The use of oscillating fins by aquatic animals for propulsion has inspired the design of corresponding propulsors for ships. Rigid and partly flexible oscillating propulsor designs were analyzed for a large ship based on two-dimensional linear and nonlinear theories. Open water efficiencies of the oscillating propulsors were 17–25% higher than those of an optimal screw propeller. Behind the ship, quasi-propulsive efficiency of the partly chordwise flexible foil was 72%, i.e. 5% higher than that of the screw propeller. The efficiency increase produced by the oscillating propulsor was attributed to the considerable increase in the propeller working area due to the wide span of the foil.
The method of propulsion evolved by many swimming animals is centered around the generation of thrust from movements of a crescent shaped fm or wing-type surface (van Dam, 1987). This includes propulsion from the tall fins of certain fish and marine mammals, carangiform (Lighthill, 1970) and thunniform (Hoar and Randall, 1978) propulsion, and also the flapping propulsion exhibited by the pectoral flippers of many marine turtle and penguin species. Maximum propulsive efficiency for a fin whale (Balaenoptera physalus) has been estimated to be about 85% (Bose and LIen, 1989). For a naval architect, natural propulsive fin geometries (Lang, 1966, van Oossanen and van Oosterveld, 1989; Bose et al 1990; Curren, 1992) and their propulsive characteristics are important as guides in design of oscillating propulsors. On the other hand, several theories of oscillating propulsion have been developed (Bose, 1992, Chopra, 1976; Kambe, 1978, Katz and Wems, 1978; Kubota et aI., 1984; Kudo et a1., 1984; Lighthill, 1960, 1970, Liu and Bose, 1993; Wu, 1961,1971). Some of them have been extended to consider propulsion usmg wave energy (Bose and Lien, 1990; Lai et aI., 1993, IsshikI and MurakamI, 1982-1984, Isshiki and Murakamu, 1986, Wu, 1972).