Numerical Prediction of the Propulsion Characteristics of Ships in Waves

The influence of regular head waves on the propulsion characteristics of a twin screw cruise ship is investigated using RANS based flow-solver. Propeller open water characteristics are determined at first by computing the propeller forces in homogeneous inflow. Then, computations of the towed model without propeller and of the self-propelled model in calm water are performed to obtain the propulsion characteristics in calm water. Afterwards, the total resistance as well as the forces of the self-propelled model in regular head waves are computed. All computations are performed using a RANS based flow-solver coupled with the six-degrees-of-freedom equations of motion. The sliding interface method is used to allow the rotation of the geometrically modelled propeller, when needed. Computations are performed using the same numerical grids to keep errors originating on different spatial and temporal discretization as small as possible. Grid studies are performed to evaluate discretization errors of each mesh region, namely the hull region and the rotating propeller region, separately. The numerical results are compared with experimental results obtained from physical tests. It is shown, that RANS is capable of investigating the propulsion behavior of a ship in regular head waves, but to the cost of high computational effort. Fair agreement between numerical and experimental results is obtained. All results show that the propulsion characteristic change in waves. It is also found, that this is mainly caused by the change of the propeller efficiency due to a different propulsion point as a consequence of the added resistance in waves.