Design of contra-rotating propellers for high-speed stern thrusters

Abstract

The paper presents a computational method developed for the design of contra-rotating propellers of high-speed craft propulsors. The method is based on Lerbs’ lifting line theory, but with some new peculiarities introduced to adapt the method, originally devised by Morgan for slow speed propellers. The differences introduced in the method regard mainly the interaction effects between the two propellers and the criteria used to select the thickness/chord distribution to match the cavitation and strength constraints. In fact, a new fully numerical method, based on the exact lifting line model of the propellers, is used to recalculate the mutual induction factors used by Morgan (1960) to consider the interference of flow induced by each propeller to the other. After the description of the theoretical and numerical method, an example of practical application of the design method to the case of a stern-podded drive for a fast-planing boat is presented and critically discussed through the analysis of the propeller geometries obtained with different design options and by comparing them with commercial propellers sets suitable for the same application example. The applicability of the method to the investigated case has been successfully verified with some outstanding points to be verified in future research activities.