Application of multi-objective optimization based design to high-speed craft propellers

The design of a propeller for a high-speed craft is addressed by using a multi-objective numerical optimization approach. By combining a fast and reliable Boundary Elements Method, a viscous flow solver based on the RANSE approximation, a parametric 3D description of the blade and a genetic algorithm, the new propeller shape is designed to improve the propulsive efficiency, reduce the cavitation extension, increase the cavitation inception speed and maximize, at the same time, the ship speed. Rather than by constraining the propeller delivered thrust, indeed, the proposed procedure works together with an engine-propeller matching algorithm that, each time a new propeller is defined, identifies the achievable maximum speed and the resulting engine functioning point that turn in additional goals for the optimization. A set of optimal propellers, obtained through the design by optimization based on potential flow calculations, are preliminary selected for additional viscous analyses in order to further validate the results of the BEM calculations and provide a deeper insight into the complex flow fields of high-speed propellers useful for choosing the optimal geometry. The improvements observed at the cavitation tunnel and the substantial increase of the maximum ship speed during sea trials on a high-speed craft provided by Azimut|Benetti prove the reliability of the design procedure.