Multi-objective hull form optimization of a SWATH configuration using surrogate models

The present study introduces a surrogate-based multi-objective hull form optimization of a SWATH configuration, enabling optimal hull design compromises between seakeeping performance and ship resistance. A parametric model of a SWATH ship is built, which has variable horizontal torpedoes semi-axis and strut angle of inclination. The displacement is assumed to be constant and is balanced with the vertical semi-axis of torpedoes. The objective seakeeping function is the amplitude of vertical movement on the ship's gangway, calculated in irregular waves. As the energy dissipation of SWATH ships are mainly generated by viscous effects, these are estimated using empirical formulas and are added to the equations of motion. The ship resistance is computed with a finite volume solver using a RANS model. Three levels of fidelity, having increasing computation costs, are considered to model ship resistance. The first low fidelity level concerns the wetted surface of the hull. Due to their geometry, SWATH ships can be destabilized by the Munk moment and be dynamically unstable, which can lead to instabilities in resistance calculations. The medium-fidelity level then considers a free sinkage and a fixed pitch to ensure the stability of the calculations. The third higher-fidelity level considers stabilizing fins to counterbalance the destabilizing moment. Trim angles of fins are solved to reach moment equilibrium and the fins drag is included in the total ship resistance, which is to be minimized. The multi-objective optimization problem is solved for these three degrees of fidelity. Results differences between fidelity level approaches are also compared, in order to highlight the impact on the optimal hull designs of using a low-fidelity method, with regard to the computational costs.