USA As Represented by Secretary of the Navy

Alan J Becnel

John G Purnell

A pump-jet type of outboard propulsion unit within a surrounding fluid mem for marine craft or the like, is protectively enclosed within cylindrical shrouding which forms an axial flow passage about the exhaust channel of the propulsion unit through which flow of the fluid medium is induced by rotation of impeller blades. Inflow of the fluid medium to the flow passage is modified at an inlet end of the shrouding by a plurality of angularly spaced vanes attached thereto and extending forwardly therefrom at an angle.

Protective shrouding with debris diverting inflow vanes for pump-jet

The invention features a unique surface configuration which fosters atmosric ventilation and thereby uncomplicatedly reduces drag which is normally associated with implementation of shrouding for a marine impeller. Typical inventive embodiments include a blunt trailing edge and a circumferentially stepped impeller shroud (below the blunt trailing edge), the harmonious union of which affords a generally smooth and unbroken surface which encourages drag-defeating air circulation. During marine navigation, circulation of the air generally describes a path wherein the air first travels linearly downward along the blunt trailing edge, then travels curvingly downward (both clockwise and counterclockwise) along the circumferential step, forming a circumferential air pocket which extends a backward distance increasing in accordance with increasing navigational speed; upon attainment of a threshold navigational speed, the air pocket (virtually) completely surrounds the shroud. According to the invention, many a conventional outboard open propeller assembly can be facilely converted to an inventive outboard shrouded impeller assembly which produces equal or superior performance.

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Annapolis, MD, United States of America

Alan J Becnel