Abstract: An air motor for small manually powered recreational boats. The overall system includes a manually powered pump which supplys pressurized air to accumulator tanks through check valves, a throttle to pass selected quantities of pressurized air from the tanks to an air motor, and the motor which converts the air pressure to rotational motion which drives an underwater propellor to move the boat. The air motor includes at least two sets of valves and power piston assemblies. A control cam and a power cam are mounted on a rotatable crankshaft which also drives the propellor. The control cam actuates the valves to direct air therefrom in a properly timed sequence. The power piston rods press on the power cam (functioning as a lever arm) to cause rotation of the crankshaft. This air motor assembly is compact, lightweight, smooth running and energy efficient.