A marine tidal turbine harvests kinetic energy from bidirectional underwater tidal currents using an axial-flow rotor. Applying actuator disk momentum theory, the fluid velocity slows from the upstream current speed (V∞) to an axial speed (V∞(1-a)) at the rotor plane, and further decelerates in the expanding downstream wake. The power conversion efficiency is bounded by the theoretical Lanchester-Betz limit of 59.3% (Cp = 16/27).