Fin for oscillating foil propulsion system
A watercraft having 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 across the central longitudinal dimension of the watercraft. As input force is applied, the fins twist to form an angle of attack for providing forward thrust while moving in both directions along the arcurate path. Each of the fine have a generally squared off top and preferably an outer area of harder rubber than the inner area. Each of the fins can be adjusted at the trailing edge to provide adjustable tensioning of the trailing edge.
1. A watercraft having propulsion means extending below the water line comprising a pair of flexible fins each having a leading edge and a trailing edge and adapted to oscillate through an arcuate path in a generally transverse direction across the central longitudinal dimension of said watercraft, and means operatively associated with said propulsion means comprising foot pedals 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;
wherein each of said fins is firmly restrained along its leading edge and adjustably restrained at its lower end near its trailing edge to provide less restraint at said trailing edge, said trailing edge being adjustably tensionable and said fin is provided with an essentially square top end to provide enhanced fin twist and more efficient propulsion;
said fins are constructed and arranged such that as input force is applied on said foot pedals, the less restrained lower end of the fins twists to assume a propeller shape and, as the fins oscillate, the fins change pitch or shape as they simultaneously reverse direction of movement at the opposite ends of their arcuate path, and
wherein the overall thickness of each of said fins decreases in the direction of its trailing edge, and has a generally peripheral area and an inner area, said peripheral area being a harder, less flexible rubber than said inner area whereby the cordwise stiffness of the fin increases from leading to trailing edge.
2. The watercraft of claim 1 wherein said inner area is of a generally sawtooth configuration.
3. The watercraft of claim 1 having the improvement wherein each of said fins is provided with means at its trailing edge to provide adjustable tensioning of the trailing edge.
4. The watercraft of claim 1 having the further improvement wherein each of said fins is provided with threaded means at its trailing edge to provide adjustable tensioning of the trailing edge.
5. The watercraft of claim 1 wherein said peripheral area has a hardness about 50 to about 90 Shore A Hardness Scale and said inner area has a hardness of about 20 to about 50 Shore A Hardness Scale.