Lips marks a first in prop and fin combination

Low fuel consumption has always been a high priority for ship owners, but demands for low propeller-induced pressure pulses and noise level may even be more important in many applications. However, maximum propeller efficiency and minimum hull pressure forces and noise level are somewhat conflicting demands. The Finnmarken is a passenger vessel with the highest comfort requirements, and the propeller excitation has a direct impact on the comfort experienced by the passengers. The integrated Lips Efficiency Rudder (formerly known as Wärtsilä Propac – see ship description, Lysblink, The Motor Ship December 2000 page 14) provides minimum fuel consumption and operational costs, and at the same time improved comfort onboard. The Efficiency Rudders for Finnmarken were chosen after model tests demonstrating 5% to 6% lower power and fuel consumption than comparable tests with conventional flap rudders. The considerable reduction of noise and vibration level was also very important. Finnmarken will join the Coastal Express, otherwise known as the Hurtigruten, along the Norwegian coast from Bergen to Kirkenes. This daily service is a vital lifeline to many small communities, and has become a popular cruise voyage for international tourists. Finnmarken was contracted by the Narvik based shipowner Ofotens og Vesteraalens Dampskipsselskap (OVDS) at the Norwegian shipyard Kleven Verft AS in Ulsteinvik. In general the gain in power due to the rudder torpedo is smaller for twin than single screw vessels. However, the efficiency gains increase with increasing hub ratio, i.e. propeller hub diameter/ propeller diameter ratio. The propeller hub ratio on Finnmarken is slightly above 30% due to the ice strengthened stainless steel propeller blades. In general the slenderness of the rudder box and the construction of the Efficiency Rudders also slightly reduce the power consumption, and also reduce the risk of cavitation at high vessel speeds. Vibration and noise reduction Due to the torpedo the propeller induced pressure impulses against hull and propeller induced vibration level is expected to be reduced by about 25%. The streamlined torpedo slightly reduces the fluctuation of the water inflow. This phenomenon is also known from the old “Costa” rudder bulbs. The torpedo eliminates hub vortices and separation, and cavitation behind propeller hub, collapsing on the rudder. The presence of the torpedo itself reduces the water velocities close to the propeller hub. Then the propeller thrust loading towards the propeller hub increase, which reduce the cavitation extent and intensity of the tip vortices. The higher pressure around the root fillet area of the propeller blades also nearly eliminating the danger of cavitation erosion. Then the propeller blade geometry can be designed to take even more thrust loading inward the propeller blades to further reduce the hull pressure and noise level. Non-optimum rotation directions In general for such slightly v-shaped aft bodies outward-turning propellers give highest efficiency, while more favourable cavitation characteristics will be achieved with inward-turning propellers. Outward-turning propellers were selected due to 3% to 4% lower power consumption than for inward-turning propellers. For Finnmarken calculations indicated about 50% lower hull pressure forces and 3 dB(A) lower propeller induced noise level, if inward-turning propellers were selected. Then with outward-turning propellers it was very important to optimise the propeller blade design to minimise the vibration and noise level. However, with contribution from the torpedo and by using new modified blade section profiles, it was no noticeable increase of propeller-induced hull pressures due to cavitation, even at the trial condition at 18 knots. In particular for fast vessels with high propeller loading, the Efficiency Rudder will reduce the vibration and noise level considerably. Integrated rudder and shaft bracket The performance of conventional propeller shaft systems on fast twin screw vessels can be improved by positioning the shaft?s main bearing behind the propeller, inside the torpedo of the Efficiency Rudder. Doing this should reduce the pressure impulses against the hull by up to 60% and decrease the noise level by more than 5 dB(A). A reduction of required propulsive power of about 5% should also achieved, though the mechanical design of this concept is not ready yet. Single screw vessels Model tests and full scale measurements have shown a gain in power at constant ship speed of at least 5% owing solely to the presence of the rudder torpedo. The gain with fixed pitch propellers seems to be about 2% lower than with controllable pitch propellers. The main reason for this increase of propulsion efficiency is the “wake gain” effect. The torpedo decreases the flow speed to the propeller in front of the torpedo by its displacement effect. The decrease of flow speed is largest in the region close to the propeller hub, but also the average water inflow speed over the propeller is reduced. This change in inflow speed contributes to increased hull efficiency, which means that less power is needed to produce the same propeller thrust. This efficiency gain can also be calculated by numerical potential theory. Model tests have shown that fitting the integrated rudder torpedo reduces pressure impulses against the hull by 35 to 40%. The propeller diameter could be increased to further increase the propulsion efficiency. * The author is hydrodynamics specialist at John Crane-Lips Norway