9 th International Conference on Hydrodynamics October 11-15, 2010 Shanghai, China 553 2010, 22(5), supplement :570-576 DOI: 10.1016/S1001-6058(09)60254-7 Design and analysis of counter-rotating propellers-comparison of numerical and experimental results Davide Grassi 1 , Stefano Brizzolara 2 , Michele Viviani 2 , Luca Savio 2 , Sara Caviglia 2 1 ZF Marine Arco s.p.a. Arco (Tn), Italy Email: davide.grassi@zf.com 2 Department of Naval Architecture and Marine Engineering (DINAV), University of Genoa, Genova, Italy Email: brizzolara@dinav.unige.it ; viviani@dinav.unige.it ; savio@dinav.unige.it ; caviglia.s@gmail.com ABSTRACT: In these last years a growing interest has been devoted to counter-rotating propellers especially for pod/stern drive applications for pleasure boats. In this context DINAV has been interested in the framework of an industrial R&D project to develop design/analysis computer codes for such kind of propulsors which is based on a modern lifting- line/lifting surface design method [1-4] . The present work is devoted to the comparison between the foregoing numerical design/analysis tools and the results of the cavitation tunnel tests. The validation study has been performed on a set counter rotating propellers designed to be installed on a commercial pod drive system.. Despite some uncertainty intrinsically connected to the nature of the test, results are very well in agreement with design points, allowing to validate the theoretical codes developed so far and to obtain semi-empirical correction factors for future application. KEY WORDS: contra-rotating, stern thrusters. 1 INTRODUCTION In these last years, a general interest has been developed in contra-rotating propellers (CRPs), in their use in outboard propulsor for fast-planing craft or in podded drives for fast ships or mega-yachts. It is quite clear that the advantage of having CRPs can be better exploited for these types of vessels usually characterized by shallow drafts and high specific power loads on propellers. The advantages for fast boat propellers, mainly derive from dividing the delivered power (and thrust) on two propellers moderately loaded instead of just one with higher load, to contain the expanded area ratio, possibly the diameter and ensure a good cavitation margin. However, there are also applications in larger scales, such as in conventional shaft line arrangements or podded propulsors for large displacement ships. So the potential applications of a CRPs design method are rather general and widely applicable. The present paper deals with the validation of a design system for optimum CRPs, which is based on the following two types of numerical computer codes: (1) A design program based on a revised lifting line/lifting surface theory for optimum CRPs; (2) A steady lifting surface program for the analysis of the designed CRP set. After a brief introduction of the theoretical and numerical aspects of the foregoing methods, an example of a practical application to the case of a fast- planing boat with stern drives propulsor is presented and critically discussed. The contra-rotating propeller set designed through the foregoing design system has been manufactured and installed on a commercial pod system in order to be tested at the University of Genoa Cavitation Tunnel for assessing global performance characteristics and cavitation behaviour; such an experimental campaign provided very useful information to validate the theoretical codes presented in the present work. 2 DESIGN METHOD FOR CONTRA- ROTATING PROPELLERS 2.1 Introduction Despite the wide existing interest about contra- rotating propellers, few articles have appeared in literature recently about design methods of CR propellers; hence the early method proposed by Morgan [7] remains still a valid option to design their optimum wake-adapted geometries. On this base, it is possible to re-consider this classical design method and develop it, by taking advantage of