14. Fore End Structures Flashcards
Where is the fore end?
The fore end relates to the structures situated in the region forward of 0.3L from the forward perpendicular. May include dry spaces (voids or cofferdams), areas that house specific equipment (bow thruster), tank areas (fore peak tank and deep tank). Includes the stem, chain locker, fore peak tank, and (when fitted) the bulbous bow.
Fore End Construction
- Abrupt changes in structure are avoided by fitting suitable transition pieces (scarfing or overlapping arrangements)
- Forward flat bottom structure is normally provided with extra strengthening to accommodate pounding (could be heavier plating, additional plates/solid floors, extra side girders, stronger attachments).
- Usually double bottoms stop at the collision bulkhead.
- If double bottom is present though, and bottom has significant rise of floor, it may be necessary to increase the height of the floors in order to allow easier access into the space.
- The center girder generally only extends past the collision bulkhead several frame spaces and is tapered down into the shell plating in order to reduce stress concentrations. One of the primary purposes of the center girder is to resist longitudinal bending stresses. Very little of this stress is found in the forward area which reduces the need for heavy longitudinal strengthening.
- An alternate construction arrangement is to carry the girder as far forward as is practical in an effort to better support the floors and lower bow area. Some ships require significant strength in the bow area (ice breaking) and extending the center girder into this space (with or without connecting it to the strengthening used in the stem) will offer this additional strenght.
Fore Peak
- The floors are usually plate type and extend completely from port to starboard, making the center girder intercostal. The height of the floors is increased and the upper edge is normally flanged or with a rider plate to stiffen the upper edge and reduce the possibility of distortion.
- Side girders are not normally carried into the structure. Vessels with a full form bow will have these side girders extend as far as practical and the side shell framing will be placed perpendicular to the plating to provide easier installation and better strength
- If the peak area forms a tank, it is customary to fit wash plates/bulkheads in the upper region to reduce the effect of liquid movement (act as baffles).
Fore Peak Area
Generally, the bottom framing of the peak area will almost always be transverse. In special cases, longitudinal framing will be considered.
Panting occurs primarily in the forward section of the ship as it pitches in and out of waves. As a result of this and operational loads and the possibility of impact loads, considerable strengthening is fitted in the bow area of the ship.
Fore Peak Construction Method 1
Horizontal stringers are fitted at approx 2 meters apart vertically to support the side framing. Struts (panting beams) are placed from port to starboard at alternate frames to support these stringers from being pushed inward. These struts are attached to the stringers by the use of brackets and are supported in the center by an angle bar pillar. Elsewhere, the stringers are bracketed to the remaining frames
Fore Peak Construction Method 2
Horizontal stringers are fitted approx 2 meters apart vertically to support the side frames. Web frames are used to support the stringers. The top of the web frame is suitably joined to a deep beam thus forming a ring structure around the girth of the vessel
Fore Peak Construction Method 3
The fitting of strengthened perforated flats (decks) no more than 2.5 m apart vertically support the side shell framing. The area of the perforations is to be no less than 10% of the total area of the flat. Edge strengthening might be required around the perforations in the flat.
Fore Peak Construction Method 4
This method using a combination of the above 3 methods.
Depending on the type of framing as well as the length and depth of the fore peak area, extra perforated flats and wash bulkheads may be fitted. This is in conjunction with suitable panting arrangements.
Fore Peak Testing
If the peaks are dry areas: testing is conducted by filling the area with water to the level of the load waterline. This subjects the structure to the type of pressure that it will encounter in the event of hold damage and subsequent flooding.
If the area forms a tank: structure testing will be conducted with a head of pressure to a height not less than 2.45 m above the crown of the tank or to the top of the overflow pipe (whichever is higher).
This structural test is done immediately after construction or when structural integrity is questioned due to heavy corrosion, impact damage or major structural alternations/repairs. Leak testing can be carried out by hydrostatic test or by pneumatic pressure testing.
Tanks are required to be visually inspected as well as leak tested every 5 years.
Types of Bow Configurations
Plumb: perpendicular at the waterline, used for cargo carrying
Raked: the angle formed by the stem to the waterline is 15-25degrees to the vertical. The rake reduces the amount of water being shipped onto the deck and gives the ship an appearance of sleekness and speed.
Meierform: raked stem but the angle changes partway down the height of the stem. Often found on icebreaking vessels
Spoon & clipped: sailing vessels, bows are formed by a curved stem from top to bottom; spoon is convex and clipper is concave.
Bulbous: a projection on the lower section of the stem, helps the ship move more easily through the water. A fast vessel can be driven at service speed with less power while consuming less fuel
Bulbous bow
Ships with a high block coefficient and those operating at high speeds are often fitted with a bulbous bow. This is a spherical or cylindrical projection fitted to the lower section of the stem forming an extension of the side shell plating.
Resistance to movement through water is caused by both frictional resistance and wave-making resistance. Frictional resistance is largely determined by the wetted surface of the hull, the roughness of the plating, and the projections found on the hull. As speed increases, the amount of friction resistance increases in a non-linear fashion. As speed increases, the tendency to make a larger bow wave increases.
In an effort to reduce the wave making tendency of a ship, bulbous bows are fitted. These bows disturb the normal characteristics of forming the bow wave and result in smoother flow of water around the hull. Bulbous bows increase the wetted surface area of the hull and are not normally fitted on slower ships. They tend to reduce the amount of pitching of a vessel.
Construction arrangements:
- Heavier shell plating (impact, anchor and cable chafing)
- Bulb area must be adequately support and integrated into the structure of the fore peak area
- Vertical transverse diaphragm plates are fitted in the transition area from the peak to the bulb
- Web frames are fitted at approx every 5 frame spaces
- Horizontal diaphragm plates are fitted approx every meter apart vertically
- Centerline wash bulkheads (large bulb) or a centerline web (small bulb) are fitted
- If the bulb is long and wide, transverse wash bulkheads may be fitted
A sufficient number of appropriately sized manholes are found in order to permit inspection/maintenance of the area
Bar Stems
Simplest form of stem, usually only found on smaller vessels. The bar is a cast or forged solid piece that may be round, square or rectangular in cross section. Sometimes it is a special shape to facilitate the attachment of the side shell plating. The bar is curved at the lower end to allow attachment to the keel plate. The side shell plating may be welded directly to the sides of the bar or the bar may have rabbat sections to allow the shell to be flush
Plate Stem
Formed by the use of either a curved plating to form the forward section OR (if the bow is sharp) two plates welded together. The fully plated stem extends from the keel plate to the upper edge of the bow. The thickness of the curved plating is usually heavier than the side shell plating in order to resist impact damage as well as chafing from the anchor cable.
To help support the stem, horizontal plates known as breast hooks and webs are installed at intervals between the decks. One or more vertical centerline stiffeners might be fitted along the inside of the stem for greater support.
Composite Stem
Combination of bar and plate stem. The bar stem is fitted extending from the keep plate to the waterline region. A cast transition piece is sometimes fitted at the top of this bar stem. The remaining portion of the stem is constructed of curved plating. Often this construction is referred to as “soft-nose” as it would tend to buckle in the event of a forward collision. Although stem damage does occur, this construction reduced the impact and consequently lessens the possibility of damage severe enough to cause loss of the vessel.
Chain Locker
Purpose: storage of the anchor chain/cable.
Most often, the aft bulkhead of the chain locker is part of the collision bulkhead. Although the chain locker is generally located forward of the collision bulkhead, it is possible to find it fitted aft of the collision bulkhead as well as higher in the structure.
The size of the locker depends on the size and length of the anchor chain to be carried. The locker does not normally extend to the ship’s sides. Often, stiffeners are positioned outside the chain locker to prevent damage from the chain. If two anchors are fitted, a centerline bulkhead divides the chain locker. This bulkhead does not extend the full height of the locker. Stiffening on the centerline bulkhead is in the form is half-round solid bar or angle bar placed with the heel out. This upper edge of the bulkhead is protected and strengthened by the fitting of a split pipe or similar arrangement. Sometimes a small platform is installed in the upper section of the locker with a short ladder leading to the access hatch at the top of the locker. Foot holes are cut into the bulkhead to facilitate access to the bottom of the locker.
Securing Arrangement for the Anchor
The securing arrangement for the anchor consists in a doubler plate with welded lugs that secure the end of the shackle. The end of the chain is securely attached to the locker by a reinforced arrangement referred to as the end clench or the bitter end. Sometimes, a release mechanism is fitted in a location where it can be operated from outside the chain locker. This allows the chain to be slipped in the event of an emergency when normal retrieval of the chain is impossible or the situation places the vessel in immediate jeopardy. Will also need to be removable for inspections, repairs, replacement.
Chain Locker Bottom
A false bottom is fitted to allow water and mud drainage (the chain and anchor should be washed as it is being taken in). This bottom is formed by perforated plating supported on floors of suitable height arranged transversally and longitudinally within the locker. A means is provided for drainage of the locker.
Chain Pipr
The anchor chain is led into the chain locker via a chain or spurling pipe. The end of this pipe is formed in such a manner as to reduce snagging of the chain and chafing of the pipe/chain.
Hawse Pipe
Hawse pipes are generally tubular in shape and are fabricated from steel plates. Modern hawse pipes have their upper and lower ends suitably shaped to reduce damage to the area when the chain is being run out or retrieved. Most ships are now fitted with a bow roller that reduces the friction of the chain on the upper end of the hawse pipe. These rollers are positioned so that the chain passes into the hawse pipe with very little contact at the top of the pipe.
The bow roller is also fitted with a chain stopper (guillotine) which is used to relax the load of the chain from the windlass. Some hawse pipes are thicker on the lower half to accommodate the erosion of the tube due to the chain. In addition to the hawse pipe strength, extra deck and side shell strength is necessary at the ends of the hawse pipes. Hawse pipes are normally fitted with seawater wash arrangements to clean the chain as it is being brought on board thus reducing the amount of debris that might enter the chain locker.
