Ring Propeller

One embodiment of the invention will now be described with reference to the accompanying drawings in which:

FIG. 1 is a perspective view of a propeller when viewed from the upstream end;

FIG. 2 is a side view of the propeller illustrated in FIG. 1;

FIG. 3 is a plan view of the propeller;

FIG. 4 is a side view at 60.degree.;

FIG. 5 is an inverted plan view; and

FIG. 6 is a section on line A--A of FIG. 5.

FIG. 7 is a side view of a modified propeller in which the apertures are formed by bores.

Referring now to the drawings, a bronze ring propeller in the embodiment described has three blades, 2 positioned at 120.degree. around a central boss 4. The annular shroud 6 is coaxial with the boss and is connected to the boss by each blade, the whole propeller being an investment casting. The blades 2 are all laid back 10.degree. which places the leading edge 8 of the shroud in the same plane as the boss 4 and the trailing edge 10, just to the rear of the boss 4. Each of the blades 2 are of constant width and of an aerofoil section which is constant between the mutually opposite ends of the blade that is they have a common chord root. The pitch of the blades is 30.degree.. The shroud 6 has a frusto-conical inner face 12 which defines a 10.degree. taper whereas the outer face 14 is a compound arcuate curve giving an aerofoil section. Each of the three sectors of the shroud is pierced by three rectangular slots 16. The slots are spaced equally from one another and from the junctions of the blades with the shroud. The axis of each slot is inclined at about 10.degree. to the axis of rotation 18 of the boss. As mentioned above, the apertures 16 pierce the shroud at an angle which is preferably from 2.degree. to 65.degree. relative to the periphery of the shroud. Since the apertures are so inclined, it will be understood that each aperture has walls which are inclined at an angle relative to a line which extends radially from the central axis of the propeller to the respective aperture.

When the propeller is driving in the forward direction which is indicated by the arrow B in FIG. 5 substantially no water is drawn through the slots 16 however when the propeller is operated in the reverse direction the leading inclined edge 20 of each slot bites into the water and thrusts jets of water on the trailing faces of each blade thereby vastly improving the moving thrust of the propeller.

While the distribution of the slots can be seen in FIG. 1 the inclination of the slots can best be seen in FIG. 2.

In another embodiment shown in FIG. 7, each slot is replaced by a series of several bores 16' each lying at the same angle to the diameter D as the single slot.

While the embodiment described refers to a propeller having three blades with three slots in each sector it will be appreciated that the invention is not so limited.