Abstract:
A hybrid power device for a marine vehicle is provided that has two hybrid prime movers, an electric motor and a combustion engine, that distribute power, for example torque, to a single or dual propulsor, such as surface drives with propellers. The prime movers can apply power singly or in unison, but maintain substantially optimum propulsive efficiency in all cases. The power outputs of the prime movers are in communication with a power transmitting device such as a gear box that may combine the power outputs to drive a single propulsor, or may have a power-splitting embodiment driving dual propulsors. In addition, multiple hybrid power devices may be deployed in other embodiments.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to marine powertrains and more specifically to a device for transmitting power from hybrid prime movers to one or more propulsion devices on a marine vehicle. 
     2. Discussion of the Related Art 
     In light of numerous environmental concerns, hybrid electric-combustion vehicles that can be powered with electrical power instead of relying solely on internal combustion engines are being used to reduce pollution, primarily in the form of reduced exhaust emissions and noise, and to improve overall fuel efficiency. As a result, such hybrid vehicles are becoming increasingly popular. To date, the most prevalent commercialized examples of this trend are found in the automobile industry. 
     Some efforts have been made to utilize electric power and hybrid drive technologies in marine vehicles. However, the most prevalent marine examples have been implemented in custom hybrid electric-combustion systems only in the largest of marine vessels, but none of these marine vehicles incorporate a power device that allows for controlled application of either or both power sources, the electric motor and the combustion engine, while not significantly impacting propulsive efficiency. 
     Because the current marine powertrains fail to provide a solution to the problems of noise, air pollution, low fuel efficiency, and reliability, a green solution was desired that would create less environmental pollution in the form of decreased noise and exhaust emission, and realize the advantages of a secondary prime mover in terms of speed and stealth while not sacrificing improved propulsive and fuel efficiency. What is needed then is a hybrid power device for a marine vehicle that is flexible and efficient—one that allows the user to rely solely on an electric motor in certain circumstances, solely on a combustion engine in other circumstances, or on both prime movers in other circumstances, while not impacting speed and propulsive efficiency. 
     SUMMARY OF THE INVENTION 
     The present invention provides a device for transmitting power from two hybrid power sources to at least one or more propulsion devices such surface propeller drives, conventional propeller installations, water jets, outdrives, pod drives, and the like. 
     According to one aspect of the invention, a propulsion system for a marine vehicle includes an electric motor having a first power transmitting element in communication with a power transmitting device whereby a first torque is transmitted to a first input of the power transmitting device. In addition, the system includes a combustion engine having a second power transmitting element in communication with the power transmitting device whereby a second torque is transmitted to a second input of the power transmitting device. At least one propulsor having a power input element is also provided, wherein a torque applied to the power input element generates a propulsive force to move the marine craft. The power transmitting device further includes at least one output in communication with the at least one propulsor&#39;s power input element, and a power transmitting assembly configured such that 1) when the first torque or the second torque is applied at any given time, there is a substantially corresponding torque of the output in communication with the at least one propulsor&#39;s power input element; and 2) when the first torque applied to the first input and the second torque applied to the second input is at substantially the same revolutions per minute, a substantially corresponding revolutions per minute of the output in communication with the at least one propulsor&#39;s power input element occurs. 
     In another aspect of this embodiment, a propulsor thruster configured for optimum efficiency when running at full power with both the first torque applied to the first input and the second torque applied to the second input at substantially the same revolutions per minute. 
     According to another aspect of this embodiment, an RPM equalizing device configured to control the power transmission from the first power transmitting element of the electric motor and from the second power transmitting element of the combustion engine such that the first input and second input are rotated at substantially the same revolutions per minute. 
     In a further aspect of this embodiment, an automatic mode selection element is configured to control the application of power from the electric motor and the combustion engine such that either power source can be used independently or in combination. 
     In yet another aspect of this embodiment, the electric motor is configured to be run over a range of power outputs that includes a power output that causes a revolutions per minute of the first input that is substantially the same revolutions per minute of the second input whereby the electric motor may be a booster. 
     According to another aspect of this embodiment, the power transmitting device is a gear box further including a gear box housing fixed with respect to a transom of a marine vehicle. The gear box also includes a power transmitting assembly including a gear train mounted within the gear box housing, the gear train accepting power from the first input and the second input and substantially halving the power into the two power transmitting device. 
     According to another embodiment, a method of propelling a marine vessel includes operating prime movers, wherein one prime mover is an electric motor and one prime mover is a combustion engine. The method further includes accepting power created by either or both prime movers into a gear train housed in a gear box, and outputting the power as either one or two power components. Next, the method includes accepting the one or two power components into corresponding one or two clutch assemblies, and selectively transmitting the one or two power components through the clutch assemblies and to corresponding one or two propulsors operably connected thereto thereby propelling a marine vehicle. 
     Being able to use an electric motor as a sole prime mover allows boats and other marine vehicles to reduce pollution from exhaust emissions and to reduce noise when at or near marinas, or other mooring locations, as may be required by waterway regulations. 
     It is further noted that in various jurisdictions, anti-idling rules and regulations are being proposed and implemented for boats and other watercraft. Some jurisdictions are proposing and implementing rules and regulations that prohibit the use of internal combustion engines, or establish maximum horsepower ratings for internal combustion engines, for certain portions of the waterways in these jurisdictions. 
     In addition marine vehicles, especially those involved in military maneuvers, may need to run as quietly as possible to avoid detection. Being able to power the marine vehicle quietly with just an electric motor in a so-called stealth mode may help avoid detection by enemy forces, thus saving lives and equipment. 
     Marine vehicles may also be required to operate at lower speeds to avoid generating wakes when traversing a no-wake designated portion of a waterway. Importantly, electric motors are not only quieter and cleaner than combustion engines, but they are more fuel efficient than combustion engines at lower speeds. 
     Alternatively, being able to use the combustion engine as the sole prime mover allows for the usage of an alternative fuel source and alternative prime mover in the event of the failure of the electric motor or the loss of electric power such as when the batteries are discharged. This redundancy allows the vehicle to be more reliable, and to continue its voyage even after the loss of either prime mover or its associated fuel. In addition, the combustion engine may recharge the discharged batteries by generating electricity when turning the electric motor. 
     Finally, being able to use both prime movers, the combustion engine and the electric motor, can allow for increased maximum speed, which is especially important for pursuit or evasion for military marine craft, government agency marine vehicles, and the like. In addition, using both prime movers may allow a planing marine craft to overcome a high resistance hump in achieving planing condition, which neither the electric motor nor the combustion engine on its own can overcome. 
     These, and other aspects and objects of the present invention will be better appreciated and understood when considered in conjunction with the following description and the accompanying drawings. It should be understood, however, that the following description, while indicating preferred embodiments of the present invention, is given by way of illustration and not of limitation. Many changes and modifications may be made within the scope of the present invention without departing from the spirit thereof, and the invention includes all such modifications. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Preferred exemplary embodiments of the invention are illustrated in the accompanying drawings in which like reference numerals represent like parts throughout, and in which: 
         FIG. 1  is a side elevation view with a cutaway section of the aft portion of a marine vehicle showing a hybrid power device in accordance with a preferred embodiment; 
         FIG. 2  is a top elevation view with a cutaway section of the aft portion of a marine vehicle showing a hybrid power device in accordance with a preferred embodiment including two propulsors (i.e., surface drives); 
         FIG. 3  is an isometric schematic representation of a hybrid power device in accordance with a preferred embodiment including two propulsors (i.e., propellers); 
         FIG. 3A  is an isometric schematic representation of a hybrid power device in accordance with a preferred embodiment including two propulsors (i.e., propellers) and a control system; 
         FIG. 4  is a schematic representation of a gear train of the gearbox of the hybrid power device of  FIGS. 2 and 3 ; 
         FIG. 5  is a schematic representation of a gear train of the gearbox of the hybrid power device of  FIG. 1 ; 
         FIG. 6  is a rear elevation, schematic representation of a marine vessel incorporating two hybrid power devices with each gearbox driving two propulsors (i.e., pairs of counter-rotating propellers); 
         FIG. 7  is a family of plots showing RPM versus propeller blade diameter for several values of motor horsepower as a diameter-horsepower-RPM chart; 
         FIG. 8  is a family of plots showing marine craft speed (knots) versus RPM for several values of propeller diameter size as a pitch-RPM-speed chart; 
         FIG. 9  is a variant of the chart shown in  FIG. 7 ; and 
         FIG. 10  is a variant of the chart shown in  FIG. 8 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     With reference now to the drawings, and particularly to  FIG. 1 , there is shown a cutaway side view of an aft portion of a marine vessel  12  that has a transom  14  and includes the hybrid power device  10  of a preferred embodiment having a single power output  30 . 
     The hybrid power device  10  utilizes two prime movers, an electric motor  16  and a combustion engine  18  that may be a diesel or gasoline powered engine. A transmission  20  is operably connected to prime mover  18 , behind or downstream of prime mover  18 . Transmission  20  is preferably an MGX-series transmission (QuickShift® transmission) or an MG-series transmission, available from Twin Disc, Inc. headquartered in Racine, Wis. and in one preferred embodiment is a 2-speed transmission. 
     Prime movers combustion engine  18  and transmission  20  are connected to a gearbox  22 , for example by way of a transmission output shaft  24 . In addition, prime mover electric motor  16  is connected to gearbox  22 , for example by way of a transmission output shaft  26 . Power transmitting device, for example gear box  22  converts power that is delivered from prime movers  16 ,  18  into one or more power components. In this illustrative embodiment there is a single power component output via power output shaft  30  for drive assembly  28 . The drive assembly  28  is preferably a marine surface drive, for example an ARNESON™ surface drive available from Twin Disc, Inc., noting that other drives, including submerged-type drives, water jets, and so forth are also contemplated and well within the scope of the invention. 
     Referring now to  FIG. 2 , there is shown cutaway top view of an aft portion of a marine vessel  12  that has a transom  14  and includes another embodiment of a hybrid power device  40  of the current invention having two power outputs  44 ,  48 . 
     The hybrid power device  40  utilizes two prime movers, an electric motor  16  and a combustion engine  18  that may be a diesel, a turbine, or gasoline powered engine. A transmission  20  is operably connected to the prime mover  18 , behind or downstream of the prime mover  18 . Transmission  20  is preferably an MGX-series transmission (QuickShift® transmission) or an MG-series transmission, available from Twin Disc, Inc. headquartered in Racine, Wis. and in one preferred embodiment is a 2-speed transmission. 
     Prime mover combustion engine  18  is connected to transmission  20  via output  25 , and transmission  20  is connected to a power splitting gearbox  42 , for example by way of a transmission output shaft  24 . In addition, prime mover electric motor  16  is connected to a power splitting gearbox  42 , for example by way of an output shaft  26 . Power splitting gearbox  42  converts power that is delivered from either or both prime movers  16 ,  18  into two output power components via power output shaft  44  for drive assembly  46  and power output shaft  48  for drive assembly  50 . The drive assemblies  46 ,  50  are preferably marine surface drives, for example ARNESON™ surface drives available from Twin Disc, Inc., noting that other drives, including submerged-type drives, water jets, and so forth are also contemplated and well within the scope of the invention. 
     Referring now to  FIGS. 2 and 3 , hybrid power device  40  with power splitting gearbox  42  provides an interface between the marine vessel  12  and the drive assemblies  46 ,  50  while inputting power from the prime movers  16 ,  18  and dividing and distributing the power (or components thereof) to the pair of drive assemblies  46 ,  50 . In this regard, hybrid power device  40  allows a marine vessel  12  with a pair of prime movers  16 ,  18  to utilize a pair of counter-rotating propellers  52 ,  54  ( FIG. 3 ). 
     Still referring to  FIGS. 2 and 3 , in one embodiment, prime mover  16  may be a two-hundred fifty horsepower electric motor, and prime mover  18  may be a diesel combustion engine with substantially the same horsepower, and the power transmitting device  42  may be a 1-to-1 ratio power splitting gear box such as that disclosed in U.S. Ser. No. 12/478,329, filed on Jun. 4, 2009, and expressly incorporated by reference herein, although other ratios are contemplated, for example, 2-to-1 to accommodate a combustion engine with a high RPM. The electric motor  16  may have its own gear box  45  for reducing the RPMs of its output shaft  26  (as shown in  FIG. 3 ), for example by a 2-to-1 or a 3-to-1 ratio, or it may connect directly to the gear box  42  (as shown in  FIG. 2 ). As well, the combustion engine  18  may have its own gear box  20 . 
     Either or both prime movers  16 ,  18  can be activated to turn corresponding outputs  26 ,  24 . With either prime mover  16 ,  18  activated the speed of the marine vehicle may be, for example, thirty-six knots. With both prime movers  16 ,  18  activated the speed of the marine vehicle may be, for example, fifty knots. With both prime movers activated the electric motor  16  may run as a booster to the combustion engine  18  in which case combustion engine gear box  20  may be a 2-speed gearbox, for example, with 1500 RPM maximum output in one speed in order to accommodate the higher RPM of the combustion engine with respect to the electric motor. Also, with both prime movers  16 ,  18  activated, the hybrid power device may be designed so that both prime movers contribute substantially equally to the power input to power transmitting gear box  42 , or a control system may equalize power inputs into the gear box  42  as described below (similarly, for the power input to power transmitting device  22  in the embodiment of  FIG. 1 ). 
     Referring more specifically to  FIG. 3 , either prime mover  16 ,  18  may transmit its power to drives  46 ,  50  by mechanically engaging the power transmission path with a clutch, for example, clutches  51 ,  53  for transmission of torque to drives  46 ,  50  respectively. Alternatively, one or more clutches may control the transmission of power from the prime mover  16 ,  18  into the gear box  42  or the gear box  10  (not shown). In addition, each prime mover  16 ,  18  has a power source: a battery pack for electric motor  16  and a fuel tank for combustion engine  18  (not shown). 
     Turning now to  FIG. 3A , a control system  15 , by monitoring various electric motor  16  and combustion engine  18  signals such as output speed, output power, output load, and the like may provide flexible control of hybrid power device  10 . The electric motor  16  and combustion engine  18  may be controlled either independently or in combination to achieve a desired result, for example, substantially equal power distribution between prime movers  16 ,  18 , optimal power, speed, and/or fuel efficiency. The control system  15  may monitor and/or control the prime movers  16 ,  18  with control interface device ECUs (engine control units)  29 ,  31 . Control and status signals may be transmitted and received by the main control unit  23  via wired connections shown as harnessing  27  (alternatively, via wireless connections not shown). The control system  15  may also have user interface  33  that provides control and status for the user which may be separate or integrated with the main control unit  23 . 
     Continuing with  FIG. 3A , control system  15  may manage the on-the-fly engagement or disengagement of either prime mover  16 ,  18  to optimize the desired result using disconnect devices  19 ,  21 , respectively. Either or both of the prime movers  16 ,  18  may be physically disconnectable from gear box  42  via various means. For example, either or both disconnect devices could be a clutch. Again, control of the power connections to shafts  26 ,  24  may be desirable to reduce drag, while only using one prime mover, or may aid in the control system&#39;s ability to seamlessly manage on-the-fly connections/disconnections of either prime mover. This may allow an emergency “limp-home” mode in the case of a failed prime mover. Also, control system  15  may manage disconnect devices  19 ,  21  to achieve other desired results; for example, to optimize speed or efficiency, allow for manual control in select situations, and so forth. 
     Control system  15  may consist of a microprocessor based ECU capable of monitoring various sensors, directly or indirectly with a bus and then communicating with both prime mover ECUs  29 ,  31  to achieve desired responses, and controlling the disconnect device, are via, for example, associated harnessing  27 . Sensors may include speed, temperature, pressure, etc. as required to obtain data to achieve the desired results. 
     Referring now to  FIGS. 2-4 , power splitting gearbox  42  at least partially contains a gear train  70  or other various components of the power splitting gearbox  42 . The gearbox housing  100  may mechanically attach and provide an interfacing structure between the drive assemblies  46 ,  50  and the transom  14 . This is because the gearbox housing  100  may attach to the transom  14 , and the final drive assemblies  46 ,  50  attach to the gearbox housing  100 . Since gearbox housing  100  connects the final drive assemblies  46 ,  50  to the transom  14 , it also distributes the application of propulsive forces delivered through the final drive assemblies  46 ,  50  as well as the weight of the power splitting gearbox  42  and drive assemblies  46 ,  50  to the transom  14 . 
     Now referring to  FIG. 4 , but also to  FIGS. 2 and 3 , gear train  70  mechanically splits power received through inputs  24 ,  26  for delivery through outputs  44 ,  48  which may drive a drive assembly  46 ,  50 . Gear train  70  includes multiple gears  60  that intermesh with each other and therefore rotate simultaneously. Gears  60  preferably have helically cut teeth and are radially aligned with each other so that every other gear  60  of gear train  70  rotates in the same direction, while gears  60  that are immediately adjacent each other rotate in opposing directions. Since adjacent, radially engaging gears rotate in opposite directions, intuitively, gears  60  that are spaced from each other by two intermediate gears (or a number of gears that is a multiple of two) will rotate in opposing directions. Correspondingly, the gear train  70  can input power into any one of gears  60  in gear train  70  and achieve counter-rotation of outputs  44 ,  48 , by delivering power through gears  60  that are spaced from each other by two intermediate gears  60  (or a number of gears that is a multiple of two). Thus, contra-rotating outputs could be alternatively connected to the center of gears  55 ,  57 , though not preferred. 
     It is contemplated that inputs  24 ,  26  and outputs  44 ,  48  need not be separate and distinct components, apart from gears  60 , but rather can be integrated with individual ones of the gears  60 . For example, input  24  can be a splined inner circumferential surface of one of the gears  60  that receives a splined end of output shaft  44 . Likewise, outputs  44 ,  48  can be splined inner circumferential surfaces of ones of the gears  60  that accept and drive splined ends of output shafts  44 ,  48  connected to drive assemblies  46 ,  50 . 
     Now referring to  FIG. 5 , but also to  FIG. 1 , gear train  70  mechanically transmits power received through inputs  24 ,  26  for delivery through outputs  30  which may drive a drive assembly  28  ( FIG. 1 ). The gear train  70  includes multiple gears  60  that intermesh with each other and therefore rotate simultaneously. Gears  60  preferably have helically cut teeth and are radially aligned with each other so that every other gear  60  of the gear train  70  rotates in the same direction, while gears  60  that are immediately adjacent each other rotate in opposing directions. Although shown with inputs connected to the outermost gears in  FIG. 5 , the gear train  70  can input power into any one of the gears  60  in the gear train  70 . Similarly, gear train  70  may deliver output from the center gear  70  to output shaft  30  as shown, or any of the other gears. 
     It is contemplated that inputs  24 ,  26  and output  30  need not be separate and distinct components, apart from the gears  60 , but rather can be integrated with individual ones of the gears  60 . For example, input  24  can be a splined inner circumferential surface of one of the gears  60  that receives a splined end of input shaft  24 . Likewise, output  30  can be splined inner circumferential surfaces of ones of the gears  60  that accept and drive splined ends of output shafts  30  connected to drive assembly  28 . 
     Turning now to  FIG. 6 , by using a pair of hybrid power devices  40  each having a power splitting gearbox  42 , a marine vessel  12  that has two pairs of prime movers  16 ,  18  and  17 ,  19  can utilize two pairs of counter-rotating propellers  52 ,  54  and  53 ,  55 , whereby four total propellers, including a pair of counter-rotating propellers at each of the starboard and port sides of the transom  14 , are incorporated into the marine vessel  12 . 
     Referring next to  FIGS. 7 and 8  and also to  FIG. 3 , an example demonstrating the dual and single prime mover engagement at two speeds is described showing the correspondence in optimally efficient propulsor geometry. Turning to  FIG. 7  in particular, in an embodiment in which a single prime mover  16  or  18  of about 250 HP is engaged and the output of the prime mover is configured to rotate at substantially 1500 RPM, the optimally efficient propeller diameter can be determined utilizing  FIG. 7  as follows: 1) locate the 1500 RPM point  112  on the vertical axis and the corresponding 1500 RPM horizontal line  114 , 2) locate or approximate the intercept of the horsepower (HP) curve with the 1500 RPM horizontal line  114 , and 3) read the propeller diameter in inches from the corresponding intercept on the horizontal axis  118 . For example, with the given prime mover of 250 HP, there is no corresponding HP curve, so locate the closest curves that are greater than and less than 250 HP. So, using  FIG. 7  locate the 200 HP curve  120  and the 300 HP curve  122 . Follow the curves  120 ,  122  through their intercept points with the 1500 RPM horizontal line  114 , points A and B. Because 250 HP is approximately midway, bisect the line between points A and B and draw a vertical line  124  from that point to intercept the horizontal axis  118  at point C. Finally, read the optimum propeller diameter from the horizontal axis point C, which in this case is approximately 23.75 inches. Thus, either 250 HP prime mover running at 1500 RPM is optimized for that rotational speed with a propeller having a diameter of about twenty-four inches. 
     Referring now more specifically to  FIG. 8 , determining the speed of the marine vehicle can be performed given the propeller pitch, RPM, and slip. For example, with a twenty-four inch pitch propeller 1) select the corresponding twenty-four inch curve  132 , 2) locate the 1500 RPM on the horizontal axis, 3) draw a vertical line  136  through the intercept of the twenty-four inch curve  132  (shown as point F), 4) draw a horizontal line  148  from the intercept F to the vertical axis showing knots, and 5) read the speed of the corresponding craft as thirty knots. As another example, the plot in  FIG. 8  shows that with a thirty-two inch pitch propeller 1) select the corresponding thirty-two inch curve  132 , 2) locate the 1500 RPM on the horizontal axis, 3) draw a vertical line  136  through the intercept of the thirty-two inch curve  132  (shown as point F′), 4) draw a horizontal line  148 ′ from the intercept F′ to the vertical axis showing knots, and 5) read the speed of the corresponding craft as forty knots, which with a 10% slip becomes thirty-six knots. 
     Similarly, when running the vessel with both prime movers  16 ,  18  engaged the power amounts to substantially 500 HP. Referring now to  FIG. 9 , the propulsor blade has a vertical line  156  indicating its diameter of twenty-three and one-hall inches at point H on the horizontal axis  152 . The intercept of the 500 HP curve  154  with the diameter is at point q. Drawing a horizontal line  150  from point q to the vertical axis  142  indicates that the propulsor blade pitch of twenty-three and one-half inches or about twenty-four inches with 500 HP will yield about 1900 RPM. Referring now to  FIG. 10  to compute speed, 1900 RPM is located on the horizontal axis and a vertical line  160  is drawn to intercept with the twenty-four inch pitch curve  162  at point I. Speed is determined by drawing a horizontal line  158  from the intercept to the vertical axis and determining the speed in knots of about 38. The propulsor blade pitch of twenty-four inches provides for 250 hp and 500 hp operation at speeds of about 30 and 38 knots respectively (corresponding to 1500 and 1900 RPM). As another example, the plot in  FIG. 10  shows that a propeller with a thirty-two inch pitch at 1900 RPM, the vertical line  160  intercepts with the thirty-two inch pitch curve  162  at point I′. Speed is determined by drawing a horizontal line  158  from the intercept to the vertical axis and determining the speed in knots of about 50 with 2% slip. 
     Referring again to  FIG. 3 , in another embodiment wherein both prime movers  16 ,  18  contribute substantially equally, the combustion engine  18  and electric motor  16  may be geared to rotate, for example, at 1500 RPM into gear box  42 . Propulsive efficiency to achieve fifty knots for a ten ton marine vehicle would be substantially 0.69 when driving propellers  52 ,  54 , which would require that propellers be sized at about 23.5 inches by thirty-two inches. Importantly, the same propeller sizing is optimally efficient with either prime mover engaged at a full speed of thirty-six knots for example. More importantly, by providing substantially equivalent power from prime movers  16 ,  18  there is a optimum propulsor output sizing (for either propellers or water jets) that is the same for maximum single prime mover speed and maximum dual prime mover speed. 
     Still referring to  FIG. 3 , note that the combustion engine prime mover  18  may have a gearbox having a low and high gear  20 , whereas the electric prime mover  16  may have no gearbox due to its substantially flat power curve. In one embodiment, the combustion engine  18  may be providing an output of about 1500 RPM into gear box  20  having a gear ratio of 1:1.25 that increases the RPMs of the gearbox output  24  to about 1500 RPM. This may yield a speed of thirty-six knots with either and only one of the two prime movers  16 ,  18  engaged. The 23.524×32 inch propeller sizing (i.e., a 23.524 inch diameter blade with a 32 inch pitch) remains optimum with a 0.69 efficiency. 
     The 23.5×32 sizing works out to a 2% slip at fifty knots and 10% slip at thirty-six knots. This 12% slip difference is in keeping with propulsion norms for both surface and submerged propellers, lower speed, higher slip. The fact that the propeller efficiency is 0.69 in both cases comes from proprietary tunnel test data for surface propellers. 
     The power device  10  need not be limited to the embodiments described above, but may include other embodiments. The scope of some of these changes is discussed above. The scope of others will become apparent from the appended claims. 
     Regardless, it is noted that many changes and modifications may be made to the present invention without departing from the spirit thereof. The scope of some of these changes is discussed above. The scope of others will become apparent from the appended statements of invention.