Dual propeller drive for ski boat

Abstract

A dual prop drive for a ski boat has a gear box having an input gear that is connected to an engine output shaft. The gear box has two laterally spaced output gears on opposite sides of the input gear, with the output gears being drivingly connected to the input gear. The dual prop dive unit has two propeller shafts, each of which is drivingly connected to one of the output gears. The output gears and propeller shafts desirably rotate in counter-rotating directions. The gear box desirably reduces the rate of rotation of the propeller shafts relative to the engine output shaft. The gear box is lubricated without a pump by a gravity feed splash oil system.

Description

CROSS-REFERENCES TO RELATED APPLICATIONS

This is a non-provisional application based on and claiming the filing priority of co-pending provisional patent application Ser. No. 60/489,746, filed Jul. 23, 2003.

BACKGROUND OF THE INVENTION

The present invention relates to an improved drive mechanism for a recreational motor boat having a planing hull, such as the type used for water skiing and the like, and a method for retrofitting an existing motor boat to incorporate the improved drive mechanism.

As used herein, reference to a “recreational motor boat” is intended to include the class of boats having planing hulls, which are larger than row boats and small boats with outboard motors. Generally such boats are smaller, than larger, non-planing cabin cruisers, yachts, and large ships. These boats are typically in the range of 16-28 feet in length and frequently are powered by a single inboard engine that turns a propeller by means of a rotating propeller shaft that extends through the bottom of the boat. The propeller shaft is journalled in bearings in a support bracket mounted on the underside of the boat.

Boats with planing hulls used for water skiing or other water sports wherein one or more persons are pulled behind the boat (all referred to herein as water skiing) require a substantial amount of low speed power to pull water skiers out of the water and a substantial amount of top-end speed in order to pull the skiers at a high rate of speed where the boat hull is planing. As an example, one well-know water ski boat that employs a 350 horsepower automobile engine is currently able to pull up to fourteen skiers or more out of the water and then reach a top speed of 42 to 51 mph at a maximum rated engine speed of 5100 rpms. Top speed for most skiing demonstrations, however, is somewhat less. Boats used for related water sports, such as wake boarding has comparable or greater performance requirements. Larger boats, such as 28-foot boats, are heavy and require greater power than smaller boats.

At the present time, the best professional water ski boats employ a single inboard engine that drives a single prop. In order to increase the performance capabilities of such a boat, it is necessary to increase the engine size substantially in order to achieve both increased power and increased top speed. This increases dramatically the cost of the ski boat. Moreover, even with a more powerful engine, a boat with a single prop can still pull no more than about fourteen skiers.

Twin props have been used on ships and on various boats for specific purposes, but they require greater power. Increased power requires a lower engine to prop gear ratio. This in turn was thought to produce unacceptably reduced top speeds, at least without the added expense of the two-speed transmission. Two prop drives also have required a redesign of the boat interior and have not been adaptable as a retrofit in an existing one prop boat. An object of the present invention is to provide an improved and cost effective drive train for a recreational motor boat of the type used in water skiing in order to improve the performance characteristics of the boat.

BRIEF SUMMARY OF THE INVENTION

In accordance with the present invention, an improvement in the drive train for a water ski boat employing a single engine comprises fitting the boat with a gear box wherein the single drive shaft on the inboard engine drives at least two laterally-spaced props at a reduced gear ratio. An additional feature of the present invention is that improved steering and handling characteristics are achieved by interconnecting the two propellers so that the propellers are counter-rotating (rotate in opposite directions).

These and other features and advantages of the present invention will here and after appear and for purposes of illustration but not of limitation a preferred embodiment of the present invention is described below and shown in the appended drawings.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic side elevational view of a motor boat employing the dual prop drive of the present invention.

FIG. 2 is a partially broken away plan view of the boat of FIG. 1, showing in phantom the construction of a single prop drive not employing the present invention.

FIG. 3 is a plan view of the gear box of the present invention.

FIG. 4 is a front elevational view of the gear box of FIG. 3.

FIG. 5 is a sectional view taken along line 5-5 of FIG. 3.

FIG. 6 is a sectional view taken along line 6-6 of FIG. 3.

FIG. 7 is a sectional view taken along line 7-7 of FIG. 3.

FIG. 8 is a plan view of the Gearbox of the present invention.

FIG. 9 is a perspective view of the gearbox with the covers removed.

FIG. 10 is a perspective view of an output gear shown in the gearbox, with a funnel-type gravity feed lubricator being positioned to receive lubrication splashed in the gear housing by the rotating gears, the lubricator conveying lubrication by gravity to the idler and input gears in the interior of the housing, the zerk fitting on the thrust bearing also being removed so the thrust bearing can receive splashed lubrication instead of requiring manual lubrication.

FIG. 11 is a perspective view showing the side of the gearbox housing facing the engine, showing a removable access door on the housing for access to the ends of the propeller shafts to fasten them into the housing.

FIGS. 12-14 are perspective views of the underside of the boat hull, showing the propeller shafts, propellers, and rudders, and showing the locations and fittings for the original single prop propeller shaft.

DETAILED DESCRIPTON OF PREFERRED EMBODIMENTS

A motor boat or powerboat 10, shown schematically, includes an inboard engine 12, such as a 350 horsepower automobile engine or larger, mounted in a conventional manner inside the hull 14 of the boat. The engine has a conventional transmission 16, which typically permits the engine to be shifted between neutral, forward, and reverse gears. In the exemplary embodiment the engine transmission includes a gear reduction ratio of 1½ to 1, which means that when the engine is rotating at 3,000 rpm, the engine output shaft is rotating at 2,000 rpms.

A conventional inboard engine (which for convenience will be deemed to include the conventional engine transmission) drives the boat by means of a propeller shaft 18, which extends downwardly and rearwardly from driving engagement with an output shaft 17 of the engine through the bottom of the boat to an outer end, on which a propeller is mounted. The propeller shaft is journalled in seal 19 in the floor of the boat and in a support bracket 20 mounted on the underside of the boat. A rudder 21 is positioned behind the propeller. This is the setup for a conventional recreational boat.

In order to achieve additional power without redesigning the whole boat and without sacrificing significant top end speed, the propeller shaft 18 and propeller 22 of a conventional system are replaced by a dual prop drive mechanism constructed in accordance with the present invention. Dual prop drive mechanism 24 comprises a gear box 26 driven by the engine and a pair of laterally-spaced propellers 28 and 30 driven by propeller shafts 32 and 34, which are in turn driven by output gears 36 and 38 in the gear box. The output gears are driven by an input gear 40 that is drivingly connected to the output shaft or drive shaft of the engine through transmission input shaft 25. Input gear 40 is connected to output gears 36 and 38 by idler gears 42, 44, and 46. There is a single idler gear between input gear 40 and output gear 36, and there are two idler gears connected together between input gear 40 and output gear 38. The extra idler gear between input gear 40 and output gear 38 causes output gear 38 to rotate in the opposite direction from output gear 36. Thus, propellers 28 and 30 rotate in opposite directions.

A separate rudder 27 and 29 is placed behind each propeller 28 and 30 for steering the boat. The main rudder 21 can be disconnected (and left in place if desired) or it can be left in operational condition.

The counter rotating props provide important steering and handling advantages. With a single prop or with a dual prop setup wherein both props rotate in the same direction, it is easier to steer in one direction than the other direction. In addition, steering in reverse is even more difficult. Typically, without counter-rotating props, a boat can be steered in reverse in only one direction. With dual, counter-rotating props, the boat can be steered equally easily in either direction, whether the boat is in forward or reverse gears.

The gears in gear box 26 are housed in a housing 48, which has a non-removable top over the center portion of the housing and removable covers 50 and 54 on the sides, which provide access to the interior of the housing. The housing is fluid tight and is provided with an oil bath for cooling and lubricating of the gears. A dip stick 51 mounted in cover 50 is used to measure the oil level in the housing.

A feature of the invention is that lubrication is circulated over the gears in the housing without the use of a pump by means of a gravity feed splash oil funnel 31 that receives oil being splashed by the gears on the sides of the housing and feeds the oil into the central interior of the housing. This gravity feed lubrication system avoids the expense and pump failure problems of a pump lubrication system.

The gear box is shaped so as to be large enough to hold the gears and yet be small enough that it fits in a conventional motor boat without substantial modification of the hull or repositioning of the engine. In practice, however, it may be necessary to modify slightly the bottom of the hull by putting a slight bulge 15 therein, in order to fit a gear box into a conventional motor boat that is retrofitted to accommodate a dual prop drive unit. As shown, the housing of the gear box preferably is a durable metal, such as cast aluminum, and the covers are securely bolted to the gear housing. This prevents injury in the event that gears should fail while the engine is operating at a high rate of speed.

Propeller shafts 32 and 34 extend inwardly through an outer wall 56 of the gear box and then through gears 36 and 38. The propeller shafts are then received in thrust bearings 58 and 60. Thrust bearings 58 and 60 receive the forward thrust exerted by the propellers on the propeller shafts and translate the forward thrust into forward movement of the boat. The zerk lubrication fittings conventionally in thrust bearings desirably are removed, leaving openings 59 that receive lubricant by gravity as the oil is splashed around the interior of the housing by the gears.

While a gear box is desired in the present invention, because of durability and high speed operation attributes, the same functional characteristics can be achieved with a roller chain and sprocket arrangement, although durability is more of a concern with a roller chain and sprocket arrangement operating at the high engine speeds employed with a gasoline engine. Also, it is possible to drive twin propeller shafts by means of a hydraulic drive mechanism, wherein the boat engine drives a hydraulic pump, and the pump drives individual hydraulic drive motors for each of the propeller shafts. Whichever drive is employed, the desired drive ratio still should be employed.

As shown in the drawings, if the present invention is retrofitted in an existing installation, it is not necessary to completely remove the center propeller shaft 18 from the engine compartment, although this is preferred if the center shaft is not used. Instead, the propeller shaft, which is sealed in a propeller shaft opening in the hull of the boat, can simply be disconnected from the engine and possibly shortened so that gear box 26 will fit in the hull behind the engine. The propeller shaft in its existing position thus makes it unnecessary to plug a new opening in the bottom of the boat. The propeller can be removed from the end of the propeller shaft in order to reduce drag. Obviously the propeller shaft can be removed in its entirety and the hole in the boat where the propeller shaft extended through the hull can be sealed. The support bracket for the main propeller shaft can be left on bottom on the bottom of the boat or can be removed as desired. As a possible alternative, the main propeller shaft can be connected to the input gear of the transmission, so that the boat operates with three propellers.

The gear ratios chosen for the present invention are significant feature of the invention. Desirably, the gear ratios of the gears employed in the gear box are selected so that the rotational speed of the engine transmission output shaft and the dual propeller shafts are about 1½ to 1, which is when the output shaft from the transmission is rotating at 3,000 rpms, the dual propeller shafts are each operating at 2,000 rpms.

With a 1½ to 1 gear ratio in the gear box and the gear box being connected to an engine having a conventional 1½ to 1 gearing in the engine transmission, the overall gear reduction from the engine to the prop is 2¼ to 1. This exact ratio can be varied, as long as the range is set to provide sufficient power to pull the desired number of skiers without losing so much top boat speed that desired boat speed or water skiing performances cannot be achieved. A range of operable gear box gear ratios is about 1.2 to 1.8 to 1, yielding an overall engine to prop ratio of about 1.8 to 2.7 to 1.

While conventional wisdom would suggest that the reduction in gear ratios employed in the present invention would cause the boat to be too slow, it has been surprisingly found that the overall speed of the boat is not substantially reduced by this reduction in gear ratios. On the other hand, the power of the boat is increased dramatically.

The propellers in the exemplary embodiment of the present invention have a diameter of 14 inches and a pitch of about 16 to 18. This is the preferred propeller size, but the exact size has not been determined to be critical. The pitch of the prop is related to speed, so a change in pitch can have the same effect as a change in gear ratio. With the 1½ to 1 gear box ratio, a pitch of 18 may produce a top speed of about 43 mph, while a pitch of 16 may produce a top speed of 38 mph. A range of propeller sizes about 10 to about 18 inches in diameter is possible. A limitation in propeller size is that the propellers must not contact each other or the hull of the boat. A propeller size of about 10 inches in diameter is desired for a three-prop installation, so the props do not overlap.

With a dual prop system employing two 14 inch props having a pitch of 16-18 and employing a gear box ratio of 1½ to 1 (2.25 to 1 overall from engine to props), it has been found that the performance increase is dramatic. Whereas a conventional single prop water ski boat can pull a maximum of about 14 skiers from the water, no matter how powerful the boat, a boat retrofitted with (or fitted in the first instance with) the gear mechanism of the present invention can pull 22 skiers from the water without any problem.

Moreover, the top speed is not substantially reduced. Whereas a single prop boat may achieve a top speed of approximately 42 to 51 mph at a maximum rated engine speed of 5,100 rpms, the same boat fitted with the dual prop drive of the present invention can achieve a top speed of 38 to 43 mph, which is generally well above the speeds necessary for most water-towing sports. With the improved drive mechanism of the present invention top boat speed is even high enough to pull barefoot skiers which usually require the highest boat speed.

While a 1½ to 1 gear box ratio is desired in the present invention and has been demonstrated to be effective, variations in the gear ratios may be employed, as desired, in order to alter the performance characteristics one way or another to achieve greater power or greater top-end speed. In order to achieve the 1½ to 1 gear ratio in the present invention, the drive mechanism can include an input gear 40 having a diameter of 4 inches and output gears 36 and 38 having a diameter of 6 inches, with the idler gears 42, 44, and 46 having diameters of 5 inches.

One of the advantages of the improved drive mechanism of the present invention is that the drive mechanism does not appear to affect the wake of the boat. In a water ski boat, it is usually desirable to minimize the wake of the boat (unless the boat is used for wake boarding or other activities). When a dual prop mechanism is employed, the wake is substantially the same. In order to achieve the additional power by using two engines, substantial additional weight would be added as well as expense, and the additional weight would increase the wake of the boat.

Another important advantage of the present invention is that the drive mechanism provides increased pulling power with little decrease in speed under load. Thus where one or a large number of skiers are pulled, the maximum speed of the boat is not reduced nearly as much as when a single prop engine is used. This increased pulling power also makes it easier to maintain a constant speed through a slalom course, even though this load varies substantially.

Likewise, when a skier is pulled from a sitting position on a dock, the tow boat does not lose momentum due to prop slip when the rope gets tight. A strong steady increase in speed is achieved, and this minimizes falls.

Another outstanding advantage of the two propeller drive is when a bare footed skier steps off his ski at 40 mph, the boat speed is not dramatically reduced like it is with a single prop.

The present invention also is advantageous for wake boarding. A wake board is different from a water skiing operation, because a larger wake is desired. In order to achieve a larger wake with a conventional ski boat is customary to add ballast to the boat, which causes the boat to ride lower in the water and thus increases the wake. More power is necessary to drive a boat when it is riding lower in the water, and the drive mechanism of the present invention provides the additional power.

While the present invention is particularly desirable with a water ski boat, the advantages of the present invention can also be achieved with larger family boats as long as about 28 feet or so in length which are generally used only recreationally for water skiing. Boats of this nature sometimes are driven by a single prop and a single motor, but these boats can be underpowered. An option for a larger boat is the use of two engines, but this is considerably more expensive and adds weight to the boat. Good performance characteristics, including good power and good top speed, can be achieved with a single engine in a boat as large as 28 feet or larger by employing the dual-prop drive mechanism of the present invention.

In the foregoing exemplary embodiment the transmission is mounted to the rear of the engine and in front of the props. The present invention also can be used in installations where the engine is mounted to the rear of the transmission, in which case the engine output shaft faces forwardly and is connected to the rear side of the transmission. The props also are connected to the rear side of the transmission.

It will be understood by one having ordinary skill in the art and by those who practice the invention that various modifications and improvements may be made without departing from the spirit of the disclosed concept. Various relational terms, including left, right, front, back, top, and bottom, for example, is used in the detailed description of the invention and in the claims only to convey relative positioning of various elements of the claimed invention. The scope of protection afforded is to be determined by the claims and by the breadth of interpretation allowed by law.

Claims

1. A method for enhancing the pulling power of a single engine recreational motor boat while maintaining desirable top speed performance, wherein the boat comprises an inboard engine that rotates a propeller through a propeller shaft, the method comprising the steps of:

disconnecting the main propeller shaft from the engine;

providing a gear box having an input gear adapted to be drivingly connected to the engine output shaft and having two laterally spaced output gears on an opposite sides drivingly connected to the input gear, the gear box being fitted in a boat hull adjacent the output shaft of the engine, with the input gear being mounted on an input shaft that is in driving engagement with the engine output shaft; and

drivingly connecting output gears to laterally spaced propellers positioned on the exterior of a boat, such that the propellers are driven by the output gears through propeller shafts that extend through laterally spaced openings formed in the bottom of the boat.

2. A method as in claim 1 wherein a gear box output shafts, on which the output gears are mounted, are drivingly connected to the gear box input shaft in such a way that the gear box causes the output shafts to rotate at substantially the same rotational speed as each other in counter-rotating directions, the gear box reducing the rate of the rotation of the output shafts relative to the rate of rotation of the input shaft, the output shafts being connected to propeller shafts that extend through the bottom of the boat from inner ends that are driven by the gear box to outer ends that are spaced laterally under the boat;

3. A recreational motor boat having a planing hull wherein the boat has a single engine and a single engine output shaft, the improvement wherein the boat includes a gear box having an input shaft connected to the engine output shaft, the gear box having two spaced output shafts that are driven in counter-rotating directions by the input shaft through gears in the gear box, the output shafts being drivingly connected to spaced propellers mounted on the exterior of the hull, the gear box reducing the rotational speed of the output shafts in comparison with the input shaft.

4. A boat as in claim 3 wherein the gear box gear ratios reduce engine output shaft rotation by a factor of 1½ to 1.