Abstract: A watercraft having propulsion means extending below the water line comprising a pair of flexible fins each adapted to oscillate through an arcurate 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 arcurate path; the improvement wherein each of the fins preferably have a generally squared off top and an outer area of harder rubber than the inner area. Preferably, each of the fins is provided with means at the trailing edge to provide adjustable tensioning of the trailing edge, preferably by threaded means at the clew of the trailing edge.