Boat propulsion system

Claim 1

In a propulsion system on a boat having: means defining a downwardly-facing, open-bottomed recess extending lengthwise along the bottom of the boat and gradually converging to provide a progressively smaller cross-section rearwardly; the improvement which comprises: an annular shroud defining a passage that is closed at the bottom of the boat and merges smoothly with the back end of said open-bottomed recess and provides a rearward extension of said recess, said passage presenting a venturi throat and having an abruptly increased cross-section behind said throat and terminating in an open back end; a propellor circumferentially enclosed in said passage just in front of said venturi throat; means defining one or more openings in said shroud leading into said shroud immediately behind said venturi throat; auxiliary passage means open at one end thereof to the outside of the boat and communicating at the other end thereof through said openings with the interior of said shroud immediately behind said venturi throat for passing water into said shroud by the venturi action of the water displaced through said throat by the propellor; said recess providing substantially horizontal and non-turbulent flow of intake water along the bottom of the boat with gradually increasing velocity to the inlet side of the propellor; and a plate-like valve member in said shroud behind said openings, said valve member being pivoted and movable between an open position substantially parallel to the direction of water flow from said propellor through said shroud and a closed position substantially transverse to said direction.