Marine drive having two counter-rotating surfacing propellers and dual propeller shaft assembly
BACKGROUND AND SUMMARY 
     The invention relates to a marine drive having two counter-rotating surface operating propellers. 
     The present invention arose during development efforts directed toward a marine drive enabling increased top end boat speed. This is achieved by raising the torpedo out of the water to reduce drag, and by using two counter-rotating surface operating propellers. Reducing torpedo drag by raising the torpedo above the surface of the water is known in the art, for example U.S. Pat. No. 4,871,334, column 3, lines 35+. 
     In one aspect of the invention, simple, effective retaining structure is provided for holding the propeller-bearing-supporting spool in the horizontal bore of the torpedo fixed for non-rotation in each of the opposite rotational directions of the counter-rotating propellers. This prevents rotational loosening of the spool in each of the rotational directions as the propellers strike and pierce the surface of the water. 
     In another aspect, structure is provided to minimize shaft wobble tendency as the propellers strike and pierce the surface of the water. 
     In another aspect, a thrust bearing is provided which transfers thrust from a hollow outer propeller shaft to an inner counter-rotating concentric propeller shaft. The thrust bearing is located between fore and aft driven gears on the propeller shafts. The propeller shafts float within their respective gears. 
     In another aspect, combinations of carbon steel and stainless steel are provided for the propeller shafts to afford bearing support and corrosion protection where needed. 
     In another aspect, propeller self-centering mounting structure is provided to maintain proper propeller mounting notwithstanding surface operation vibration as the propellers strike and pierce the surface of the water. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a side elevation view of a marine drive in accordance with the invention. 
     FIG. 2 is a partial sectional view of a portion of the structure of FIG. 1. 
     FIG. 3 is an enlarged view of a portion of the structure of FIG. 2. 
     FIG. 4 is an exploded perspective view of a portion of the structure of FIG. 1. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 shows a marine drive 10 having two counter-rotating surface operating propellers 12 and 14. The drive is mounted to the transom 16 of a boat 18 according to the usual marine stern drive mounting arrangement. An input shaft 20, FIG. 2, is driven by an engine (not shown) in the boat. Input shaft 20 is coupled through a universal joint 22 to an upper gear and clutch mechanism 24 which is known in the art, as shown in U.S. Pat. Nos. 4,630,7I9, 4,679,682, and 4,869,121, incorporated herein by reference. Universal joint 22 allows trimming and steering of the drive. 
     Drive housing 26 has a horizontal bore 28 and an intersecting vertical bore 30 therein. Upper gear mechanism 24 drives a vertical driveshaft 32 positioned in vertical bore 30. Horizontal bore 28 is in the portion of the drive housing called the torpedo 34, FIG. 4. In the preferred embodiment, FIG. 1, torpedo 34 is spaced slightly above the bottom 36 of the boat such that torpedo 34 is slightly above the surface of the water. A spool assembly 38, FIG. 3, is positioned in horizontal bore 28 of housing 26 and supports a first inner propeller shaft 40 and a second hollow outer propeller shaft 42. Propeller shaft 42 is positioned concentrically over propeller shaft 40. The propeller shafts rotate in opposite rotational directions. Surface operating propeller 12 is mounted to propeller shaft 40. Surface operating propeller 14 is mounted to propeller shaft 42. One of the propellers is a right hand rotating propeller, and the other propeller is a left hand rotating propeller. 
     Retaining structure is provided for holding the spool assembly 38 fixed for non-rotation within horizontal bore 28 in both rotational directions as the propellers strike and pierce the surface of the water. The retaining structure is provided by a first right hand thread set 44, FIG. 3, and a second left hand thread set 46. The thread sets are spaced along the rotational axis of the propeller shafts. Right hand thread set 44 prevents right hand rotational loosening of the spool assembly. Left hand thread set 46 prevents left hand rotational loosening of the spool assembly. The spool assembly includes a cylindrical bearing support housing 48, FIGS. 3 and 4, having a mounting thread 50 thereon for engagement with a mounting thread 52 in bore 28 of housing 26. The spool assembly also includes a cylindrical ring locking member 54 having a left hand thread 56 for engagement with a mating thread 58 in bore 28 of housing 26, for clamping against bearing support housing 48 to fix the rotational and axial position of both bearing support housing 48 and locking member 54 in horizontal bore 28, whereby rotation of the spool assembly is prevented in each rotational direction. A locking tab washer 60 is provided between locking member 54 and bearing support housing 48, and O-ring 62 provides a seal between bearing support housing 48 and drive housing 26 preventing entry of water forwardly into bore 28. Flats 61 on washer 60 engage flats 49 on housing 48 to lock the washer 60 against rotation relative to housing 48. Tabs 63 on washer 60 are bent outwardly into slotted recesses 55 on locking member 54 to prevent rotation of member 54 relative to washer 60, which in turn prevents rotation of member 54 relative to housing 48. Housing 48 is then locked into bore 28 by the noted reverse threads 50 and 56. Spool retaining structure for submerged drives using a set screw is known in the prior art, for example U.S. Pat. No. 4,897,058. 
     The spool assembly includes an aft bearing support portion 64, FIG. 3, extending rearwardly outwardly from housing 26. Needle bearing 66 is positioned between propeller shaft 42 and bearing support housing 48 in extended bearing support portion 64 such that the propeller shafts are supported over a length to prevent bending of the propeller shafts during the surface operation of the propellers as the propellers strike and pierce the surface of the water. Bearings in rearwardly extended spool portions are known in the prior art, for example U.S. Pat. No. 4,897,058. One or more seals 68, 70 are positioned between propeller shaft 42 and extended bearing support portion 64 of the spool assembly and aft of bearing 66 to prevent entry of water forwardly into the space between propeller shaft 42 and the spool assembly. 
     A pinion driving gear 72, FIG. 3, is mounted on the lower end of vertical driveshaft 32 in splined relation and is held thereon by nut 74. A fore driven gear 76 is fixed on inner propeller shaft 40 in splined relation and is engaged by pinion gear 72 for drivingly rotating inner propeller shaft 40. An aft driven gear 78 is fixed on outer propeller shaft 42 in splined relation and is engaged by pinion gear 72 for drivingly rotating outer propeller shaft 42 in the opposite rotational direction as inner propeller shaft 40. A tapered roller thrust bearing 80 supports driven gear 76 for rotation in bore 28 of housing 26. Bearing 80 has an inner race 82 engaging gear 76, and an outer race 84 engaging housing 26. A shim 86 may be provided if desired for adjusting axial positioning. A second tapered roller thrust bearing 88 supports the aft driven gear for rotation in bore 28 of housing 26. Bearing 88 has an inner race 90 engaging gear 78, and an outer race 92 engaging housing 26. Outer race 92 has a rearward end portion 94 facing the spool assembly and held thereby against axial movement, to prevent rearward movement of gear 78. Spacer washer 96 is provided between rearward end 94 of outer race 92 and forward end 97 of bearing support housing 48 of spool assembly 38. 
     A thrust bearing assembly 98, FIG. 3, engages between the propeller shafts such that thrust from outer propeller shaft 42 is transferred to inner propeller shaft 40 during rotation of the propeller shafts in opposite directions. Inner propeller shaft 40 extends through fore driven gear 76 and aft driven gear 78. Outer propeller shaft 42 extends through aft driven gear 78. Inner propeller shaft 40 has an annular shoulder 100 against which the thrust from outer propeller shaft 42 is transferred. Thrust bearing assembly 98 is mounted between shoulder 100 and the forward axial end 102 of outer propeller shaft 42. Thrust bearing assembly 98 is located axially between fore driven gear 76 and aft driven gear 78, such that thrust is transferred from outer propeller shaft 42 to inner propeller shaft 40 at an axial position on the propeller shafts located between gears 76 and 78. Thrust bearing assembly 98 includes a thrust bearing 104 engaging shoulder 100 of inner propeller shaft 40, and an annular cup-shaped thrust member 106 engaging thrust bearing 104 and the forward end 102 of outer propeller shaft 42 to transfer thrust from outer propeller shaft 42 to inner propeller shaft 40. Snap ring 108 stops rearward movement of the shaft in the reverse direction. The propeller shafts are allowed to slide fore and aft within their respective gears 76 and 78 along their respective splines, providing a floating shaft arrangement, without loading the gears. Thrust bearing assembly 98 is a double speed bearing and accommodates the opposite rotational directions of the propeller shafts. 
     One or more annular seals 110, 112, FIG. 3, are positioned between inner propeller shaft 40 and outer propeller shaft 42 at the propeller mounting end of propeller shaft 40 such that water is prevented from entering forwardly into the space between the propeller shafts. A needle bearing 114 is positioned between inner propeller shaft 40 and outer propeller shaft 42 and forward of seals 110, 112. Propeller shaft 40 has a stainless steel outer surface 116 rearward of seals 110, 112, and a carbon steel outer surface 118 forward of the seals at bearing 114. In one embodiment, inner propeller shaft 40 is a two piece member formed by a forward carbon steel piece and a rearward stainless steel piece welded to each other at a weld joint 120 between bearing 114 and the seals 110, 112. In another embodiment, inner propeller shaft 40 is a stainless steel member having a carbon steel sleeve therearound at bearing 114. Outer propeller shaft 42 has a stainless steel outer surface 122 rearward of seals 68, 70, and a carbon steel outer surface 124 forward of the seals at bearing 66. In one embodiment, outer propeller shaft 42 is a two piece member formed by a forward carbon steel piece and a rearward stainless steel piece welded to each other at a weld joint 126. In another embodiment, outer propeller shaft 42 is a stainless steel member having a carbon steel sleeve therearound at bearing 66. 
     Self-centering mounting structure is provided for the propellers on each propeller shaft. Inner propeller shaft 40 has a tapered shoulder outer surface 128, FIG. 3, a threaded outer surface 130 axially spaced rearwardly of tapered outer surface 128, and a driving spline 132 therebetween and drivingly engaging propeller 12 in splined relation. An annular ring 134 of a material, e.g. bronze, non-fretting relative to stainless steel, has an inner tapered surface 136 engaging tapered outer surface 128 of inner propeller shaft 40. Ring 134 has a tapered outer surface 138. An internally threaded nut 140 of a material, e.g. bronze, non-fretting relative to stainless steel, threadingly engages threaded outer surface 130 of inner propeller shaft 40. Nut 140 has a tapered outer surface 142. Propeller 12 is mounted on inner propeller shaft 40 between ring 134 and nut 140 and is engaged forwardly at tapered outer surface 138 of ring 134, and is engaged rearwardly at tapered outer surface 142 of nut 140. Tapers 138 and 142 provide a tight self-centering fit and mounting of the propeller to the propeller shaft. Splines 132 do not provide a tight fit, but merely rotational drive. 
     Outer propeller shaft 42 has a tapered shoulder outer surface 144, FIG. 3, a threaded outer surface 146 axially spaced rearwardly of tapered outer surface 144, and a driving spline 148 therebetween for drivingly engaging propeller 14 in splined relation. A ring 150 of a material, e.g. bronze, non-fretting relative to stainless steel, has a tapered inner surface 152 engaging tapered outer surface 144 of outer propeller shaft 42. Ring 150 has a tapered outer surface 154. An internally threaded nut 156 of a material, e.g. bronze, non-fretting relative to stainless steel, threadingly engages threaded outer surface 146 of outer propeller shaft 42. Nut 156 has a tapered outer surface 158. Propeller 14 is mounted on outer propeller shaft 42 between ring 150 and nut 156 and is engaged forwardly at tapered outer surface 154 of ring 150, and is engaged rearwardly at tapered outer surface 158 of nut 156. Tapers 154 and 158 provide a tight self-centering fit. Splines 148 do not provide a tight fit, but only rotational drive. 
     Vertical driveshaft 32, FIG. 2, is supported at its top end by a needle bearing 160 as in the above incorporated patents. The driveshaft is supported at its lower end by a needle bearing 162. Driveshaft 32 is centrally supported in bore 30 by tapered roller thrust bearing 164 retained by threaded ring 166. Driveshaft 32 is also supported by needle bearing 168 in upper spool 170 mounted at threads 172 in bore 30, and also having a needle bearing 174 supporting gear 176 of upper gear assembly 24. Reference is made to commonly owned co-pending U.S. application Ser. No. 07/889,495, filed on even date herewith, entitled &#34;Counter-rotating Surfacing Marine Drive&#34;. 
     Cooling water for the engine is supplied from water intake 178 in skeg 180. The water flows through skeg passage 182, torpedo nose passage 184 and then through housing passage 186 and then to the engine in the usual manner. After cooling the engine, the water and the engine exhaust are exhausted in the usual manner through an exhaust elbow and through the drive housing and are discharged at exhaust outlet 188 above torpedo 34 and into the path of the propeller blades in the upper portion of their rotation, as in U.S. Pat. No. 4,871,334. Oil circulates from the lower gears upwardly through passages 190 and 192 to the upper gears and then downwardly through passage 194 to the lower gears at passages 196 and 197. Passage 196 supplies oil through passage 198 in the spool assembly to bearings 88 and 66, and through passage 199 in outer propeller shaft 42 to bearing 114. Passage 197 supplies oil to the forward end of bearing 88. 
     It is recognized that various equivalents, alternatives and modifications are possible within the scope of the appended claims.