Abstract:
A gear-based airboat transmission is provided for driving a pair of coaxial, counter-rotating propellers. The transmission includes a fore output gear rotatably affixable within a housing and affixable to the inner shaft for rotating the outer propeller. An aft output gear is rotatably affixable within the housing and is affixable to the outer shaft coaxial with the inner shaft for rotating the inner propeller opposite the outer. The aft output gear is generally coaxial with the fore output gear. An intermediate gear shaft has a fore end and an aft end, and both ends are opposedly and rotatable affixable within the housing. This bracing confers additional stability to the transmission, conferring longer life and decreased vibration. An intermediate gear is mounted thereon, and is positioned in driving relation to the fore output gear. A drive gear is rotatably affixable within the housing and is affixed for corotation with a drive shaft. The drive gear has a fore portion that is positioned in driving relation to the intermediate gear and an aft portion that is positioned in driving relation to the aft output gear. Improved stability characteristics are imparted by supporting the drive shaft at two points within the case and also by positioning the drive and the output shafts in vertical alignment. The adaptability of the gear-based transmission to being coupled with an automobile engine confers improved noise and efficiency characteristics.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a CIP of application Ser. No. 09/133,583, “Airboat Transmission, Lubrication System, and Associated Method,” filed Aug. 13, 1998, now U.S. Pat. No. 6,053,782. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to airboat propulsion mechanisms and, more particularly, to gear-based transmissions for airboats. 
     2. Description of Related Art 
     Airboats are often driven over land and water at high speeds. Airboats typically have employed aircraft engines operating at approximately 2500-3000 revolutions per minute (rpm) connected to solid direct-drive shafts, which rotate a single propeller. Aircraft engines are extremely expensive, and it is a general practice therefore to mount a used aircraft engine to an airboat to save on costs. 
     The steering apparatus of an airboat usually comprises a pair of rudders, with trim tabs added to correct for the torque that results from the rotation of the propeller, this torque tending to keep the boat from maintaining a level attitude. 
     Extreme gyroscopic forces can occur when airboats are turned rapidly, and these forces are borne, among other structures, by the driven shaft. 
     Previously known airboat systems utilize belt-driven transmissions, which are inefficient owing to power losses caused by belt friction, especially at higher rotational velocities. Belt breakage in these systems is a source of failure. Another disadvantage of belt-driven systems is their inability to permit reduction ofpropeller speed, since the driven shaft used to effect such a reduction would have to be too small to be practicable. Thus it would be advantageous to utilize a different transmission method in an airboat to enable engine speed reduction without loss of efficiency. 
     Propeller breakage is also a major source of failure, since at 3000 rpm extremely high forces are experienced at the propeller hub. It would therefore be desirable to reduce the load on the propeller. 
     It has been taught by Becker et al. (U.S. Pat. No. 4,932,280, dated Jun. 12, 1990) to use coaxial drive shaft systems for driving multiple outputs from a single input in an aircraft. Gearing means are disclosed for driving two outputs at different speeds. 
     The use of a gear-based transmission for airboats has been taught by Kaye (U.S. Pat. No. 5,807,149), including a transmission for driving a pair of counter-rotating coaxial shafts, to each of which is affixed a propeller. Such an arrangement can be used with an automobile engine, which is far more economical than an aircraft engine. This transmission has been shown to reduce noise and torque, to permit varying gear ratios, to increase fuel efficiency and engine life, and to be less expensive to operate. 
     An improved gear-based transmission for airboats has also been disclosed by Jordan (U.S. Pat. No. 5,724,867, the entire disclosure of which is incorporated herein by reference). 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide an airboat transmission that has improved strength, efficiency, noise, and stability characteristics for driving a pair of counter-rotating propellers. 
     It is a further object to provide such an airboat transmission having a minimum number of gears for driving two coaxial counter-rotating shafts. 
     It is another object to provide such an airboat transmission having a compact configuration to optimize space utilization. 
     It is an additional object to provide such an airboat transmission having reduced weight. 
     These and other objects are achieved by the airboat transmission of the present invention, which is for driving a pair of coaxial, counter-rotating propellers. 
     When the transmission is in use on an airboat, a drive shaft is mated at one end to a motor crank extending from and rotated by an engine. The opposite end of the drive shaft extends into the transmission from the fore side. As mentioned above, previously known airboats utilized aircraft-type engines; however, with the transmission of the present invention, it has been found that an automobile engine can be used. This has a benefit of reducing cost, as well as other benefits to be discussed below. 
     An inner shaft also extends into the interior space of the housing, typically from the aft side. The inner shaft is for rotating an outer propeller, that is, the propeller farther from the airboat body. 
     A hollow outer shaft likewise extends into the interior space of the housing and is further positioned in surrounding, generally coaxial arrangement to the inner shaft. The outer shaft is shorter than the first, and both ends protrude beyond the ends of the inner shaft. This outer shaft is for rotating an inner propeller, that is, the propeller closer to the airboat body. 
     The transmission of the present invention comprises a fore output gear rotatably affixable within a housing and affixable to the inner shaft for rotating the outer propeller in a first direction. An aft output gear is rotatably affixable within the housing and is affixable to the outer shaft coaxial with the inner shaft for rotating the inner propeller in a second direction opposite the first direction. The aft output gear is generally coaxial with the fore output gear. 
     An intermediate gear shaft has a fore end and an aft end, and both ends are opposedly affixable for rotation within the housing. This bracing on both ends confers additional stability to the transmission, conferring longer life and decreased vibration. An intermediate gear is mounted on the intermediate gear shaft, and is positioned so that the intermediate gear is in driving relation to only one of the fore or the aft output gear. 
     A drive gear is rotatably affixable within the housing and is affixed for corotation with the drive shaft. The drive gear has a first portion that is positioned in driving relation to the intermediate gear and a second portion that is positioned in driving relation to the other of the fore or the aft output gear, whichever is not being driven by the intermediate gear. 
     In a particular embodiment, the drive gear further has a central portion that has a diameter smaller than a diameter of the fore and the aft portions, and the central portion is positioned axially between the fore and the aft portions and further is radially coplanar with and in spaced relation to the output gear that is being driven by the intermediate gear. 
     Also in this particular embodiment, the intermediate gear is positioned axially in spaced relation from the output gear that is not being driven thereby and the drive gear by which it is not being driven. 
     The rotation of the drive shaft in one direction achieves, owing to the interposition of the intermediate gear between the drive gear and the output gear it is driving, a counter-rotation of the two output shafts and thus imparts counter-rotation to propellers attached thereto. 
     Using a gear-driven transmission permits driving an automobile engine at the point of maximum horsepower, which generally implies a motor crank rotational speed approximately in the range of 5000-5200 rpm, and then gearing down the rotational speed to roughly 1000-1800 rpm, a significantly quieter speed at which to run the propellers. In addition, the use of a gear-based transmission permits driving counter-rotating propellers a different speeds if desired. 
     The invention is not, of course, limited to the use of an automobile engine; in fact, the use of gears enables the user to optimize for efficiency and noise characteristics by altering gear ratios as desired. 
     The features that characterize the invention, both as to organization and method of operation, together with further objects and advantages thereof, will be better understood from the following description used in conjunction with the accompanying drawing. It is to beexpressly understood that the drawing is for the purpose of illustration and description and is not intended as a definition of the limits of the invention. These and other objects attained, and advantages offered, by the present invention will become more fully apparent as the description that now follows is read in conjunction with the accompanying drawing. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates a side cross-sectional view of a first embodiment of the airboat transmission. 
     FIG. 2 illustrates an axial cross-sectional view the embodiment of FIG. 1, taken through line  2 - 2 ′. 
     FIG. 3 illustrates an axial cross-sectional view the embodiment of FIG. 1, taken through line  3 - 3 ′. 
     FIG. 4 illustrates an axial cross-sectional view the embodiment of FIG. 1, taken through line  4 - 4 ′. 
     FIG. 5 illustrates a side cross-sectional view of a second embodiment of the airboat transmission. 
     FIG. 6 illustrates a side cross-sectional view of the lubrication portion of the airboat propulsion system. 
     FIG. 7 llustrates a side cross-sectional view of a third embodiment of the airboat transmission. 
     FIG. 8 illustrates a side cross-sectional view of a fourth embodiment of the airboat transmission. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A description of the preferred embodiments of the present invention will now be presented with reference to FIGS. 1-8. 
     The four embodiments of the airboat transmission  10 ,  10 ′,  10   a , and  10   a ′ of the present invention to be discussed herein are shown from the side in FIGS. 1,  5 ,  7 , and  8  and are designed to drive a pair of coaxial, counter-rotating propellers  20  and  30 . The transmission comprises a housing  50  that has an interior space  502 , a fore side  504 , and an opposed aft side  506 . The fore side  504  is affixable to the engine&#39;s bell housing, or may be an integral part thereof. The aft side  506  has an opening  505  for admitting propeller shafts  22 , 32 ; the fore side  504  has an opening  503  for admitting a drive shaft  12 . It is preferred that the housing&#39;s exterior be aerodynamically shaped in order to confer good airflow characteristics to the transmission  10  and to the propellers  20 , 30  during use at high speeds. 
     The drive shaft  12  extends into the interior space  502  of the housing  50  through the fore side  504 . The drive shaft  12  is rotatable in a first direction. The drive shaft  12  is preferably configured as a “through” shaft with respect to the housing  50 , and is thus rotatably supportable via bearings and brackets  508 , 509  affixed on the inside of both the fore  504  and the aft  506  sides, respectively, of the housing&#39;s interior space  502 . This dual support of all gears and shafts confers exceptional stability to the systems  10 ,  10 ′,  10   a ,  10   a′.    
     The outer propeller  20  is mounted via propeller mount  202  to the aft portion  224  of, and is rotated by, an inner shaft  22 , which in a preferred embodiment is hollow, that extends from the aft side  506  into the interior space  502  of the housing  50 . The fore end  222  of the inner shaft  22  is rotatably supported via a bearing and bracketing  510  on the inside of the housing&#39;s fore side  504 . 
     The inner propeller  30  is mounted via propeller mount  302  to the distal portion  324  of, and is rotated by, a hollow outer shaft  32  that extends from the aft side  506  into the interior space  502  of the housing  50 . The outer shaft  32  is shorter than and is positioned in surrounding, generally coaxial arrangement to the inner shaft  22 . These relative lengths permit the fore end  222  and the aft portion  224  of the inner shaft  22  to protrude, respectively, beyond the fore end  322  and the aft portion  324  of the outer shaft  32 . The outer shaft  32  is supported on the interior of the housing&#39;s aft side  506  by two bearings and bracketing  510 , 511 . 
     In apreferred embodiment  10 , as shown in FIGS. 2-4, the longitudinal axes of the drive shaft  12  and the inner  22  and outer  32  shafts are positioned generally in vertical alignment when the transmission  10  is substantially level. This positioning confers improved stability during use, as the gyroscopic forces balance in this configuration, which reduces torque and improves performance. 
     The airboat transmission of the present invention further comprises a gear system within the housing  50  for driving the shafts  22 , 32 . All the embodiments  10 ,  10 ′,  10   a ,  10   a ′ shown in FIGS. 1-5,  7 , and  8  contain at least four gears: a fore output gear, an aft output gear, an intermediate gear, and a drive gear. The configurations and shapes of these gears have been optimized for minimum volume and maximum stability and are believed to represent a considerable improvement in efficiency and wear characteristics over airboat transmissions previously known in the art. 
     Three common elements of the four embodiments are: a fore output gear  40 , an aft output gear  41 , and an intermediate gear shaft  42 . The fore output gear  40  is affixed to the inner shaft  22  adjacent the fore end  222  for imparting rotational motion thereto. The aft output gear  41  is affixed to the outer shaft  32  coaxially with the inner shaft  22  adjacent the fore end  322  for imparting rotational motion thereto. The aft output gear  41  is generally coaxial with the fore output gear  40 . 
     The intermediate gear shaft  42  has a fore end  422  and an aft end  424 , which are opposedly affixed for rotation, respectively, to the inside of the fore  504  and aft  506  sides of the housing  50  via bearings and bracketing  420 , 421 . The “trough” nature ofthis shaft  42  confers exceptional stability to the transmission, which has not been achieved with previously known designs. 
     Airboat Transmission First Embodiment 
     In a first embodiment of the airboat transmission of the present invention, illustrated in FIGS. 1-4, the drive gear  43 , which is mounted for rotation upon the drive shaft  12  within the housing interior space  502 , comprises three sections: a fore drive gear  432 , a central generally tubular portion  433  aft of the fore drive gear  432 , and an aft drive gear  434  aft of the central portion  433 . The fore  432  and the aft  434  drive gears each has a diameter larger than that of the central portion  433 . The aft drive gear  434  is dimensioned and positioned, by being axially and radially adjacent, for driving the aft output gear  41  in an opposite sense from the incoming rotational direction. The fore drive gear  432  is axially positioned in spaced relation to, and thus is not in position to drive, the fore output gear  40 . The central portion  433  is radially coplanar with and in spaced relation to the fore output gear  40 , and thus can be seen to serve as a “spacer” between the drive gear sections  432 , 434 . 
     An intermediate gear  44 , which is mounted for rotation on the intermediate gear shaft  42 , is dimensioned and positioned for being driven by the fore drive gear  432  and for driving the fore output gear  40 , thus preserving the rotational direction of the former to the latter. In this embodiment the intermediate gear  44  comprises a fore gear section  442  having a diameter and a width sufficiently large to engage the fore drive gear  432  and an aft gear section  444  having a diameter and a width sufficiently large to engage the fore output gear  40 . The intermediate gear  44  further comprises a generally tubular aft portion  446  having a diameter smaller than that of the fore gear section  442 . The aft portion  446  is positioned radially coplanar with the drive gear central portion  433  and in spaced relation therefrom. The aft portion  446  is further positioned axially in spaced relation from the aft output gear  41  and aft drive gear  434  and thus is in driving relation to neither. 
     The intermediate gear  44  design and positioning permits the improved compactness of the present system  10 , since it obviates the need for additional planetary gears as disclosed in previous gear-based transmissions known in the art. 
     Additionally, it is known that, when gears are in use, there is a force component tending to drive the gears apart, causing a portion of this component to be experienced by the gear support shaft. The intermediate gear  44  of the present invention, since it is interacting with two other gears, experience a net force component from those two other gears, which makes the ability to mount the intermediate gear  44  on a through shaft  42  even more important. Similarly, the drive gear  43  and the output gears  40 , 41  also experience exceptional stability owing to their being mounted on through shafts supported at both ends. 
     Airboat Transmission Second Embodiment 
     In a second embodiment of the airboat transmission of the present invention, illustrated in FIG. 6, the drive gear  43 ′ is mounted for rotation upon the drive shaft  12  within the housing interior space  502 . As compared with the drive gear  43  of the first embodiment, drive gear  43 ′ is wider so that it can drive both the aft output gear  41  and the intermediate gear  44 ′. Its aft portion  434 ′ is dimensioned and positioned, by being axially and radially adjacent, for driving the aft output gear  41  in an opposite sense from the incoming rotational direction. Its fore portion  432 ′ is axially positioned in spaced relation to, and thus is not in position to drive, the fore output gear  40 . The drive gear  43 ′ obviously has a diameter larger than that of the drive shaft  12 , on which it is mounted. A portion of the drive shaft  12  fore of the drive gear  43 ′ is radially coplanar with and in spaced relation to the fore output gear  40 , a nd thus is not in driving relation thereto. 
     The intermediate gear  44 ′, which is mounted on the intermediate gear shaft  42 , is dimensioned and positioned for being driven by the fore portion  432 ′ of the drive gear  43 ′ and for driving the output gear  40 , thus preserving the rotational direction of the former to the latter. In this embodiment the intermediate gear  44 ′ comprises an aft gear section  444 ′ having a diameter and a width sufficiently large to engage the fore portion  432 ′ of the drive gear  43 ′ and a fore gear section  442 ′ having a diameter and width sufficiently large to engage the fore output gear  40 . The intermediate gear  44 ′ further comprises a generally tubular aft portion  446 ′ having a diameter smaller than that of the fore gear section  442 ′. The aft portion  446 ′ is positioned radially coplanar with the aft output gear  41  and in spaced relation therefrom. 
     Airboat Transmission Third Embodiment 
     In a third embodiment of the airboat transmission of the present invention, illustrated in FIG. 7, the drive gear  43   a , which is mounted for rotation up on the drive shaft  12  within the housing interior space  502 , comprises three sections: a fore drive gear  432   a , a central generally tubular portion  433   a  aft of the fore drive gear  432   a , and an aft drive gear  434   a  aft of the central portion  433   a . The fore  432   a  and the aft  434   a  drive gears each has a diameter larger than that of the central portion  433   a . The fore drive gear  432   a  is dimensioned and positioned, by being axially and radially adjacent, for driving the fore output gear  40  in an opposite sense from the incoming rotational direction. The aft drive gear  434   a  is axially positioned in spaced relation to, and thus is not in position to drive, the aft output gear  41 . The central portion  433   a  is radially coplanar with and in spaced relation to the aft output gear  41 , and thus can be seen to serve as a “spacer” between the drive gear sections  432   a ,  434   a.    
     An intermediate gear  44   a , which is mounted for rotation on the intermediate gear shaft  42 , is dimensioned and positioned for being driven by the aft drive gear  434   a  and for driving the aft output gear  41 , thus preserving the rotational direction of the former to the latter. In this embodiment the intermediate gear  44   a  comprises an aft gear section  442   a  having a diameter and a width sufficiently large to engage the aft drive gear  434   a  and a fore gear section  444   a  having a diameter and a width sufficiently large to engage the aft output gear  41 . The intermediate gear  44   a  further comprises a generally tubular aft portion  446   a  having a diameter smaller than that of the aft gear section  442   a.    
     The intermediate gear  44   a  design and positioning permits the improved compactness of the present system  10   a , since it obviates the need for additional planetary gears as disclosed in previous gear-based transmissions known in the art. 
     Additionally, as above, the intermediate gear  44   a  of the present invention, since it is interacting with two other gears, experiences a net force component from those two other gears, which makes the ability to mount the intermediate gear  44   a  on a through shaft  42  even more important. Similarly, the drive gear  43   a  and the output gears  40 , 41  also experience exceptional stability owing to their being mounted on through shafts supported at both ends. 
     Airboat Transmission Fourth Embodiment 
     In a fourth embodiment of the airboat transmission of the present invention, illustrated in FIG. 8, the drive gear  43   a ′ is mounted for rotation upon the drive shaft  12  within the housing interior space  502 . As compared with the drive gear  43   a  of the third embodiment, drive gear  43   a ′ is wider so that it can drive both the fore output gear  40  and the intermediate gear  44   a ′. Its fore portion  432   a ′ is dimensioned and positioned, by being axially and radially adjacent, for driving the fore output gear  40  in an opposite sense from the incoming rotational direction. Its aft portion  434   a ′ is axially positioned in spaced relation to, and thus is not in position to drive, the aft output gear  41 . The drive gear  43   a ′ obviously has a diameter larger than that of the drive shaft  12 , on which it is mounted. A portion of the drive shaft  12  aft of the drive gear  43   a ′ is radially coplanar with and in spaced relation to the aft output gear  41 , and thus is not in driving relation thereto. 
     The intermediate gear  44   a ′, which is mounted on the intermediate gear shaft  42 , is dimensioned and positioned for being driven by the aft portion  434   a ′ of the drive gear  43   a ′ and for driving the aft output gear  41 , thus preserving the rotational direction of the former to the latter. In this embodiment the intermediate gear  44   a ′ comprises a fore gear section  442   a ′ having a diameter and a width sufficiently large to engage the aft portion  434   a ′ of the drive gear  43   a ′ and an aft gear section  444   a ′ having a diameter and width sufficiently large to engage the aft output gear  41 . The intermediate gear  44   a ′ further comprises a generally tubular aft portion  446   a ′ having a diameter smaller than that of the aft gear section  444   a ′. The aft portion  446   a ′ is positioned radially coplanar with the fore output gear  40  and in spaced relation therefrom. 
     The combination of radial and axial spacings of the above-listed components, which are illustrated in side cross-section in FIGS. 1,  5 ,  7 , and  8  and in axial cross-sections in FIGS. 2-4, permits an optimized, compact arrangement of a minimum number of gears within the housing  50 . Compactness and lower weight translate into improved efficiency in terms of fuel efficiency and better wear characteristics. It has been estimated that an increase of 25-30% in fuel efficiency will be attained, as well as a 50% increase in engine life. Further, there is significantly less noise produced. 
     In either of the above-detailed embodiments or their equivalents it may be seen that the gear system can be adapted to drive the propellers at different speeds, which has been shown to provide improved thrust characteristics and reduced noise. In a particular embodiment the gear ratios vary so that the propeller velocity ratio ranges from 0.85:1 to 1:0.85. In a preferred embodiment the velocity of the outer propeller  20  is greater than that of the inner  30  by a ratio of 1:0.85. This gearing allows for a velocity gain, as the air for the inner propeller  30  is accelerated toward the outer propeller  20 , which makes it advantageous to rotate the outer propeller  20  at a higher speed to “catch” faster-moving air. 
     Airboat Propulsion Lubrication System 
     An additional aspect of the present invention comprises a lubrication system for delivering lubricant to the elements of the propulsion system. A particular element of the lubrication system illustrated in FIG. 6, comprises a lubrication driving gear  61 , which is positioned for being driven by the aft output gear  41  (see FIG.  4 ). The motion of the lubricating output gear  61  drives lubricant from the well  62 , which is positioned adjacent the gear  61 , through tubing  63 , and through a hole  64  in collar  65  into the interior thereof. Collar  65  is believed to represent a novel advance, and is positioned in surrounding relation adjacent the fore end  222  of the inner shaft  22 , between the fore side  504  of the housing  50  and the fore output gear  40 . Collar  65  floats on the inner shaft  22 , which has a plurality of holes  227  beneath the collar  65 . The holes  227  enable lubricant to be delivered under pressure via a substantially stationary element (the collar  65 ) to a rotating body (the inner shaft  22 ). 
     Lubricant proceeds from the holes  227  into the interior of the inner shaft  22  and in a generally aft direction, and then out through holes  228  in the aft portion of the shaft  22  to enter the space between the shafts  22 , 32 , where there are positioned a plurality of floating cylindrical bearings  66 , which maintain the distance between the shafts  22 , 32  and also assist to distribute lubricant. In a preferred embodiment there are three of these bearings  66  positioned in spaced relation from each other along the shafts  22 , 32  (two are shown in FIG.  6 ), and the material comprises brass. 
     The bearings  66  themselves represent a novel lubrication element, being designed to maximize lubricant return in the fore direction. In a preferred embodiment each bearing  66  has a series of generally helical grooves  662  cut in the outer surface, through which the lubricant may return toward the source. 
     It may be appreciated by one skilled in the art that additional embodiments may be contemplated, including variable numbers and sizes of gears, which may be positioned and configured to permit variable relative speeds of the two counter-rotating propellers. 
     In the foregoing description, certain terms have been used for brevity, clarity, and understanding, but no unnecessary limitations are to be implied therefrom beyond the requirements of the prior art, because such words are used for description purposes herein and are intended to be broadly construed. Moreover, the embodiments of the apparatus illustrated and described herein are by way of example, and the scope of the invention is not limited to the exact details of construction. 
     Having now described the invention, the construction, the operation and use of preferred embodiment thereof, and the advantageous new and useful results obtained thereby, the new and useful constructions, and reasonable mechanical equivalents thereof obvious to those skilled in the art, are set forth in the appended claims.