Abstract:
An airboat and selectable drive system for an airboat are described. The airboat may be configured for mounting an engine either high or low in the hull of an airboat. The selectable drive system is connected to the engine. The selectable drive system includes a transmission interconnected to an air propeller drive. A clutch has three positions to include forward, neutral, and reverse. The air propeller drive rotates the air propeller in one of two directions based upon the selected position of the clutch. The drive system has the capability to rotate air propellers at different speeds. The reverse speed of rotation of the air propellers may be higher or lower than the forward speed of rotation. The modular design and simplified drive system is easier to assemble and align for both configurations.

Description:
BACKGROUND OF INVENTION 
   1. Field of the Invention 
   The present invention is directed to a drive system, and more particularly to a selectable drive system for controlling the direction of an air propeller that drives 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 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 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 transmissions are directly driven by the engine. 
   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, there is a fundamental problem and limitation with the known prior art airboat drive systems. The transmissions are not selectable and do not provide a reverse capability. As a result, the prior art drives fail to provide thrust for maneuverability and control of an airboat. 
   Therefore, there is a need for an improved drive system in an airboat that is selectable between a forward, neutral, and reverse direction when the engine is running. 
   SUMMARY OF INVENTION 
   The present invention has many advantages. Operating the selectable drive system in neutral prevents motion of the airboat when the engine is running. Operating the drive system in forward propels the airboat in a forward direction. Operating the drive system in reverse when the airboat is in forward motion provides an air break to slow the airboat speed. Operating the drive system in reverse further propels the airboat in a reverse direction. The modular design of the selectable drive system may be applied to the case where the engine is mounted high in the hull or when the engine is mounted low in the boat. When the engine is mounted high in the hull, the transmission and air propeller drive are  secured together, or are mounted in close proximity by a supporting structure and interconnecting frame. When the engine is mounted low in the hull, the transmission and air propeller drive are separated by a distance and supported by an interconnecting frame. The modular design and simplified drive is easier to assemble and align. 
   In one broad aspect of the present invention, an airboat is provided. The airboat comprises a hull, an engine mounted in the hull, at least one air rudder, at least one air propeller, and a selectable drive system. The selectable drive system includes an air propeller drive, a transmission, and a clutch. The engine engaging the transmission providing in use operational power to the transmission. The transmission in use providing operational power to the air propeller drive. The at least one air propeller rotated, in use, by the air propeller drive. The clutch operable, in use, between a forward position and a reverse position. The clutch in the forward position directs rotation of the at least one air propeller in a first direction for moving the airboat in a forward direction and the clutch in the reverse position directions rotation of the at least one air propeller in a second opposite direction for moving the airboat in a reverse direction. 
   The airboat may further include a first differential drive. The first differential drive providing, in use, a first differential rotational speed to a first air propeller and a second differential rotational speed to a second air propeller in a second opposite direction. 
   The airboat may further include a second differential drive. The second differential drive providing, in use, a reverse rotational speed to a first air propeller when the clutch is in the reverse position, and the first differential drive providing, in use, a different forward rotational speed to a second air propeller when the clutch is in the forward position.  
   In an embodiment of the invention, the air propeller drive is a counter rotating air propeller drive for simultaneous counter rotation, in use, of a first air propeller in a first direction and a second air propeller in a second opposite direction. 
   In an embodiment of the invention, the counter rotating air propeller drive includes a first drive, a second drive, and a third drive. The first air propeller rotated, in use, by the second drive. The second air propeller rotated, in use, by the third drive. The first drive rotating, in use, the second drive in a first direction and the first drive simultaneously rotation, in use, the third drive in an opposite second direction. 
   In an embodiment of the invention, the second drive extends through the third drive. 
   In an embodiment of the invention, the first drive includes a first bevel gear and a first housing member. The first bevel gear mounted in the first housing. The first bevel gear rotated, in use, by transmission members. 
   In an embodiment of the invention, the second drive includes a second bevel gear, a second housing member, and an air propeller drive shaft. The second bevel gear mounted in the second housing. The second bevel gear having a mount for receiving an air propeller drive shaft. The second air propeller rotated, in use, by the air propeller drive shaft. Teeth on the second bevel gear engaging teeth on the first bevel gar for rotation, in use, of the second bevel gear by the first bevel gear. The second housing mounted in the counter rotation air propeller drive about a horizontal axis. 
   In an embodiment of the invention, the third drive includes a third bevel gear, and a third housing member. The third bevel gear mounted in the third  housing member. The third bevel gear for rotating, in use, the first air propeller. Teeth on the third bevel gear engaging teeth on the first bevel gear for rotation, in use, of the third bevel gear by the first bevel gear, and the third housing mounted in the counter rotating air propeller drive about the horizontal axis. 
   In an embodiment of the invention, the transmission includes a fourth drive and a fifth drive. The fifth drive rotating, in use, the fourth drive. 
   In an embodiment of the invention, the transmission includes a fourth drive, and a fifth drive for rotating, in use, the fourth drive and the fourth drive rotating, in use, the first drive. 
   In an embodiment of the invention, the fourth drive includes a fourth bevel gear and a fourth housing member. The fourth bevel gear mounted in said fourth housing member. The fourth bevel gear and the first bevel gear connected by an interconnecting drive shaft. 
   In an embodiment of the invention, the fifth drive includes a fifth bevel gear and a sixth bevel gear. The fifth bevel gear for rotating, in use, the fourth bevel gear in a first direction when the clutch is in the forward position and the sixth bevel gear for rotating, in use, the fourth bevel gear in a second opposite direction when the clutch is in the reverse position. 
   In an embodiment of the invention, teeth on the fifth bevel gear engage teeth on the fourth bevel gear, and teeth on the sixth bevel gear engage the teeth on the fourth bevel gear. 
   In an embodiment of the invention, the airboat includes a first friction cone clutch. The first friction cone clutch having a neutral position and an engaging  position providing, in use, the operational power to the air propeller drive in a first forward direction. 
   In an embodiment of the invention, the airboat includes a second friction cone clutch. The second friction cone clutch having a neutral position and an engaging position providing, in use, the operational power to the air propeller drive in a second reverse direction. 
   In an embodiment of the invention, the airboat includes a fourth bevel gear, a fifth bevel gear, a sixth bevel gear, and a clutch member. The fourth bevel gear providing, in use, the operational power to the air propeller drive. The fifth bevel gear engaging the fourth bevel gear. The sixth bevel gear engaging the fourth bevel gear. The first friction cone clutch intermediate a driven shaft and the fifth bevel gear and the second friction cone clutch intermediate the driven shaft and the sixth bevel gear. 
   In an embodiment of the invention, the airboat includes a fist clutch coupler, and a second clutch coupler. The first clutch coupler intermediate the fifth bevel gear and the clutch member, and the second clutch coupler intermediate the sixth bevel gear and the clutch member. 
   In an embodiment of the invention, wherein the first friction cone clutch is a first external cone surface on the clutch member and a first internal cone surface on the first clutch coupler and the second friction cone clutch is a second external cone surface on the clutch member and a second internal cone surface on the second clutch coupler. 
   In another broad aspect of the present invention, a selectable drive system for an airboat is provided. The selectable drive system comprises an air propeller drive, a transmission, and a clutch. The transmission having an interface for  connection to an airboat engine. The transmission, in use, providing operational power to the air propeller drive. The air propeller drive for rotating, in use, at least one air propeller. The clutch operable, in use, between a forward position and a reverse position. The clutch in the forward position directs rotation of the at least one air propeller in a first direction for moving the airboat in a forward direction and the clutch in the reverse position directs rotation of the at least on air propeller in a second opposite direction for moving the airboat in a reverse direction. 
   The selectable drive system may further include a first differential drive. The first differential drive providing, in use, a first differential rotational speed to a first air propeller and a second differential rotational speed to a second air propeller in a second opposite direction. 
   The selectable drive system may further include a second differential drive providing, in use, a reverse rotational speed to a first air propeller when the clutch is in the reverse position, and the first differential drive providing, in use, a different forward rotational speed to a second air propeller when the clutch is in the forward position. 
   In an embodiment of the selectable drive system, the air propeller drive is a counter rotating air propeller drive for simultaneous counter rotation, in use, of a first air propeller in a first direction and a second air propeller in a second opposite direction. 
   In an embodiment of the selectable drive system, the counter rotating air propeller drive includes a first drive, a second drive, and a third drive. The first air propeller rotated, in use, by the second drive. The second air propeller rotated, in use, by the third drive. The first drive rotating, in use, the second drive in a first direction and the first drive simultaneously rotating, in use, the third drive in an opposite second direction.  
   In an embodiment of the selectable drive system, the second drive extends through the third drive. 
   In an embodiment of the selectable drive system, the first drive includes a first bevel gear, and a first housing member. The first bevel gear mounted in the first housing. The first bevel gear rotated, in use, by transmission members. 
   In an embodiment of the selectable drive system, the second drive includes a second bevel gear, second housing member, and an air propeller drive shaft. The second bevel gear mounted in the second housing, the second bevel gear having a mount for receiving an air propeller drive shaft. The second air propeller rotated, in use, by the air propeller drive shaft. Teeth on the second bevel gear engaging teeth on the first bevel gear for rotation, in use, of the second bevel gear by the first bevel gar, and the second housing mounted in the counter rotating air propeller drive about a horizontal axis. 
   In an embodiment of the selectable drive system, the third drive includes a third bevel gear, a third housing member, and an air propeller mount. The third bevel gear mounted in the third housing member. The third bevel gear for rotating, in use, the first air propeller. Teeth on the third bevel gear engaging teeth on the first bevel gear for rotation, in use, of the third bevel gear by the first bevel gear, and the third housing mounted in the counter rotating air propeller drive about the horizontal axis. 
   In an embodiment of the selectable drive system, the transmission includes a fourth drive, and a fifth drive. The fifth drive rotating, in use, the fourth drive.  
   In an embodiment of the selectable drive system, the transmission includes a fourth drive, and a fifth drive. The fifth drive for rotating, in use, the fourth drive and the fourth drive rotating, in use, the first drive. 
   In an embodiment of the selectable drive system, the fourth drive includes a fourth bevel gear, and a fourth housing member. The fourth bevel gear mounted in the fourth housing member, the fourth bevel gear and the first bevel gear connected by an interconnecting drive shaft. 
   In an embodiment of the selectable drive system, the fifth drive includes a fifth bevel gear, and a sixth bevel gear. The fifth bevel gear for rotating, in use, the fourth bevel gear in a first direction when the clutch is in the forward position, and the sixth bevel gear for rotating, in use, the fourth bevel gear in a second opposite direction when the clutch is in the reverse position. 
   In an embodiment of the selectable drive system, teeth on the fifth bevel gear engage teeth on the fourth bevel gear, and teeth on the sixth bevel gear engage the teeth on the fourth bevel gear. 
   In an embodiment of the selectable drive system, further including a friction cone clutch. The first friction cone clutch having a neutral position and an engaging position providing, in sue, the operational power to the air propeller drive in a first forward direction. 
   In an embodiment of the selectable drive system, further including a second friction cone clutch. The second friction cone clutch having a neutral position and an engaging position providing, in use, the operational power to the air propeller drive in a second reverse direction.  
   In an embodiment of the selectable drive system, further including a fourth bevel gear, a fifth bevel gear, a sixth bevel gear, and a clutch member. The fourth bevel gear providing, in use, the operational power to the air propeller drive. The fifth bevel gear engaging the fourth bevel gear. The sixth bevel gear engaging the fourth bevel gear. The first friction cone clutch intermediate a driven shaft and the fifth bevel gear, and the second friction cone clutch intermediate the driven shaft and the sixth bevel gear. 
   In an embodiment of the selectable drive system, further including a first clutch coupler, and a second clutch coupler. The first clutch coupler intermediate the fifth bevel gear and the clutch member, and the second clutch coupler intermediate the sixth bevel gear and the clutch member. 
   In an embodiment of the selectable drive system, the first friction cone clutch is a first external cone surface on the clutch member and a fist internal cone surface on the first clutch coupler, and the second friction cone clutch is a second external cone surface on the clutch member and a second internal cone surface on the second clutch coupler. 

   
     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 diagrammatic perspective view of an airboat and a counter rotating air propeller drive system with the engine mounted high in the hull, 
       FIG. 2  is a diagrammatic perspective view of an airboat and a counter rotating air propeller drive system with the engine mounted low in the hull, 
       FIG. 3  is a diagrammatic cross sectional view of the drive system for an embodiment of the invention when the engine is mounted high in the hull, 
       FIG. 4  is a diagrammatic cross sectional view of the drive system for an embodiment of the invention when the engine is mounted low in the hull,  
       FIG. 5  is a diagrammatic cross sectional view of the drive system for an alternative embodiment of the invention when the engine is mounted low in the hull, 
       FIG. 6  is a diagrammatic perspective view of the counter rotating air propeller drive housing, 
       FIG. 7  is a diagrammatic cross sectional side view of the counter rotating air propeller drive illustrating the first drive, second drive, and third drive, 
       FIG. 8  is a diagrammatic cross sectional side view of the second drive, 
       FIG. 9  is a diagrammatic cross sectional side view of the first drive, 
       FIG. 10  is a diagrammatic cross sectional side view of the third drive, 
       FIG. 11  is a diagrammatic perspective view of the transmission housing, 
       FIG. 12  is a diagrammatic cross sectional side view of the transmission illustrating the fifth drive and double cone friction clutch, 
       FIG. 13  is a diagrammatic cross sectional side view of the fifth housing member illustrating the connection between the engine and transmission, 
       FIG. 14  is a diagrammatic cross sectional side view of the fifth drive and the double cone friction clutch, and 
       FIG. 15  is a diagrammatic cross sectional side view of the fourth drive. 
   

   DETAILED DESCRIPTION 
   The present invention is described in accordance with an 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 high in the hull  12  of the airboat  10  by a supporting structure and frame. The engine  14  is connected through a drive shaft (not shown) to a selectable drive system  16  (schematic representation). The selectable drive system  16  includes a transmission  22  and counter rotating air propeller drive  26  (see  FIG. 3 ). 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 maneuver the airboat  10 . 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. 
   Another embodiment of the invention is illustrated in  FIG. 2 . Again, the airboat is generally indicated at  10 . In this embodiment, the engine  14  is mounted low in the hull  12  of the airboat  10 . The engine  14  is connected through a drive shaft (not shown) to a selectable drive system  16  (schematic representation). The selectable drive system  16  includes a transmission  22 , interconnecting frame  24 , and counter rotating air propeller drive  26  (see  FIG. 4 ). 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 maneuver the airboat. 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. 
   The transmission  22  may be selected to drive the air propellers  18  in a first direction and maneuver the airboat  10  in a forward direction. Alternatively, the transmission may be selected to drive the air propellers  18  in a second opposite direction. When the air propellers  18  are operating in the second opposite direction and if the airboat has a forward motion, the air propellers  18  act as an air break to slow the forward motion of the airboat  10 . When the air propellers  18  are operating in the second opposite direction and if the airboat  10  is at rest, the air propellers act to thrust and maneuver the airboat  10  in a reverse direction. 
   Those skilled in the art will appreciate that alternatively the airboat  10  could be equipped with a single air rudder. Alternatively, the air propeller drive  16  could be for a single air propeller, or a multiple air propellers rotating in the same direction.  
   Referring now to  FIG. 3 , the drive system  16  of the present invention for the embodiment where the engine is mounted high above the hull ( FIG. 1 ) is further described. The engine  14  (not shown) is connected to the transmission  22  through the input drive shaft  310 . The transmission  22  is connected to the counter rotating air propeller drive  26  through a short interconnecting drive shaft  30  to transmit power from the transmission  22  to the counter rotating air propeller drive  26 . The counter rotating air propeller drive  26  has a pair of air propeller hubs ( 64 ,  68 ) for mounting a first and second air propeller. For this embodiment, the counter rotating air propeller drive  26  is mounted and secured to the transmission  22  through a plurality of fasteners, for example, nuts and bolts. In addition, a supporting structure and frame (not shown) extends between the airboat  10  and the drive  16 . 
   Referring now to  FIG. 4 , the drive system  16  of the present invention where the engine is mounted low in the hull of the airboat  10  ( FIG. 2 ) is further described. The engine  14  is connected to the transmission  22  through an input drive shaft  310 . 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 . The counter rotating air propeller drive  26  has a pair of air propeller hubs ( 64 ,  68 ) for mounting a first and second air propeller. 
   Referring now to both embodiments illustrated in  FIG. 3  and  FIG. 4 , the transmission  22  is disposed about a first horizontal axis  39  and the counter rotating air propeller drive  26  is disposed about a second horizontal axis  37 . The  second horizontal axis  37  is above the first horizontal axis  39  and the second horizontal axis  37  is substantially parallel to the first horizontal axis  39 . Preferably, the first horizontal  39  axis is in alignment with the engine. The second horizontal axis  37  may be in close proximity to the first horizontal axis  39  for the embodiment of  FIG. 3 , or the second horizontal axis  37  may be spaced away from the first horizontal axis  39  for the embodiment of  FIG. 4 . 
   The transmission  22  is connected to the counter rotating air propeller drive  26  about a third axis which is substantially perpendicular to the first axis and second axis. The third axis is intermediate the ends of the transmission  22  and the counter rotating air propeller drive  26 . The interconnecting drive shaft  30  is disposed about the vertical axis  41 . 
   Referring now to  FIG. 5 , 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 splined shaft that interfaces to the fourth drive  32  in the transmission  22  through the mount  184  (see  FIG. 12 ) 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 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 splined shaft. The spines cooperate to rotate both members while permitting a vertically sliding joint  284  between the primary drive section  278  and the secondary drive section  286 .  
   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 splined shaft that interfaces to the mount  94  of the first drive  36  in the counter rotating air propeller drive  26  (see  FIG. 9 ). 
   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. 6 , 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. 7 , 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  as 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  37  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  41 . 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  36 . 
   Referring now to  FIG. 9 , 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  58  of the second bevel gear  56  (not shown) and teeth  62  of the third bevel gear  60  (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 the threaded cylindrical section on the first bevel gear  52 . The retainer  106  contacts a surface of the bearing  84 . A nut  90  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. 8 , 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 third 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  126  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 the ends and 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 a second 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 second air propeller hub  68 . The mount  66  provides rotation of the second air propeller hub  68  with the air propeller shaft  42 . 
   An end of the air propeller shaft  42  includes a threaded end  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. 10 , 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. 7 ,  9 ,  8 , and  10 , 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. A first differential drive is formed by differing the gear ratios and interaction of the first drive  26 , second drive  38 , and third drive  40 . The first differential drive causes rotation of the second drive  38  at one speed, and simultaneous rotation of the third drive  40  at a different speed providing differential speed between the pair of air propellers at the same time.  
   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. 11 , 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 housing member  72  to the top member  238  by a plurality of fasteners, for example bolts. 
   The second central housing  70  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 housing member  74 . A plurality of threaded bores  252  are provided in the front member  240  for securing and sealing the fifth housing member  74  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. 12 , the transmission  22  is further described. The fourth drive  32  is mounted on the second central housing, generally indicated at  70 . The fourth housing member  72  is secured to the second central housing  70  by a plurality of fasteners (not shown), for example bolts. The fifth drive, generally indicated at  34  is mounted in the second central housing  70 . Mounted  intermediate the ends of the fifth drive  34  is a double cone friction clutch, generally indicated at  300  (see  FIG. 14 ). The fifth housing member  74  is mounted on an end of the second central housing  70  and secured by a plurality of fasteners, for example bolts (not shown). The fifth housing member  74  retains and aligns an input drive shaft  310  to the shaft  314 . One end of the input drive shaft  310  is configured for mounting to an engine  14  (not shown). The other end of the input drive shaft  310  is configured for connecting to an end of the shaft  314 . For example, the connection between the input drive shaft  310  and the shaft  314  is a spline to spline connection with a splined coupler  311 . However, persons skilled in the art appreciate that other connections are possible, for example, through a universal joint arrangement. 
   In an embodiment of the invention, the second central housing  70  includes another housing member generally indicated at  312 . This housing member is releasably secured by fasteners to the second central housing  70 . This permits access and installation of the fifth drive  34  in the second central housing. 
   The input drive shaft  310  and fifth drive  34  are mounted and retained about a substantially horizontal axis  39  by the second central housing  70  and the fifth housing member  74 . The fourth drive  32  is mounted and retained about a substantially vertical axis  41  by the second central housing  70 . This provides operational alignment between the fourth drive  32  and the fifth drive  34 . 
   Preferably the gear ratios between the fourth bevel gear  600 , fifth bevel gear  340 , and sixth bevel gear  440  are 1:1. However other gear ratios are contemplated to provide either a speed reduction, speed increase, or differential speed to the fourth bevel gear  600 . A second differential drive is formed by differing the gear ratios and interaction of the fourth bevel gear  600 , fifth bevel gear  340 , and sixth bevel gear  440 . The second differential drive causes rotation of the fourth bevel gear  600  at one speed when the clutch is in a first position, and  rotation of the fourth bevel gear  600  at a different speed when the clutch is in a second position providing a different speed to the air propeller drive and air propellers for forward thrust and reverse thrust. 
   For example, if the gear ratio between the fourth bevel gear  600  and the fifth bevel gear  340  is different from the fourth bevel gear  600  and the sixth bevel gear  440 , then the speed for reverse is different for the speed in forward because the fourth bevel gear  600  will rotate at different speeds based upon the different gear rations to the fourth bevel gear  600 . 
   The radius of the teeth  342  of the fifth bevel gear  340  could be different, for example, larger from the radius of the teeth  442  of the sixth bevel gear  440 . The teeth  602  of the fourth bevel gear  600  are wider and provide two separate areas. The first area providing a larger radius of teeth than the second area which provides a smaller radius of teeth. The teeth  342  of the fifth bevel gear  340  engage first area of the teeth  602  of the fourth bevel gear  602 . The teeth  442  of the sixth bevel gear  440  engage the second area of the teeth  602  of the fourth bevel gear  602 . 
   Referring now to  FIG. 14 , the fifth drive generally indicated at  34  and the double cone friction clutch, generally indicated at  300 , of the transmission are further described. The double cone friction clutch  300  includes a clutch member  320 , first internal cone surface  330  on the first clutch coupler  328 , and second internal cone surface  334  on the second clutch coupler  332 . The clutch member  320  has a central splined axial bore  318 . The central splined axial bore  318  is of a diameter for complimentary engagement with splines  316  intermediate the ends of the shaft  314 . This splined interface between the clutch member  320  and shaft  314  permit axial movement of the clutch member  320  and rotational movement of the clutch member  320  when the shaft  314  is rotated by the engine.  
   The double cone friction clutch  300  also includes a first friction cone clutch  324 . The first friction cone clutch  324  is formed by the first external cone surface  336  and the first internal cone surface  330  on the first clutch coupler  328 . 
   The double cone friction clutch  300  also includes a second friction cone clutch  326 . The second friction cone clutch  326  is formed by the second external cone surface  338  and the second internal cone surface  334  on the second clutch coupler  332 . 
   The clutch member  320  includes a central annulus  322 . The central annulus  322  engages a shift member (not shown) for axial movement of the clutch member  320  between a neutral position, a forward position, and a reverse position. The neutral position locates the clutch member  320  intermediate the first clutch coupler  328  and the second clutch coupler  332  such that the first external cone surface  336  does not engage the first internal cone surface  330  and the second external cone surface  338  does not engage the second internal cone surface  334 . The forward position moves the clutch member  320  axially to engage the first external cone surface  336  with the first internal cone surface  330  for operating the fourth drive  32  in a first direction. The reverse position moves the clutch member  320  axially to engage the second external cone surface  338  with the second internal cone surface  334  for operating the fourth drive  32  in a second opposite direction. 
   Preferably, the double cone friction clutch  300  is a metal to metal clutch running in an oil bath. 
   The fifth drive  34  includes a fifth bevel gear  340  and the first clutch coupler  328 . Teeth  342  on the fifth bevel gear  340  engage teeth  602  on the fourth bevel gear  600  (see  FIGS. 12 and 15 ). The fifth bevel gear  340  has an axial lengthwise bore  350 . The bore  350  permits the shaft  314  to extend through the  fifth bevel gear  340  without engaging the gear  340 . The fifth bevel gear  340  has an interface  344  for receiving the first clutch coupler  328 . In an embodiment of the invention, the interface  344  is a threaded bore for complimentary engagement with threads on a cylindrical end of the first clutch coupler  342 . The interface  344  provides locking engagement between the first clutch coupler  328  and the fifth bevel gear  340 . The first clutch coupler  328  has an axial lengthwise bore  352 . The bore  352  permits the shaft  314  to extend through the first clutch coupler  328  without engaging the coupler  328 . 
   A bearing  346  is disposed in the bore  352  intermediate the first clutch coupler  328  and the shaft  314 . A flat plate thrust bearing  607  is disposed intermediate the fifth bevel gear  340  and the second central housing  70 . The flat plate thrust bearing  607  engages an underside of the fifth bevel gear  340 . This prevents in operation the fifth bevel gear  340  from axially moving when the clutch member  320  engages the first clutch coupler  328 . Another bearing  348  is disposed about a cylindrical surface of the fifth bevel gear  340  intermediate the fifth bevel gear  340  and the second central housing  70 . The bearing  348  and bearing  346  permit rotational movement of the fifth bevel gear  340  and the first clutch coupler  328 . The bearing  348 , fifth bevel gear  340 , first clutch coupler  328 , and bearing  346  provide support for one end of the shaft  314 . 
   Alternatively, the bearing  348 , for example a roller bearing, and the flat plate thrust bearing  607  could be replaced by a two row tapered roller bearing at the same location as bearing  348 . The tapered roller bearing prevents axial movement of the fifth bevel gear  340  in operation of the clutch. 
   The fifth drive  34  also includes a sixth bevel gear  440  and the second clutch coupler  332 . Teeth  442  on the sixth bevel gear  440  engage teeth  602  on the fourth bevel gear  600  (see  FIGS. 12 and 15 ). The sixth bevel gear  440  has an axial lengthwise bore  450 . The bore  450  permits the shaft  314  to extend through  the sixth bevel gear  440  without engaging the gear  440 . The sixth bevel gear  440  has an interface  444  for receiving the second clutch coupler  332 . In an embodiment of the invention, the interface  444  is a threaded bore for complimentary engagement with threads on a cylindrical end of the second clutch coupler  332 . The interface  444  provides locking engagement between the second clutch coupler  332  and the sixth bevel gear  440 . The second clutch coupler  332  has an axial lengthwise bore  452 . The bore  452  permits the shaft  314  to extend through the second clutch coupler  332  without engaging the coupler  332 . 
   A bearing  446  is disposed in the bore  452  intermediate the second clutch coupler  332  and the shaft  314 . A second flat plate thrust bearing  607  is disposed intermediate the sixth bevel gear  440  and the second central housing  70 . The second flat plate thrust bearing  607  engages an underside of the sixth bevel gear  440 . This prevents in operation the sixth bevel gear  440  from moving axially when the clutch member  320  engages the second clutch coupler  332 . Another bearing  448  is disposed about a cylindrical surface of the sixth bevel gear  440  intermediate the sixth bevel gear  440  and the second central housing  70 . The bearing  448  and bearing  446  permit rotational movement of the sixth bevel gear  440  and the second clutch coupler  332 . The bearing  448 , sixth bevel gear  440 , second clutch coupler  332 , and bearing  446  provide support for a second end of the shaft  314 . 
   Alternatively, the bearing  448 , for example a roller bearing, and the second flat plate thrust bearing  605  could be replaced by a two row tapered roller bearing at the same location as bearing  448 . The tapered roller bearing prevents axial movement of the sixth bevel gear  440  in operation of the clutch. 
   The shaft  314  includes splines  315  on one end of the shaft  314  for engagement with a coupler  311  and connection to the input drive shaft  310  (see  FIG. 12 ).  
   Referring now to  FIG. 13 , the fifth housing  74  and input drive assembly are further described. The fifth housing  74  is generally hollow as indicated at  500 . The fifth housing  74  is mounted to an end of the second central housing  70  by a mounting surface  524  and plurality of openings  502  that receive fasteners, for example bolts to tightly seal and secure the fifth housing  74  to a complimentary mounting surface  248  of the second central housing  70 . A bore  522  is provided in an end of the fifth housing  74  for receiving the input drive shaft  310 . A seal  518  is mounted in the bore  522  to keep a lubricating fluid, for example oil, within the transmission. 
   The input drive shaft  310  has an engine mount  520  on one end to engage the engine  14  (not shown) for rotational movement of the drive shaft  310 . The input drive shaft  310  is substantially cylindrical. The other end of the input drive shaft is splined  530  to cooperate with a splined coupler  311  for connection to the splined end of the shaft  314  (see  FIG. 14 ). The splined coupler  311  includes an internal splined surface  528 . A first spacer  516 , bearing  514 , second spacer  512 , and another bearing  510  are mounted over the input drive shaft  310 . Intermediate the ends of the input drive shaft  310  is a threaded portion  504  to receive a nut  506 . The first spacer  516  and nut  506  keep the bearings  514 ,  510  and input drive shaft  310  in operational alignment. A cylindrical bore  526  and ledge formed in the fifth housing  74  receive the bearings ( 514 ,  510 ). A nut  508  keeps the bearings in tight engagement with the fifth housing  74  and keeps the input drive shaft  310  in axial alignment with the shaft  314  located in the second central housing  70 . 
   Preferably, the bearings  510 ,  514  are tapered roller bearings to prevent axial movement of the input drive shaft  310  in operation. 
   Referring now to  FIG. 15 , the fourth drive of the transmission is further described. The fourth drive housing  72  has a cylindrical axial opening. An outwardly extending ledge  620  of the housing  72  extends into the cylindrical axial  opening. A bearing  612  is located in a cylindrical recess and the bearing engages one side of the ledge  620 . A second bearing  614  is located in another cylindrical recess and the bearing engages a opposite side of the ledge  620 . A third flat plate thrust bearing  603  is disposed intermediate the fourth bevel gear  600  and the housing  72 . The third flat plate thrust bearing  605  engages an underside of the fourth bevel gear  600 . This prevents in operation the fourth bevel gear  600  from moving axially in operation. The fourth bevel gear  600  has a cylindrical section that engages both the first bearing  612  and the second bearing  614 . One end of the cylindrical section is threaded  608  to receive a nut  610 . The fourth bevel gear  600  and nut  610  cooperate to keep the bearing  614  and bearing  612  in place within the housing  72 . The fourth bevel gear  600  is rotatable about a vertical axis. The fourth bevel gear  600  includes an axial bore. The axial bore has a centering section  604  for receiving an end of the interconnecting drive shaft (not shown) and a larger diameter splined section  606 . The splined section  606  cooperates with a splined section of the interconnecting drive shaft to rotate the interconnecting drive shaft upon rotation of the fourth bevel gear  600 . A bearing retainer cap  616  and seal  618  are provided in an end of the housing  72 . 
   Alternatively, the bearings  614  and  612 , for example a roller bearings, and the third flat plate thrust bearing  603  could be replaced by a two row tapered roller bearing. The tapered roller bearing prevents axial movement of the fourth bevel gear  600  in operation. 
   Referring now to  FIG. 4 , 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 hull frame mount  226  is provided to secure the interconnecting frame  24  to an inside surface of the hull  12  of the airboat. A transom 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 (not shown). 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 . 
   Referring now to  FIGS. 7 and 12 , assembly of the drive system is described. The air propeller drive  26  is assembled as a unit. The first drive  36 , second drive  38  and third drive  40  are assembled and installed into the central housing  31  of the counter rotating air propeller drive  26 . The central housing  31  is then filled to a predetermined level with a lubricating fluid. 
   The transmission  22  is also assembled as a unit. The fourth drive  32 , fifth drive  34 , and double cone friction clutch  300  are assembled and installed into the  second central housing  70 . The input drive shaft  310  and fifth drive housing  74  are assembled and installed to the second central housing  70 . The second central housing  70  is then filled to a predetermined level with a lubricating fluid. 
   For the embodiment illustrated in  FIG. 1  where the engine  14  is mounted high in the hull  12  of the airboat  10 , the transmission  22  and air propeller drive  26  may be secured together by a plurality of fasteners and connected by a short interconnecting drive shaft  30  (see  FIG. 3 ). 
   For the embodiment illustrated in  FIG. 2  where the engine  14  is mounted low in the hull  12  of the airboat  10 , the transmission  22  and air propeller drive  26  are secured to the interconnecting frame  24  by a plurality of fasteners and connected by a longer interconnecting drive shaft  30  (see  FIG. 4 ). 
   Referring now to  FIGS. 7 and 12 , operation of the selectable drive is further described. The engine  14  of the airboat  10  rotates the input drive shaft  310  which through the splined connection, rotates the shaft  34 . The shaft  34  rotates the clutch member  320 . With the double cone friction  300  in the neutral position, the fifth bevel gear  342  and the sixth bevel gear  442  do not rotate. Therefore, the fourth bevel gear  600 , interconnecting drive shaft  30 , third bevel gear  40 , second bevel gear  38 , and first bevel gear  36  do not rotate and the air propellers remain stationary. 
   When the double cone friction clutch  300  is placed into a first position, for example forward, the rotating shaft  314  and double cone friction clutch  300  rotate the fifth bevel gear  342 , but not the sixth bevel gear  440 . This rotates the fourth bevel gear  600 , interconnecting drive shaft  30  and third bevel gear  40 . The third bevel gear  40  rotates the second bevel gear  38  in a first direction and the first bevel gear  36  in a second opposite direction. The air propellers counter rotate in a direction that provides forward thrust to the airboat  10 .  
   When the double cone friction clutch  300  is placed into a second position, for example reverse, the rotating shaft  314  and clutch  300  rotate the sixth bevel gear  440 , but not the fifth bevel gear  342 . This rotates the fourth bevel gear  600 , interconnecting drive shaft  30 , and third bevel gear  40  in an opposite direction. The third bevel gear  40  rotates the second bevel gear  38  in an opposite first direction and the first bevel gear  36  in an opposite second direction. The air propellers counter rotate in a direction that provides reverse thrust to 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 
         30 —interconnecting drive shaft 
         31 —central housing 
         32 —fourth drive 
         34 —fifth drive 
         36 —first drive 
         37 —first horizontal axis 
         38 —second drive 
         39 —second horizontal axis 
         40 —third drive 
         41 —vertical axis 
         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 
         82 —second bearing 
         84 —bearing 
         86 —fastener 
         88 —lock washer 
         90 —nut 
         92 —retainer 
         94 —mount 
         96 —cylindrical recess 
         98 —second cylindrical recess  
         100 —flange 
         102 —ledge 
         104 —third cylindrical recess 
         106 —retainer 
         108 —second bearing 
         110 —bearing 
         112 —flange 
         114 —first cylindrical recess 
         116 —second cylindrical recess 
         118 —ledge 
         120 —third cylindrical recess 
         122 —mount 
         124 —retainer 
         126 —nut 
         128 —lock washer 
         130 —retainer 
         132 —bearing 
         134 —flange 
         136 —bolts 
         138 —central hub 
         140 —lock washer 
         142 —nut 
         144 —threaded end  
         146 —central axial 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 —central hub 
         168 —fastener 
         170 —washer 
         172 —nut 
         184 —mount 
         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 —frame member 
         258 —bores 
         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 
         300 —double cone friction clutch 
         310 —input drive shaft 
         311 —spline coupler 
         312 —housing member 
         314 —shaft 
         315 —spline 
         316 —spline 
         318 —central splined axial bore 
         320 —clutch member 
         322 —annulus 
         324 —first friction cone clutch 
         326 —second friction cone clutch 
         328 —first clutch coupler 
         330 —first internal cone surface 
         332 —second clutch coupler 
         334 —second internal cone surface 
         336 —first external cone surface 
         338 —second external cone surface 
         340 —fifth bevel gear  
         342 —teeth 
         344 —interface 
         346 —bearing 
         348 —bearing 
         350 —axial bore 
         352 —axial bore 
         440 —sixth bevel gear 
         442 —teeth 
         444 —interface 
         446 —bearing 
         448 —bearing 
         450 —axial bore 
         452 —axial bore 
         500 —hollow opening 
         502 —opening 
         504 —threaded portion 
         506 —nut 
         508 —nut 
         510 —bearing 
         512 —second spacer 
         514 —bearing 
         516 —spacer 
         518 —seal  
         520 —engine mount 
         522 —bore 
         524 —mounting surface 
         526 —bore 
         528 —internal spline surface 
         530 —spline 
         600 —fourth bevel gear 
         602 —teeth 
         603 —third flat plate thrust bearing 
         604 —centering section 
         605 —first flat plate thrust bearing 
         606 —splined bore 
         607 —second flat plate thrust bearing 
         608 —threads 
         610 —nut 
         612 —bearing 
         614 —second bearing 
         616 —bearing retainer cap 
         618 —seal 
         620 —ledge