Question

(a) With the aid of a sketch describe the method of attachment for a bilge keel and hence explain why provision is made to reduce the possibility of the hull being punctured in the event of damage to the keel. (6)

(b) State why the keel does not extend for the length of the ship. (5)

(c) Evaluate the effectiveness of bilge keels for large wall sided vessels. (5)

Answer 1

Part (a)

Method of Bilge keel attachment to the hull:

Bilge keels are fitted port and starboard at the turn of the bilge. They do not extend outside the lines of the side and bottom shell. The bilge keels are attached directly in line with an internal stiffening member such as a girder or longitudinal. The bilge keel comprises a flat bar doubler welded directly to the shell, and an offset bulb plate (OBP) with 'scallops' cut in it is welded to the flat bar doubler. The ends of the bilge keels are tapered (minimum 3 in 1) and will end in line with transverse internal stiffening, such as a frame. Using a hull doubler protects the hull in case of damage to the keel, as the crack would not extend into the hull.

Part (b)

Bilge keels are not fitted for the full length of the vessel because:

The 'lever' to the ship's axis of rotation is reduced at the ends;
The ineffectiveness, the closer the bilge keel is to the rolling centre, increased resistance and more likelihood of damage.
The hydrodynamic effect would cause a large increase in resistance and fuel consumption;
At the aft end, the boundary layer is much thicker, and since the keel would not project through, it would have a much reduced effect.

✓ Verified Model Answer (Text Solution)

Method of Bilge keel attachment to the hull:

Bilge keels are not fitted for the full length of the vessel because: