Costa bulb (Propulsion bulb)

Limited to single screw vessels (usually) the bulb is a simple but effective device for recovering energy from losses aft of the propeller. It consists of a fabricated stream lined steel shell, manufactured in two halves and welded onto the rudder immediately aft of the propeller boss with its centre line continuous with the tail shaft.

The bulb eliminates vortici created due to turbulent flow and sudden contraction of the water which trails from the boss. Thsi contraction is caused by the suden release of the very large volumes of air released under normal operational conditions when the water passes through the prop
It has a tranquillising effect on the flow of water behind a propeller. Reduces prop vibration, stiffens rudder, increases buoyancy and improves steering.
Increase in propulsive efficiency is about 0.5%