A patented semi-balanced rudder developed by Wärtsilä Corporation. The Efficiency Rudder is characterised by a streamlined “torpedo”-shaped bulb on the rudder horn immediately abaft and coaxial with the propeller hub. The bulb ensures a more homogeneous water flow both in front of the propeller and in the propeller slipstream. The bulb increases propulsion efficiency by the “wake gain” effect, which means that it reduces the water speed into the propeller so that less power is needed to produce the same propeller thrust. The bulb also reduces propeller-induced hull noise and vibration by eliminating hub vortices and separation, and cavitation behind the propeller hub collapsing on the propeller. Single-screw vessels equipped with the Efficiency Rudder have 5-7% improved propulsive efficiency, pressure impulses are reduced by 30-45%. See also Energopac.