Evaluation of an energy saving device via validation speed/power trials and full scale CFD investigation

Energy saving devices (ESD)s designed to improve the propulsive efficiency of a ship are often designed and validated using CFD tools and model tests. Evaluation at full scale is however still required to understand extrapolation methods and scale effects. This paper describes the evaluation of an
ESD by means of dedicated speed/power trials just prior and directly after the installation of the device in dry dock. The energy saving device is in the form of three stator fins located at the port side just ahead of the propeller creating a pre-swirl in the flow into the propeller which reduces rotational losses of the propulsion. The stator was build and retrofitted by Uljanik shipyard in Croatia on a new build 52,000 DWT bulk carrier. Trials were done prior and after retrofitting of the ESD in almost ideal weather conditions. Comparison of the trial results revealed a fuel saving effect of 6.8% in power. Cavitation observations of the stator and propeller showed a removal of the cavitating hub vortex of the propeller after the installation of the fins. This can be detected and confirmed in the CFD computations as well. Full scale CFD investigation employing the RANS-BEM coupling method to simulate the propeller effect gives a power saving of more than 5%, which is in good agreement with the trial results. The geometry used in the CFD simulation is based on the 3D in-situ geometry measured via laser scan technique after the retrofitting of the fins.