Patent Abstract:
An airboat and drive system for operating a pair of counter rotating air propellers that propel an airboat are described. An engine is mounted low in the hull of an airboat to lower the center of gravity and provide a more stable airboat. The drive system is connected to the engine through a drive shaft. The drive system includes a transmission, interconnecting frame, and counter rotating air propeller drive. The interconnecting frame mounts the counter rotating air propeller drive above the transmission providing proper clearance for the air propellers with the hull of the airboat. The transmission and counter rotating air propeller drive are connected through an interconnecting drive shaft. The amount of noise from a conventional belt drive is reduced. The modular design and simplified drive system is easier to assemble and align.

Full Description:
BACKGROUND OF INVENTION 
   1. Field of the Invention 
   The present invention is directed to a drive system, and more particularly to a drive system for operating a pair of counter rotating air propellers that propel an airboat. 
   2. Background Information 
   Drive systems for propelling airboats are known in the art. For example, U.S. Pat. No. 6,540,570 B1 issued to Eakin on Apr. 1, 2003. This reference illustrates an airboat where the engine is mounted high above the hull. A small compact transmission is driven by the engine through a belt connection located between a drive shaft and a sprocket. A number of gears transmit power from the engine and rotate the air propellers. 
   Other examples of prior art airboat drive systems include:
         (a) U.S. Pat. No. 5,839,926 issued to K-Way on Nov. 24, 1998,   (b) U.S. Pat. No. 6,053,782 issued to Bell on Apr. 25, 2000,   (c) U.S. Pat. No. 6,478,641 issued to Jordan on Nov. 12, 2002,   (d) U.S. Pat. No. 5,724,867 issued to Jordan on Mar. 10, 1998, and   (e) U.S. Pat. No. 6,299,485 issued to Jordan on Oct. 9, 2001.       

   All of these references illustrate an engine mounted high above the hull of an airboat. A single compact transmission is driven by a direct connection to the engine through a drive shaft. A number of gears are provided to operate a pair of output drive shafts. One of the output drive shafts is hollow and surrounds the other solid drive shaft to rotate the air propellers. 
   The prior art airboat drive system references consistently teach mounting an engine high above the hull of an airboat in combination with a single compact transmission. Keeping the engine high and the transmission compact provides the necessary clearance between the tips of the air propellers and the hull of the airboat. 
   However, there are a number of problems with the prior art airboat drive systems. The transmissions are overly complex with many gears. They require multiple long output drive shafts, including a solid output drive shaft located within a hollow output drive shaft. Mounting the engine and drive system high above the hull creates a high center of gravity. The compactness, and horizontal input and output shafts of these transmissions renders the transmission completely unusable in situations when an engine is mounted low in the hull of an airboat. 
   Another example of a prior art drive system is shown in the Husky™ Nattiq™ airboat (www.huskyairboats.com). This airboat has the engine mounted low in the hull of an airboat. The existing drive system is an elongated belt extending from a pulley mounted on an end of a drive shaft of an engine to another pulley mounted on a air propeller drive shaft. However, external belt drives are very noisy, prone to wear, and can become dangerous upon failure of the belt. 
   Other examples of prior art marine drive systems with counter rotating water propellers are also known. These include inboard stern drives and lower units for outboard motors such as:
         (a) U.S. Pat. No. 5,890,938 issued to Brunswick on Apr. 6, 1999,   (b) U.S. Pat. No. 5,529,520 issued to Kaisha on Jun. 25, 1996, and   (c) U.S. Pat. No. 5,558,498 issued to Kaisha on Sep. 24, 1996       

   However, the marine based prior art solutions also suffer from overly complex transmissions, specific hydrodynamic housings for application in a water environment, and multiple drive shafts (again, a solid drive shaft inside a hollow drive shaft). Assembly of these units is difficult and time consuming. This area of prior art is simply not adaptable to airboats for operating a pair of air propellers. 
   Therefore, there is a need for a drive system for counter rotating a pair of air propellers when an engine is mounted low in the hull of an airboat. 
   SUMMARY OF INVENTION 
   The present invention has many advantages. Mounting the engine low in the hull of the airboat provides a more stable airboat with a lower center of gravity. The drive system elevates the propellers to provide the proper clearance between the tips of the air propellers and the hull of the airboat. The amount of noise from a conventional belt drive is reduced. The modular design and simplified drive is easier to assemble and align. The second hollow drive shaft and the complexities associated with it are eliminated. 
   In one broad aspect of the present invention, a counter rotating air propeller drive system is provided. The counter rotating air propeller drive system includes a housing, a first drive, a second drive, and a third drive. The housing supporting the first drive in engaging relationship with the second drive. The housing also supporting the first drive in engaging relationship with the third drive. The third drive including an air propeller mount for receiving a first air propeller. The second drive coupled to an air propeller output drive shaft for receiving a second air propeller. In operation, the first drive operates the second drive in a first direction and the first drive operates the third drive in an opposite direction for counter rotating the air propeller mount and the air propeller output drive shaft. 
   In another broad aspect of the present invention, an airboat is provided. The airboat includes a hull, an engine disposed in the hull, an air rudder, and a counter rotating air propeller drive system. The counter rotating air propeller drive system includes a housing, a first drive, a second drive, and a third drive. The housing supporting the first drive in engaging relationship with the second drive. The housing also supporting the first drive in engaging relationship with the third drive. The third drive including an air propeller mount for receiving a first air propeller. The second drive coupled to an air propeller output drive shaft for receiving a second air propeller. In operation, the first drive operates the second drive in a first direction and the first drive operates the third drive in an opposite direction for counter rotating the air propeller mount and the air propeller output drive shaft. 
   The first drive includes a first housing member retaining the first drive in relationship with the second drive and the third drive. 
   The second drive includes a second housing member retaining the second drive in relationship with the first drive. 
   The third drive includes a third housing member retaining the third drive in relationship with the first drive. 
   The first drive is a first bevel gear, the second drive is a second bevel gear and the third drive is a third bevel gear. Teeth on the first bevel gear engage teeth on the second bevel gear. Teeth on the first bevel gear engage teeth on the third bevel gear for counter rotating the second bevel gear and the third bevel gear. 
   In an embodiment, the third drive includes a mount for receiving a first air propeller hub and the first air propeller is coupled to the first air propeller hub. Alternatively, the first air propeller is coupled directly to the third drive. 
   In an embodiment, the air propeller output drive shaft includes a mount for receiving a second air propeller hub and the second air propeller is coupled to the second air propeller hub. Alternatively, the second air propeller is coupled directly to the third drive. 
   Preferably, teeth on the first bevel gear engage teeth on the second bevel gear and teeth on the first bevel gear engage teeth on the third bevel gear for rotating the air propellers at the same speed. Alternatively, a first region of teeth on the first bevel gear engage teeth on the second bevel gear and a second region of teeth on the first bevel gear engage teeth on the third bevel gear for rotating the air propellers at differential speeds. 
   The drive system further includes a second housing, a fourth drive, a fifth drive, an interconnecting drive shaft, and a frame. The second housing supporting the fourth drive in engaging relationship with the fifth drive. The interconnecting drive shaft engaging the fourth drive and the interconnecting drive shaft engaging the first drive wherein rotating the fourth drive rotates the interconnecting drive shaft for rotating the first drive. 
   The fourth drive includes a fourth housing member retaining the fourth drive in relationship with the fifth drive. 
   The fifth drive includes a fifth housing member retaining the fifth drive in relationship with the fourth drive. 
   The fourth drive includes a fourth bevel gear. The fifth drive includes a fifth bevel gear. Teeth on the fourth bevel gear engage teeth on the fifth bevel gear for rotating in operation the interconnecting drive shaft. 
   The fifth bevel gear includes a mount for receiving an input drive shaft for rotating in operation the fifth drive. 

   
     BRIEF DESCRIPTION OF DRAWINGS 
     Exemplary embodiments of the present invention will now be described with reference to the accompanying drawings, in which: 
       FIG. 1  is a perspective view of an airboat and a counter rotating air propeller drive system, 
       FIG. 2  is a cross sectional side view of a drive system illustrating a transmission, interconnecting frame, and counter rotating air propeller drive, 
       FIG. 3  is a perspective view of the counter rotating air propeller drive, 
       FIG. 4  is a cross sectional side view of the counter rotating air propeller drive illustrating the first drive, second drive and third drive, 
       FIG. 5  is a cross sectional side view of the first drive, 
       FIG. 6  is a cross sectional side view of the second drive, 
       FIG. 7  is a cross sectional side view of the third drive, 
       FIG. 8  is a perspective view of the transmission, 
       FIG. 9  is a cross sectional side view of the transmission, 
       FIG. 10  is an exploded cross sectional side view of the transmission illustrating the fourth drive and fifth drive, 
       FIG. 11  is an end view of the drive system illustrating the transmission, interconnecting frame, and counter rotating air propeller drive, and 
       FIG. 12  is a cross sectional side view of a drive system illustrating an alternative embodiment of the interconnecting drive shaft. 
   

   DETAILED DESCRIPTION 
   The present invention is described in accordance with a preferred embodiment as illustrated with reference to  FIG. 1 . An airboat is generally indicated at  10 . The airboat  10  is capable of operating on water, ice, and snow. The airboat  10  includes a hull  12 . An engine  14  is mounted low in the hull  12  of the airboat  10  to keep the center of gravity low. The engine  14  is connected through a drive shaft (not shown) to a drive system  16  (schematic representation). The drive system  16  includes a transmission, interconnecting frame, and counter rotating air propeller drive (see  FIG. 2 ). A pair of air propellers  18  are connected to the counter rotating drive and in operation, the air propellers rotate in opposite directions to provide the necessary thrust to propel the airboat  10  forward. A pair of air rudders  20  are provided in order to maneuver the airboat  10  in operation by re-directing the flow of air to turn the airboat  10  either left or right. Those skilled in the art will appreciate the airboat  10  could be alternatively equipped with a single air rudder. 
   Referring now to  FIG. 2 , the drive system of the present invention is further described. The engine  14  (only a diagrammatic portion is illustrated) is connected to the transmission  22  through a drive shaft  28 . The transmission  22  is mounted low in the hull  12  of the airboat  10  in alignment with the engine  14 . The counter rotating air propeller drive  26  is disposed above the transmission  22  by the interconnecting frame  24  at a suitable height to provide clearance for the air propellers  18  (not shown) with the hull of the airboat  10 . The transmission  22  and counter rotating air propeller drive  26  are connected through an interconnecting drive shaft  30  to transmit power from the transmission  22  to the counter rotating air propeller drive  26 . The transmission  22  and the counter rotating air propeller drive  26  are retained in operational alignment by the frame  24 . 
   Referring now to  FIG. 12 , an alternative embodiment of the present invention is described with respect to the interconnecting drive shaft  30 . In this embodiment, the interconnecting drive shaft  30  includes a primary drive interconnect  290 , a primary universal joint  282 , a primary drive section  278 , a secondary drive section  286  and secondary drive interconnect  288 . 
   The primary drive interconnect  290  has a long splined shaft that interfaces to the fourth drive in the transmission  22  through the mount  184  (see  FIG. 10 ) and is retained with the fourth drive. A primary universal joint  282  connects the primary drive interconnect  290  to the primary drive section  278  at one end. This provides a first flexible joint between the transmission  22  and the counter rotating air propeller drive  26 . The primary drive section  278  includes a long splined shaft at another end to interface with a secondary drive section  286 . The secondary drive section  286  has a complimentary splined mount for receiving the long splined shaft. The spines cooperate to rotate both members while permitting a vertically sliding joint between the primary drive section  278  and the secondary drive section  278 . 
   The secondary drive section  286  is connected to the secondary drive interconnect  288  by a secondary universal joint  280 . This provides a second flexible joint between the transmission  22  and the counter, rotating air propeller drive  26 . An end of the secondary drive interconnect  288  includes a long splined shaft that interfaces to the mount  94  of the first drive  36  in the counter rotating air propeller drive  26 . 
   The alternative embodiment of the interconnecting drive shaft  30  provides two flexible joints and one vertically sliding joint when mounting the counter rotating air propeller drive  26  at differing vertical and horizontal alignments from the transmission  22  while providing rotation movement of the interconnecting drive shaft  30 . 
   Referring now to  FIG. 3 , the central housing  31  of the counter rotating air propeller drive  26  is described. The central housing  31  includes a bottom member  260 . The bottom member  260  has a mounting surface  264  and a central opening  262 . The central opening  262  and surface  264  receive for mounting the first drive  36  (not shown). A plurality of threaded bores  266  are provided in the bottom member  260  for securing and sealing the first drive  36  to the bottom member  260  by a plurality of fasteners, for example bolts. 
   The central housing  31  also includes a front member  268 . The front member  268  has a mounting surface  272  and a central opening  270 . The central opening  270  and surface  272  receive for mounting the second drive  38  (not shown). A plurality of threaded bores  274  are provided in the front member  268  for securing and sealing the second drive  28  to the front member  268  by a plurality of fasteners, for example bolts. 
   The central housing  31  also includes a back member  276 . The back member is substantially the same as the front member  268  (for example, the size of the central opening may be a different diameter, larger in the preferred embodiment). The back member has a mounting surface (not shown) and central opening (not shown). The third drive  40  is received for mounting by the surface and central opening of the back member  276 . A plurality of threaded bores (not shown) are provided in the back member  276  for securing the third drive  40  to the back member  276  by a plurality of fasteners, for example bolts. 
   The central housing  31  also includes a frame mount member  256 . The frame mount member  256  has a plurality of bores  258 . The frame mount member  256  and bores  258  are for mounting and securing the central housing  31  in the interconnecting frame  24 . 
   Referring now to  FIG. 4 , a cross sectional view of the counter rotating air propeller drive  26  is further described. A first drive, generally indicated at  36  is illustrated mounted in the first opening  262  of the bottom member  260  as previously described. 
   A second drive, generally indicated at  38  is illustrated mounted in the second opening  270  of the front member  268  as previously described. The second drive  38  is connected to an air propeller output drive shaft  42 . Rotation of the second drive  38  causes rotation of the air propeller output drive shaft  42 . A second air propeller hub  68  is mounted on an end of the air propeller output drive shaft  42  for connecting a second air propeller (not shown) to the drive  26 . 
   A third drive, generally indicated at  40 , is illustrated mounted in the third opening of the back member  276  s previously described. In the preferred embodiment, a first air propeller hub  64  is mounted directly to the third drive  40  for connecting a first air propeller (not shown) to the drive  40 . Alternatively, a first air propeller may be mounted to the third drive  40  without an air propeller hub  64 . 
   The central housing  31  supports and retains the first drive  36 , the second drive  38 , and the third drive  40  in operational relationship such that rotation of the first drive  36  rotates the second drive  38  in one direction and the first drive  36  also operates the third drive  40  in an opposite direction for driving the counter rotating air propellers. The second drive  38  and the third drive  40  are retained in axial alignment about a lengthwise horizontal axis by the central housing  31 . This alignment is obtained by a inner surface of the second opening  270  in the front member  272  engaging a complimentary sidewall inner surface of the second housing member  48 , and a surface of the third opening (not shown) in the back member  276  engaging a complimentary sidewall surface of the third housing member  50 . The first drive  36  is retained about a substantially perpendicular vertical axis. A inner surface of the first opening  262  in the bottom member  260  engages a complimentary sidewall surface of the first housing member  46  provides alignment of the first drive. 
   Referring now to  FIG. 5 , the first drive  36  of the counter rotating air, propeller drive  26  is further described. 
   The first drive  36  has a first housing member generally indicated as  46 . The first housing member  46  is a separate member from the central housing  31 . The first housing member  46  is formed by a main body with a central axial opening. A first cylindrical recess  96  is located in one end of the first housing member  46  for receiving a retainer  106 . Optionally, a seal may be provided between the retainer  106  and the cylindrical recess  96 . Persons skilled in the art understand a seal may be provided in other locations to keep a lubricant in the air propeller drive during operation. A second cylindrical recess  98  is formed in the first housing member  46  for receiving a bearing  84 . A third cylindrical recess  104  is formed in the first housing member  46  for receiving a second bearing  82 . A ledge  102  extends outwardly towards the central opening between the recesses ( 98 ,  104 ) separating the second cylindrical recess  98  and the third cylindrical recess  104 . The ledge  102  provides support and a seat for the bearings ( 82 ,  84 ). The first housing member  46  has an outwardly extending flange  100  with a plurality of spaced openings for receiving fasteners. The outwardly extending flange  100  engages a surface  264  (see  FIG. 4 ) for sealing engagement with the housing  31 . A plurality of fasteners  86  secure and seal the first housing member  46  the central housing  31 . Optionally, a seal (“O” ring or gasket) may be provided between the first housing member  46  and the central housing  31 . 
   The first drive  36  also has a first bevel gear  52 . The first bevel gear  52  has a plurality of teeth  54  for engaging teeth of the second bevel gear (not shown) and teeth of the third bevel gear (not shown). One end of the first bevel gear  52  includes a smaller diameter cylindrical threaded portion for receiving a nut  90 . The other end of the first bevel gear  52  includes a surface for cooperating with a retainer  92 . 
   A mount  94  is provided to engage the first bevel gear  52  with the interconnecting drive shaft  30 . The mount  94  includes a toothed spline on one end of the interconnecting drive shaft  30  and a complimentary toothed spline on the inside surface of the central axial opening of the first bevel gear  52 . The central axial opening of the first bevel gear  52  extends the length of the first bevel gear  52 . Those skilled in the art will appreciate the mount  94  is not limited to toothed splines. Alternatively for example, a pair of slots and key could be used in the mount  94 . The mount  94  provides rotation of the first bevel gear  52  with the interconnecting drive shaft  30  in operation. 
   The first bevel gear  52  is secured to the interconnecting drive shaft  30  by the retainer  92  and a shoulder on an end of the interconnecting drive shaft  30 . While the retainer  92  is illustrated as a member with fasteners located on an end of the interconnecting drive shaft  30 , other retainers may be applied. For example, an end of the interconnecting drive shaft  30  may be threaded to receive a nut for securing the first bevel gear  52  to the interconnecting drive shaft  30 . 
   The first bevel gear  52  and the first housing member  46  are assembled to form the first drive  36 . The bearing  82  is placed, or pressed, into the cylindrical recess  104  until it bottoms out and seats on a surface of the flange  102 . The other bearing  84  is placed, or pressed, into the cylindrical recess  98  until it bottoms out and seats on an opposite surface of the flange  102 . A cylindrical shaft of the first bevel gear  52  is inserted into the openings of the bearings ( 82 ,  84 ) until a ledge of the bevel gear  52  contacts a surface of the bearing  82 . This locates the first bevel gear  52  in the central opening of the first housing member  46 . The retainer  106  is placed over threaded cylindrical section on the first bevel gear  52 . The retainer  106  contacts a surface of the bearing  84 . A nut is placed on the threaded cylindrical section and tightened to retain the assembly in the first housing member  36 . A lock washer  88  keeps the nut tight. 
   Referring now to  FIG. 6 , the second drive  38  of the counter rotating air propeller drive  26  is further described. 
   The second drive  38  has a second housing member generally indicated as  48 . The second housing member  48  is separate from the central housing  31 . The second housing member  48  is formed by a main body with a central opening. A first cylindrical recess  114  is located in one end of the second housing member  48  for receiving a retainer  130 . Optionally, a seal may be provided between the retainer  130  and the cylindrical recess  114 . Persons skilled in the art understand a seal may be provided in other locations to keep a lubricant in the air propeller drive during operation. A second cylindrical recess  116  is formed in the second housing member  48  for receiving a bearing  110 . A third cylindrical recess  120  is formed in the second housing member  48  for receiving a second bearing  108 . A ledge  118  extends outwardly towards the central axial opening of the second housing member  48  separating the second cylindrical recess  116  from the third cylindrical recess  120 . Opposite sides of the ledge  118  provide support and a seat for the bearings ( 110 ,  108 ). The second housing member  48  has an outwardly extending flange  112  with a plurality of spaced openings for receiving fasteners. The outwardly extending flange  112  engages a surface  272  (see  FIG. 7 ) for sealing engagement. A plurality of fasteners  86  secure and seal the second housing member  48  and the housing  31 . Optionally, a seal may be provided between the second housing member  48  and the housing  31  such as an “O” ring or gasket. 
   The second drive  38  also has a second bevel gear  56 . The second bevel gear  56  has a plurality of teeth  58  for engaging teeth of the first bevel gear  52  (not shown). One cylindrical end of the second bevel gear  56  includes threads for receiving a nut  126 . 
   A mount  122  is provided to engage the second bevel gear  56  with the air propeller drive shaft  42 . The mount  122  includes a toothed spline on one end of the air propeller drive shaft  42  and a complimentary toothed spline on the inside surface of the central axial opening of the second bevel gear  56 . The central axial opening extends the length of the second bevel gear  56 . Those skilled in the art will appreciate the mount  122  is not limited to toothed splines. Alternatively for example, a key could be used in the mount  122 . The mount  122  provides rotation of the second bevel gear  56  with the air propeller drive shaft  42  in operation. 
   The second bevel gear  56  is secured to the air propeller drive shaft  42  by the retainer  124  (illustrated as a nut and lock washer) and a shoulder on another end of the air propeller drive shaft  42 . 
   The second bevel gear  56  and the second housing member  48  are assembled to form the second drive  38 . The bearing  108  is placed, or pressed, into the cylindrical recess  120  until it bottoms out and seats on a surface of the ledge  118 . The other bearing  110  is placed, or pressed, into the cylindrical recess  116  until it bottoms out and seats on an opposite surface of the ledge  118 . A cylindrical shaft of the second bevel gear  56  is inserted into the central opening of the bearings ( 108 ,  110 ) until a ledge of the second bevel gear  56  contacts a surface of the bearing  108 . This locates the second bevel gear  56  in the central opening of the second housing member  48 . The retainer  130  is placed over a cylindrical threaded section of smaller diameter on the second bevel gear  56 . The retainer  130  contacts a surface of the bearing  110 . A nut is placed on the treaded section and tightened to retain the assembly in the second housing member  48 . A lock washer  128  keeps the nut tight. 
   A bearing  132  is located intermediate on an outside surface of the air propeller drive shaft  42 . The bearing  132  provides support and permits rotational movement between the air propeller drive shaft  42  and the third drive  40  (not shown). 
   The air propeller shaft  42  includes a mount  66  for receiving an air propeller hub  68 . The mount  66  includes a toothed spline on one end of the air propeller shaft  42  and a complimentary toothed spline on an inside surface of a central axial opening in the air propeller hub  68 . The mount  66  provides rotation of the air propeller hub  68  with the air propeller shaft  42 . 
   An end of the air propeller shaft  42  includes threads  144 . The air propeller hub  68  is secured to the air propeller shaft  42  by the nut  142  and a shoulder on the air propeller shaft  42 . A lock washer  140  keeps the nut tight. The air propeller shaft  42  includes an outwardly extending flange  134  and a central hub  138 . An air propeller is centered and mounted over the central hub and secured to the flange  134  by a plurality of fasteners  136 , for example bolts. 
   Referring now to  FIG. 7 , the third drive  40  of the counter rotating air propeller drive  26  is further described. 
   The third drive  40  has a third housing member  50  and a third bevel gear  60 . The third housing member  50  is separate from the central housing  31 . The third bevel gear  60  has teeth  62  for engaging complimentary teeth  54  on the first bevel gear  52  (see  FIG. 5 ). A central axial opening  146  extends lengthwise through the third bevel gear  60 . The air propeller output drive shaft  42  extends through the central axial opening  146  (see  FIG. 4 ). The bearing  132  engages an inner surface of the central opening  146  to support the air propeller output drive shaft  42 . The third bevel gear  60  has a substantially cylindrical section. A first diameter portion receives the bearings ( 156 ,  158 ). A second smaller diameter portion includes the mount  44  for mounting the air propeller hub  64 . Alternatively, an air propeller (not shown) may be mounted directly to the mount  44 . In either embodiment, the air propeller is mounted to the third drive  40  by a drive shaftless connection. A third and smallest diameter portion includes threads for receiving the nut  172 . 
   The third housing member  50  has a cylindrical recess  148 , a ledge  152 , and another cylindrical recess  150 . The cylindrical recess  148  receives the bearing  156  and the other cylindrical recess  150  receives the bearing  158 . The ledge  152 ′ provides separation, support, and a seat for the bearings ( 156 ,  158 ). Optionally, a seal  160  is mounted in the cylindrical recess  148 . Alternatively, a seal may be provided in other locations of the assembly. An outwardly extending flange  154  includes a plurality of spaced openings to receive fasteners  86 . The third drive  40  is mounted in an opening in the back member  276  and secured by the fasteners  86 . Optionally, a seal (“O” ring or gasket) is provided to seal the third drive  40  with the central housing  31 . 
   In a preferred embodiment, a mount  44  on the third bevel gear  60  receives the propeller hub  64 . The mount  44  includes a toothed spline on an end of the third bevel gear  60  and a complimentary toothed spline on an inner surface of a central axial opening on the first propeller hub member  162 . A washer  170  and the nut  172  secure the propeller hub member  162  on the third bevel gear  60 . A second propeller hub member  164  fits over the first propeller hub member  162 . The second propeller hub member  164  includes a central opening for passing the air propeller output drive shaft and a central hub  166 . The central hub  166  centers an air propeller (not shown) on the hub  166 . The air propeller and second propeller hub member  164  are secured to the first propeller hub member  162  by a plurality of fasteners  168 . 
   In assembly, the bearing  158  is placed or pressed into the cylindrical recess  150  until it seats on a surface of the ledge  152 . The other bearing  156  is placed or pressed into the cylindrical recess  148  until it seats on an opposite surface of the ledge  152 . The cylindrical section of the third bevel gear  60  is placed through the openings of the bearings ( 158 ,  156 ) until a surface of the third bevel gear  60  contacts a surface of the bearing  158 . The seal  160  is optionally placed in the cylindrical recess  148 . The first propeller hub member  162  is placed over the third bevel gear  60  on the mount  44  until and end of the first propeller hub member  162  engages a surface of the bearing  156 . The washer  170  and nut  172  are placed on the threaded end of the third bevel gear  60 . The nut is tightened to retain the assembly with the third housing member  50 . The washer  170  keeps the nut  172  tight. 
   Referring now to  FIGS. 4 ,  5 ,  6 , and  7 , the gear ratio between the first drive  36  and second drive  38  is preferably 1:1. The gear ratio between the first drive  36  and the third drive  40  is preferably 1:1. Those skilled in the art will appreciate that in the alternative, the two gear ratios may be different. For differing gear ratios, rotation of the first drive  36  causes rotation of the second drive  38  at one speed, and causes simultaneous rotation of the third drive  40  at a different speed providing differential speed between the pair of air propellers. 
   For example, the radius of the teeth  62  of the third bevel gear  60  could be different from the radius of the teeth  58  of the second bevel gear  56 . The teeth  54  of the first bevel gear  52  must be wide enough to engage the teeth  54  on a first region or portion of the teeth  54 , and to engage the teeth  58  on a second region or portion of the teeth  54 . This provides different gear ratios between the second and third drives. 
   Referring now to  FIG. 8 , the second central housing  70  of the transmission is described. The second housing includes a top member  238 . The top member  238  has a mounting surface  242  and a central opening  244 . The central opening  244  and surface  242  receive for mounting the fourth drive  32  (not shown). A plurality of threaded bores  246  are provided in the top member  238  for securing and sealing the fourth drive  32  to the top member  238  by a plurality of fasteners, for example bolts. 
   The second housing also includes a front member  240 . The front member  240  has a mounting surface  248  and a central opening  250 . The central opening  250  and surface  248  receive for mounting the fifth drive  34  (not shown). A plurality of threaded bores  252  are provided in the front member  240  for securing and sealing the fifth drive  34  to the front member  240  by a plurality of fasteners for example bolts. The front member  240  also includes a plurality of threaded bores  254  for mounting the second central housing  70  to the interconnecting frame  24  by a plurality of fasteners, for example bolts. 
   Referring now to  FIG. 9 , the transmission, generally indicated at  22  is described. The transmission  22  includes a second central housing  70 , a fourth drive  32 , and a fifth drive  34 . The second central housing  70  retains the fourth drive  32  and the fifth drive  34  in operational alignment. The fourth drive  32  is retained about a vertical axis and the fifth drive  34  is retained about a substantially perpendicular horizontal axis. The engine (not shown) is connected to the fifth drive  34  through the drive shaft  28 . The fifth drive  34  is connected to the fourth drive  32  which in turn is connected through the interconnecting drive shaft  30  to first drive  36  of the counter rotating air propeller drive  26  through the interconnecting drive shaft  30 . Rotation of the drive shaft  28  rotates the fifth drive  34 , which rotates the fourth drive  32 , which rotates the interconnecting drive shaft  30 . The gear ratio between the fifth drive  34  and the fourth drive  32  are 1:1, however, those skilled in the art will appreciate different gear ratios may be applied. 
   Referring now to  FIG. 10 , the fourth drive  32  is further described as shown in the exploded view. The fourth drive  32  includes the fourth housing member  72  and the fourth bevel gear  76 . The fourth housing member  72  is separate from the second central housing  70 . The fourth bevel gear  76  includes teeth  182  for engaging complimentary teeth on the fifth bevel gear  78 . The fourth bevel gear  76  has a first cylindrical portion for receiving the bearings ( 186 ,  188 ) and a second cylindrical portion with threads for receiving a nut  194 . A mount  184  is provided in the central axial opening of the fourth bevel gear  76 . The central axial opening extends the length of the fourth bevel gear  76 . The mount  184  includes a spline formed on the surface of the central opening and a spline formed on an end of the interconnecting drive shaft  30 . Those skilled in the art appreciate that the mount  184  could also be a pair of slots and key arrangement. The mount  184  is a floating mount that permits lengthwise movement of the interconnecting drive shaft  30  with the fourth bevel gear  76 . This permits a degree of height adjustment between the transmission  22  and the air propeller drive  26 . 
   The fourth housing member  72  includes a central axial opening, a first cylindrical recess  178  for receiving a bearing  186 , and a second cylindrical recess  174  for receiving another bearing  188 . An outwardly extending ledge  176  is formed between the recesses ( 178 ,  174 ) and provides a positive stop and seat for the bearings ( 186 ,  188 ). The nut  194  is tightened on the threads of the fourth bevel gear  76  with the retainer  196  to keep the fourth bevel gear  76  in the fourth housing member  72 . The washer  192  keeps the nut tight. Optionally, a seal is provided at an end of the fourth housing member  72  between the retainer and the cylindrical recess. 
   The fourth drive  32  is assembled by placing or pressing the bearing  186  into the cylindrical recess  178  of the fourth housing member  72  until it seats. Another bearing  188  is placed or pressed into the cylindrical recess  174  of the fourth housing member  72  until it seats. The fourth bevel gear  76  is placed into the central openings of the bearing ( 186 ,  188 ). The retainer  196 , washer  192  and nut  194  are placed on the threaded end of the fourth bevel gear  76 . The nut  194  is tightened retaining the fourth bevel gear  76  with the fourth housing member  72 . 
   The fourth housing member  72  fits into an opening  244  of the top member  238 . The outwardly extending flange  180  includes a plurality of openings (not shown) and is secured and sealed by a plurality of fasteners (not shown). Optionally, a seal (“O” ring or gasket) is provided between the fourth housing member  72  and the second housing  70 . 
   Referring now to  FIG. 10 , the fifth drive  34  is further described as shown in the exploded view. The fifth bevel gear  78  has a central axial opening for receiving the drive shaft  28 . A mount  200  is provided between the central axial opening of the fifth bevel gear  78  and the drive shaft  28 . The mount  200  is a spline on the surface of the central opening and a complimentary spline on the outer surface of the drive shaft  28 . Alternatively, the mount  200  could be a slot and key arrangement. The fifth bevel gear  78  includes a first cylindrical section for receiving the bearings ( 212 ,  214 ), and a second threaded smaller diameter cylindrical section for receiving the nut  220 . Teeth  198  engage complimentary teeth  182  on the fourth bevel gear  76 . A retainer  202  secures the drive shaft  28  with the fifth bevel gear  78 . The retainer  202  is a chamfered washer and nut combination, however, other forms of a retainer  202  may be applied. 
   The fifth housing member  74  includes a first cylindrical recess  204  for receiving a bearing  214 , and a second cylindrical recess  208  for receiving a bearing  212 . The fifth housing member  74  is separate from the second central housing  70 . An outwardly extending ledge  206  separates, supports, and provides a seat for the bearings ( 212 ,  214 ). In assembly, the bearing  214  is placed or pressed into the cylindrical recess  204  of the fifth housing member  74  until it seats on a surface of the ledge  206 . Another bearing  212  is placed or pressed into the cylindrical recess  208  of the fifth housing member  74  until it seats on an opposite surface of the ledge  206 . The cylindrical shaft of the fifth bevel gear  78  is inserted into the openings of the bearings ( 212 ,  214 ). The retainer  216 , washer  218 , and nut  226  are placed over the threaded end of the fifth bevel gear  78 . The nut  220  is tightened to retain the fifth bevel gear  78  in the fifth housing member  74 . 
   The fifth housing member  74  mounts in an opening  250  of the front member  240  and rests in place on the outwardly extending flange  210 . The outwardly extending flange  210  includes a plurality of openings for receiving a plurality of fasteners  86  for securing and sealing the fifth housing member  74 . Optionally, a seal (“O” ring or gasket) is located between the fifth housing member  74  and the second housing  70 . 
   Referring now to  FIGS. 2 and 11 , the interconnecting frame  24  is described. The interconnecting frame  24  includes an engine mount  222  for securing the interconnecting frame  24  to the back of the engine. Another frame mount  226  is provided to secure the interconnecting frame  24  to an inside surface of the hull  12  of the airboat. Another frame mount  228  is provided to secure the interconnecting frame  24  to the transom of the airboat  10 . 
   The interconnecting frame  24  has a number of upright members  230  connected to a number of horizontal members  232  that form a substantially rectangular box like structure. The box like structure is further strengthened by a number of diagonal members  234 . The interconnecting frame  24  may be welded together, or fastened together with fasteners such as nuts and bolts. Alternatively, the interconnecting frame  24  could be a cast or an enclosed structure. Alternatively, the interconnecting frame  24  could be part of the air propeller cage. The interconnecting frame  24  has a central vertical opening for receiving the interconnecting drive shaft  30  between the transmission  22  and the air propeller drive  26 . 
   The interconnecting frame  24  has a transmission mount  224  located at one end of the frame and a air propeller drive mount  236  located at another end of the frame. The transmission mount  224  and the air propeller drive mount  236  are located in the interconnecting frame  24  such that the first drive  36  and the fourth drive  32  are in alignment when connected by the interconnecting drive shaft  30 . The transmission  22  is secured to the transmission mount  224  by fasteners and the air propeller drive  26  is also secured to the air propeller drive mount  236  by fasteners. In the preferred embodiment, the transmission  22  is mounted to the interconnecting frame  24 . Alternatively, the transmission  22  may be mounted to an inside surface of the hull  12  of the airboat  10 . 
   It will, of course, be understood that the above description has been given by way of example only and that modifications in detail may be made within the scope of the present invention. 
   Nomenclature for the Figures: 
   
       
         10 —airboat 
         12 —hull 
         14 —engine 
         16 —drive system 
         18 —pair of air propellers 
         20 —air rudders 
         22 —transmission. 
         24 —interconnecting frame 
         26 —counter rotating air propeller drive 
         28 —drive shaft 
         30 —interconnecting drive shaft 
         31 —central housing 
         32 —fourth drive 
         34 —fifth drive 
         36 —first drive 
         38 —second drive 
         40 —third drive 
         42 —air propeller output drive shaft 
         44 —air propeller mount 
         46 —first housing member 
         48 —second housing member 
         50 —third housing member 
         52 —first bevel gear 
         54 —teeth 
         56 —second bevel gear 
         58 —teeth 
         60 —third bevel gear 
         62 —teeth 
         64 —air propeller hub 
         66 —mount 
         68 —second air propeller hub 
         70 —second central housing 
         72 —fourth housing member 
         74 —fifth housing member 
         76 —fourth bevel gear 
         78 —fifth bevel gear 
         80 —mount 
         82 —bearing 
         84 —bearing 
         86 —fastener 
         88 —washer 
         90 —nut 
         92 —retainer 
         94 —mount 
         96 —cylindrical recess 
         98 —cylindrical recess 
         100 —flange 
         102 —ledge 
         104 —cylindrical recess 
         106 —retainer 
         108 —bearing 
         110 —bearing 
         112 —flange 
         114 —cylindrical recess 
         116 —cylindrical recess 
         118 —ledge 
         120 —cylindrical recess 
         122 —mount 
         124 —retainer 
         126 —nut 
         128 —washer 
         130 —retainer 
         132 —bearing 
         134 —flange 
         136 —bolts 
         138 —hub 
         140 —washer 
         142 —nut 
         144 —threaded end 
         146 —central opening 
         148 —cylindrical recess 
         150 —cylindrical recess 
         152 —ledge 
         154 —flange 
         156 —bearing 
         158 —bearing 
         160 —seal 
         162 —first propeller hub member 
         164 —second propeller hub member 
         166 —mount 
         168 —fastener 
         170 —washer 
         172 —nut 
         174 —cylindrical recess 
         176 —ledge 
         178 —cylindrical recess 
         180 —flange 
         182 —teeth 
         184 —mount 
         186 —bearing 
         188 —bearing 
         190 —seal 
         192 —washer 
         194 —nut 
         196 —retainer 
         198 —teeth 
         200 —mount 
         202 —retainer 
         204 —cylindrical recess 
         206 —ledge 
         208 —cylindrical recess 
         210 —flange 
         212 —bearing 
         214 —bearing 
         216 —retainer 
         218 —washer 
         220 —nut 
         222 —engine mount member 
         224 —transmission mount 
         226 —hull frame mount 
         228 —transom frame mount 
         230 —upright members 
         232 —horizontal members 
         234 —diagonal members 
         236 —air propeller drive mount 
         238 —top member 
         240 —front member 
         242 —mounting surface 
         244 —central opening 
         246 —threaded bores 
         248 —mounting surface 
         250 —central opening 
         252 —threaded bores 
         254 —threaded bores 
         256 —mount member 
         258 —openings 
         260 —bottom member 
         262 —central opening 
         264 —mounting surface 
         266 —threaded bores 
         268 —front member 
         270 —central opening 
         272 —mounting surface 
         274 —threaded bores 
         276 —back member 
         278 —primary drive section 
         280 —secondary universal joint 
         282 —primary universal joint 
         284 —sliding joint 
         286 —secondary drive section 
         288 —secondary drive interconnect 
         290 —primary drive interconnect

Technology Classification (CPC): 1