A device including propulsion means extending below the water line comprising a pair of flexible fins each adapted to oscillate through an arcuate path in a generally transverse direction with respect to the central longitudinal dimension of said watercraft, and means operatively associated with said propulsion means for applying input force to said propulsion means whereby as input force is applied said flexible fins can twist to form an angle of attack for providing forward thrust with respect to the longitudinal dimension of the watercraft while moving in both directions along said arcuate path, each said fin being carried at its upper trailing edge by a fixed pivot and at its upper leading edge by a retainer which is normally engaged while being disengageable when said leading edge strikes a resistance element allowing said fin to pivot aft to clear the resistance element and re-engageable as the fin rotates forward and resumes producing thrust.
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