A robotic mechanical fin, having a motor housing containing a plurality of rib rotation motors, rib spars, and a plurality of ribs, mechanically movable and communicatively coupled to the plurality of rib rotation motors and shafts, where the plurality of ribs are rotationally coupled to and actuated by the plurality of rib rotation motors and shafts. The mechanical fin further includes a flexible fin casing, within which the ribs reside, forming the complete actively controlled curvature robotic propulsion and steering apparatus. The mechanical fin, is connected to a plurality of control electronics circuits and a computer processor programmed with actuation code that when executed by the computer processor causes automated actuation of simultaneous propulsion and steering maneuverability of the actively controlled curvature, robotic, mechanical fin.