The selection of the propulsion system is made in the concept design phase, based on a trial design point. Evaluation of the vessel design and the propulsion units has been based on model scale measurements for decades. Recent developments of CFD simulations have brought these numerical simulations to a sufficient mature level, to be considered as a good alternative for the model scale measurements. Comparisons of CFD results for model scale and full scale have revealed new interesting insights in the Reynolds scaling effects.
The actual operating conditions of offshore vessels are significantly different from the calm water trial condition. The optimized propulsion units for the trial condition are not per se the optimum solution for the actual operating conditions. A comparison has been made of two azimuth thruster types: pulling thruster with open propeller and a pushing thruster with ducted propeller. It has been shown that the performance in calm water is comparable for both type of thrusters. However, when the vessel is operating at increased loading, the benefits of the ducted propeller units become more pronounced. This is in the end reflected in an increased capability of keep a certain transit sailing speed in more harsh conditions.
