Dual-operating-point blade optimization for high-speed propellers

Propeller blade design for fast ships is often driven by cavitation constraints. A tradeoff exists, where larger chord lengths and section thicknesses typically improve cavitation performance but result in lower efficiency. Typically, chord lengths are optimized for the design condition (ship endurance speed), with some specified margin to prevent cavitation off-design (at maximum ship speed). Cavitation performance at the maximum speed is considered post-facto, and blade shape often needs to be modified for cavitation considerations in high-speed operation. This paper presents an improved method for blade shape optimization. The present method simultaneously considers the cavitation performance at the endurance speed design point and a maximum speed off-design point, and blade chord lengths and thicknesses are set to prevent cavitation at both operational conditions. During the present design optimization routine, the on-design load distribution is optimized, and the off-design performance is determined, such that the chord lengths can be set to a minimum that still prevents cavitation at both the on- and off-design conditions. A case study is presented, considering the notional design of a propeller for the U.S. Navy DDG51 destroyer-class ship. Propellers designed using standard chord/thickness optimization procedures are compared to those designed using the present procedures. Cavitation performance is compared for the two design methods.