Abstract: A ring propeller has a frusto-conical shroud disposed co-axially about a boss, and a plurality of blades extending between the boss and the shroud thereby dividing the shroud into equal sectors. The blades have parallel edges and are of constant corss section between the boss and the shroud. The reversing characteristics of the propeller are substantially improved by having a plurality of apertures disposed equally around the shroud and passing through it. There is at least one aperture in each sector, and the apertures are disposed at an angle to the central axis of the propeller of from 2.degree. to 65.degree.. The walls of the apertures are disposed at an angle to the outer surface of the shroud to thrust water on the trailing faces of the blades when the propeller is rotated in a reverse direction.

Abstract: A marine ring propeller comprising blades, each having parallel edges and a constant cross-section, and a ring in the form of an annular shroud fixed to the outer ends of the blades. The shroud is typically frusto-conical on its inner surface and curved on its outer surface to provide an aerofoil cross-section, apart from the leading edge which is provided with a concave bevel on either or both of the inner and outer surfaces.