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.

• 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.

Increased Roll Period:

• Bilge keels increase the ship's roll period (the time it takes for the vessel to complete one roll). This is achieved by increasing the moment of inertia (K) of the vessel. A longer roll period means the vessel rolls more slowly, thus reducing the amplitude of the roll.

$$T_{r}=2\pi\:\frac{k}{\sqrt{g\times GM}}$$

Where:

• g: acceleration due to gravity
• GM: metacentric height
• k: mass moment of inertia

By increasing k, the ship's stability during roll improves.

Hydrodynamic Resistance:

• As the vessel rolls, the bilge keels move through the water, creating pressure differences. Water pressure on one side of the keel opposes the rolling motion, providing a damping effect. This is due to the interaction between the bilge keel and the water, creating hydrodynamic forces that counteract the rolling motion.