Propeller drive sleeve with asymmetric shock absorption
BACKGROUND AND SUMMARY 
     The invention relates to marine drives, and more particularly to a shock absorbing drive sleeve mounting the propeller to the propeller shaft. 
     The invention arose during continuing development efforts relating to marine drive sleeves such as shown in U.S. Pat. Nos. 5,244,348, 5,322,416, 5,484,264, incorporated herein by reference. 
     A shock absorbing drive sleeve protects the propeller shaft and the marine drive, including the gear train, when the propeller strikes an object. It is desirable to provide a drive sleeve which is sufficiently soft, i.e. low spring rate, to permit the propeller shaft to continue to rotate through a limited angle after the propeller has been stopped by an object, yet sufficiently strong or stiff, i.e. high spring rate, to support enough torque to propel the boat, including under high load conditions. These are conflicting design goals which require a trade-off between a low spring rate for shock absorption versus a high spring rate for torque bearing capability. The inventions of the above incorporated patents address and solve the noted trade-off, and afford both shock absorption and high load capability. 
     The present invention addresses the difference in torque bearing capability of a marine drive between forward and reverse. The forward drive components of the marine drive gear train, such as drive gears and clutches, have higher strength and greater torque bearing capability than the reverse drive components. This is because high load conditions, i.e. high speed, requiring high torque to propel the boat are encountered in the forward direction, but not in the reverse direction. Because the reverse drive components of the marine drive have lower strength, there is a need to protect same at lower torque levels, and hence it is desirable to provide a softer spring rate for reverse, i.e. a differential spring rate between reverse and forward. The present invention addresses and solves this need, and provides asymmetric shock absorption, i.e. different shock absorption characteristics between forward and reverse. In preferred form, this is accomplished by simple modification to the drive sleeves of the above noted incorporated patents. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Prior Art 
     FIGS. 1-27 are taken from incorporated U.S. Pat. No. 5,484,264. 
     FIG. 1 shows a marine drive. 
     FIG. 2 shows an exploded perspective view of a portion of FIG. 1, and illustrates propeller mounting structure in accordance with incorporated U.S. Pat. No. 5,244,348. 
     FIG. 3 is a perspective view partially cut away of the drive sleeve of FIG. 2. 
     FIG. 4 is a cross sectional view of a portion of the structure of FIG. 2 in assembled condition. 
     FIG. 5 is a sectional view taken along line 5--5 of FIG. 4. 
     FIG. 6 is a sectional view taken along line 6--6 of FIG. 4. 
     FIG. 7 is a view like FIG. 6 and shows a further rotated position. 
     FIG. 8 is a sectional view taken along line 8--8 of FIG. 4 and also shows a further modification in accordance with incorporated U.S. Pat. No. 5,322,416. 
     FIG. 9 is a view like FIG. 8 and shows a further rotated position. 
     FIGS. 10-14 are graphs showing rotation versus torque. 
     FIG. 15 is similar to FIG. 4 and shows a further embodiment. 
     FIG. 16 is a sectional view taken along line 16--16 of FIG. 15. 
     FIG. 17 is similar to FIG. 15 and shows a further embodiment. 
     FIG. 18 is a sectional view taken along line 18--18 of FIG. 17. 
     FIG. 19 shows an exploded perspective view of a portion of FIG. 1, and illustrates propeller mounting structure in accordance with incorporated U.S. Pat. No. 5,484,264. 
     FIG. 20 is a perspective view partially cut away of the drive sleeve adapter of FIG. 19. 
     FIG. 21 is a perspective view partially cut away of the drive sleeve of FIG. 19. 
     FIG. 22 is a cross-sectional view of a portion of the structure of FIG. 19 in assembled condition. 
     FIG. 23 is a sectional view taken along line 23--23 of FIG. 22. 
     FIG. 24 is a sectional view taken along line 24--24 of FIG. 22. 
     FIG. 25 is a view like FIG. 24 and shows a further rotated position. 
     FIG. 26 is a sectional view taken along line 26--26 of FIG. 22. 
     FIG. 27 is a view like FIG. 26 and shows a further rotated position. 
     Present Invention 
     FIG. 28 shows an exploded perspective view of a portion of the structure of FIG. 1, and illustrates propeller mounting structure in accordance with the present invention. 
     FIG. 29 is a cross-sectional view of a portion of the structure of FIG. 28 in assembled condition, taken along line 29--29 of FIG. 37. 
     FIG. 30 is a view like FIG. 29 but taken along line 30--30 of FIG. 37. 
     FIG. 31 is a perspective view of the drive sleeve of FIG. 28. 
     FIG. 32 is like FIG. 31 but partially cut away. 
     FIG. 33 is a rear view in elevation of the drive sleeve of FIG. 31. 
     FIG. 34 is a sectional view taken along line 34--34 of FIG. 30. 
     FIG. 35 is a sectional view taken along line 35--35 of FIG. 30. 
     FIG. 36 is a view like FIG. 35 but shows a further rotated position for the forward direction of the boat, namely clockwise rotation of the propeller shaft for a right hand rotation propeller. 
     FIG. 37 is a sectional view taken along line 37--37 of FIG. 30. 
     FIG. 38 is like FIG. 37 but shows a further rotated position in the forward direction, namely clockwise rotation of the propeller shaft for a right hand rotation propeller. 
     FIG. 39 is a view like FIG. 35 but shows a further rotated position in the reverse direction of the boat, namely counterclockwise for a right hand rotation propeller. 
     FIG. 40 is like FIG. 37, but shows a further rotated position for the reverse direction of the boat, namely counterclockwise for a right hand rotation propeller. 
     FIG. 41 is a schematic illustration of the rotation in FIG. 36. 
     FIG. 42 is a schematic illustration of the rotation in FIG. 38. 
     FIG. 43 is a schematic illustration of the rotation in FIG. 39. 
     FIG. 44 is a schematic illustration of the rotation in FIG. 40. 
    
    
     DETAILED DESCRIPTION 
     Prior Art 
     FIGS. 1-27 are taken from incorporated U.S. Pat. No. 5,484,264, which is a continuation-in-part of U.S. Pat. No. 5,322,416, which is a continuation-in-part of U.S. Pat. No. 5,244,348. FIGS. 1-27 show a marine drive as described in the &#39;264 patent beginning at column 2, line 46, and which description is repeated here for ease of review. 
     FIG. 1 shows a marine drive 10 having a propeller with a propeller hub 14. The marine drive includes a driveshaft housing 16 with a lower gearcase 18 and torpedo housing 20. Propeller shaft 22, FIG. 2, extends rearwardly from torpedo housing 20 and has a splined portion 24 and a rear threaded portion 26. Propeller hub 14 is mounted to splined portion 24 between front and rear thrust hubs or washers 28 and 30, as in U.S. Pat. No. 5,022,875, incorporated herein by reference. Front thrust washer 28 bears against tapered transition portion 32, FIG. 4, of the propeller shaft. Hub 14 has an inner hub portion 34 connected to outer hub portion 36 by a plurality of radial spokes such as 38. The forward end of inner hub portion 34 bears against thrust washer 28. Rear thrust washer 30 is internally splined and received on portion 24 of the propeller shaft and engages the rear end of inner propeller hub portion 34 to mount the propeller hub to the propeller shaft. Rear thrust washer 30 has a plurality of outer slots 40, FIG. 2, spaced circumferentially around propeller shaft 22. A nut 42 is threaded onto rear threaded portion 26 of the propeller shaft. A locking tab washer 44 is around the propeller shaft between nut 42 and rear thrust washer 30. Locking tab washer 44 has an inner hex configured surface 46, FIG. 4, recessed forwardly and receiving hex nut 42 therein and preventing rotation of nut 42 relative to locking tab washer 44. Locking tab washer 44 has a plurality of tabs 48 extending radially outwardly therefrom and bendable forwardly into slots 40 to prevent rotation of locking tab washer 44 relative to rear thrust washer 30 which in turn is prevented from rotating relative to propeller shaft 22 by the noted splined mounting thereto at portion 24. For further reference relative to the mounting structure thus far described, reference is made to incorporated U.S. Pat. No. 5,022,875. 
     Propeller hub 14 is mounted to propeller shaft 22 at splined portion 24 by drive sleeve 50, FIGS. 2-4. Drive sleeve 50 has a rearward inner diameter portion 52 keyed to and engaging the propeller shaft in driven relation. It is preferred that inner diameter portion 52 have a plurality of molded-in splines or teeth 54 engaging the propeller shaft in splined driven relation at splined portion 24. Drive sleeve 50 has a forward inner diameter portion 56 axially spaced forwardly from inner diameter portion 52 and spaced radially outwardly of and disengaged from propeller shaft 22, FIG. 4. Forward inner diameter portion 56 is axially tapered, FIG. 4, to increasing radial separation from propeller shaft 22 with increasing axial distance from rearward inner diameter portion 52. Drive sleeve 50 has an annular thickness which is largest at rearward inner diameter portion 52, and which annular thickness decreases along forward inner diameter portion 56 with increasing axial distance from rearward inner diameter portion 52. The minimum annular thickness of the drive sleeve is at the forward axial end of forward inner diameter section 56 spaced axially farthest from rearward inner diameter section 52. 
     Drive sleeve 50 has a rearward outer diameter portion 58 radially aligned with rearward inner diameter portion 52. Drive sleeve 50 has a forward outer diameter portion 60 radially aligned with forward inner diameter portion 56 and axially spaced forwardly from inner diameter portion 52 and outer diameter portion 58. 
     The outer diameter of drive sleeve 50 is tapered to have a larger outer diameter at forward portion 60 than at rearward portion 58. Forward outer diameter portion 60 snugly engages the propeller hub at portion 34 in keyed relation, such as by providing a polygonal shape to each, FIG. 5. Rearward outer diameter portion 58 is spaced slightly radially inwardly of propeller hub portion 34 by a small gap 62 such that rearward outer diameter portion 58 of the drive sleeve may partially rotate relative to the propeller hub in response to rotation of the propeller shaft drivingly engaging inner diameter portion 52. 
     When the propeller strikes an object, the shock is absorbed by torsional twisting of drive sleeve 50 wherein rearward inner diameter portion 52 and rearward outer diameter portion 58 continue to rotate to a further rotated position, FIG. 7, than the position of forward outer diameter portion 60, whereafter splines 54 of the drive sleeve shear. FIG. 6 shows the normal position of rearward outer diameter portion 58 of drive sleeve 50 relative to propeller hub portion 34. When the propeller strikes an object, portion 58 continues to rotate to the further rotated position shown in FIG. 7. The torsional twisting of the drive sleeve followed by shearing of splines of teeth 54 protects the propeller shaft and the drive. The magnitude of the shear force is set by the axial extent of splines 54 of the drive sleeve. The torsional twisting deformation of the drive sleeve absorbs shock when the propeller strikes an object. 
     During assembly, drive sleeve 50 is slid rearwardly into the propeller until its rearward shoulder 64 is stopped against shoulder 66 of propeller hub portion 34. The hub is then slid forwardly onto the propeller shaft until the forward end of hub portion 34 engages forward thrust washer 28. Rearward thrust washer 30, locking tab washer 44 and nut 42 are then assembled and torqued down. Forward molded spacing bumps 68 on drive sleeve 50 are compressed during torquing down of nut 42 and provide tolerance compensation. 
     The outer surface of drive sleeve 50 and the inner surface of propeller hub 34 are tapered relative to each other such that the radial gap 62 therebetween increases as one moves axially rearwardly. At the forward end of the drive sleeve, there is a snug fit with no gap, FIG. 5. At the rearward end of the drive sleeve, radial gap 62 has increased to its largest dimension, FIG. 8. When the propeller strikes an object, and the propeller shaft 22 and the rearward portion of drive sleeve 50 continue to rotate to further rotated positions, outer tips such as 102, FIG. 7, on the drive sleeve engage the inner surface of inner propeller hub 34. The axial length of such engagement increases with increasing angular rotation of propeller shaft 22 and the rearward portion of drive sleeve 50. For example, referring to FIG. 4, assume that the propeller has struck an object and that propeller shaft 22 and the rearward portion of drive sleeve 50 continue to rotate even though inner propeller hub 34 has stopped. After 1° of rotation, not only does drive sleeve 50 engage propeller hub 34 at forward area 104, but also at area 106 slightly rearwardly thereof, due to the noted rotation. Without such rotation, there is a small gap at area 106 between drive sleeve 50 and propeller hub 34. Continuing with this example, after 2° of rotation, the drive sleeve outer tips 102 have now been rotated into contact with propeller hub 34 at area 108. After 3° of rotation, the point of such interference contact now extends rearwardly to area 110, such that the axial length of interference engagement now extends from forward area 104 rearwardly to area 110. After 5° of rotation, there is contact at area 112, which is illustrated in FIG. 7. The noted example is illustrative, and the points of contact may vary depending upon tolerances and taper. 
     It is believed particularly important to minimize shock load to the drive within the first 5° of rotation of the propeller shaft after the propeller is stopped by an object. Hence, it is preferred that the tapered gap 62 between drive sleeve 50 and propeller hub 34 be such as to allow at least 5° of rotation before drive sleeve 50 engages inner propeller hub 34 in interference fit relation for at least some axial distance of such fit, such as from area 104 to area 112, FIG. 4. 
     Further rearward areas such as 114, FIG. 4, rotate to even further rotated positions than area 112. For example, FIG. 8 shows that gap 62 is wider at area 114 than at area 112, FIG. 6. Accordingly, the rearward portion of drive sleeve 50 at area 114 rotates to a further rotated position, FIG. 9. As each area 106, 108, 110, etc., FIG. 4, of the drive sleeve progressively engages propeller hub 34 in interference fit, the remaining rearward portion of drive sleeve 50 continues to rotate by torsionally twisting. The engaged area may continue to rotate slightly due to compression of tip 102, and torsional twisting relative to areas forward thereof. 
     The noted torsional twisting is a function of the structural configuration and the material of drive sleeve 50. As noted above, it is desirable that the drive sleeve material be sufficiently soft, i.e. have a low spring rate, particularly at low angles of rotation, i.e. less than 5°, to permit continued rotation of the drive sleeve and propeller shaft for at least 5° after the propeller stops. As noted above, it is believed particularly important to minimize shock load during this initial range of rotation, to dampen initial impact. On the other hand, it is desirable that the drive sleeve material be sufficiently strong, i.e. high spring rate, to support enough torque for high load conditions, preferably at least 1,000 lb.ft. of torque. 
     Numerous drive sleeve materials were tried, and the test results are plotted in FIGS. 10-12 showing torque in lb.ft. versus degrees of rotation of propeller shaft 22 after the propeller has been stopped. In general, the plots show increasing torque with increasing rotation as drive sleeve 50 torsionally twists, followed by decreasing torque with increasing rotation corresponding to failure of the drive sleeve, i.e. shearing of splines 54 and sometimes cracking and splintering of the drive sleeve. The various drive sleeve materials represented by the plots in FIGS. 10-12 are unacceptable because: a) the torque increased too quickly at low angles of rotation, e.g. the torque rose above 500 lb.ft. at less than 5° of rotation; and/or b) the drive sleeve would not support 1,000 lb.ft. torque before failure. If the torque increases too quickly at low angles of rotation, the drive material is too stiff, i.e. the spring rate is too high. This is unacceptable because, as above noted, it is believed that most damage occurs within 5° of rotation after the propeller stops, and hence a low spring rate is desired, at least for the first 5° of rotation. The noted second ground for unacceptability of the drive sleeve material is its torque bearing capability. In order to propel the boat under high load conditions, it is desired that the drive sleeve be able to support at least 1,000 lb.ft. torque before failure. 
     Trace 120, FIG. 10, shows the test results for a drive sleeve composed of an acetal plastic material known in the trade as Delrin 500. This material is unacceptable because at 5° of rotation of propeller shaft 22 after the propeller has stopped, the torque has already risen to 500 lb.ft., as indicated at 120a, and hence such material is too stiff, and is deemed not soft enough to absorb shock and prevent damage as desired, i.e. the spring rate is too high. Furthermore, the highest torque bearing capability is only about 600 lb.ft., as indicated at 120b, whereafter the torque decreases as shown at 120c indicating failure of the drive sleeve. 
     Trace 122 shows the test results for a drive sleeve composed of acetal plastic material known in the trade as Delrin 100, which material is unacceptable because its maximum torque bearing capability is only about 400 lb.ft., as shown at 122a. 
     Trace 124 is for a drive sleeve composed of polypropylene including 15% glass and 20% mica by weight, which material is unacceptable because of its high initial spring rate providing rapidly increasing torque to over 500 lb.ft. at less than 5° of rotation, and because it only supports a maximum torque of about 600 lb.ft. before failure. 
     Trace 126 is for a drive sleeve of polyester PBT material with no glass, and is unacceptable because of the high spring rate at angles less than 5° causing the torque to increase too quickly, and because it supports only about 700 lb.ft. torque before failure. 
     Trace 128 is for Nypel Type 6 nylon material with 15% glass, and is unacceptable because of the high initial spring rate causing the torque to increase above 500 lb.ft. at angles less than 5°, and because it supports a maximum torque of only about 900 lb.ft. before failure. 
     Trace 130 is for thermoplastic polyurethane TPU with 30% glass, and is unacceptable because it supports a maximum torque of only about 500 lb.ft. before failure. 
     Trace 132 is for polypropylene with no glass, and is unacceptable because it supports a maximum torque of only about 300 lb.ft. before failure. 
     Trace 134 is for acetal polyplenco bar stock material known in the trade as Delrin 150E, and is unacceptable because of its high initial spring rate causing the torque to rise too quickly to about 1,000 lb.ft. at 5° rotation, and hence does not provide the damage protection desired. The material of trace 134 does have a desirable torque bearing capability of nearly 1,200 lb.ft. before failure. 
     Trace 136, FIG. 11, is for polyester PBT with 10% glass, known in the trade as Celanex 5200 and 2002, and is unacceptable because of its initially high spring rate causing the torque to increase to about 500 lb.ft. at 5° rotation, and because it supports a maximum torque of only about 700 lb.ft. before failure. 
     Trace 138 is for acetal plastic material with 10% glass known in the trade as Celcon GC25A and M25, and is unacceptable because of its initially high spring rate causing the torque to increase too quickly to about 500 lb.ft. at 5° of rotation. 
     Trace 140 is for nylon copolymer NC with no glass, and is unacceptable because it supports a maximum torque of only about 700 lb.ft. 
     Trace 142 is for nylon blend 30% glass and copolymer and 15% the noted type 6, and is unacceptable because of its initially high spring rate causing the torque to increase too quickly to about 500 lb.ft. at 5° rotation. 
     Trace 144 is for acetal plastic 25% glass filled material, and is unacceptable because of its initially high spring rate causing the torque to increase too quickly to about 500 lb.ft. at 5° rotation. 
     Trace 146 is for acetal plastic Delrin ST material, and is unacceptable because it supports maximum torque of only about 400 lb.ft. 
     Trace 148 is for acetal plastic blend material including 50% Delrin 100 and 50% Delrin ST, and is unacceptable because it supports maximum torque of only about 700 lb.ft. 
     Trace 150 is for acetal blend material including Delrin 11 150E and 25% Delrin 100 ST, and is unacceptable because it supports maximum torque of only about 700 lb.ft. 
     Trace 152, FIG. 12 is for acetal blend material including Delrin 11 150E and 10% Delrin 100 ST, and is unacceptable because it supports maximum torque of only about 700 lb.ft. 
     Trace 154 is for acetal plastic material known in the trade as Celcon UV90, and is unacceptable because the torque is already about 500 lb.ft. at about 5% of rotation. 
     Trace 156 is for polypropylene material with 30% glass, and is unacceptable because it supports maximum torque of only about 300 lb.ft. 
     Trace 158 is for polyester PBT material known in the trade as Celanex 2002, and is unacceptable because the initial spring rate is too high since the torque is already about 500 lb.ft. at 5° of rotation, and because it supports maximum torque of only about 700 lb.ft. before failure. 
     Trace 160 is for Delrin 150E material with 1% Activex 535 foaming agent, and is unacceptable because of its initially high spring rate causing the torque to increase above 500 lb.ft. at less than 5% of rotation. 
     Trace 162 is for acetal plastic material known in the trade as Delrin 11 900, and is unacceptable because the torque is already about 500 lb.ft. at 5° of rotation, and because it supports maximum torque of only about 700 lb.ft. before failure. 
     Trace 164 is for a nylon 6-12 Zytel GRZ 77633 material, and is unacceptable because the initial spring rate is too high, causing the torque to increase above 500 lb.ft. at less than 5° of rotation. 
     Trace 166 is for PET Rynite 545 material, and is unacceptable because the initial spring rate is too high, causing the torque to rapidly increase above 500 lb.ft. at less than 5° of rotation. 
     Trace 170, FIG. 13, shows the test results for a drive sleeve composed of extrudable grade acetal resin plastic material known in the trade as Delrin II 150E. As shown by the trace, this material is found to provide at least 5° of torsional twisting angular rotation at torque loads less than 500 lb.ft., and has a spring rate less than 100 lb.ft. per degree for torsional twisting from 0° to 5°. Furthermore, the drive sleeve torsionally twists to angular rotations greater than 5° and supports over 1,000 lb.ft. torque before failure. From 5° to 10°, the spring rate from point 170a to point 170b is greater than 100 lb.ft. per degree. The sleeve supports about 1,200 lb.ft. torque at point 170c at about 20° rotation, before failure as indicated at 170d. 
     As shown in FIG. 13, the noted material has an initially low spring rate causing the torque to rise only to about 300 lb.ft. at point 170a at 5° of rotation. This spring rate is less than 100 lb.ft. per degree of rotation, and is low enough to provide a soft enough material, at least during initial rotation, to absorb shock, in accordance with the above noted belief that it is particularly important to minimize shock load within the first 5° of propeller shaft rotation after the propeller has been stopped. 
     From 5° to 10° rotation, FIG. 13, the torque increases from about 300 lb.ft. at point 170a to about 900 lb.ft. at point 170b, which is a spring rate greater than 100 lb.ft. per degree. This is desirable because it causes the torque to rise more rapidly after the initial 5° of rotation, which increase is desirable so that the ultimate torque supported by the sleeve can increase above 1,000 lb.ft. The torque supported by the drive sleeve increases to about 1,000 lb.ft. at about 12°-13° of rotation, and ultimately reaches about 1,200 lb.ft. at about 20° of rotation before failure. 
     The noted drive sleeve structural configuration and material in combination provide torsional twisting angular rotation at a first spring rate less than 100 lb.ft. per degree from 0° to 5° rotation, a second higher spring rate from 5° to 10° rotation, and supports over 1,000 lb.ft. torque before failure. In accordance with the parent and present invention, it is desirable that the initial spring rate be as low as possible up to about 5° of rotation, and then increase rapidly to support over 1,000 lb.ft. torque. If most of the shock can be absorbed within the first 5° of rotation, it is desirable to then rapidly increase the torque load bearing capability of the drive sleeve. It is desirable that the drive sleeve continue to rotate to at least about 15° or 20° of rotation before failure, though this is not deemed critical since most of the shock should already have been absorbed prior thereto, i.e. from 0° to 5° rotation. 
     As an experiment, the drive sleeve of the noted material performing as shown in FIG. 13 was modified by filling gap 62 between drive sleeve 50 and propeller hub 34 with epoxy, and letting same harden. This was done in order to prevent torsional twisting. The test results on the latter drive sleeve are shown at trace 172 in FIG. 14, wherein the torque rapidly rises well above 1,000 lb.ft. at less than 5° rotation. A comparison of FIGS. 13 and 14 illustrates the difference between the torsional twisting of the invention and a rigid non-twisting drive sleeve or a drive sleeve with a high initial spring rate. 
     FIG. 15 shows an alternate embodiment, and uses like reference numerals from FIG. 4 where appropriate to facilitate understanding. Drive sleeve 180 has a flat rearward end 182 spaced slightly forwardly of shoulder 66 of propeller hub 34 by an axial gap 184. The tapered fit of the drive sleeve within the propeller hub at forward portion 60 limits rearward movement of the drive sleeve and forces bumps 68 against forward thrust washer 28 as noted above, forward molded spacing bumps 68 on the drive sleeve are compressed during torquing down of nut 42 and provide tolerance compensation. As shown in FIG. 5, bumps 68 are spaced about the propeller shaft in a balanced pattern providing balanced axial loading and uniform axial registration of the drive sleeve within propeller hub 34 and on propeller shaft 22 to prevent cocking of the drive sleeve relative thereto and maintaining alignment of the drive sleeve parallel to propeller hub 34 and parallel to propeller shaft 22. The drive sleeve may include rearward reduced diameter extended portion 186 as in FIG. 4, or the latter portion may be omitted as shown in FIG. 15, for example to change the axial length of drive sleeve splines 54 to alter the ratio thereof to the overall axial length of the drive sleeve. In the preferred embodiment, the drive sleeve inner diameter portion engaging propeller shaft 22 has an axial length which is about 40% to 50% of the overall axial length of the drive sleeve. 
     Another modification in the embodiment of FIG. 15 is the step reduction at 188 of the outer diameter of the drive sleeve between forward outer diameter portion 60 and rearward outer diameter portion 190. Step reduction 188 reduces the outer diameter of the drive sleeve along a sharper taper than the taper of the inner diameter of propeller hub 34 to provide a gap 192 between propeller hub 34 and rearward outer diameter portion 190. In this manner, when the propeller stops, the drive sleeve continues to rotate and there is no progressive interferences fit engagement of the propeller hub by the outer tips 102 of the rearward portion of the drive sleeve as in FIG. 4. Instead, the drive sleeve in FIG. 15 torsionally twists along its axial extent rearward of step 188 until a rearward portion such as 194 engages propeller hub 34 as shown at outer tip engagement point 196, FIG. 16. In this manner, the continued rotation of propeller shaft 22 and torsional twisting of drive sleeve 180 does not have to additionally compress the outer tips or corners such as 102, FIGS. 7 and 9, of the drive sleeve, which tips engage propeller hub 34 and progressively compress at progressive areas 106, 108, 110, etc., to permit continued rotation of propeller shaft 22 and torsional twisting of the drive sleeve. The embodiment of FIG. 4 is preferred because it is believed that such additional corner compression at tips 102 aids in increasing the spring rate between points 170a and 170b, FIG. 13. It is believed that the embodiment in FIG. 15 will also provide increased spring rate during rotation, but not until higher degrees of rotation. The latter may be desirable if it is desired to absorb shock and provide further minimization of shock load beyond the initial 5° of rotation. It is believed that the embodiment of FIG. 15 will provide progressive spring rates as in FIG. 4, but will enable a lower spring rate to be maintained beyond the initial 5° of rotation, followed by a higher spring rate. 
     FIG. 17 shows a further embodiment and uses like reference numerals from FIGS. 4 and 15 where appropriate to facilitate understanding. Drive sleeve 200 has a step 202 to a reduced outer diameter 204 spaced from propeller hub 34 by a radial gap 206 which is wide enough such that outer tips 208, FIG. 18, of outer diameter portion 204 of the drive sleeve do not engage propeller hub 34 in interference fit relation, but instead are free to rotate therein. In this embodiment, the increasing spring rate after an initial range of rotation relies entirely upon the drive sleeve material itself, without relying upon compression of outer tips such as 102, FIGS. 7 and 9, or 218, FIG. 16, to additionally increase the spring rate. In the embodiment of FIGS. 4 and 15, respective radial gaps 62 and 192 normally extend continuously around the entire periphery of the rearward outer diameter portion of the drive sleeve, unless the propeller strikes an object, where upon selected sections of the gap are closed, FIGS. 7, 9 and 16, at respective points 102 and 196, as the rearward outer diameter portion of the drive sleeve rotates through a given angle relative to the propeller hub and engages the propeller hub in interference fit at selected sections such as 102 and 196. The rearward outer diameter portion of the drive sleeve includes a plurality of radiused corners spaced radially inwardly of the propeller hub and rotatable through the given angle relative to the propeller hub to engage the propeller hub at the noted selected sections. In the embodiment of FIG. 17, the outer tips such as 208 of the drive sleeve at the edges of the radiused corners do not engage the propeller hub even upon continued rotation of the propeller shaft and drive sleeve after the propeller has been stopped. 
     FIGS. 19-27 use the same reference numerals from FIGS. 1-18 where appropriate to facilitate understanding. FIG. 19 illustrates the propeller mounting structure in accordance with U.S. Pat. No. 5,484,264. Propeller shaft 22 extends rearwardly from torpedo housing 20 and has a splined portion 24 and a rear threaded portion 26. Propeller hub 14 is mounted to splined portion 24 between front thrust washer or hub 28 and rear thrust washer or hub 302 of drive sleeve adapter 304, FIGS. 19 and 20. Front thrust washer 28, FIG. 19, bears against tapered transition portion 32, FIG. 22, of propeller shaft 22. Hub 14 has an inner hub portion 34 connected to outer hub portion 36 by a plurality of radial spokes such as 38, FIG. 23. Hub 14, FIG. 19, receives drive sleeve 308, FIGS. 19 and 21, within inner hub portion 34, FIG. 22. The forward end of inner hub portion 34 bears against thrust washer 28. 
     Drive sleeve adapter 304 is internally splined and received on splined portion 24 of propeller shaft 22 and engages the rear end of inner propeller hub portion 34 for mounting propeller hub 14 and drive sleeve 308 to propeller shaft 22. Drive sleeve adapter 304 has rear thrust hub 302 forwardly facing on the rear end of drive sleeve adapter 304. Rear thrust hub 302 of adapter 304 has a plurality of outer slots 306, FIG. 19, rearwardly facing and spaced circumferentially around propeller shaft 22. Locking tab washer 44 is placed over propeller shaft 22 and against drive sleeve adapter 304. Nut 42 is threaded onto rear threaded portion 26 of propeller shaft 22. Locking tab washer 44 has an inner hex configured surface 46, FIGS. 19 and 22, recessed forwardly and receiving hex nut 42 to prevent rotation of nut 42 relative to locking tab washer 44. Locking tab washer 44 also has a plurality of tabs 48 extending radially outward and bendable forward into slots 306 of adapter 304 to prevent rotation of locking tab washer 44 relative to rear thrust hub 302 of adapter 304 which in turn is prevented from rotating relative to propeller shaft 22 by the noted splined mounting thereto at portion 24. 
     Propeller hub 14 is mounted to propeller shaft 22 at splined portion 24 by drive sleeve adapter 304 and drive sleeve 308. Drive sleeve adapter 304, FIG. 20, has an inner diameter portion 310 keyed to and engaging the propeller shaft in driven relation. It is preferred that inner diameter portion 310 have a plurality of splines 312 engaging the propeller shaft in splined driven relation at splined portion 24, FIG. 22. Drive sleeve 308 and adapter 304 have an overall axial length, and inner diameter portion 310 engaging propeller shaft 22 has an axial length which is 40% to 50% of said overall axial length. Drive sleeve adapter 304 has outer diameter portion 314, FIG. 20, keyed to and engaging drive sleeve 308 as later described. Preferably outer diameter portion 314 has a plurality of outer splines 316 to engage drive sleeve 308. 
     Drive sleeve 308, FIGS. 21 and 22, has a rearward inner diameter portion 318 keyed to and engaging drive sleeve adapter 304 in driven relation. It is preferred that inner diameter portion 318 have a plurality of inner splines 320 engaging the adapter outer splines 316, FIG. 20, in splined driven relation. Drive sleeve 308 has a forward inner diameter portion 322 spaced axially forward of drive sleeve adapter 304, and spaced radially outward of drive shaft 22, FIG. 22. Forward inner diameter portion 322 begins at inner diameter transition portion 323, FIGS. 21 and 22, and tapers radially outward as it extends axially forward increasing the radial separation from propeller shaft 22 with increasing axial distance from inner diameter transition portion 323. Drive sleeve 308 has an annular thickness which is largest at inner diameter transition portion 323, and decreases along tapered forward inner diameter portion 322 with increasing axial distance from rearward inner diameter portion 318. The narrowest annular thickness of drive sleeve 308 is at the forward axial end of forward inner diameter portion 322 spaced axially farthest from rearward inner diameter portion 318. 
     Drive sleeve 308, FIGS. 21 and 22, has a rearward outer diameter portion 324 radially aligned with rearward inner diameter portion 318. Drive sleeve 308 has a forward outer diameter portion 326 radially aligned with forward inner diameter portion 322 and axially spaced forward from inner diameter portion 318 and outer diameter portion 324. 
     The outer diameter of drive sleeve 308 is tapered to have a larger outer diameter at forward portion 326 than at rearward portion 324. Forward outer diameter portion 326 snugly engages the propeller hub at portion 34, FIGS. 22 and 23, in keyed relation at area 104. Preferably, the outer diameter of drive sleeve 308 is polygon shaped, FIG. 23. As shown in FIG. 24, rearward outer diameter portion 324 is spaced slightly radially inwardly of inner propeller hub portion 34 forming a small radial gap 62 such that rearward outer diameter portion 324 of drive sleeve 308 may partially rotate relative to inner propeller hub 34 in response to rotation of propeller shaft 22 drivingly engaging adapter 304 which drivingly engages drive sleeve 308. Propeller shaft 22 drives adapter 304 by splined portion 24 engaging adapter inner splines 312 of adapter 304. Adapter 304 drives drive sleeve 308 by adapter outer splines 316 engaging drive sleeve inner splines 320 of drive sleeve 308. 
     The outer surface of drive sleeve 308 and the inner surface of inner propeller hub 34 are tapered relative to each other such that the radial thickness of gap 62 increases as it extends axially rearward as shown in FIG. 22. At the forward end of drive sleeve 308, FIG. 23, there is a snug fit between drive sleeve 308 at outer diameter portion 326 and inner propeller hub 34 such that there is no radial gap. At the rearward end of drive sleeve 308, radial gap 62 is at its largest dimension, FIGS. 22 and 26. 
     When propeller 12, FIG. 1, strikes an object, propeller shaft 22, FIGS. 24 and 25, drive sleeve adapter 304, and the rearward portion of drive sleeve 308 continue to rotate to a further rotated position as shown in FIG. 25. Outer tips 328 of drive sleeve 308 engage inner propeller hub portion 34 of propeller hub 14. The axial length of the engagement increases with increasing angular rotation of propeller shaft 22, adapter 304, and the rearward portion of drive sleeve 308. For example, referring to FIG. 22 and assuming that the propeller has struck an object, propeller shaft 22, drive sleeve adapter 304, and the rearward portion of drive sleeve 308 continue to rotate even though inner propeller hub 34 has stopped. After 1° of rotation, drive sleeve 308 engages inner propeller hub 34 not only at forward area 104, but also at area 106 slightly rearward thereof, as a result of the 1° rotation. Prior to the propeller striking the object there is no torsional rotation and a small radial gap exists between drive sleeve 308 and inner propeller hub 34 at area 106. After 2° of torsional rotation resulting from the propeller striking an object, drive sleeve outer tips 328, FIG. 24, are rotated into contact with inner propeller hub 34 at area 108, FIG. 22. After 3° of rotation, the point of interference contact extends rearwardly to area 110, such that axial length of interference engagement extends from forward area 104 rearward to area 110. FIG. 25 illustrates the 3° torsional rotation example at area 110 with outer tips 328 engaging inner propeller hub 34. Comparing FIG. 24 which illustrates no torsional rotation and FIG. 25 which illustrates 3° of torsional rotation, it becomes evident that while inner propeller hub 34 has stopped rotating, propeller shaft 22 and adapter 304 are permitted to continue rotating to a certain rotated angle. After 5° of rotation, there is contact at area 112, FIG. 22. These noted examples are illustrative and the points of contact may vary depending upon tolerances and taper. 
     It is believed particularly important to minimize shock load to the drive within the first 5° of rotation of the propeller shaft after the propeller is stopped by an object. Therefore, it is preferred that the tapered gap 62 between drive sleeve 308 and inner propeller hub 34 allow at least 5° rotation before drive sleeve 308 completely engages inner propeller hub 34 in interference fit. For example, 5° torsional rotation will cause interference fit between drive sleeve 308 and inner propeller hub 34 from area 104 to area 112, FIG. 22. Further torsional rotation, past 5°, will cause the interference fit to extend to area 114 and eventually result in drive sleeve 308 cracking and/or splintering at approximately 25° of torsional rotation which allows propeller shaft 22 to continue rotating to prevent damage to the marine drive. 
     Further rearward areas such as 114, FIG. 22, allow rotation to even further rotated positions than area 112. For example, FIGS. 22 and 26 show that radial gap 62 is wider at area 114 than at area 110, FIG. 24. Accordingly, the rearward portion of drive sleeve 308 at area 114 rotates to a further rotated position as shown in FIG. 27. As each area 106, 108, 110, etc., FIG. 22, of drive sleeve 308 progressively engage inner propeller hub 34 in interference fit, the remaining rearward portion of drive sleeve 308 which is not yet in contact with inner propeller hub 34 continues to rotate by torsional twisting. The engagement area may continue to rotate slightly due to compression of outer tips 328, FIGS. 25 and 27, and the torsional twisting of the drive sleeve areas already in contact with inner propeller hub 34, The noted torsional twisting is a function of the structural configuration and the material of drive sleeve 308, as previously explained. Preferably, drive sleeve 308 is composed of extrudable grade resin plastic known in the trade as Delrin 11 150E, as above described, and drive sleeve adapter 304 is preferably composed of brass. 
     Referring to FIG. 19, during assembly, drive sleeve 308, is slid rearwardly into propeller hub 14 until rear shoulder 330, FIG. 22, of drive sleeve 308 is in contact with shoulder 66 of inner propeller hub 34. Hub 14, FIG. 19, drive sleeve 308, and forward thrust washer 28 are then slid forwardly onto propeller shaft 22, until the forward end of inner hub portion 34 engages forward thrust washer 28, FIG. 22, and forward thrust washer 28 engages tapered transition portion 32 of propeller shaft 22. Drive sleeve adapter 304, FIG. 19, with integral rear thrust hub 302 is slid forwardly into propeller hub 14 and drive sleeve 308 and over propeller shaft 22 engaging splines 24 until integral rear thrust hub 302 is stopped against rear hub shoulder 334 of inner hub 34. Locking tab washer 44, FIGS. 19 and 22, and nut 42 are then assembled and torqued down. Forward molded spacing bumps 332 on drive sleeve 308 are compressed against forward thrust washer 28, FIG. 22, during the torquing of nut 42 to provide tolerance compensation. 
     Present Invention 
     FIGS. 28-44 use like reference numerals from FIGS. 1-27 where appropriate to facilitate understanding. 
     FIG. 28 shows torpedo housing 20 of marine drive 10 of FIG. 1, similarly to FIG. 19, including propeller shaft 22, splined portion 24, and threaded portion 26. Propeller 12 is mounted at propeller hub 14 to splined portion 24 between front thrust washer or hub 28 and rear thrust washer or hub 302 of drive sleeve adapter 304. Front thrust washer 28 bears against tapered transition portion 32, FIGS. 22 and 29, of propeller shaft 22. Hub 14 has an inner hub portion 34 connected to outer hub portion 36, FIG. 4, by a plurality of radial spokes such as 38, FIG. 23. Hub 14, FIG. 28, receives drive sleeve 402, FIGS. 28-32, within inner hub portion 34, FIGS. 29 and 30. The forward end of inner hub portion 34 bears against thrust washer 28. 
     Drive sleeve adapter 304 is internally splined and received on splined portion 24 of propeller shaft 22 and engages the rear end of inner propeller hub portion 34 for mounting propeller hub 14 and drive sleeve 402 to propeller shaft 22. Drive sleeve adapter 304 has rear thrust hub 302 forwardly facing on the rear end of drive sleeve adapter 304. Rear thrust hub 302 of adapter 304 has a plurality of outer slots 306, FIGS. 19 and 28, rearwardly facing and spaced circumferentially around propeller shaft 22. Locking tab washer 44 is placed over propeller shaft 22 and against drive sleeve adapter 304. Nut 42 is threaded onto rear threaded portion 26 of propeller shaft 22. Locking tab washer 44 has an inner hex configured surface 46, FIGS. 22, 29, 30, recessed forwardly and receiving hex nut 42 to prevent rotation of nut 42 relative to locking tab washer 44. Locking tab washer 44 also has a plurality of tabs 48 extending radially outward and bendable forwardly into slots 306 of adapter 304 to prevent rotation of locking tab washer 44 relative to rear thrust hub 302 of adapter 304 which in turn is prevented from rotating relative to propeller shaft 22 by the noted splined mounting thereto at portion 24. 
     Propeller hub 14 is mounted to propeller shaft 22 at splined portion 24 by drive sleeve adapter 304 and drive sleeve 402. Drive sleeve adapter 304, FIG. 20, has an inner diameter portion 310 keyed to and engaging the propeller shaft in driven relation. It is preferred that inner diameter portion 310 have a plurality of splines 312 engaging the propeller shaft in splined driven relation at splined portion 24, FIGS. 22, 29, 30. Drive sleeve 402 and adapter 304 have an overall axial length, and inner diameter portion 310 engaging propeller shaft 22 has an axial length which is 40% to 50% of said overall axial length. Drive sleeve adapter 304 has an outer diameter 314, FIG. 20, keyed to and engaging drive sleeve 402. Preferably outer diameter portion 314 has a plurality of outer splines 316 to engage drive sleeve 402. 
     Drive sleeve 402, FIGS. 28-32, has a rearward inner circumference portion 404 keyed to and engaging drive sleeve adapter 304 in driven relation. It is preferred that inner circumference portion 404 have a plurality of inner splines 406 engaging the adapter outer splines 316, FIG. 20, in splined driven relation. Drive sleeve 402 has a rearward outer circumference portion 408 radially aligned with rearward inner circumference portion 404. Drive sleeve 402 has a forward outer circumference portion 410 axially forward of outer circumference portion 408 and inner circumference portion 404. Drive sleeve 402 has a forward inner circumference portion 412 radially aligned with outer circumference portion 410 and axially forward of inner circumference portion 404 and outer circumference portion 408. As inner circumference portion 412 extends axially rearwardly it tapers radially inwardly toward propeller shaft 22. The outer circumference of drive sleeve 402 tapers radially inwardly as it extends rearwardly, as do the drive sleeves described above. 
     Drive sleeve 402 absorbs shock by torsional twisting. Rearward inner circumference portion 404 is keyed to rotate with propeller shaft 22. Rearward outer circumference portion 408 is radially aligned with rearward inner circumference portion 404 and is radially spaced inwardly of inner propeller hub portion 34 by a gap, like drive sleeves 50, 180, 200, 308. Forward outer circumference portion 410 is keyed to rotate with the propeller hub at inner hub portion 34. As shown in FIG. 34 which is a sectional view through the forward portion of the sleeve along section line 34--34 of FIG. 30, there is a tight keyed fit of forward outer circumference portion 410 of drive sleeve 402 in inner hub portion 34, with no gap therebetween. As seen in FIGS. 35 and 37, which are taken along respective section lines 35--35 and 37--37 of FIG. 30, the rear outer circumference portion 408 is spaced radially inwardly of inner hub portion 34 by a radial gap, which gap increases as the sleeve extends rearwardly, as illustrated at gap 414 in FIG. 35 and larger gap 416 in FIG. 37. The construction thus far described is comparable to the drive sleeves described above in connection with FIGS. 1-27. 
     When propeller 12 strikes an object, the propeller inner hub portion 34 and forward outer circumference portion 410 of drive sleeve 402 stop rotating, and sleeve 402 torsionally twists to enable rearward inner circumference portion 404 and rearward outer circumference portion 408 to continue to rotate to a further rotated position than forward outer circumference portion 410. The torsional twisting is illustrated in FIG. 36 for the section along line 35--35 of FIG. 30. As shown in FIG. 36, for a right hand rotation propeller, propeller shaft 22, adapter 304, inner circumference portion 404 and outer circumference portion 408 of the drive sleeve continue to rotate clockwise to a further rotated position relative to inner hub portion 34. As shown in FIG. 41, the propeller shaft continues to rotate in the clockwise direction through a rotational angle 418 to a further rotated position relative to the hub. The further rotated position of FIG. 36 is shown in FIG. 41 at solid line at 420. The unloaded or nontorsionally twisted position of drive sleeve 402 as shown in FIG. 35 is illustrated in FIG. 41 at dashed line 422. 
     Further rearward portions of the drive sleeve torsionally twist even further to further rotated positions. This is illustrated in FIG. 38 showing the torsional twisting of the rearmost portion of the drive sleeve along section line 37--37 of FIG. 30. At this rearward section, propeller shaft 22, adapter 304, inner circumference portion 404 and outer circumference portion 408 of drive sleeve 402 continue to rotate in the clockwise direction through rotational angle 424, FIG. 42, to the further rotated position relative to inner hub portion 34 as shown in FIG. 38. The further rotated position of FIG. 38 is shown in FIG. 42 at solid line 426. The unloaded nontorqued nontorsionally twisted position of the drive sleeve as shown in FIG. 37 is illustrated in FIG. 42 at dashed line 428. 
     Drive sleeve 402 has an asymmetric spring rate such that the sleeve has a higher spring rate in one direction of rotation of propeller shaft 22 than in the other direction of rotation of propeller shaft 22. The sleeve has a higher spring rate with greater torque bearing capability for the forward direction of the boat, which is clockwise rotation of propeller shaft 22 as viewed from the rear, for a right hand rotation propeller. The sleeve has a lower spring rate and absorbs more shock for the reverse direction of the boat, which is counterclockwise rotation of propeller shaft 22 as viewed from the rear, for a right hand rotation propeller. 
     Drive sleeve 402 absorbs shock by permitting propeller shaft 22 to continue to rotate to a further rotated position than the propeller hub when the propeller strikes an object. The sleeve mounts the propeller hub to the propeller shaft in asymmetric relation such that: in the clockwise direction of rotation, when the propeller strikes an object, propeller shaft 22 continues to rotate clockwise through a forward rotational angle 418, FIG. 41, to a further forward rotated position 420, FIGS. 41 and 36, relative to the hub; and in the counterclockwise direction of rotation, FIGS. 39, 40, 43, 44, to be described, when the propeller strikes an object, propeller shaft 22 continues to rotate counterclockwise to a reverse rotational angle 430, FIG. 43, relative to the hub. Reverse rotational angle 430, FIG. 43, is greater than forward rotational angle 418, FIG. 41. This differential in the amount of rotation angle between forward and reverse provides the noted asymmetric spring rate. 
     Referring to FIGS. 35 and 39, when the marine drive is in reverse and the propeller strikes an object, inner propeller hub portion 34 and outer circumference portion 410 of the drive sleeve stop rotating, and propeller shaft 22, adapter 304, inner circumference portion 404 and outer circumference portion 408 of drive sleeve 402 continue to rotate counterclockwise through angle 430, FIG. 43, to the solid line position 432 shown in FIG. 43. The unloaded nontorsionally twisted position of the drive sleeve as shown in FIG. 35 is illustrated in FIG. 43 at dashed line 422. The counterclockwise rotated position as shown in FIG. 39 is illustrated in FIG. 43 at solid line 432. 
     The counterclockwise rotated positions of FIGS. 39 and 43 are for the portion of the drive sleeve along section line 35--35 of FIG. 30. Further rearward portions of the drive sleeve rotate counterclockwise to even further rotated positions as the sleeve torsionally twists. The rearmost portion of the drive sleeve torsionally twists and rotates counterclockwise through rotational angle 436, FIG. 44, to the solid line position shown at 438, corresponding to FIG. 40. The unloaded nontorsionally twisted starting position of the rearmost section of the drive sleeve as shown in FIG. 35 is illustrated in FIG. 44 at dashed line 428. 
     The asymmetry of forward and reverse shock absorption is illustrated by comparing FIGS. 41 and 43. Reverse counterclockwise rotational angle 430 of FIG. 43 is greater than forward clockwise rotational angle 418 of FIG. 41. This is also illustrated by comparing FIGS. 42 and 44, wherein reverse counterclockwise rotational angle 436 of FIG. 44 is greater than forward clockwise rotational angle 424 of FIG. 42. 
     Drive sleeve 402 and inner hub portion 34 are asymmetrically configured relative to each other to provide the above noted differential rotation between forward and reverse. In preferred form, the sleeve and hub are asymmetrically tapered relative to each other such that when the propeller strikes an object, the sleeve progressively engages more of the hub in one direction of rotation than in the other direction of rotation. For ease of manufacturability, it is preferred that the taper and asymmetry be formed on the drive sleeve. Sleeve 402 extends in the axial direction, i.e. along the axis of propeller shaft 22, from outer circumference portion 410 rearwardly to outer circumference portion 408. The sleeve has relief lines 442 and 444, FIG. 31, along outer circumference portion 408 defining a relief 446 therebetween spaced radially inwardly of hub inner portion 34 by a gap 448, FIG. 37, enabling angle 436, FIG. 44, to be greater than angle 424, FIG. 42. 
     Relief lines 442 and 444 extend at different angles relative to the noted axial direction. Relief line 442 extends at divergence angle 448 relative to the axial direction. Relief line 444 extends at divergence angle 450 relative to the axial direction. Divergence angle 448 is greater than divergence angle 450. Relief line 442 tapers in the circumferential direction along divergence angle 448, and relief line 444 tapers in the opposite circumferential direction along divergence angle 450. Relief lines 442 and 444 taper away from each other as they extend rearwardly from outer circumference portion 410, such that the width of relief 446 in the circumferential direction increases as the relief extends rearwardly away from outer circumference portion 410. Both of relief lines 442 and 444 taper radially inwardly as they extend rearwardly away from outer circumference portion 410, as shown in FIGS. 30 and 29, respectively. 
     Relief lines 442 and 444 define different size step transitions 452 and 454, respectively, in radius of outer circumference portion 408. The size or height of step 452 is greater than that of step 454. The difference between the step sizes increases as relief lines 442 and 444 extend rearwardly away from outer circumference portion 410. The size of step 452 increases as relief line 442 extends rearwardly. The size of step 454 is substantially constant as relief line 444 extends rearwardly. Steps 452 and 454 diverge away from each other as relief lines 442 and 444 extend rearwardly. Step 452 diverges at the noted divergence angle 448, and step 454 diverges at the noted divergence angle 450. Relief 446 tapers radially inwardly as it extends rearwardly from outer circumference portion 410. The radial inward taper or slope of relief 446 as it extends rearwardly is greater than the radial inward taper or slope of relief line 442 as it extends rearwardly, such that the radial height or size of step 452 increases as it extends rearwardly. The radial inward taper or slope of relief line 444 as it extends rearwardly is substantially the same as that of relief 446. 
     As above noted, drive sleeve 402 has an unloaded position wherein rearward outer circumference portion 408 is not torsionally twisted relative to forward outer circumference portion 410, as shown in FIGS. 35 and 37. Relief lines 442 and 444 define respective outer shoulders 456 and 458 spaced by respective gaps 460 and 462 from hub inner portion 34 in the noted unloaded position of the drive sleeve. Gap 460 is smaller than gap 462. Shoulder 456, FIG. 38, engages hub inner portion 34 in the noted clockwise rotated position of drive sleeve 402. Shoulder 458, FIG. 40, engages hub inner portion 34 in the noted counterclockwise rotated position of drive sleeve 402. Shoulders 456 and 458 taper radially inwardly as they extend rearwardly away from outer circumference portion 410. The radially inward taper or slope of shoulder 458 as it extends rearwardly is greater than that of shoulder 456, and hence gap 462 is greater than gap 460. Shoulders 456 and 458 taper in the circumferential direction, FIG. 31, away from each other as they extend rearwardly. Shoulder 456 extends at divergence angle 448, and shoulder 458 extends at divergence angle 450. 
     In preferred form, drive sleeve 402 is keyed to propeller shaft 22 by drive sleeve adapter 304 having an inner circumference 310 at splines 312 engaging and keyed to propeller shaft 22 at splines 24 in driving relation, and an outer circumference 314 at outer splines 316 keyed to and engaging inner circumference portion 404 at splines 406 of drive sleeve 402 in driving relation. Alternatively, inner circumference portion 404 of drive sleeve 402 at splines 406 may directly engage propeller shaft 22 at splines 24 in keyed driving relation, like drive sleeves 50, 180, 200, above. 
     It is recognized that various equivalents, alternatives and modifications are possible within the scope of the appended claims.