Resistance of a ship increases in waves, that is, so called resistance increase of a ship due to waves. However, an oscillatory hydrofoil attached to the ship bow generates thrust. Under a certain condition, the ship can be driven by wave power alone.
Generally speaking, resistance of a ship becomes bigger in waves than in calm water. This phenomenon is well known to naval architects as the resistance increase of a ship In waves. It is, however, possible to convert wave energy Into propulsive one by utilizing thrust generated by hydrofoil placed at the bow under water. Under a certain condition, a ship may advance at pretty high speed only by wave energy alone. Surprisingly enough, in 1895, Linden (The Naval Architect, 1973) filed a British patent on a ship which has a flexible hydrofoil at the bow and moves by utilizing wave energy. According to the contemporary report, he really built two boat of 13 f tand 24 ft In length named Autonaut as shown In Fig. 1, and the boat could move of their own motions due to waves against wind and wave at a speed of three to f our knots. Recently, Jakobsen (1981) has reported very interesting experiential results. According to his report, a model ship of 1.03 m in length could move against a wave at a speed of about 1 m/s. The wave length and height were 2.25 m and 0.1 m respectively. Terao (1982) has also given an experimental report on a floating body which moves against waves. On the other hand, Wu (1972) has shown theoretically that propulsive efficiency of an active-type hydrofoil propulsor driven by an external energy can become more than one or even minus In a wave as shown In Fig. 3. The negative propulsive efficiency means that an extra energy other than the propulsive one is taken outside.
