Research on Prevention of Root Erosion by Fin

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(1). Purpose of the Present Project:The propeller shafts for high-speed ships and crafts are equipped to the ship hull with an inclined angle to the hull surface and the incoming flow to the propeller becomes an oblique flow. It is well known that this type of flow generates detrimental unsteady cavitation near the root of propellers due to large variation of inflow angles during one revolution of the propeller and cloudy cavitation on the propeller working in the oblique flow often causes severe erosion in the vicinity of propeller root even during the speed trial. In the previous research, the root erosion was reduced remarkably by equipping small fins in front of the propeller. On the other hand, no optimum theoretical design methods of these fins have not been developed yet. This project has been carried out to develop the optimum theoretical design tools for the propeller with small fins and to acquire reliable validation data for both theoretical analytical and design tools.(2). … More Progress of the Project:In order to develop the theoretical analysis method and the design tools, first of all, the effects of the fin to the propeller performance and to the cavity collapsing behavior were examined by not only paint erosion test but also flow visualization using an ultra-high speed video camera around 100,000 fps. The flow field around the forward fin propeller with several configurations of fins was measured by Laser Doppler Velocimeter (LDV) to obtain the validation data for theoretical analytical tools.In the second year of the present project, a new propeller and a fin were designed and manufactured. The performance of this propeller and other one was measured without fins and in several combinations with two kinds of fins. The effects of the fins on the cavitation characteristics around the root of the propellers were examined not only in uniform flow but also in the 10deg-oblique flow. Two analytical methods for the propellers working in the oblique flows were developed, which are the rigorous surface panel methods in time domain and the practical surface panel methods in frequency domain. Cavitation observation and erosion tests were carried out on the propellers with several fin configurations. Data analysis on the erosion test results and the computations on the blade pressure distribution around the root of propellers was performed to find the correlation factor between the erosion severity and the fin-propeller configuration including the cases without fin. Finally the reasonable design method for the optimum arrangement of the propeller with anti-root erosion fins has been developed. Less