Abstract:
A variable-pitch marine propeller system has a propeller unit for mounting on a drive shaft, and a power unit including a stationary annular hydraulic cylinder for operating the propeller unit, a hydraulic remote control unit being fluid-coupled to the power unit. The propeller unit is provided with a shear pin assembly that is serviceable for replacing sheared pins without removing the propeller unit from the drive shaft. An annular piston of the hydraulic cylinder is coupled to a ring-shaped actuator yoke by a roller thrust bearing, the actuator yoke axially displacing a mating yoke of the propeller unit with which the actuator yoke is allowed to rotate. The piston operates in a sealed environment for the exclusion of water from the separately sealed surfaces of the cylinder itself. In one configuration, the propeller unit is replaceable without disturbing the sealed environment of the annular piston. The control unit includes a hydraulic control cylinder that is operated by a threaded piston rod. In a preferred configuration of the control unit, the rod is a ballscrew having an antifriction ballnut fastened to the control piston, and the control unit can be provided with a position encoder. The control unit can also be motorized and equipped with a clutch control knob for manual operation with the motor decoupled.

Description:
This application is a continuation-in-part of Ser. No. 08/949,021 filed Oct. 10, 1997 now U.S. Pat. No. 5,967,750. 
    
    
     BACKGROUND 
     The present invention relates to propeller propulsion devices, and more particularly to variable pitch propeller devices for marine craft such as inboard and outboard pleasure boats, yachts and fishing boats. 
     Variable pitch aircraft propellers are well known, implementations including hydraulic actuators being disclosed, for example, in U.S. Pat. Nos. 2,425,261 to Murphy et al., 2,554,611 to Biermann, and 4,362,467 to the present inventor. The &#39;467 patent, which is incorporated herein by this reference, discloses a mounting flange for mounting to the propeller shaft flange of an engine, a hub for pivotally supporting a plurality of blades on respective radial axes, and a stationary annular hydraulic cylinder and piston between the mounting flange and the hub, the piston being connected by a yoke and transverse pin to a longitudinal rack member that engages respective pinions of the blades to rotate same through a wide angle of approximately 90°. 
     Typical marine propeller installations include a rearwardly extending propeller shaft on which is mounted a one-piece propeller having an annular hub portion, the shaft extending through the hub and threadingly engaging a retainer nut. The hub is secured against rotation relative to the shaft such as by splined engagement or by one or more keys or shear pins. 
     The aircraft propeller implementation of the &#39;467 patent, while having certain advantages including the stationary annular hydraulic cylinder, is unsuitable for use in typical marine applications for a number of reasons. For example: 
     1. The shaft interferes with placement and movement of the yoke pin and the rack member; 
     2. The rear of the hub, including a biasing spring mechanism therein, interferes with access to the nut whereby the hub would be secured to the shaft; 
     3. The hub and blades would be difficult to remove for servicing and/or replacement in case of damage by underwater hazards; and 
     4. The device would be subject to water damage in that hub is unsealed, and the piston seals would have to operate in a wet environment. 
     A further problem exhibited in the prior art relates to the need in marine applications for means to decouple the propeller in case of impact with potentially damaging foreign objects such as submerged rocks and logs. Typically, such a device couples the propeller to it&#39;s shaft by a “shear pin” that transmits normal driving torques but which is supposed to sever when the propeller strikes an obstacle. The shear pins of the prior art are difficult to replace in that the propeller must be removed from the shaft, typically with significant difficulty resulting from interference with jagged edges of the severed shear pin. Moreover, the difficulty with which the propeller is removed significantly increases the risk of it&#39;s being dropped into the water. 
     Thus there is a need for a variable pitch marine propeller that is effective for providing a wide angular range, that is compatible with existing fixed-pitch installations, that is easy to service, repair, and replace, and is resistant to water damage. 
     SUMMARY 
     The present invention meets this need by providing a modular variable pitch system configuration of propeller and stationary annular actuator for facilitating assembly, servicing and replacement particularly of parts most subject to damage by under-water hazards. The system is adapted for marine drives including a driven shaft having a locating surface, a torque-transmitting surface, and a retainer surface for engagement by a retaining device, the shaft extending from a base structure such as a drive housing. In one aspect of the invention, a propeller system having an easily serviceable torque limiting safety device includes a propeller unit having a hub rotatably mountable on the driven shaft for supporting a plurality of radially projecting blade members, a hub passage being formed in structure rigidly fixed relative to the hub; a sleeve member mountable on the driven shaft in engagement with the torque-transmitting surface and having a sleeve passage formed in one wall thereof, the sleeve passage being alignable with the hub passage for placement of a shear pin in engagement with the hub and sleeve passages; and a retainer member removably mountable in covering relation to at least one of the hub and sleeve passages when the shear pin is placed therein for retaining the shear pin, whereby torque is transmittable from the sleeve member through the shear pin to the hub until fracture of the shear pin in response to occurrence of a predetermined limiting torque, allowing the hub to rotate relative to the driven shaft, and when the retainer member is removed from covering the at least one of the hub and sleeve passages, the shear pin is removable from the passages for replacement without the hub being removed from the driven shaft. The system can be assembled with the shear pin located in engagement with the hub and sleeve passages, the retainer member being removably fastened in fixed relation to the hub. 
     Preferably the blade members are movably supported relative to the hub, a yoke member being axially movable relative to the hub in response to a power actuator, and means for moving the blade members in variable pitch relation to the hub in response to axial movement of the yoke member. The means for moving can include each blade member being rotatably mounted on a respective radially extending axis of the hub and having a pinion fixedly connected thereto, and the propeller yoke including axially extending rack elements engaging corresponding ones of the pinions. 
     The power actuator can include an annular hydraulic cylinder rotatably supportable relative to the spindle and having a fluid port and means for preventing rotation of the cylinder by mechanical coupling to the base structure, an actuator piston sealingly axially movable in the hydraulic cylinder for coupling fluid flow relative to the port, a thrust bearing for transmitting axial force between the actuator piston and the yoke member, whereby the yoke member moves axially relative to the spindle in response to fluid flow into the port, and the spindle, the hub, and the yoke member can be rotated by the shaft while the cylinder and the piston are being prevented from rotation by the coupling to the base. 
     The shaft is operable submerged in water, and the system can further include power unit seal means for excluding water from the actuator piston and the thrust bearing. Preferably the propeller unit is separable from the drive shaft without disturbing the power actuator. 
     The yoke member can be a propeller yoke and the power actuator can be in a power unit, the power unit further including a spindle for coupling to the shaft and rotation therewith, a piston yoke for contacting the propeller yoke, a first thrust bearing for transmitting axial force between the spindle and the cylinder, and a second thrust bearing for transmitting axial force between the piston and the piston yoke. The piston yoke moves axially relative to the spindle in response to fluid flow into the port, and the spindle and the piston yoke can be rotated by the shaft while the cylinder and the piston are being prevented from rotation by the coupling to the base. The power unit is locatable adjacent the propeller unit opposite the retainer device whereby the axial force is transmitted from the locating surface, through the power unit to the propeller yoke by the piston yoke, and through the means for moving and the hub to the retainer device. The axial movement of the propeller yoke causes the blade members to move from a first position toward a second position relative to the hub in response to the fluid flow into the port. 
     Preferably the spindle is adapted for being clamped between the locating surface and the hub by the retainer device. Preferably the system further includes an antifriction radial bearing for concentrically supporting the cylinder relative to the spindle. 
     The system can further include a hydraulic control unit being fluid-connectable to the fluid port and including a primary hydraulic cylinder, a control piston sealingly axially movable in the primary cylinder, a lead screw rotatably supported in the housing and having an antifriction nut assembled thereto, the nut being threadingly engaged with the lead screw by means of a plurality of rollingly interposed elements, the nut being rigidly connected to the control cylinder, the lead screw being axially supported within the housing by an antifriction thrust bearing for advancing the control piston at high mechanical advantage in response to rotation of the lead screw, whereby the pitch of the blades is adjustable in response to rotation of the lead screw. The control unit preferably includes an encoder coupled to the lead screw for signaling positions thereof to an external device. The control unit can further include a control motor coupled to the lead screw for driving same to an externally determined setpoint position in response to the encoder. The control unit can further include a manual control knob and a clutch coupled between the control knob and the lead screw and the control motor for selectively decoupling the control motor from the lead screw, the lead screw being manually operable by the control knob when the control motor is in a decoupled condition. 
     The system can further include means for biasing the piston against the fluid flow into the port for retraction of the piston relative to the cylinder when fluid is allowed to flow out of the port, in response to reduced fluid pressure, the blade members correspondingly moving toward the first position. The biasing means can include a spring for urging the propeller yoke axially toward the piston yoke relative to the hub. 
     The means for preventing rotation can include the fluid port being formed for engagement by a hydraulic fitting having a conduit extending therefrom, and a mechanical connection between the conduit and the base. The system can include two of the blade members that project from opposite sides of the hub, or three of the blade members that project in equally spaced relation to the hub. 
    
    
     DRAWINGS 
     These and other features, aspects, and advantages of the present invention will become better understood with reference to the following description, appended claims, and accompanying drawings, where: 
     FIG. 1 is a perspective view of a variable pitch propeller system according to the present invention, the system being installed on an existing outboard watercraft engine; 
     FIG. 2 is an axial sectional view of a propeller unit portion of the propeller system of FIG. 1; 
     FIG. 3 is a fragmentary lateral sectional view of the propeller system of FIG. 1 on line  3 — 3  in FIG. 2; 
     FIG. 4 is a rear view of the propeller unit of FIG. 2; 
     FIG. 5 is a lateral sectional view of a control unit portion the propeller system of FIG. 1; 
     FIG. 6 is a sectional view as in FIG. 5, showing an alternative configuration of the control unit; 
     FIG. 7 is a lateral sectional view showing an alternative configuration of the control unit portion of FIG. 6, in a manual mode condition; 
     FIG. 8 is a sectional view as in FIG. 7, showing control unit of FIG. 7, in a powered mode condition; 
     FIG. 9 is a sectional view on line  9 — 9  of FIG. 7; 
     FIG. 10 is a sectional view as in FIG. 2, showing an alternative configuration thereof; and 
     FIG. 11 is a lateral sectional view showing another alternative configuration of the propeller unit of FIG.  2 . 
    
    
     DESCRIPTION 
     The present invention is directed to a variable pitch propeller system that is particularly effective in marine environments. With reference to FIGS. 1-5 of the drawings, a propeller system  10  for a shaft drive  12  includes a power unit  14 , a propeller unit  16 , and a control unit  18  being fluid-connected to the power unit as described below. The shaft drive  12  is representative of typical existing hardware, having a propeller shaft  20  rearwardly extending from a base structure  22  that can be a hull member or an outboard drive housing. The shaft  20  is formed with an engagement surface  24  having a tapered portion  25  for locating a conventional propeller (not shown), a cylindrical portion  26 , splined portion  27  for transmitting torque to the conventional propeller, and a threaded portion  28  for engagement by a retainer nut  29  by which the conventional propeller is clamped against the tapered portion  25 . The power unit  14  of the propeller system  10  includes a spindle  30  for coupling to the shaft  20  by location on the tapered portion  25  and the cylindrical portion  26  of the engagement surface, optionally by using an adapter sleeve  31  for facilitating use of a singly configured spindle  30  with a plurality of differently configured propeller shafts  20 . An annular hydraulic cylinder  32  is supported in concentric relation to the spindle  30 , the cylinder  32  having a piston  34  being axially slidable in sealed relation therewith, the cylinder  32  also having a port member  36  fixedly extending therefrom. The port member  36  has a threaded fluid port  37  formed therein for receiving a suitable hydraulic fluid as further described below, the port  37  being fluid-connected to the cylinder  32  by a port passage  38  for axially displacing the piston  34 . The port member  36  can be integrally formed with the cylinder  32 , or fastened thereto as shown in the drawings, an O-ring  39  sealing the passage  38 . 
     The power unit  14  also includes a piston yoke  40  for operating the propeller unit  16  as described below, and antifriction bearings for transmitting axial forces while permitting rotation of the spindle  30  and the piston yoke  40  with the shaft  20  while the hydraulic cylinder  32  and the piston  34  are restrained from rotation. A first needle or roller thrust bearing  42  is located between the spindle  30  and the hydraulic cylinder  32 ; a second such thrust bearing  44  is located between the piston  34  and the piston yoke  40 , for transmitting axial force to the piston yoke  40 ; and a radial needle bearing  46  is located within the cylinder  32  for engagement by the cylindrical portion  26  of the propeller shaft  20  (preferably via the adapter sleeve  31  as discussed above) to thereby maintain concentricity of the cylinder  32  with the shaft  20 . 
     The piston  34  is provided with respective outside and inside ring seals  48  and  49  that sealingly contact corresponding finished surfaces of the hydraulic cylinder  32  in a conventional manner. According to the present invention, the power unit has further seals for excluding water and foreign matter from the surfaces contacted by the ring seals  48  and  49 . In one exemplary configuration and as shown in FIG. 3, a rotary first seal  50  is supported by the cylinder  32  for sealingly contacting a front portion of the spindle  30 ; a rotary second seal  51  is supported by an inside surface of the piston yoke  40  for sealingly contacting a rear portion of the spindle  30 ; and an axial third seal  52  is supported by the cylinder  32  for sealingly contacting an outside surface of the piston yoke  40 . 
     The threaded port  37  is provided with a feed fitting  54  by which the control unit  18  is fluid-connected to the power unit  14  through a suitable conduit  56  (schematically shown in FIG.  3 ). The port member  36  (alone or in combination with the fitting  54 ) provides a mechanical connection point for restraining the hydraulic cylinder  32  from rotating with the spindle  30 . For example, rotational restraint can be achieved by the fitting  54  extending between opposite walls of the base member  22 , or by anchoring the conduit  56  to the base member  22  proximate the fitting  54 . The port member  36  is also formed for supporting a bleed valve  58  in fluid communication with the passage  38 , by which air can be bled from control unit  18 . 
     The propeller unit  16  includes a hub  60 , a flange member  62  for coupling the hub  60 , preferably through a torque limiting safety device, designated shear pin assembly  61  and further described below, to the splined portion  27  of the shaft  20 , the flange member  62  being affixed to the hub  60  by a plurality of flange fasteners  63 . A plurality of radially extending blade members  64  are rotatably supported by the hub  60 , each blade member  64  having a pinion  66  on a stem portion  68  thereof. The propeller unit  16  also includes a ring-shaped rack member  70  having a plurality of radial rack sections  72  formed thereon for engaging corresponding ones of the pinions  66 , the rack member  70  being axially slidably supported on a portion of the flange member  62  that extends within the hub  60 . A retainer ring  73  is assembled to the flange member  60  proximate a front extremity thereof for limiting forward movement of the rack member  70 . The rack member  70  is formed with an annular groove  74  for engaging a complementary annular projection  76  of the piston yoke  40 , the groove  74  and the projection  76  acting to help maintain concentricity of the yoke  40  and rack member  70  relative to the hub  60  and the flange member  62 . Each of the blade members  64  is supported in the hub  60  by a respective bearing member  78  that rotatably engages the corresponding stem portion  68 , each bearing member  78  having a spaced pair of internal O rings  80  for sealingly retaining a suitable lubricant such as grease therebetween. The blade members  64  are axially secured in the bearing members  78  by the pinions  66  being pinned to the stem portions using respective pin members  82 . Inward portions of the bearing members  78  are formed as enlarged flange portions  79  for retention by counterbored portions of the hub  60  as best shown in FIG. 2, thereby securing the blade members  64  against outward movement from the hub  60 . Inward movement of the blade members  64  (and the bearings  78 ) is blocked by respective flattened portions of the flange member  62  contacting end extremities of the stem portions  68  as shown in FIGS. 2 and 3, the flattened portions also providing clearance for the pinions  66 . Rotational alignment of the rack member  70  relative to the hub  60  for maintaining geared engagement of the pinions  66  by the rack sections  72  is maintained by the flange portions  79  of the bearing members  78  contacting the rack member  70  opposite respective ones of the rack sections  72 . 
     The extension of flange member  62  through the hub  60  axially contacts the spindle  30  of the power unit  14 , the spindle  30  and the flange member  62  being clamped between the tapered portion  25  of the shaft  20  and the retainer nut  29 . The propeller unit  16  is removable from the shaft  20  (following removal of the retainer nut  29 ) without disturbing the power unit  14 . Advantageously, the sealing of the combination of the hydraulic cylinder  32  and the piston  34  by the seals  50 ,  51  and  52  remains intact during removal and replacement of the propeller unit  16 . 
     Axial movement of the piston yoke  40  in response to pressure fluid flow into the hydraulic cylinder  32  produces corresponding axial movement of the rack member  70 , and proportional rotation of the blade members  64  relative to the hub  60 , the rotation resulting from geared engagement of the pinions  66  with the radial rack sections  72  of the rack member  70 . The propeller unit  16  is also provided with a plurality of compression springs  84  for oppositely rotating the blade members  64  while returning the rack member  70 , the yoke  40  and the piston  34  toward the passage  38  when fluid pressure is released therefrom. Opposite ends of each compression spring  84  are located in respective flange and yoke cavities  85  and  86  that are formed in the flange member  62  and the rack member  70 . The retainer ring  73  sets the maximum forward angular orientation or pitch of the blade members  64 , and prevents axial movement of the rack member  70  out of the hub  60 , thereby keeping the propeller unit  60  intact when it is removed from the shaft  20 . In a preferred implementation, the maximum forward pitch at the tips of the blade members  64  is approximately 54 degrees. (A standard fixed-pitch 140 HP propeller has a tip angle of approximately 44 degrees.) At the opposite extremity of the axial movement, a maximum reverse pitch of 25 degrees is attained. The compression springs  84  provide a total of approximately 300 pounds of biasing against movement of the piston  34 . Additionally (or alternatively), the blade members  64  are formed to provide rotational torque reactions against the piston  34  in response to advancement in a water (or air) fluidic medium. 
     The preferred shear pin assembly  61 , introduced above, includes an internally splined bushing  162  as best shown in FIG. 3, the bushing engaging the splined portion  27  of the propeller shaft  20  and being freely rotatable in the flange member  62 . The bushing  162  has radially oriented slots  164  formed therein for receiving end portions of respective shear pins  166 . The flange member  62  is also formed with radially oriented slots, designated  170 , for receiving outwardly extending portions of the shear pins  166 , the pins transmitting a predetermined maximum torque between the flange member  62  and the bushing  162 . Excessive torque, such as might be caused by blade members  64  striking a submerged tree stump or the like, results in shearing of the pins  166  so that the hub of the propeller unit  16  can freely rotate relative to the propeller shaft  20 , thereby avoiding damage to other components of the system  10 . For this purpose, the material and dimensions of the shear pins  166  is selected for fracture at a predetermined fail-safe torque. It will be understood that the number of shear pins to be employed is also variable within the scope of the present invention. A thrust plate  168  is interposed between the retainer nut  29  and the flange member  62  for retaining the pins  166  in the slots and  170 , the thrust plate being fastened to the flange member  62  by a plurality of cap screws  172 . As shown in FIG. 4, pairs of the cap screws  172  are located on opposite sides of respective shear pins  166  for convenient securing by suitable safety wire  174 . 
     As further shown in FIGS. 1-3, the propeller unit  16  is provided with tubular front and rear shrouds  88  that promote smooth fluid flow from the base member  22  and past the blade members  64 . Each of the shrouds  87  and  88  is appropriately notched to clear the bearing members  78  of the blade members  64 , being fastened to the hub  60  by a plurality of shroud fasteners  89 . Also, the hub  60  is segmented for facilitating fabrication thereof and for facilitating assembly of the propeller unit  16 . The exemplary configuration of the propeller unit shown in FIGS. 1 and 2 has a pair of the blade members  64  extending radially from opposite sides of the shaft  20 , the blade members  64  being controllably rotatable within the radially oriented bearings  78  as described above for altering the pitch of the blade members. As further described below, the propeller unit  16  can be provided with three or more of the blade members. 
     An exemplary configuration of the control unit  18 , depicted in FIG. 5, corresponds generally to a control device as described in the above-referenced U.S. Pat. No. 4,362,467. The control unit  18  includes a hydraulic control cylinder  90  having a control piston  92  therein and having a threaded piston rod  94  extending therefrom. A rotatably supported barrel member  96  threadingly engages the piston rod for axially positioning the piston  92  in the cylinder  90 . The piston rod  94  has a longitudinal groove  98  formed therein, a key pin  100  slidably engaging the groove  98  for preventing rotation of the rod  94 . The cylinder  90  has a head portion  91  opposite the piston rod  94 , counterparts of the fitting  54  and the bleed valve  58  being mounted on the head portion  91  in fluid communication with the cylinder  90 , the conduit  56  being connected to the fitting  54 . 
     The barrel member  96  has an outwardly extending flange portion  102 , one face of which rotatably engages an anchor sleeve  104  of the cylinder  90 , a roller thrust bearing  106  that is retained in the anchor sleeve  104  by a conventional retainer ring  108 . The sleeve  104  is adapted for mounting through a stationary member, such as a control panel  105 . A handle collar  110  is fixably mounted on the barrel member  96  and having an L-shaped crank member  112  rigidly extending therefrom for facilitating manual rotation of the barrel member  96 . The thrust bearing  106  axially supports the barrel member during forced advancement of the control piston  92  toward the fitting  54 , movement in the opposite direction being generally unopposed in that the springs  84  of the propeller unit  16  are effective for driving the blade members  64 , the yokes  40  and  70 , and the piston  34  to produce fluid flow into the control cylinder  90  during retraction of the control piston  92 . The piston rod  94  has a stem extremity  114  that projects from the barrel member  96  for indicating relative positions of the piston  92 , thereby providing visual indications of propeller pitch settings of the system  10 . The stem extremity  114  can have colored striping for designating particular pitch ranges such as forward (high and low pitch), neutral, and reverse. 
     With further reference to FIG. 6, an alternative configuration of the control unit, designated  18 ′, includes a housing  120 , a counterpart of the control cylinder, designated  90 ′ being formed at one end thereof, a smaller counterpart of the cylinder, designated  90 ″ being axially spaced from the cylinder  90 ′. A counterpart of the piston, designated  92 ′, has sealed sliding engagement with the cylinders  90 ′ and  90 ″, the housing  120  also having respective feed and bleed ports  122  and  124  formed in opposite walls thereof for correspondingly receiving the fitting  54  and the bleed valve  58  as described above in connection with FIG.  5 . The ports  122  and  124  are in fluid communication with a fluid chamber  125 , a volume thereof varying by an axial travel distance of the piston  92 ′ multiplied by that portion of the area of the cylinder  90 ′ that is outside of the cylinder  90 ″. A ballscrew  126  is rotatably supported in the housing by a thrust bushing  127 , a suitable antifriction thrust bearing  128  being interposed between the bushing and the housing  120 . The ballscrew  126  has a ballnut  130  assembled thereto, the ballnut being rigidly connected to the piston  92 ′ by threaded engagement therewith for advancing the piston  92 ′ toward the bearing  128  against fluid pressure in the chamber  125  in response to rotation of the ballscrew  126 . If necessary or desired, suitable means such as an axially oriented pin can be used for preventing rotation of the piston  92 ′ relative to the housing  120 , such pin being anchored to one of the housing  120  and the piston and having sliding engagement with the other. The ballscrew  126  may be made from a length of commercially available stock, designated R-308 (⅜ inch diameter×0.125 lead), a suitable ballnut for use as the ballnut  130  being available as No. 8103-448-003 (R-0308 without flange or wiper) from Warner Electric Brake &amp; Clutch Co. of South Deloit, Ill. 
     The housing  122  has fastener openings  132  for mounting to suitable structure such as the control panel  105  (not shown in FIG. 6) and/or a thrust plate  133 . A control knob  134  is fixedly mounted to the ballscrew  126  for advancing the piston  92 ′ at high mechanical advantage and low frictional resistance. A stop screw  135  with an accompanying large pattern stop washer  136  prevents movement of the ballnut  130  beyond the free end of the ballscrew  126 . Also, one or more calibration washers  137  are interposed between the ballnut  130  and the washer  136  for adjusting a full-scale hydraulic volume displacement of the control unit  18 ′ to match that of the propeller unit  16 . An end plate  138  and a retainer ring  139  therefor are included in the control unit  18 ′ for excluding dust and other contamination from the ballnut  130  and from otherwise exposed portions of the cylinder  90 ′. 
     With further reference to FIGS. 7-9, an alternative configuration of the control unit  18 ′, designated  18 ″, is selectively operable in both manual and powered modes as described herein. A motor plate  140  is mounted to the housing  120  in place of the thrust plate  133 , and a clutch bushing  142  is substituted for the control knob  134  on the ballscrew  126 . A counterpart of the control knob, designated clutch knob  134 ′, is slidably supported on the clutch bushing  142 , being coupled for rotation therewith by a pair of axially oriented dowel pins  144 . A control motor  146  is mounted to the motor plate in parallel spaced relation to the ballscrew  126 , a drive gear  148  being mounted to an output shaft  149  of the motor and engaging a driven gear  150  that is freely rotatably supported on the clutch bushing  142  when the clutch knob is in an axially withdrawn first position relative to the bushing  142  as shown in FIG.  8 . Thus, when the knob  134 ′ is in the first position, the motor  146  is effectively disengaged, the control unit  18 ″ being in a manual mode and operable in like manner as the control unit  18 ′ of FIG. 6, described above. The clutch knob  134 ′ projects through a gear cover  152  that is fastened to the motor plate  140 , enclosing the gears  148  and  150 . 
     A pair of coupling pins  153  project from the clutch knob  134 ′ for engaging the driven gear  150  when the knob is in an axially inwardly displaced second position relative to the bushing  142  as shown in FIG.  7 . Thus, when the knob  134 ′ is in the second position, operation of the control unit  18 ″ is in a powered mode in response to suitable electrical signals to the control motor  146 . In the first position of the knob, the pins  153  are withdrawn clear of the driven gear  150  as shown in FIG.  8 . The clutch knob  134 ′ is provided with an angularly spaced plurality of springballs  154  that detent in respective axially spaced grooves  155  of the clutch bushing  142  for releasably holding the knob  134 ′ in the corresponding first and second positions thereof. 
     The motor  146  is connected in a conventional control circuit (not shown) that is responsive to operator input and feedback from a rotary position encoder  156  that includes an encoder drum  158  that is mounted to the ballscrew  126  in place of the stop washer  137 , and a sensor unit  160  that projects through the housing  120  for signaling the passage of suitable reflective indicia on the drum  158  that can be axially extending lines according to conventional incremental encoder practice. It will be understood that an absolute position reference is obtainable by operating the motor  146  at low power until the ballnut  130  reaches a physical stop, such as the calibration washers  138 , also according to conventional practice. Thus the motor  146  can drive the ballscrew  126  to an externally determined setpoint in response to the encoder  156 . A 12 volt DC gear reduction motor suitable for use as the motor  146  is available as No. 455A104-2 from TRW Globe Motor Division, Dayton, Ohio. 
     With further reference to FIG. 10, an alternative configuration of the propeller unit, designated  16 ′, has three equally spaced counterparts of the blade members  64 , the pinions, and the bearings  78 . A counterpart of the hub, designated  60 ′, is formed as three segments that are joined at the bearings  78  for facilitating fabrication and assembly as described above relative to the configuration of FIGS. 1-4. A counterpart of the flange member, designated  62 ′, has three equally spaced flattened regions that provide clearance for the pinions  66  and for blocking the inward movement of the blade members  64  as described above in connection with FIGS. 2 and 3. It will be appreciated that the bearings  78  can be formed integrally with the hub  60 ′ (of bronze, for example, when salt water operation is contemplated). Also, a row of bearing balls can be interposed between the pinions  66  and the bearings  78  (whether same are formed integrally with the hub or not), a suitable concave raceway being formed in one or both of the pinion  66  and the bearing  78  for maintaining the balls in a captured condition. 
     With further reference to FIG. 11, another alternative configuration of the system  10  has a counterpart of the rotary second seal, designated  51 ′, supported on a counterpart of the hub, designated  60 ″. The seal  51 ′ sealingly contacts a cylinder extension  33  that projects from a counterpart of the annular hydraulic cylinder, designated  32 ′. The cylinder extension advantageously permits the sealing contact to be at reduced diameter for correspondingly reduced frictional drag by the seal  51 ′, and for permitting a more compact seal to be utilized than otherwise. As further shown in FIG. 11, the flange member  62 ″ can be formed for being fixedly connected to a counterpart of the spindle, designated  30 ′, such as by threaded engagement that preferably forms a water-tight connection. The power unit  14  and the propeller unit  16  being thus connected, they would be installed and removed from the shaft  20  as a single unit, thereby maintaining the sealed environment of the piston  34 . It will be understood that the configuration of FIG. 11 can, and preferably does, include the shear pin assembly of FIGS. 3 and 4. 
     The system  10  of the present invention thus avoids rotating oil seals that are subject to leakage and have short life spans. The O rings  48  and  49  operating in the stationary hydraulic cylinder  32  have minimum travel; and no rotation, and the needle thrust bearing  44  next to the piston  34  and having high loading capacity and requiring very little lubrication, allows the propeller blade members  64 , which are rotating, to be positioned by the stationary hydraulic cylinder. The blade members  64  can be feathered, a particularly advantageous feature for sail boats. For bass fishing, by being able to lower the pitch of the propeller blades, sufficiently low boat speeds are practical that an extra trolling motor is not needed. 
     Although the present invention has been described in considerable detail with reference to certain preferred versions thereof, other versions are possible. For example, the hydraulic cylinder  32  can be rigidly anchored to the base member  22 , the first thrust bearing  42  not being required. More than one outboard or inboard engine can be provided with counterparts of the system  10 , using a single control unit  18  (having dual hydraulic cylinders), even if the propellers operate in opposite directions. Also, larger marine propellers may be positioned with the control unit  18  utilizing an engine-driven hydraulic pump and having a pressure regulator. Further, the central opening of the thrust plate  168  can be enlarged such that the nut  29  (and its associated washer, if any) bears directly against the splined bushing  162  for permitting free rotation of the flange member  62  in the event that the shear pins  166  are fractured. Moreover, the control unit  18 ′ of FIG. 6 can be equipped with the position encoder  156  of FIGS. 7 and 8 for driving a suitable electronic or electromechanical indicator display. Alternatively, a graduated thimble can be mounted to the ballscrew  126 , projecting through the end plate  138  for providing a direct visual indication of the position of the control piston  92 ′. Therefore, the spirit and scope of the appended claims should not necessarily be limited to the description of the preferred versions contained herein.