A propeller having a hub (11) and a plurality of blades (13) fixed to the hub (11) and extending outwardly to terminate in an annular ring (14). Portion (21) of the ring (14) on the trailing side (19) of the blades (13) is relieved so as to allow outward flow of water upon rotation of the blade (13) to relieve ring pressure and break up the rotational boundary layer. As an alternative, the ring (14) may be replaced by discrete end portions at the blade tips (15).
