A reciprocating marine propulsion engine is disclosed which includes a hollow, propulsion cylinder with a cylindrical axis, an intake end for admitting ambient water and an opposed discharge end for discharging water. A reciprocating shaft coaxial with the propulsion cylinder, transmits a reciprocating drive force. A ring member is mounted perpendicularly on a first end of the shaft and slideably engages the interior surface of the propulsion cylinder. Two semicircular vanes of a butterfly valve are pivotally mounted on the ring member for movement about an axis perpendicular to the cylindrical axis. A hollow ram cylinder has a first end mounted to and coaxial with the intake end of the propulsion cylinder, with a ram piston mounted on a second end of the shaft which slideably engages the interior surface of the ram cylinder. A first intake port on a second end of the ram cylinder opposite from the first end thereof, admits a working fluid during a thrust cycle, for applying a thrust force to move the ram piston in a thrust direction from the second end toward the first end of the ram cylinder. This moves the ring member from the intake end to the discharge end of the propulsion cylinder, the reactive force of the water within the propulsion cylinder causing the first and second vanes of the butterfly valve to close against the ring member, thereby forcing the water within the propulsion cylinder out through the discharge end thereof. This results in the application of a reactive propulsive force on the engine opposite to the thrust direction.
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