Single and Multiobjective Design Optimization of a Fast Multihull Ship: numerical and experimental results

The numerical optimization of the initial design of a fast catamaran (HSSL-B) is carried out through Simulation Based Design (SBD) framework, based on advanced free-surface Unsteady Reynolds-averaged Navier-Stokes (URANS) solver and Potential flow solver, and Global Optimization (GO) algorithms. The Potential flow CFD SBD is used to guide more expensive URANS CFD SBD. The fluid-dynamic analysis of the flow past the catamaran proved that the use of URANS solver was fundamental in dealing with the multihull interference problem. In the case investigated the separation distance was small and the viscous flow quite distorted by the proximity of the hulls, so that only viscous solvers could correctly capture the flow details. Sinkage and trim effects, due to the high-speed range and again to the small separation distance investigated, are also relevant. The initial HSSL-B geometry and three optimization problems, including single and multi- objective optimization problems, proposed by designers from Bath Iron Works, are successfully optimized/solved and finally an experimental campaign is carried out to validate the optimal design. A new Verification and Validation methodology for assessing uncertainties and errors in simulation based optimization is used based on the trends, i.e. the differences between the numerically predicted improvement of the objective function and the actual improvement measured in a dedicated experimental campaign, including consideration of numerical and experimental uncertainties. Finally, the success of the optimization processes was confirmed by the experimental measurements, and trends for total resistance, sinkage and trim between the original and optimal designs are numerically and experimentally verified and validated.