Background Art Stem mounted, or outboard marine propulsion systems have a number of advantages over conventional systems like the fixed propeller systems in which an onboard engine drives a fixed propeller shaft through a marine transmission and steering is provided by a rudder, and over purely outboard systems in which the entire engine, drivetrain and propeller are located aft of the transom. Stem mounted drives combined with inboard engines offer more mobility than fixed propeller systems, and greater horsepower than purely outboard units. The term"outboard drive"refers to the fact that the entire drive unit apart from the engine and trans- mission are located overboard, on the transom of the boat. This feature is critical to the vessel's trim, tilt and steering operations. With this type of system propulsion is achieved when rotation is transmitted from an inboard mounted engine through some form of drive train to a propeller located below the water line. Instead of a rudder setup, steering is executed by changing the angle of the entire unit in a plane parallel to the water surface. By varying this angle, propeller thrust is redirected and the vessel's course altered. The ability to direct propeller thrust makes the vessel respon- sive and extremely maneuverable, a feature that appeals to both commercial and pleasure boat owners.

In existing stem drive propulsion systems, rotation from the inboard power plant is reduced by the transmission and then directly coupled to the outboard leg using a universal joint. Power is then transmitted through an arrangement of clutches, bevel gears and shafts to the propeller located below the water surface.

Such fixed gear ratio arrangements tend not to use fuel to the utmost efficiency. For example, accelerating a boat from a standstill requires more horsepower than any other time during operation, and this occurs when the engine is running at low rpm and producing very little horsepower. At that time engines are overfuelled in order to create more horsepower. However most of this excess fuel that is delivered to the engine is exhausted and not used. Also, particular engines, and particularly diesel engines, have a peak performance within a narrow rpm range, so in fixed ratio systems, the engine will be operating efficiently in a limited number of boat speeds and so most often will be operating with reduced fuel efficiency, causing increased costs and pollution.

Currently the largest outboard drives on the market are made of cast aluminum and do not adequately withstand the magnitude or the duration of the torque required by larger commercial boats (greater than 400 ft. -lb. of torque). In existing designs, an increase in torque would mean that the running gear would have to be made substantially more robust, and so could no longer be contained within a streamlined lightweight casing. Instead, the case would have to be made larger and more bulky in an attempt to withstand the inherent side thrust associated with bevel gears. In higher speed applications where this unit is desirable, such a massive leg would compromise fuel efficiency with its increased mass and multiplied drag. As a result, current manufacturers have designed outboard drives that are more suited to high speed (3000-5000 rpm), gasoline-fuelled engines with relatively low torque.

These restrictions have shaped their trim, lightweight drives to be suitable for pleasure boats and light duty commercial vessels with low operating hours. However, the maneuverability of these drives still appeals to customers operating heavier boats under more abusive conditions. With such operating benefits, larger commercial operators still choose to purchase these lightweight units which results in the need for costly repairs after very low hours.

Using thrust vectoring to steer the boat, rather than the traditional fixed propeller and rudder setup has considerable advantages in maneuverability. However

there are some disadvantages. The universal joint which must penetrate the transom of the boat is both a weak link in the drive train, as well as a difficult area to seal.

Consequently various designs have been proposed wherein the inboard engine is used to drive a hydraulic pump, and the hydraulic pump provides hydraulic fluid under pressure to an outboard reversible hydraulic motor which drives the propeller shaft. For example, in United States Patent no. 3,139, 062 Keefe issued June 30,1964 ; 3,587, 595 Buddrus issued June 28,1971 ; 3,599, 595 James issued August 17,1971 ; and 3,847, 107, propulsion units are disclosed in which a hydraulic motor is mounted on the propeller shaft, below the water line. Such designs result in large drag factors due to the volume of the housing which is below the water line.

Other designs such as shown in U. S. patents 2,486, 049 Miller, issued October 25, 1949; 3,673, 978 Jeffrey et al. issued July 4,1972, all provide the hydraulic motor above the water line, and connect the motor to the propeller shaft through bevel gears. The disadvantage of such designs however is that again for such bevel gear connections, the running gear would have to be made substantially more robust for high torque applications, and so could no longer be contained within a streamlined lightweight casing. The case would have to be made larger and more bulky in an attempt to withstand the inherent side thrust associated with bevel gears. In higher speed applications where this unit is desirable, such a massive leg would compromise fuel efficiency with its increased mass and multiplied drag.

There is therefore a need for an outboard drive system which will satisfy the higher horsepower/torque requirements of larger commercial vessels, as well as compete with existing outboard drives.

Disclosure of Invention The present invention therefore provides an outdrive designed to surpass the horsepower and torque limitations set by current state-of-the-art units.

In order to meet these objectives the outdrive was designed to incorporate a multi-

strand roller chain drive, replacing the conventional bevel gear arrangement. Using a chain drive in this application serves to increase the horsepower rating of this type of drive and increase the durability of the outdrive, while keeping the outer casing very streamlined.

The invention therefore provides a marine propulsion system, compris- ing: i) a transom; ii) an engine inboard of the transom; iii) a steerable screw propulsion unit outboard of the transom, and comprising a propeller shaft mounted for rotation in the propulsion unit and having a propeller mounted thereon; iv) a drive shaft mounted for rotation in the propulsion unit parallel to the propeller shaft; v) means for transferring power from the engine to rotate the drive shaft; and vi) flexible belt means for coupling the drive shaft to the propeller shaft and thereby transferring rotational energy from the drive shaft to the propeller shaft. Preferably the means for transferring power from the engine to rotate the drive shaft comprises a hydraulic pump inboard of the transom and coupled to be driven by the engine, a reversible hydraulic motor mounted on the propulsion unit and coupled to drive the drive shaft, and fluid conduits communicating between the hydraulic pump and the hydraulic motor to transfer pressurized fluid from the hydraulic pump to the hydraulic motor.

Brief Description of Drawings In drawings illustrating a preferred embodiment of the invention: Figure 1 is a schematic illustration of a prior art stem drive system; Figure 2 is a schematic illustration of the marine propulsion system according to the present invention; Figure 3 is a side elevation, partially cut-away, of the drive unit for the marine propulsion system according to the present invention; and Figure 4 is a rear view of the drive unit shown in Figure 3.

Figure 5 is a side elevation, partially cut-away, of a second embodiment of the drive unit for the marine propulsion system according to the present invention;

Figure 6 is a side elevation, partially cut-away, of the second embodi- ment of the drive unit for the marine propulsion system shown in Figure 5, in a normally tilted-up position; Figure 7 is a side elevation, partially cut-away, of the second embodiment of the drive unit for the marine propulsion system shown in Figure 5, in an emergency tilted-up position; and Figure 8 is an end view, in cross-section, showing the drive chain arrange- ment of the second embodiment of the drive unit for the marine propulsion system shown in Figure 5.

Best Mode (s) For Carrying Out the Invention In the prior art propulsion system shown in Fig. 1, an internal combustion engine 10, mounted inboard, has a fixed ratio transmission 12 mounted directly in line with it. The outboard drive unit 14 is connected through the transom 16 to the transmission 12 via a universal joint 18.

In the marine propulsion system of the present invention shown in Fig.

2, internal combustion engine 20, mounted inboard, drives a variable displacement pump 22. Pump 22 provides hydraulic fluid under pressure, through hydraulic conductors 24, to a reversible hydraulic motor 26 mounted on the outboard drive 28, outboard of the transom 29. A suitable hydraulic pump and motor system for use with a 250 horsepower engine producing 500 foot-pounds of torque is the EATON" Hydrostatic Transmission Model 76 pump and Model 54 motor.

The drive unit of the invention is shown in more detail in Figures 3 and 4. While in the preferred embodiment of this system, rotation is transmitted from the inboard engine 20 to the top shaft 30 of the outdrive by a variable hydraulic transmission including a reversible hydraulic motor 26 as described above, other sources of rotation from the source, either inboard or outboard, can be used to couple the rotation to the top shaft of the outdrive, such as by a conventional

engine/gear-type system. The top shaft 30 comprises a multi-strand sprocket 31 supported by an arrangement of bearings 32. The propeller shaft 34, located in the lower portion of the drive 28, comprises another multi-strand sprocket 35 and is supported by an arrangement of bearings 36. Propeller 38 is mounted on shaft 34.

Linking the two parallel shafts 30,34 is a fixed ratio chain reduction that may vary depending on the application, whether conventional or hydraulic. This reduction is achieved using a durable, multi-strand roller chain 40 such as a DIAMONDm multi- strand chain. The chain is lubricated by an oil bath (not shown) or pressurized stream lubrication, and kept taut using a standard form of idler arrangement as in Fig.

8. Typically, multi-strand chains consist of two more lengths of roller chain that are joined side by side to form a wide belt. By adding more strands of chain or changing the pitch (link size) of the chain, the amount of torque transmitted can be greatly increased without making the outer dimensions of the chain case any wider. Also by replacing the prior art bevel gear drive with a chain, the loads in the drive train become pure radial loads as opposed to combined radial and thrust loads. These pure radial loads require only radial-type bearings which are much smaller in diameter than a similar combined radial and thrust bearing used with bevel gear system. The advantages over the prior art systems are greater torque capacity, longer bearing life and improved durability, without compromising the streamlined casing profile.

The outer casing 43 may be fabricated or cast to include all appropriate hydrodynamic features, such as steering and planing fins. The drive unit 28 is mounted on the transom 29 by mounting bracket 42. Steering and trim are accom- plished using standard hydraulic steering cylinders 44 and trim cylinders 46.

By combining the aforesaid chain drive unit design with the hydraulic coupling from the engine to the drive unit, further benefits are obtained. Increasing or lowering the speed of the boat can be achieved through adjustment of the pump's flow control rather than varying the engine speed. The bi-directional hydraulic motor permits immediate shifting into reverse, and unlimited propeller speed is possible in both forward and reverse. This arrangement allows the engine to be operated

constantly in its most efficient range of engine speeds from the standpoint of greatest torque and fuel efficiency. This is generally a relatively low rpm, which prolongs engine life and reduces unburned fuel emissions. Also the universal joint connection is eliminated and the pump and outboard-located motor are connected only by hydraulic lines, which improves the design flexibility for location of the engine and pump within the boat and reduces the area of openings through the transom. Two outboard drives can be powered by a single inboard engine.

A second embodiment of the drive unit 150 of the invention, which provides an emergency tilt-up feature and a reduction gear on the chain drive, is shown in Figures 5 through 8. Drive unit 150 has upper casing 115, lower casing 128, propeller torque stop 126 and cavitation plate 134. The primary drive shaft 116 is driven by hydraulic motor 112 and comprises a multi-strand sprocket 101 supported by upper sprocket bearing 133. The propeller shaft 100 comprises multi-strand final drive sprocket 103, propeller shaft forward and reverse thrust bearing 121 and propeller shaft retaining nut 122 and is supported by propeller shaft load carrying roller 129. Seal carrier 127 holds the propeller shaft seal. Intermediate shaft 106 comprises primary reduction sprocket 104 and reduction sprocket 105 and is mounted on bearings 107 held in bearing carriers 124. Linking the two parallel shafts 116, 106 is the primary drive chain 119. Final drive chain 109 links parallel shafts 106, 100. Lubricating oil collects in lube oil sump 130 through lube oil drain holes 81.

Figure 8 illustrates that the primary reduction sprocket 104 is larger than reduction sprocket 105 and primary sprocket 101 and final drive sprocket 103.

A chain tensioner 102 bears against the return side of the chain, and can be switched from one side to the other depending on the direction of propeller rotation. This chain drive construction thereby permits a gear reduction of the engine speed to permit higher torque to be applied to the propeller.

Steering of the drive unit is accomplished in the usual way by steering cylinder 131, steering tiller arm 110 and kingpin 141, mounted in kingpin upper

bearing 113 and kingpin lower bearing 114. Activation of trim cylinder 171 causes tilting of the drive unit, as shown in Fig. 6. This embodiment also has an emergency tilt-up system 140 which permits the drive unit to pivot upwardly about pivot joint 220 when a submerged obstacle is struck, as shown in Fig. 7. When the drive unit is in forward drive, end 172 of trim cylinder 171 sits in open forward thrust socket 107. If a submerged obstacle is hit, end 172 of trim cylinder 171 is forced out of forward thrust socket 107, and drive unit 151 pivots upwardly on emergency tilt-up arm 132 which is attached at one end to end 172 of trim cylinder 171 and rotates at its other end in pivot joint 220. When the drive unit is in reverse, reverse lockup cylinder 118 is activated to close hook 160 and thereby lock end 172 of trim cylinder 171 in forward thrust socket 107.

Also according to the invention, an electrically-powered system can be substituted for the hydraulically powered system. In this case, an electric generator, driven by the internal combustion engine 20, is substituted for the pump 22.

Electrical conductors are substituted for the hydraulic conductors 24. A small DC electric motor, preferably reversible, of the type manufactured by General Electric or Siemens, is substituted for the hydraulic motor 26. Otherwise the operation of the electrical version of the invention is the same.

As will be apparent to those skilled in the art in the light of the foregoing disclosure, many alterations and modifications are possible in the practice of this invention without departing from the spirit or scope thereof. Accordingly, the scope of the invention is to be construed in accordance with the substance defined by the following claims.