A swim fin may include a foot pocket configured to receive a foot of a swimmer and a fin blade extending from the foot pocket. A relatively rigid substrate chassis may be bonded to the foot pocket. The fin blade may be relatively flexible. Fin rails may extend along the lateral edges of the fin blade. The fin rails may include a fin spine comprising a plurality of fin spine segments in linear configuration. The fin spine may be configured to provide a swim fin with predetermined hydrodynamic characteristics. The swim fin may flex within a maximum angle of attack that may be variable and dynamically changed, within a predetermined maximum attack angle range, as a function of the kicking force generated by a swimmer during a kicking cycle.