Patent Publication Number: US-6210110-B1

Title: Propeller having a stress relief flare arrangement

Description:
BACKGROUND OF THE INVENTION 
     The invention relates generally to outboard and stern drive engines, and more particularly, to apparatus for preventing gases (e.g., exhaust, air) from flowing into a propeller blade. 
     Through-propeller exhaust type engines include an exhaust casing extending from a power head, and a lower unit secured to the exhaust casing. The lower unit includes a gear case which supports a propeller shaft, and a propeller is engaged to the shaft. The propeller includes an outer hub through which exhaust gases are discharged. 
     During operation, a region of low pressure is developed rearwardly of the propeller. A thin low pressure boundary layer around the hub can also develop. The low pressure condition rearwardly of the hub has a tendency to join with the low pressure boundary layer, and exhaust gas migrates forwardly along the propeller hub between the blades and along the rear, or low pressure, face of the propeller blades, thereby causing conditions of “cavitation” or “ventilation”. Such conditions prevent the propeller blade from biting into the water and result in an efficiency loss. In addition, excessively low pressure in the region rearwardly of the propeller hub results in a drag on the forward movement of the engine through the water. 
     Known propeller structures for preventing ventilation include diverging flare rings and converging rings at the rear end of the propeller hub. The rings affect the flow of water over the hub and prevent migration of the exhaust gases along the hub. For example, with an aluminum propeller, and after die cast operations, the ring is formed by welding, swaging, or attaching a full-circle ring to the hub. 
     With such rings, and even during minor underwater impacts, the rings can be damaged and even lost. That is, the rings can be separated from the propeller hub and then sink to the bottom of the river, lake, or ocean. Damage and loss of such rings can result in customer dissatisfaction. 
     In addition, and with some ring configurations, slots are formed in the ring during fabrication. Formation of the slots in the ring results in high stress areas adjacent the slots, i.e., at the edges of the slots. Such high stress areas, i.e., the edges, are susceptible to cracking and breaking off. Such cracked or broken off edges are not aesthetically acceptable and can result in customer complaints. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention, in one aspect, is a propeller including a flare having a sinusoidal, or tulip, shape at a trailing edge of the propeller. The sinusoidal flare shape of the propeller trailing edge has less stress concentration than the stress concentration associated with at least some known flare rings. Specifically, stress is reduced in the sinusoidal flare shape due to smooth trailing surface and the uneven edge of the flare. As a result, potential for cracking the trailing edge of the flair is reduced. In addition, the flare has greater strength as compared to at least some known flare rings in that stresses are more evenly distributed along the tulip shaped trailing edge. The reduced stress concentration also enables expanding the flare more than is possible with some known flare rings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of an outboard engine. 
     FIG. 2 is a side, partial cross sectional view of a portion of a propeller constructed in accordance with one embodiment of the present invention. 
     FIG. 3 is a back view, i.e., the trailing edge, of a portion of the propeller shown in FIG.  2 . 
     FIG. 4 is a side, partial cross sectional view of a portion of a propeller constructed in accordance with an alternative embodiment of the present invention. 
     FIG. 5 is a side, partial cross sectional view of a portion of a propeller constructed in accordance with yet another alternative embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention is not limited to practice in connection with a particular engine, nor is the present invention limited to practice with a particular propeller configuration. The present invention can be utilized in connection with many engine and propeller configurations. Therefore, although the invention is described below in the context of an exemplary outboard engine and propeller configuration, the invention is not limited to practice with such engine and propeller. For example, the invention can be used in connection with both outboard engines and stern drive type engines having through-propeller exhaust arrangements. Also, the present invention can be used in connection with engines having through-propeller air (or any other gas) arrangements. 
     Referring now particularly to the drawings, FIG. 1 is a perspective view of an exemplary outboard engine  10 , such as an outboard engine commercially available from Outboard Marine Corporation, Waukegan, Ill. Engine  10  includes a cover  12  which houses a power head (not shown), an exhaust housing  14 , and a lower unit  16 . Lower unit  16  includes a gear case  18  which supports a propeller shaft  20 . Gear case  18  includes a bullet, or torpedo,  22  and a skeg  24  which depends vertically downwardly from torpedo  22 . 
     A propeller  100 , constructed in accordance with one embodiment of the present invention, is secured to shaft  20 . FIG. 2 is a perspective view of a portion of a propeller  100 , and FIG. 3 is a trailing end view of propeller  100 . Propeller  100  includes a central hub  102  and a plurality of blades  104  (e.g., three of four blades). Propeller  100  further includes a flare  106  having a trailing edge  108  which is a continuous surface having a sinusoidal, or tulip, shape. 
     As shown in FIG. 3, flare  106  includes flare portions  110  which extend, or flare out, from hub  102 . Flare portions  110  affect the flow of water over hub  102  and prevent migration of exhaust gases along hub  102 . The number of flare portions  110  is typically selected to correspond to the number of blades  104 . As shown in FIG. 3, and for a four blade propeller, a center line of each flare portion  110  is about 45° out of phase with respect to a center line of adjacent blades  104 . For a three blade propeller, a center line of each flare is about 60° out of phase with respect to a center line of adjacent blades. However, fewer or more flares than the number of blades could also be utilized. 
     A length L, or the extent to which each flare portion  110  extends from center hub  102  is selected depending upon the particular engine in which propeller is to be used and the desired operating characteristics of propeller  100 . An advantage of the tulip, or sinusoidal, shaped flare  106  is that such length can be selected from within a broad range of lengths because stress concentrations are not formed in flare  106 . An exemplary range of length L is 0.25 to 2.50 inches. 
     Propeller  100  is fabricated using known aluminum die cast operations. During fabrication, and as blade edge  108  is trimmed, edge  108  is flared with a swagging cone tool to form the sinusoidal, or tulip, shape. During the fabrication process, the smooth surface of trailing edge  108  prevents formation of high stress areas. In addition, the stresses on propeller trailing edge  108  are evenly distributed along edge  108 . Therefore, in addition to an even distribution of stresses, the peak stresses are lower than the peak stresses, i.e., the high stress concentration areas, in some known flare ring configurations. 
     Flare  106  is illustrated as having a sinusoidal shape. In accordance with other embodiments of the invention, the flare has other shapes. For example, FIG. 4 illustrates a propeller  150  including a hub  152 , blades  154  and a flare  156  having a trailing edge surface  158  with a parabolic shape. Alternatively, FIG. 5 illustrates a propeller  160  including a hub  162 , blades  164  and a flare  166  having a trailing edge surface  168  with an elliptical shape. Generally, and in accordance with the present invention, the trailing edge surface of the flare is curved and continuous to avoid formation of high stress concentration areas yet also is effective for preventing migration of exhaust gases along the propeller hub. 
     Rather than being integral with hub  102 , flare  106  can be separately formed as a ring and then welded to hub  102 , as is known in the art. Forming flare  106  integral with hub  102  provides the advantage that flare  106  generally cannot be separated from hub during operation, which avoids customer complaints. However, even if flare  106  is formed separate from hub  102 , such flare  106  provides the advantage of less stress concentration than at least some known flare rings. 
     In addition, propeller  100  can be fabricated from material other than aluminum. For example, material such as bronze, or any other material that can be used in a die cast operation, can be used to fabricate propeller  100 . Further, material that can be used in injection molding processes, such as plastic, can be used to fabricate propeller  100 . 
     From the preceding description of various embodiments of the present invention, it is evident that the objects of the invention are attained. Although the invention has been described and illustrated in detail, it is to be clearly understood that the same is intended by way of illustration and example only and is not to be taken by way of limitation. Accordingly, the spirit and scope of the invention are to be limited only by the terms of the appended claims.