Abstract: An anti-cavitation semi-cylindrical shroud and rudder combination is described which attaches to the transom of a boat so as to partly enclose a propeller which extends rearwardly of the boat. The axis of the propeller is selected to provide a line of thrust which intersects the approximate center of hull resistance to provide more stable running characteristics. The shroud itself encompasses about 37% of the periphery of the propeller and the lower edges of its side skirt portions lie in a plane which is parallel to the keel of the boat. The configuration of the shroud is such that it conforms to the bottom hull configuration at the front end and shrouds the propeller at the rear end. The shroud itself is configured to provide minimal flow resistance to the water.Yoke-mounted flanking rudders are pivoted ahead of the center of area of the rudder with the center of area of the rudders lying approximately in the fore and aft plane of the propeller. This facilitates turning particularly at low speeds.