Design and application of propeller boss cap fins to ducted propellers

The interest in Energy Saving Devices (ESD) has grown significantly during the last years. Recent regulations, like the IMO EEOI (Energy Efficiency Operational Indicator) and EEDI (Energy Efficiency Design Index), impose further restrictions on emissions for both new designs and existing vessels. Moreover, these regulations encourage the development and the application of efficient, unconventional, propulsive solutions to fulfil these new requirements. The paper presents a design approach, based on the paradigm of optimization applied to Propeller Boss Cap Fin (PBCF) energy saving devices for ducted propellers. PBCF, indeed, were extensively studied in the case of conventional propellers, but nowadays there are only very few researches regarding the combination of these devices with unconventional propulsors, such as ducted propellers. Since the particular functioning of propellers inside nozzles can influence significantly the entire flow field aft the propeller, normally exploited by the PBCF for energy losses recovery, ad-hoc design procedures are required. By using a combination of RANSE analyses, a parametric description of the PBCF blade and an optimization algorithm, the paper illustrates the results of the design of PBCFs for propellers operating both in accelerating and decelerating nozzles, discussing the effectiveness of such kind of devices in case of ducted propellers.