Abstract: A swim fin that has the streamlined design of an aquatic fish or mammal with the foot pocket and flexible shaft forming a proportional and similar shape to the body of a fish. The fin is a wing-like shape separate but connected to the shaft similar to the caudal fin of a fish. This streamlined design provides an efficient form of propulsion using only power strokes and the principles of lift similar to fish. Also the tips of the stiff caudal fin create vortices to reduce drag. The flexible shaft is made of a material with a physical "memory" so that it springs back to its original manufactured shape after it flexes to produce a secondary "kick" for the swimmer. This overall "fish or whale" shape also allows for the possibility of a modular construction, if desired, where the flexible caudal shaft and stiff caudal fin can be exchanged with ones of alternative size, shape, material and design so that the fin can be adapted to each swimmer's body type, muscle type, swimming style, and aquatic conditions.