Multi-objective optimization of structural parameters in outboard waterjet propulsion using NSGA-II and entropy-weighted TOPSIS method

Abstract

This study investigates how key flow-channel and blade parameters affect the hydrodynamic performance of an outboard waterjet propulsion system. Using a Box–Behnken design within a response surface methodology (RSM) framework, predictive models were developed for three performance metrics: hydraulic head, thrust, and propulsive efficiency. The design variables were the blade root pitch angle, blade tip pitch angle, and nozzle attack angle. A hybrid multi-objective optimization framework combining NSGA-II and entropy-weighted TOPSIS was then applied to identify a Pareto-optimal parameter set. The RSM models showed strong predictive capability, with predicted $R^2$ values of 0.9267, 0.9464, and 0.9635 for head, thrust, and efficiency, respectively. Increasing the pitch angles and nozzle attack angle improves head and thrust but reduces propulsive efficiency. The interaction between blade tip pitch angle and nozzle attack angle is the dominant coupling effect governing propulsion characteristics. The optimal parameter combination was 49.99° (root pitch), 28.32° (tip pitch), and 10.04° (nozzle attack angle). The proposed framework enables balanced improvement of propulsion performance and energy efficiency and provides practical guidance for the design of compact outboard waterjet systems.