Question

Describe a method for attachment of bilge keel

State 3 reasons for not extending bilge keel to the entire length of the vessel

Explain 2 principle of roll damping that bilge keel exploit

Answer.

A bilge keel consist of a projection from the turn of bilge over 30% to 50% of the ships length in the midship region. To help prevent damage to the bilge keels they are fitted within the lines of bottom and side shell. To be effective the keel should however project beyond the boundary layer.

The bilge keels are attached directly in line with an internal stiffening member such as a girder or longitudinal. On thicker shell plating (such as merchant ships) the bilge keel is made up of 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 and if the keels are positioned so that they follow the streamlines at the service speed of the ship, then the added resistance is only about 2%.

If the keel is fitted for the full length of the vessel it would be more difficult to construct and would be difficult to align with flow along the hull, particularly at the fore end, causing 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 limited effect.

The simplest form of stabilisation, bilge keels, are often fitted even when other more sophisticated methods of roll reduction are used. Cheap and easy to maintain, they can provide up to 30% roll amplitude reduction and are effective when a ship is at rest or making headway. Fitting the bilge keel to the parallel mid-body means that it is a simple construction and gives the best damping effect since it maximises the lever to the axis of rotation.

A 'full form ship has a higher natural damping coefficient than a Tine' form ship. Thus, a bilge keel fitted to a full form ship will have less effect-than on a fine form. Furthermore, since the bilge keel must be between the lines of side and bottom shell, it will be  constrained to a smaller size on full form ships.