Airboat transmission

A gear-based airboat transmission is provided for driving a pair of coaxial, counter-rotating propellers. A drive shaft couplable to an engine crank shaft extends rearward into the transmission case, and a pair of coaxial hollow driven shafts extend rearward out of the transmission case, to which are attachable a pair of propellers. A first gear train, containing an even number of gears, reverses the rotational direction of the engine; a second gear train, containing an odd number of gears, retains the rotational direction of the engine. Improved stability characteristics are imparted by supporting the drive shaft at two points within the case and also by positioning the drive and the driven shafts in vertical alignment. The adaptability of the gear-based transmission to being coupled with an automobile engine confers improved noise and efficiency characteristics.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to airboat propulsion mechanisms and, more 
particularly, to gear-based transmissions for airboats. 
2. Description of Related Art 
Airboats are often driven over land and water at high speeds. Airboats 
typically have employed aircraft engines operating at approximately 
2500-3000 revolutions per minute (rpm) connected to solid direct-drive 
shafts, which rotate a single propeller. Aircraft engines are extremely 
expensive, and it is a general practice therefore to mount a used aircraft 
engine to an airboat to save on costs. 
The steering apparatus of an airboat usually comprises a pair of rudders, 
with trim tabs added to correct for the torque that results from the 
rotation of the propeller, this torque tending to keep the boat from 
maintaining a level attitude. 
Extreme gyroscopic forces can occur when airboats are turned rapidly, and 
these forces are borne, among other structures, by the driven shaft. 
Current airboat systems utilize belt-driven transmissions, which are 
inefficient owing to power losses caused by belt friction, especially at 
higher rotational velocities. Belt breakage in these systems is a source 
of failure. Another disadvantage of belt-driven systems is their inability 
to permit reduction of engine speed, since the shaft used to effect such a 
reduction would have to be too small to be practicable. Thus it would be 
advantageous to utilize a different transmission method in an airboat to 
enable engine speed reduction without loss of efficiency. 
Propeller breakage is also a major source of failure, since at 3000 rpm 
extremely high forces are experienced at the propeller hub. It would 
therefore be desirable to reduce the load on the propeller. 
It has been taught by Becker et al. (U.S. Pat. No. 4,932,280, dated Jun. 
12, 1990) to use coaxial drive shaft systems for driving multiple outputs 
from a single input in an aircraft. Gearing means are disclosed for 
driving two outputs at different speeds. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide an airboat transmission 
that has improved strength and stability characteristics for driving a 
pair of counter-rotating propellers. 
It is a further object to provide such an airboat transmission having a 
single input shaft for driving both gear trains that in turn drive the two 
output driven shafts. 
It is another object to provide an airboat transmission for driving coaxial 
counter-rotating propellers that is drivable at variable speeds. 
It is an additional object to provide such an airboat transmission with 
which it is possible to drive the counter-rotating propellers at different 
speeds to provide additional thrust and reduce noise output. 
These and other objects are achieved by the airboat transmission of the 
present invention, which is for driving a pair of coaxial, 
counter-rotating propellers. The transmission generally is housed in a 
case that has an interior space. 
A drive shaft extends from the outside of the case into the interior space 
and is rotatable in a first direction. When the transmission is in use on 
an airboat, the drive shaft is mated at one end to a motor crank extending 
from and rotated by an engine. As mentioned above, previously known 
airboats utilized aircraft-type engines; however, with the transmission of 
the present invention, it has been found that an automobile engine can be 
used. This has a benefit of reducing cost, as well as other benefits to be 
discussed below. 
A first hollow driven shaft also extends into the interior space of the 
case, typically from a side opposite that of the drive shaft. The first 
hollow driven shaft is for rotating an outer propeller, that is, the 
propeller farther from the airboat body. 
A second hollow driven shaft likewise extends into the interior space of 
the case and is further positioned in surrounding, generally coaxial 
arrangement to the first hollow driven shaft. The second driven shaft is 
shorter than the first, and both ends protrude beyond the ends of the 
first driven shaft. This second driven shaft is for rotating an inner 
propeller, that is, the propeller closer to the airboat body. 
A first gear train for driving the first hollow shaft is housed in the 
interior space of the case. In its simplest configuration, the first drive 
train comprises two gears: a drive gear and a first driven gear. The drive 
gear is coaxially affixed to the drive shaft. The first driven gear is 
coaxially affixed to the first hollow shaft in such a position and 
configured so as to be rotatable by the drive gear. Thus, when the drive 
shaft rotates in the first direction, the drive gear is rotated in the 
first direction. This causes the first driven gear to be rotated in a 
second direction opposite in sense to the first direction, which 
consequently drives the first hollow shaft in the second direction. 
In an alternate embodiment, additional intermediate driven gears may be 
interposed between the drive gear and the first driven gear, so long as 
the total number of intermediate gears is an even number. 
A second gear train is also housed in the interior space of the case. This 
second gear train also includes the drive gear. In an alternate embodiment 
the drive gear may comprise a pair of drive gears, one for driving the 
first gear train and one for driving the second gear train. 
There are two driven gears in the second gear train: A second driven gear 
is affixed to the case and is axially spaced from the first driven gear. 
The second driven gear is positioned and configured so as to be rotatable 
by the drive gear. The third driven gear is also axially spaced from the 
first driven gear and is coaxially affixed to the second hollow shaft, 
positioned and configured so as to be rotatable by the second driven gear. 
Thus in use when the drive shaft rotates in the first direction, the drive 
gear is rotated in the first direction, the second driven gear is rotated 
in the second direction, the third driven gear is rotated in the first 
direction, and the second hollow shaft is rotated in the first direction. 
Thus it can be seen that the rotation of the drive shaft in one direction 
achieves, through the action of the two gear trains, a counter-rotation of 
the two coaxial hollow driven shafts and thus imparts counter-rotation to 
propellers attached thereto. 
Using a gear-driven transmission permits driving an automobile engine at 
the point of maximum horsepower, which generally implies a motor crank 
rotational speed approximately in the range of 5000-5200 rpm, and then 
gearing down the rotational speed to roughly 2500-2800, a quieter speed at 
which to run the propellers. 
The invention is not, of course, limited to the use of an automobile 
engine; in fact, the presence of the gear trains enables the user to 
optimize for efficiency and noise characteristics by altering gear ratios 
as desired.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
A description of the preferred embodiments of the present invention will 
now be presented with reference to FIGS. 1-3. 
The airboat transmission 10 of the present invention, shown from the side 
in FIG. 3, which is designed to drive a pair of coaxial, counter-rotating 
propellers 20 and 30, comprises a case 50 that has an interior space 502, 
a front side 504, and a back side 506. Depending upon the configuration of 
the gear trains, which will be discussed in the following, the shape of 
the case 50 as seen from the back may be symmetrical, as in FIG. 1(b), or 
asymmetrical, as in FIG. 2(b). (Here "from the back" is taken to means a 
view from the propeller side toward the engine side.) It is preferred that 
the case exterior be aerodynamically shaped in order to confer good 
airflow characteristics to the transmission 10 during use at high speeds. 
A drive shaft 12 extends into the interior space 502 of the case 50 through 
the case's front side 504. The drive shaft 12 is rotatable in a first 
direction, shown here as counterclockwise when viewed from the front. 
Typically the drive shaft's proximal portion 122, which extends outside 
the case 50, contains a coupler 124 for mating with the crank shaft 62 
from the airboat engine 60, which generates the rotational motion. The 
drive shaft 12 is preferably configured as a through shaft with respect to 
the case 50, and is thus supportable via brackets 508,509 affixed on the 
inside of both the front 504 and the back 506 sides, respectively, of the 
case interior space 502. This dual support confers exceptional stability 
to the drive shaft 12. 
The outer propeller 20 is mounted via propeller mount 202 to the distal 
portion 224 of, and is rotated by, a first hollow driven shaft 22 that 
extends from the back side 506 into the interior space 502 of the case 50. 
The proximal end 222 of the outer propeller 20 is supported via bracketing 
510 on the inside of the case's front side 504. 
The inner propeller 30 is mounted via propeller mount 302 to the distal 
portion 324 of, and is rotated by, a second hollow driven shaft 32 that 
extends from the back side 506 into the interior space 502 of the case 50. 
The second hollow shaft 32 is positioned in surrounding, generally coaxial 
arrangement to the first hollow driven shaft 20 and is shorter than the 
first driven shaft 22. These relative lengths permit the proximal end 222 
and the distal portion 224 of the first driven shaft 22 to protrude, 
respectively, beyond the proximal end 322 and the distal portion 324 of 
the second driven shaft 32. The second driven shaft 32 is supported on the 
interior of the case's back side 506 by bracketing 511. 
In a preferred embodiment, as shown in FIGS. 1 and 2, the longitudinal axes 
of the drive shaft 12 and the first 22 and second 32 hollow driven shafts 
are positioned generally in vertical alignment. This positioning confers 
improved stability during use, as the gyroscopic forces balance in this 
configuration. 
The airboat transmission 10 of the present invention further comprises two 
gear trains housed within the case 50, one for driving each of the hollow 
driven shafts 22,32. The first gear train 40 comprises an even number of 
gears for changing the incoming rotational direction. The embodiment shown 
in FIG. 1(a) contains two gears: a drive gear 402 coaxially affixed to the 
drive shaft 12 and a first driven gear 404 coaxially affixed to the first 
hollow shaft 22. The first driven gear 404 is positioned and configured so 
as to be rotatable by the drive gear 402. Thus, when the drive shaft 12 
rotates in the first direction, here shown as counterclockwise, the drive 
gear 402 is rotated in the same direction, and the first driven gear 404 
is rotated in a second direction opposite in sense to the first direction, 
that is, clockwise. Thus the first hollow shaft 22 is driven in a 
clockwise direction also, as would be an attached propeller 20. 
The second gear train 42 comprises an odd number of gears for maintaining 
the incoming rotational direction. The second gear train 42 comprises the 
drive gear 402, a second driven gear 424, and a third driven gear 426. The 
second 424 driven gear is rotatably affixed to the front side 504 of the 
case 50 and is axially spaced from the first driven gear 404. The second 
driven gear 424 is positioned and configured so as to be rotatable by the 
drive gear 402, and the third driven gear 426, which is coaxially affixed 
to the second hollow shaft 32, is positioned and configured so as to be 
rotatable by the second driven gear 424. Therefore, in use, when the drive 
shaft 12 rotates in the first direction, the drive gear 402 is rotated in 
the first direction, the second driven gear 424 is rotated in the second 
direction, the third driven gear 426 is rotated in the first direction, 
and the second hollow shaft 32 is rotated in the first direction, 
conferring counter-rotational movement to the inner propeller 30 with 
respect to the outer propeller 20. 
In an alternate embodiment 46 of the second gear train, the axis of 
rotation of second driven gear 424 is not aligned with those of the drive 
gear 402 and the third driven gear 426. In this embodiment the case 52 has 
an asymmetric cross-sectional shape, with a bulge 522 needed to 
accommodate the second driven gear 424. 
In an alternate embodiment 44 of the first gear train, shown in FIG. 2(a), 
two intermediate driven gears are provided. Both the fourth 442 and the 
fifth 444 driven gears are rotatably affixed to the inside of the front 
side 504 of the case 50 and are axially spaced from the second 424 and the 
third 426 driven gears. The fourth driven gear 442 is positioned and 
configured so as to be rotatable by the drive gear 402, and the fifth 
driven gear 444 is positioned and configured so as to be rotatable by the 
fourth driven gear 442 and to rotate the first driven gear 404. 
In this embodiment the first driven gear 404 is rotatable by the drive gear 
402 via intermediate rotations of the fourth 442 and the fifth 444 driven 
gears. 
In another embodiment of the transmission 14, the drive gear see FIG. 
3(b)! comprises a first drive gear 406 and a second drive gear 408 axially 
separated therefrom. Both are coaxially affixed to the drive shaft 12. The 
first drive gear 406 is positioned and configured so as to be in rotating 
relationship with the first driven gear 404; the second drive gear 408 is 
positioned and configured so as to be in rotating relationship with the 
second driven gear 424. 
In any of the above-detailed embodiments it may be seen that the first and 
the second gear trains can be adapted to drive the propellers at different 
speeds, which has been shown to provide improved thrust characteristics 
and reduced noise. 
It may be appreciated by one skilled in the art that additional embodiments 
may be contemplated, including variable numbers and sizes of gears, which 
may be positioned and configured to permit variable relative speeds of the 
two counter-rotating propellers. 
In the foregoing description, certain terms have been used for brevity, 
clarity, and understanding, but no unnecessary limitations are to be 
implied therefrom beyond the requirements of the prior art, because such 
words are used for description purposes herein and are intended to be 
broadly construed. Moreover, the embodiments of the apparatus illustrated 
and described herein are by way of example, and the scope of the invention 
is not limited to the exact details of construction. 
Having now described the invention, the construction, the operation and use 
of preferred embodiment thereof, and the advantageous new and useful 
results obtained thereby, the new and useful constructions, and reasonable 
mechanical equivalents thereof obvious to those skilled in the art, are 
set forth in the appended claims.