8. Bottom Structures and Keel Construction Flashcards
3 basic keel types
- Flat plate keel
- Duct keel
- Bar keel
What is the keel?
The keel is considered the backbone of a ship and is a main strength member used to reduce stress from longitudinal bending (hogging and sagging) and local loads (grounding, dry-docking).
Flat plate keel
Consists of:
- Flat plate on the bottom
- Vertically placed center girder located on the centerline of the vessel
- Horizontal plate on top of the center girder
9 structural members for single bottom flat plate keel
- Side frame (offset angle bar)
- Shell plating
- Flat keel
- Center girder/ keelson
- Limber holes (for drainage)
- Floor
- Side girder/ keelson
- Rider plate (adds rigidity to center keelson)
- Pillar
Double bottom flat plate keel
Average width of the keel is 1-2 m. Thickness must be more than the rest of the bottom plating in order to provide additional strength (against grounding and to resist longitudinal bending) and resist corrosion.
At the forward end, the keel plate attaches to the stem by a shoe plate.
At the aft end, the attachment to the sternframe is called the coffin plate.
The center girder is continuously welded to the keel plate and topped with the center strake of the inner bottom plating.
Duct keel construction
Consists of a flat plate on the bottom, two vertical girders and a top plate. The complete structure forms a box-shaped tunnel. The spacing of the girders is not more than 2 metres apart. Inside the duct keel are transverse stiffeners fitted in line with every alternate transverse floor. These stiffeners are attached to the vertical girders by the use of brackets.
The duck keel usually extends from the forward engine room bulkhead to the forward collision bulkhead, and serves as a passage to run piping forward to the various tanks. Access to the space can be done via manholes in the bulkheads or in the tank top. It is an encloses space - may be dangerous.
Keels forward of the collision bulkhead and aft of the forward machinery space bulkhead are flat plate keels. The overlap between the flat plate keel and the duct keel is generally 3 frame spaces to reduce the stress caused by an abrupt change in structure.
Duct keel construction members
- Transverse stiffeners
- Brackets
- Floor
- Side girders
- Keel plate
- Limber holes
- Tank top plating
Bar keel construction
Generally only on small vessels since it does not provide adequate longitudinal resistance for larger ships. Effect of a bar keel is an increase in extreme draft with no increase in cargo-carrying capacity.
The bar keel may be either internal or external in nature. The steel bar might have the garboard strakes attached to each side allowing the bar to extend into the structure of the hull. Another arrangement might find the bar welded to the outside of a flat plate keel.
Bilge keel construction
Serve to reduce the ship’s tendency to roll.
Fitted along the bilge radius on both sides of the ship. A means of fastening to the hull is employed which will break off the bilge keel without damage to the hull in the event of fouling or collision.
Can have stiffening within the bilge keel structure to keep it from wobbling.
Hydrodynamic keels
Recognition only.
Seen in sailing vessels and small crafts.
Ballast/full keel.
Fin keel
Skeg
Bilge keels
Centerboard
Single bottom construction
No inner bottom or tank top is fitted. Solid floors are found at every frame space. Floors are flanged or have a rider plate at their upper edges to provide extra strength.
Can be found forward of the collision bulkhead and aft of the forward engine room bulkhead or aft of the aft peak bulkhead.
If a bar keel is used, the floors may be made continuous across the width of the ship. If a plate type of keel is used, the floor are intercostal (not continuous) to the longitudinal center girder).
Side girders are fitted and are intercostal to the floors. Rule of thumb: breadth < 10 m = 1 port, 1 starboard; breath > 10 m <17 m = 2 port, 2 starboard.
Center and side girders extend as far as possible forward and aft
Double bottom construction
An inner bottom or tank top is fitted on top of the ship’s bottom structure. Generally will extend longitudinally from the collision bulkhead to the aft peak bulkhead. Transverse extent of the double bottom is from part to starboard and may go directly to the ship’s sides or to a margin plate arrangement.
Advantages:
1. Adds a significant degree of safety against flooding
2. Additional longitudinal strength against hogging and sagging
3. Tank top, center girders, side girders and watertight floors act as boundaries to form talks
4. Flat tank top allows easier cleaning of the area below the engine room deck plates.
Transversally framed double bottoms
- Center girder (continuous throughout the length of the ship, continuous welding, minimum 650 mm height and 6 mm thick but thickness may be reduced by 85% at forward and aft ends due to overlap0
- Inner bottom
- Side girders (breadth < 20 m = 1 port, 1 starboard; breadth > 20 m = 2 port, 2 starboard; intercostal to watertight and solid floor)
- Floors (solid/plate, bracket, watertight
- Brackets
- Margin plates (optional)
- Vertical structs
- Margin brackets or tank side brackets
The center girder extends from a few frame spaces forward of the collision bulkhead to a few frame spaces aft of the aft peak bulkhead (overlap to minimize stress points). To maintain a certain degree of strength, it is not permitted to cut lightening holes in the amidships region of the center girder.
All floors are intercostal (at 90deg) to the center girder.
Solid floors (transversely framed)
Solid/plate type floors are fabricated from steel plate. They have lightening holes that are not more than 50% of the depth of the double bottom for floor integrity. These lightening holes reduce the weight of the floor without a major loss in strength. They allow passage through the double bottom for inspection purposes so they may be referred to as manholes. Drain holes are found at the bottom of these floors to permit liquid passage from one side of the floor to the other. Air (limber) holes are cut in the upper part of the solid floor to ensure that gases can escape thereby reducing the possibility of explosion or asphyxiation.
Solid floors are found at every frame space in the following areas: machine spaces, pounding region, under boiler seats, under transverse bulkheads, under toes of brackets supporting stiffeners in deep tanks, under any change in double bottom depth, in some cargo areas.
Elsewhere, these floors are spaces at a distance of approx 3 m apart but the actual distance is dictated by the side frame spacing.
Watertight floors
Fabricated from steel plates and are completely sealed to give a watertight or oil-tight seal. Found under watertight bulkheads and at the end of tanks. Approx 2 mm thicker than solid floors to provide more strength. If the height of the floor is over 915 mm, then vertical stiffeners are welded to the floor. Passage of pipes and inspection doors through these watertight floors must be made watertight.
Bracket floors
Found at every frame space where solid and watertight floors are not fitted. Do not provide the equivalent strength as other types of floors. Have a bracket placed at the center girder and at the margin plate or ship’s hull. The width of the bracket is 75% of the depth of the double bottom. Flanged at their free edge to increase strength. Angle or bulb bar are welded along the inner and outer bottom plating between the two brackets. Vertical struts (angle, channel or bulb bar) are placed between the transverse angle bars to provide additional strength where unsupported span is excessive.
Intersection of bracket floor and side girder is stiffened by welding in a flat bar.
Margin plate
Longitudinal plate found inside the ship and placed at the bilge radius strake. Continuous and will extend as far forward and aft as possible. Welding is watertight creating a tank boundary. Space between margin plate and hull is called the bilge area and this is where the bilge pumping system will draw suction. Margin plate is appro 20% thicker than the middle strake of tank top plating as it’s a major contributor to longitudinal strength and subjected to a great deal of corrosion.
Margin brackets
Fitted to the margin plate and ship’s side plating at every frame. Upper edge is flanged or fitted with a flat bar. Lightening and drain holes are cut into the bracket to reduce weight and allow drainage. Tank top plating will extend to the margin plates.
Longitudinally framed double bottom
Vessels > 120 m in length
Consists of:
- Center girder
- Side girders (breadth > 14m < 21m = 1 port, 1 starboard; breath > 21 m = 2 port, 2 starboard)
- Floors
- Margin plates (optional)
- Margin brackets/ tank side brackets
- Inner and outer bottom longitudinals (bulb bar or inverted angle bar, welded along the inner and outer bottom plating parallel to the center girder; if length > 215, longitudinals are continuous through watertight floors; if length < 215 m, longitudinals are intercostal)
- Stiffeners
- Brackets
- Vertical structs (if longitudinals are unsupported for 2+ frame spaces)
Floors (longitudinally framed)
Every frame space:
- Engine room spaces
- Under boiler seats
- Under transverse bulkheads
- Under the toes of brackets supporting stiffeners in deep tanks
Elsewhere, spaced 3.7 m apart.
Bracket floors are found at the intermediate frame spaces at the ship’s sides and may be spaced as far apart as 1.25 m at the center girder. These brackets extend to the first longitudinal stiffener and are flanged at their free edge. The outer and inner brackets do not necessarily line up transversely.
Tank tops extended to the ship’s sides
Passenger vessels are fitted with tank tops directly to the ship’s sides as an added safety precaution against flooding. The plating thickness is increased in the machinery spaces, under hatchways and in certain cargo areas.
This method eliminates the use of margin plates. Continuity of transverse strength must be maintained either by cutting the frame short of touching the tank top and installing a bracket welded to the frame and the tank top OR by extending the frame completely to the tank top where it is welded with a small bracket may be inserted in line with the flange on the frame.
Pounding/ slamming
Normal area of concern is 5%-30% of the length from the forward perpendicular. The bottom shell plating in this area is usually increased in thickness and all welding must be continuous in nature.
Transverse framing:
1. Solid floors at every frame space
2. Intercostal side girders max 2.2 m apart
3. Intercostal half height girders fitted midway between side girders
Longitudinally framing:
1. Solid floors at every OTHER frame space
2. Intercostal side girders max 2.1 m apart
3. Increasing longitudinal scantling size and reduce spacing
4. Unsupported span of longitudinals < 1.85 m (add floor or strut)
