Optimization Design of Novel Afterbody Shape for a Cruise Ship

A novel stern hull form is presented to improve both fuel economy and comfort performance of a cruise ship propelled by twin pod propulsion units. The particular afterbody shape is characterized by the shallow tunnel around the area over each propeller and its adjacent wedge-shaped tail. The design optimization is implemented with application of Simulation-Based Design (SBD) technique. Based on the original hull with a well optimized conventional afterbody, the configurations of shallow tunnel and local wedge are established via a partially parametric modeling method, seven important parameters are selected to be design variables in the optimization process. For every automatically modified hull, wave resistances at two typical speeds are calculated using a fully nonlinear potential flow method. A multi-objective genetic algorithm is adopted as the optimizer to explore the design space. The optimization results are further numerically verified by use of high-fidelity RANS codes. Dedicated model tests are finally carried out in towing tank. The validated reduction of total resistance in model scale is 1.2% and 1.6% separately at the two speeds, meaning a full scale power saving of 2.0% and 2.7% respectively. In addition, both the area of the axial wake peak and the value of the maximum wake fraction at the propeller disc of the optimized hull are obviously decreased. The results indicate that the presented novel afterbody shape has advantages of energy saving and wake uniformity over the conventional stern hull form.