Kuroshio is one of the ocean currents in the north Pacific, passing through the east of Taiwan. Kuroshio current has a steady flow in which the direction of 75-80% heads the north and the northeast. It is still difficult to harvest Kuroshio energy due to the conditions of deep seabed more than 400m and surface wave of current flow affected by season winds and typhoons. In order to obtain higher efficiency of Kuroshio energy, a power-free underwater vehicle, which works under the sea to carry generation turbines, was developed. This underwater vehicle can move upward and downward by means of changing rudders without applying power supply. In this study, to find the optimal rudder profile for the power-free underwater vehicle several symmetric profiles of the rudder in accordance with National Advisory Committee for Aeronautics (NACA) airfoil designation are selected to investigate the lifting and drag force in the Kuroshio current. Results indicate that the optimum rudder profile is NACA0008-L5 by considering the lift force and mechanical strength of the rudder. The rudder profile NACA0008-L5 at a 30° attack angle in 1.0 m/s uniform flow offers a 19.1% increment of lifting force, more efficient than the other rudder profiles.
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