Abstract: 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.