In the event that the primary hydraulic unit fails to operate, this pump will be able to rig the fin into or out of position. The emergency circuit has a safety valve and manual distributors, which allow the fin to be reset to the zero angle position, the fin to be rigged in or out, and the fin lock to be enabled. This is all made possible thanks to the emergency circuit. LCD touch screens are utilized by the system in order to facilitate automatic control of the extension and storage (rigging) of the fins. The following three positions have the ability to exercise control over this:  Bridge control panel, including operations for starting and shutting down the entire system. Main control panel (which can be found in the ECR): Used for starting up and shutting down the entire system, as well as controlling individual fins for purposes of maintenance. Local control panels: Individual fin control for maintenance purposes. In the event that power fails, the control system also allows for the storage of the fins. This can either take place automatically upon the connection of power to the emergency motor starter or be controlled from the various local control panels located around the facility. Must be remembered that if either the port or starboard LOCAL CTRL indicators are on, that fin cannot be controlled from the bridge. In the bridge and main control panels LOCAL CTRL is displayed if local control is on. In the event that the stabilizer power is switched off with the fin not housed, the stabilizer control system will trigger the ship’s general alarm via the FIN NOT IN alarm relay. The fin control can be transferred from the bridge to the main control panel in the ECR, even when the fins are in operation, by switching the main control panel selector switch to the MCU position. It is important to note that when the fins are out, they extend beyond the beam of the ship and are angled downwards, with the tip near the keel line. As a result the fins can create a navigational hazard if extended when the vessel is operating in shallow or confined water. Normally, the system provides the following features to reduce the risk of damage to the fins: Key locked power control. Power ON interlocks that can prevent operation of the stabilizers at the same time as the thrusters. Alarm indicators that appear on the bridge control panel if the ability to stow the stabilizers is impaired. Power OFF and fin NOT IN alarm contacts can activate an alarm if the key switch is turned off while the fin is out. An alarm that sounds when the ship’s speed drops below a pre-set level. The control system can be configured to automatically rig in the fin when the ship’s speed drops below a pre-set level.

Question 230

Explain the purpose and construction of Bilge keel

Answer 230

This plating projects from the hull and is arranged at the bilge to lie above the line of the bottom shell and within the breadth of the ship, thus being partially protected against damage. The depth of the bilge keels depends to some extent on the size of the ship, but there are two main factors to be considered: 1.The web must be deep enough to penetrate the boundary layer of water travelling with the ship. 2.If the web is too deep, the force of water when rolling may cause damage. Bilge keels 250–400 mm in depth are fitted to ocean-going ships. The keels extend for about one-half of the length of the ship and take in the midships section. They should be continuous and are tapered gradually at the ends with the ends terminated on an internal stiffening member. A “bilge keel” is one of a pair of longitudinal plates that, like fins, project from the sides of a ship or boat and run parallel to the centre keel. They are intended to check rolling. On large ships the outward projections of the bilge keels may be slight; on small yachts they may be comparatively deep. Most ships are fitted with some form of bilge keel, the prime function of which is to help damp the rolling motion of the vessel. Other relatively minor advantages of the bilge keel are protection for the bilge on grounding, and increased longitudinal strength at the bilge. It is carefully positioned on the ship so as to avoid excessive drag when the ship is under way, and to achieve a minimum drag, various positions of the bilge keel may be tested on the ship model used to predict power requirements. This bilge keel then generally runs over the midship portion of the hull, often extending further aft than forward of amidships and being virtually perpendicular to the turn of the bilge.

Question 231

What are Tank stabilisers?

Answer 231

Tank stabilizers generate anti-rolling forces by phased flow of appropriate masses of fluid, usually water, in tanks installed at suitable heights and distances from the ship's centre line. Fluid transfer may be by open flume or from and to wing tanks connected by cross ducts. The tank/fluid combination constitutes a damped mass elastic system having its own natural period and capable of developing large forces at resonance with the impressed wave motion. Since the fluid can only flow downhill and has inertia, it cannot start to move until the ship has rolled a few degrees. The natural restoring forces limit the maximum roll angle and initiate roll in the opposite sense. In the meantime the fluid continuing to flow downhill, piles up on the still low side and provides a moment opposing the ship motion. As the ship returns and passes its upright position, fluid again flows downhill to repeat the process. The fluid flow tends to lag a quarter of a cycle behind the ship motion, a phase lag of approximately 90 deg , to generate a continuing stabilizing moment. This is due, mainly, to the transfer of the centre of gravity of the fluid mass away from the centre line of the ship. The transverse acceleration of the fluid generates an inertia force and thereby a moment, about the roll centre, which reduces the gravity moment when the tanks are below the roll centre and increases it when they are above. In practice, tanks may be placed 20% off the beam below the roll centre without serious loss of performance. Above the roll centre, other factors associated with the phase of fluid motion prevent augmentation of the gravity stabilizing power being realized. The phase lag may be increased, within limits, by placing orifice plates or grillages in the fluid flow path, to increase the damping. There are three basic systems of roll-damping using free surface tanks: 1.Passive tanks. 2.Controlled passive tanks. 3.Active controlled tanks. These systems do not depend upon the forward movement of the ship and are therefore suitable for vessels such as drill ships. In introducing a free surface to the ship, however, there is a reduction in stability which must be considered when loading the ship.

Question 232

With the aid of sketch es describe What are Passive tanks?

Answer 232

Two wing tanks are connected by a duct having a system of baffles (Figure 11.14). The tanks are partly filled with water. When the ship rolls, the water moves across the system in the direction of the roll. As the ship reaches its maximum angle and commences to return, the water, slowed by the baffles, continues to move in the same direction. Thus a moment is created, reducing the momentum of the ship and hence the angle of the subsequent roll (Figure 11.15). The depth of water in the tanks is critical and, for any given ship, depends upon the metacentric height. The tank must be tuned for any loaded condition by adjusting the level, otherwise the movement of the water may synchronise with the roll of the ship and create dangerous rolling conditions. Alternatively the cross-sectional area of the duct may be adjusted by means of a gate valve.

Question 233

With the aid of a sketch describe What are Controlled passive tanks

Answer 233

The principle of action is the same as for the previous system, but the transverse movement of the water is controlled by valves operated by a control system similar to that used in the fin stabiliser. The valves may be used to restrict the flow of water in a U-tube system, or the flow of air in a fully enclosed system (Figure 11.16).

Question 234

With the aid of a sketch describe What are Active controlled tanks

Answer 234

In this system the water is positively driven across the ship in opposition to the roll. The direction of roll, and hence the required direction of the water, changes rapidly. It is therefore necessary to use a uni-directional impeller in conjunction with a series of valves. The impeller runs continually and the direction of the water is controlled by valves which are activated by a gyroscope system similar to that used for the fin stabiliser (Figure 11.17).

Question 235

What's the main advantage of podded drives (Azipod)?

Answer 235

One of the unique features of Azipod propulsion is that it works by making propellors pull a ship forward, rather than pushing, like a conventional shaft and propellor This is one of the main features, to create high efficiency. And the reason is that when the water enters the propeller, the water flow is undisturbed meaning reduced wakefield, there is nothing disturbing it, compared to a conventional shaft-line propulsion where the shaft is protruding through the hull, and in many cases there is parts of the shafts and supports outside of the vessel hull. And they are creating turbulence to the flow, which then enters through the propeller and then the propeller is not functioning as efficiently Essentially it's an electric motor that rotates a propeller. And then that has just been integrated into And then the rudder part of the whole Azipod can turn 360 degrees around its axis. So, then we are able to create thrust to any direction. And by that, having much better manoeuvrability of the vessel and such characteristics.

Question 236

With the aid of sketches Explain what is wake field and how it affects working of a propeller and Explain the Fluctuating Forces Caused by the Propeller Wake Field

Answer 236

When the ship is sailing ahead, the friction of the hull will create a boundary layer of water around the hull. The velocity of the water on the surface of the hull is equal to that of the ship, but is reduced by the distance from the surface of the hull. The thickness of the boundary layer increases with its distance from the bow. The layer is therefore the thickest at the end of the hull. It means that there will be a certain wake velocity caused by friction along the sides of the hull. Additionally, the displacement of water by the ship will also cause wake waves both fore and aft. All this results in the propeller behind the hull is working in non-uniform water flow called wake-field. Propellers work in an adverse environment created by the varying wake field produced by the after end of the ship at the propeller disc. Figure 11.1 shows a typical wake distribution for a single screw ship. High wake fractions indicate that the water is being carried along at almost the same speed as the ship. Thus the propeller is working in almost dead water. The lower fractions indicate that the water is almost stationary and therefore has a high speed relative to the propeller. As the propeller blade passes through these different regions it is subject to a fluctuating load. These variations in loading cause several problems. For example, consider a four-bladed propeller turning in the wake field shown in Figure 11.1, two of the blades are lightly loaded and two heavily loaded when the blades are in the position shown in the figure. When the propeller turns through 90° the situation is reversed. The resulting fluctuations in stress may produce cracks at the root of the blades and vibration of the blades. The fluctuating loads might be reduced by changing the number of the blades or cleaning up the wake field. A three-bladed propeller, for example, will have only one blade fully loaded or lightly loaded at any one time, while five, six and seven blades produce more gradual changes in thrust and torque per blade and hence reduce the possibility of vibration due to this cause. An alternative method of reducing the variation in blade loading is to fit a skewed propeller (Figure 11.2) in which the centreline of each blade is curved to spread the distribution of the blade area over a greater range of wake contours. In these propellers there is also less cavitation produced and under some conditions there are efficiency gains as well as a reduction in vibration. The thrust of a propeller depends upon the acceleration of a mass of water within its own boundary of influence. If a propeller is on the centreline of a ship, it lies within the wake field moving past the hull and therefore it accelerates water which is already moving. The disturbed water moving towards the blade will create a fluctuating pressure entering the propeller disc as it rotates (Figure 11.1). The fluctuating pressure will set up vibration and also differencing effectiveness in the thrust produced across the propeller blade. A propeller which is off the centreline lies only partly within the wake field and therefore has a wider variation in pressure differential to contend with as some of the blade is working in slower moving water which is outside of the influence of the hull. For this reason, single screw ships can be slightly more efficient than twin screw ships for similar conditions.The main advantages of twin screw ships are their increased manoeuvrability and the duplication of propulsion systems leading to improved safety. Set against this is the considerable increase in the cost of the construction of the after end, whether the shaft support is by A-frames or by spectacle frames and bossings, compared with the sternframe of a single screw ship. In a single screw ship, the rudder is also more effective since it lies directly in the outflow from the propeller and hence the velocity of water at the rudder is increased, producing increased rudder force. Conversely, many twin screw ships are fitted with twin rudders in line with the propellers, further increasing their manoeuvrability at the expense of increased cost of steering gear. The variation in wake in a twin screw ship can be less than with a single screw ship, due to the propellers working in smoother water, and the blades are therefore less liable to vibrate due to fluctuations in thrust. The support of the shafting and propeller is less rigid, however, and vibration may occur due to the deflection of the support.

Question 237

Explain the purpose and working principle of bow and stern thrusters?

Answer 237

The purpose of the thruster units is to turn the vessel when is operating at slow speeds or when is not under way, to keep the ship in position in a cross wind and to move the ship towards or away from a mooring position as required. The thrust is produced by rotation of a propeller unit which is housed in a transverse cylindrical ducting, where the propeller unit is rotated by means of a vertical electric motor via bevel gears. The propeller blade pitch is controllable in order to obtain the desired magnitude and direction of thrust. The thruster comprises of a number of separate sections: The electric motor unit with drive shaft and bevel gearing driving the propeller unit hub; The propeller unit with blades mounted in the hub; The hydraulic unit which changes the pitch of the propeller blades; The control system which regulates the blade pitch in accordance with demand from the bridge. Power is transmitted from the electric motor through the flexible coupling, input shaft and bevel gears to the propeller shaft, rotating the propeller in a single direction. The propeller part, usually consists of four propeller blades and a propeller hub. The propeller hub and gear case house a hydraulic servomotor and sliding block mechanism. The propeller blades are connected to blade carriers by blade bolts, and this ensures easy exchange of blades in the thruster tunnel. The gear case, which carries the propeller parts, is connected to the thruster tube by bolts and this ensures easy overhauling of all parts inside the thruster tube. The power transmission gear is located inside the gear case and consists of the vertical input shaft, the right angle reduction bevel gear and the horizontal propeller shaft, and serves to transfer the power from the prime mover to the propeller. The bevel gear and individual bearings are lubricated by the gravity oil filling the gear case. The hydraulic power pack unit provides oil under pressure and this is used to change the pitch of the thruster unit blades. The oil is drawn from the gravity tank, through the suction filter and into the oil service pump. The pressurized oil is pumped to the solenoid valve via the check valve and the flow of oil is controlled by the solenoid valve. The hydraulically operated solenoid valve is a changeover valve for the distribution of the hydraulic oil to the respective servo cylinders depending on the command entered at the active control panel. When the command is entered on the control panel, the solenoid valve is actuated and pressurized hydraulic oil is supplied to one of the hydraulic circuits down the oil tube, through the feed ring and oil entry tube to the servomotor, causing displacement of the crosshead piston. The reciprocating movement of the piston is converted into a turning movement by the sliding block mechanism and this turns the propeller blades. The vent side of the servomotor piston drains to the oil bath in the thruster body via a solenoid valve. From this pressurized oil bath, oil returns to the header tank. The main actuator power pack pump takes oil from the header tank and supplies it to the thruster unit via the solenoid control valves. A shaft sealing mechanism is attached to the gear case in order to prevent leakage of oil out of the system. When a pitch change command is entered, the propeller will tend to move excessively. The pilot check valve prevents any excessive movement of the propeller whilst changing pitch. Operation of the bow thrusters requires starting a large induction motor and the power requirement of this electric motor is high, requiring that additional generators are started in order to avoid the risk of a vessel blackout. It is important to note that, on modern vessels if there is insufficient power capacity available at the switchboard an additional generator is started by the power management system. The thruster drive motor cannot be started until sufficient power is available at the switchboard. Usually, the main switchboard includes a bow thruster control panel, which includes a control position selection switch, lock-out relay trip reset, motor control and an ammeter. A series of status indicators are also included for monitoring the condition of the VCB, gravity tank, pump pressure etc. If any warning lamp is illuminated, the cause of the fault should be determined and remedied before operation of the bow thruster. Feeder protection for the bow thrusters is achieved by means of the protection and monitoring panel located on the main switchboard bow thruster panel and offers both measurement and protection for the bow thruster drive motor. The bevel gear and all the bearings inside the gear case are lubricated by the bath lubricating method. The lubrication oil in the gear case is slightly pressurized by the connection with the gravity tank which is positioned above the waterline to prevent sea water from leaking into the oil system. The thruster unit includes a feedback system for transmitting the angle of the propeller blades to the remote control panel located on the bridge. As the oil entry varies, the stroke of the oil entry tube also varies. The movement of the oil entry tube causes movement of the feedback lever. This movement is transmitted via the feedback chain to the blade angle transmitter located outside the thruster gear casing. This mechanical movement is then converted to an electrical signal by the blade angle transmitter and transmitted to the angle indicator on the bridge and local control panels. To ensure safe, reliable operation of the bow thrusters, limits are imposed on the vessel’s speed and draught. If there is insufficient draught, the thruster will suffer a reduction in performance along with cavitation and the possibility of air drawing. The result of this will be increased vibration which may cause damage. Similarly, at speeds greater than 5 knots there is a risk of drawing air into the thruster, particularly when operating at shallow waters. This will degrade the performance and can cause cavitation damage and it can be detected by hunting of the main motor ammeter and should be avoided. If the vessel’s speed is below 5 knots and air drawing is occurring, reducing the propeller pitch will prevent further air drawing from taking place. The main motor must only be started when the blades are in the neutral position (zero pitch), or in the allowable zone (blade pitch of ±3°). The system is interlocked to prevent the main motor from starting if the blade pitch is outside of the set limits. The interlock switches also prevent the main motor from starting when: The cooling fan is stopped; The power pack gravity tank level is low; The control oil pressure is low Under normal circumstances the main power supply is activated by the engineering department and after that the thruster operation and control is undertaken by the deck department from the bridge panels. The main switch at the local thruster control panel should be set at REMOTE in order to allow for this. Control of the thruster on the bridge is either at the wheelhouse control stand or the control stands on the bridge wings. It is important to note that, especially on bow thruster, when the hydraulic pump is started the fan is also started and the FAN RUN indicator in the panel will be illuminated. The main motor is interlocked with the fan and oil pump and will not start unless they are running. Also is important to remember that there are EMERGENCY STOP pushbuttons in the wheelhouse panel, forward mooring station and in the bridge wing panels.

Question 238

Explain the construction and purpose of Double Bottom system

Answer 238

Ocean-going ships (with the exception of tankers, which now have to be double hulled) and most coastal vessels are fitted with a double bottom system of construction, which extends from the fore peak bulkhead almost to the after peak bulkhead. The double bottom consists of the outer shell and an inner skin or tank top between 1 m and 1.5 m above the keel. This provides a form of protection in the event of damage to the bottom shell, and it also provides protection to the environment from any oil or contaminants that may be in the bilges at the time of a breach of the hull. However, the International Convention for the Prevention of Pollution from Ships (MARPOL) specifies whether, and how, fuel and lubrication oil is permitted to be stored in ‘double bottom’ tanks.The tank top, being continuous, contributes to the hull girder strength. The double bottom space contains a considerable amount of scantlings and is therefore unsuitable for carrying much cargo. Double bottom tanks may be used for the carriage of oil fuel, fresh water and water ballast. They are subdivided longitudinally and transversely to reduce any free surface effect. Double bottom tanks can be filled or emptied with the different liquids that are required to be carried, and they can also be used to correct the heel of a ship or to change the trim. Access to these tanks is arranged in the form of manholes with watertight covers and care must be taken when entering these tanks as they are dangerous spaces and could have an oxygen-depleted or poisonous atmosphere.In the majority of ships only one watertight longitudinal division, a centre girder, is fitted, but many modern ships are designed with either three or four tanks across the ship A cofferdam must be fitted between a fuel tank and a fresh water tank to prevent contamination of one with the other, if a failure occurs in one tank. The tanks are tested by pressing them up until they overflow. Since the overflow pipe usually extends above the weather deck, the tank top is subject to a tremendous head which in most cases will be sufficient as a test for water tightness and will be greater than the weight of the cargo pressing down from the hold.The tank top plating must be thick enough to prevent undue distortion when the cargo is loaded. In bulk carriers, if it is anticipated that cargo will be regularly discharged by grabs or by forklift trucks, it will be necessary to fit either additional protection to the ceiling of the tank or heavier flush plating. Under hatchways, where the tank top is most liable to damage, the plating or protection must be increased to the tank’s ceiling, and the plating is at least 10% thicker in the engine room. In the lower part generally considered to be the bilges, the tank top may be either continued straight out to the shell, or knuckled down to the shell by means of a tank margin plate set at an angle of about 45° to the tank top and meeting the shell almost at right angles. It has the added advantage, however, of forming a bilge space into which water may drain and has, in the past, proven to be popular. If no margin plate is fitted it is necessary to fit drain hats or wells in the after end of the tank top in each compartment so that the bilges can be pumped dry.

Question 239

With the aid of a sketch describe what are Duct keels or pipe tunnels?

Answer 239

Some ships might still be fitted with a tunnel or tunnels which is a convenient method of routing any pipework that is required to supply services to the forward part of the vessel. These are known as duct keels (see Figure 4.5) and as long as they are of equal strength, they can be used in place of the centre girder explained here and here. The pipe tunnel extends from within the engine room along the length of the vessel to the forward holds. This arrangement then allows the pipes to be carried beneath the hold spaces and are thus protected against cargo damage. Access into the duct is arranged from the engine room. The pipes can then be inspected and repaired at any time independent of the weather (within working constraints) and cargo operations. At the same time it is possible to carry oil and water pipes in the duct, preventing contamination which could occur if the pipes passed through tanks. Duct keels are particularly important in insulated ships, allowing access to the pipes without disturbing the insulation. Ducts are not required aft since the pipes may be carried through the shaft tunnel. The duct keel is formed by two longitudinal girders up to 1.83 m apart. This distance must not be exceeded as the girders must be supported by the keel blocks when docking. The structure on each side of the girders is the normal double bottom arrangement. The keel and the tank top centre strake must be strengthened either by supporting members in the duct or by increasing the thickness of the plates considerably. It is vital that the duct space is treated as an enclosed space and great care must be taken before any person enters the area.

Question 239

What are solid floors?

Answer 239

In ships less than 120 m in length the bottom shell and tank top are supported at intervals of not more than 3 m by transverse plates known as solid floors. The name slightly belies the structure since large lightening holes are cut in them. In addition, small air release and drain holes are also cut at the top and bottom, respectively. These holes are most important since it is essential to have adequate access and ventilation to all parts of the double bottom. There have been many cases of personnel entering tanks which have been inadequately ventilated, with resultant gassing or suffocation. These tanks must still be regarded as enclosed spaces.The solid floor is usually fitted as a continuous plate extending from the centre girder to the margin plate. The side girder is therefore broken on each side of the floor plate and is referred to as being intercostal. Solid floors are required at every frame space in the machinery room, in the forward quarter length and elsewhere where heavy loads are experienced, such as under bulkheads and boiler bearings. The shell and tank top between the widely spaced solid floors are stiffened by bulb angles or similar sections running across the ship and attached at the centreline and the margin to large flanged brackets. Additional support is given to these stiffeners by the side girder and by intermediate struts which are fitted to reduce the span.

Question 239

With reference to Internal structure of the hull, What are continuous centre girders and side girders?

Answer 239

The hull girder strength can be enhanced with the inclusion of a continuous centre girder and/or side girders, these extending longitudinally from the fore peak to after peak bulkhead. The centre girder is usually watertight except at the extreme fore and after ends where the ship is narrow. Centre girders must also provide sufficient strength to withstand the docking loads and additional ‘docking brackets’ may need to be included in the design. A pipe tunnel may be substituted for a centre girder as long as the construction of the tunnel is of sufficient strength. Additional longitudinal side girders are fitted (at a maximum of 5 m apart) depending upon the breadth of the ship but these are neither continuous nor watertight, having large manholes or lightening holes in them. Special consideration must be given to providing side girders under the machinery space and/or the thrust block seating. The tanks are divided transversely by watertight floors, which in most ocean-going ships are required to be stiffened vertically, to withstand the liquid pressure.

Question 240

What are resin chocks?

Answer 240

resin’ chocks, and resilient mountings are used on small or medium sized engines. The resilient mountings are able to soak up more vibration from the engines, thus making the machinery space and the ship quieter. Resilient mountings can be manufactured from rubber and may also incorporate some form of springing.

Question 241

With the aid of a sketch describe what are web frames?

Answer 241

Web frames may be fitted in the machinery space and connected to strong beams or pillars in an attempt to reduce vibration (Figure 4.12). These web frames are about 600 mm deep and are stiffened on their free edge. It is usual to fit two or three web frames on each side of the ship, a smaller web being fitted in the 'tween decks. The exact scantling requirements will be determined by the strength calculations completed by the designer.