Flow through marine propeller
BACKGROUND AND SUMMARY 
     The invention relates to marine propellers, and more particularly to flow through type propellers. 
     Flow through type marine propellers discharge engine exhaust gases through the propeller and beneath the water level at a location behind the boat. Propellers of this type include an inner hub, which is driven by the propeller shaft, and an outer hub which is spaced radially outwardly of the inner hub and is connected thereto by a series of radial spokes legs. The space between the hubs defines an exhaust passage through which the engine exhaust gas is discharged beneath the water level. A plurality of blades are formed integrally with the outer hub. 
     To improve engine and boat performance, it is known in the prior art to provide a diffuser ring at the trailing end of the outer hub. In Cleary U.S. Pat. No. 5,158,433, incorporated herein by reference, the outer hub has an integral aft skirt portion flared outwardly to a trailing end to provide a diffuser ring. The diffuser ring assists in exhaust gas flow and provides a pressure barrier that helps prevent exhaust gases from feeding back into the propeller blades. This enhances engine and boat performance. 
     In one aspect of the present invention, structural design improvements are provided enhancing the forming operation of the flare, and reducing cracking and fatigue. 
     In another aspect of the invention, structural design improvements are provided further enhancing engine and boat performance by preventing exhaust in the exhaust passage from seeking the negative pressure backside surfaces of the propeller blades. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an isometric view of a marine propeller in accordance with the invention. 
     FIG. 2 is an end view from the rear of the propeller of FIG. 1. 
     FIG. 3 is a sectional view taken along line 3--3 of FIG. 2. 
     FIG. 4 is a side view, partially in section, of a portion of the propeller of FIG. 1. 
     FIG. 5 is a view like FIG. 4 and shows an alternate embodiment. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 shows a marine propeller 10 having an inner hub 12, FIG. 2, and an outer hub 14 spaced radially outwardly of inner hub 12 by a plurality of radial spokes or legs 16, 18, 20, to provide an exhaust passage 22 between the inner and outer hubs. Propeller blades 24, 26, 28 extend outwardly from outer hub 14. Outer hub 14 has an integral aft skirt portion 30 flared outwardly to a trailing end 32, FIG. 4. A plurality of slots 34, 36, 38, 40, 42, 44 extend forwardly from trailing end 32 along integral aft outwardly flared skirt portion 30 and divide skirt portion 30 into a plurality of circumferentially spaced segments 46, 48, 50, 52, 54, 56 separated from each other at trailing end 32 by respective slots therebetween and integrally joined to each other at outer hub 14 forwardly of the slots. The slots are formed in the outer hub during casting of the propeller, and skirt portion 30 is flared in a flaring operation as described in incorporated U.S. Pat. No. 5,158,433. 
     Slots 34, 36, 38, 40, 42, 44 have a length extending from trailing end 32 forwardly to an annular transition surface 58. Integral aft outwardly flared skirt portion 30 begins to flare outwardly at annular transition surface 58. The slots are V-shaped with tapered sides, such as 40a and 40b, FIGS. 2 and 4, diverging aft. The slots facilitate the noted flaring operation, and minimize cracking and fatigue in skirt portion 30 including at annular transition surface 58. 
     The propeller blades have a positive pressure frontside surface such as 28a, FIGS. 1 and 4, and a negative pressure backside surface such as 28b, for a right hand rotation, i.e. clockwise, propeller as shown in FIG. 1. Frontside surface 28a meets backside surface 28b at an outer blade tip 28c extending around the blade from a forward leading blade tip 28d to an aft trailing blade tip 28e. Aft trailing blade tip 28e merges with outer hub 14 at a point 60, FIG. 4, offset from the slots to prevent exhaust in exhaust passage 22 from seeking the negative pressure backside surface 28b through a slot. Blade 28 merges with outer hub 14 along a line of curvature 62 extending from leading blade tip 28d to trailing blade tip 28e along outer hub 14. An aft extension 64, shown in dashed line in FIG. 4, of line of curvature 62 continuing the same curvature along integral aft outwardly flared skirt portion 30 meets trailing end 32 of outer hub 14 at a point 66 spaced generally centrally between respective slots 40 and 42. 
     In the embodiment of FIG. 4, the slots extend axially forwardly from trailing end 32 along integral aft skirt portion 30. In an alternate embodiment as shown in FIG. 5, the slots such as 68 and 70 curve in the direction of rotation of the propeller as the slots extend forwardly from trailing end 72. Slots 68 and 70 extend forwardly from trailing end 72 along respective lines of curvature parallel to line of curvature 62 including aft extension 64. This aspect of the invention is applicable to a propeller having an outwardly flared skirt portion 30, FIG. 4, and to a propeller having a non-flared or straight skirt portion 74, FIG. 5. In each embodiment, the integral aft skirt portion 30 or 74 has a trailing end 32 or 72 aft of the point 60 where the trailing blade tip 28e merges with outer hub 14. In preferred form, the propeller has three blades and six slots, each slot being offset from a point of merger of the respective trailing blade tip with the outer hub, though other numbers and combinations of blades and slots may be used. 
     In a further alternate embodiment, one of the slot sides, for example as shown in dashed line at 42a in FIG. 2, is spaced from the axis 76 of rotation of the propeller by a greater radial distance than the other slot side 42b, which drafts any exhaust gas flow therethrough at a point offset from the negative pressure backside surface of the propeller blade. Slot side 42a leads slot side 42b during rotation of the propeller, i.e. the propeller in FIG. 2 rotates clockwise. 
     It is recognized that various equivalents, alternatives and modifications are possible within the scope of the appended claims.