Wartsila NSD Corporation has completed shipboard trials with the new Wartsila NSD PropacRudder which more than confirms its benefits in terms of reduced propulsive power requirement, with reduced hull vibration and noise, while providing good maneuverability.
The trials were undertaken in October and November 1999 on board the new 5175 tdw paper carrier ‘Lysbris’. The vessel is equipped with a Wartsila NSD Propac propulsion package, comprising a 6300 kW Wartsila 6L46C medium-speed diesel engine, 4.5 m-diameter Wartsila CP propeller, gearbox and the PropacRudder. This is, in fact, the second PropacRudder to enter service, with the first being in the sister ship ‘Lysvik’ delivered in November 1998.
The PropacRudder 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.
Model tests showed that solely by applying the PropacRudder, the presence of the bulb gives a large reduction in power of about ten per cent for the same ship speed compared with using a conventional flap rudder. The full-scale trials actually demonstrated an even greater power saving than was predicted from the model tests.