8.(a) Describe, with the aid of a sketch, the method of attachment of a Bilge Keel to a vessel’s hull, explaining the reason for the longitudinal position of the bilge keel, relative to the hull.(7)
(b) State the advantages and disadvantages of fitting a bilge keel compared with other methods of stabilisation.(3)
(a) Method of Attachment of a Bilge Keel & Longitudinal Position (7 marks)
Attachment method:
- The bilge keel is a long, narrow fin fitted externally at the bilge region of the vessel.
- Constructed from rolled steel plate.
- Attached to the hull plating by a continuous line of welding or by brackets and doubling plates, ensuring no sharp discontinuities.
- Ends are tapered (streamlined) to reduce resistance and avoid high stresses.
- Often attached along a strengthened section of the bilge (frame or longitudinal girder) to distribute loads.
Reason for longitudinal position:
- Bilge keels are fitted along the turn of the bilge, approximately at right angles to rolling motion.
- They are placed over the midship half-length of the vessel, where rolling movement (angular velocity and water flow) is greatest.
- Not extended too close to the bow or stern to prevent damage in docking/grounding and to avoid vibration.
- Optimum position = maximum damping effect on roll without compromising hull strength or hydrodynamics.
Sketch (exam style):
- Cross-section of hull showing:
- Hull plating.
- Bilge keel plate welded at bilge radius.
- Internal bracket/doubling for strength.
- Arrows showing roll damping action.
(b) Advantages and Disadvantages vs Other Stabilisation Methods (3 marks)
Advantages:
- Simple, robust, and inexpensive – no moving parts or complex systems.
- Low maintenance – fitted permanently, minimal upkeep.
- Effective passive roll reduction – reduces roll amplitude by up to 40%.
Disadvantages:
- Less effective than active systems (e.g., fin stabilisers or anti-roll tanks).
- Increase in resistance/drag – small but constant fuel penalty.
- Cannot be adjusted – effectiveness fixed once installed.