Oscillating foils for ship propulsion

The high propulsive efficiency, the fast manoeuvrability and the low noise production of the propulsion of marine animals inspired the development of a new ship propeller. This text describes the design of a flapping foil ship propeller and the experiments performed on it. The flapping foil propeller mimics the tail fin of fish that swim at high speed, like tunas or sharks, in at least two ways: the hydrodynamics and the resonant driving mechanism. The motion of the foil is a combination of a heaving and a pitching oscillation, with a phase difference. The wake behind the tail of a fish has a special structure called the reversed von Karman street. If the motion parameters are well chosen, the wake behind the flapping foil has a similar structure, resulting in positive thrust force and high propulsive efficiency. The driving mechanism uses flexibility to exclude the need for one of the two actuators. The influence of the free surface and the oscillation frequency on the performance are investigated.