BACKGROUND OF THE INVENTION The standard outboard motor is widely used for marine or riverine propulsion. However, when all that is required is some propulsion means to propel a boat, or some other marine craft, relatively short distances, the purchase of an outboard motor is often an expensive option.

For example, owners of yachts or similar vessels who must moor their vessel in deep water use some form of tender to access their moored vessel. At present, the tender used can be an oar propelled rowing boat or a boat powered by an outboard motor. Upon reaching the moored vessel, the tender is attached to the mooring to prevent it drifting away. When an outboard motor is used to propel the tender, the outboard motor is usually lifted off the tender and loaded onto the yacht to ensure that an opportunistic thief does not disappear with the unattended tender and outboard motor. However, due to the weight of the motor, this is often a difficult and laborious task. On the other hand, this may be less tiring than physically rowing out to the mooring site in a rowing boat. Further, the upkeep and running costs for an outboard motor are relatively high considering that it is primarily being used merely to propel the tender to the moored vessel, not to mention the cost of purchasing an outboard motor for this purpose initially.

OBJECT OF THE INVENTION The object of the present invention is to disclose a portable, light-weight marine or riverine propulsion device that is thought to be cheaper to buy than a typical outboard motor.

SUMMARY OF THE INVENTION The object is preferably achieved with a pre-existing brush cutter device of the user who is then able to purchase a propeller attachment at low cost and then assemble a modified brush cutter device including the propeller. Preferably, the tube housing the shaft of the propulsion device is mounted on a swivel attached to the stern of the craft. This permits the person controlling the propulsion device to vary the angle at which the propeller shaft engages the water. The swivel allows rotation of the tube in azimuth relative to the swivel in order to steer the craft. Therefore, there is no need for a rudder. The depth at which the propeller sits in the water may also be varied by adjusting the altitude angle of the tube. This is particularly useful when travelling across shallow water. Additionally, the swivel preferably permits the tube to slide longitudinally through the swivel to bring the propeller either closer to, or further from, the"stern"of the craft.

The attachment for a brush cutter device is preferably able to be disconnected from the shaft attached to the engine of the brush cutter. In this way, the main component of the brush cutter containing the engine and to which the marine propulsion attachment is connected has a dual purpose in that either the abovementioned propeller attachment or a conventional lawn or vegetation trimming blade cutting attachment can be connected.

Furthermore, the use of this marine propulsion device is not limited to the propulsion of boats and has equal application in the propulsion of surfboards, surf skis and like craft.

Traditionally, craft within this category have relied on a user paddling, the wind or the waves to propel them through the water. This can often be a laborious or time consuming method of propulsion.

In accordance with the first aspect of the present invention there is disclosed a marine propulsion attachment for a brush cutter device comprising an engine and a first shaft rotatably mounted within a tube and rotatable by said engine, said attachment comprising a propeller connected to a second shaft, a protective shroud for said propeller and rotatably supporting said second shaft, and connection means to interconnect said shafts to permit their simultaneous rotation by said engine.

According to the second aspect of the present invention there is disclosed a marine or riverine craft propulsion device, said device comprising a brush cutter having an engine, a shaft rotatably mounted within a tube and rotatable by said engine, a propeller rotatably mounted within a protective shroud for said propeller and rotatable by said shaft, a mounting swivel adapted to be mounted on said craft receiving said tube and permitting longitudinal movement of said tube relative to said swivel, rotation of said tube in an azimuth plane relative to said swivel, and rotation of said tube in altitude relative to said swivel.

According to the third aspect of the present invention there is disclosed a surfboard, surf ski or like craft having an upper person supporting surface and an under surface, a brush cutter device having an engine, a shaft rotatably mounted within a tube and rotatable by said engine, a propeller rotatably mounted within a protective shroud for said propeller and rotatable by said shaft, said tube and shaft being substantially aligned with the longitudinal axis of said craft and passing therethrough between said surfaces, said engine being located above said upper surface and said propeller being located below said lower surface, whereby said propeller is rotatable by said engine to advance said craft through water, and said craft is steerable by altering the weight distribution of the person using said surfboard, surf-ski or like craft.

DESCRIPTION OF THE FIGURES Three embodiments of the present invention will now be described with reference to the drawings in which: Fig. 1 is a perspective view of a prior art brush cutter with its the brush cutting attachment disconnected from the engine-driven shaft, Fig. 2 is a perspective view of a first embodiment of the marine propulsion attachment, Fig. 3 is a side perspective view of the attachment of Fig. 2, Fig. 4 is an end view of the attachment, Fig. 5 is a cross-sectional view of the attachment taken along the line V-V of Fig. 4, Fig. 6 is an enlarged view of the interior of a drive reduction gear box able to be located within the attachment of Fig. 2-5, Fig. 7 is a partial view of an alternative pair of bevelled gears for use in the gear box of Fig. 6, Fig. 8 is a side view of a dingy embodiment in use, Fig. 9 is a plan view of the dingy of Fig. 8, Fig. 10 is a side view of a canoe embodiment in use, Fig. 11 is a side view of a surf-ski embodiment, Fig. 12 is a plan view of the surf-ski of Fig. 11, Fig. 13 is a partial inverted plan view of the surf-ski of Fig. 11, Fig. 14 is a partial plan view of the front of the surf-ski of Fig 11, Fig. 15 is a view of a swivel for use in the canoe or dingy embodiments.

DETAILED DESCRIPTION Turning now to Fig. 1, it is evident that the prior art brush cutter 1 takes the form of a shaft 2 housed within a tube 3 and connected such that the shaft 2 is rotatable by an engine 4. The engine drives the rotation of the shaft 2 via a centrifugal clutch (conventional-but not illustrated). A brush cutting attachment 5 can be connected to, and disconnected from, the free end 6 of the shaft 2 by various conventional means including threaded engagement or fasteners of various types (not illustrated).

A first form of a marine propulsion attachment 7 is illustrated in Figs. 2-5. The attachment 7 takes the form of a twin blade low pitch propeller 10 fixed to a propeller shaft 8. The shaft 8 is rotatably mounted in an end bearing 9 supported by a three-armed spider 11. A pair of longitudinally spaced cylindrical shrouds 12 support the spider 11 and protects persons from injury by coming into contact with the tips of the blades 13 of the propeller 10.

Additionally, the protective shrouds 12 and their supporting struts 19 guard the blades 13 against damage from coming into contact with submerged objects while the propeller 10 is rotating.

A sleeve 15 having a bearing 16 for the shaft 2, is retained in the free end of the tube 3 by a mechanical clamp 14 in an interference fit. The shaft 2 is secured to the propeller shaft 8 by means of a grub screw 17, or similar. As the engine 4 drives the rotation of the shaft 2 which is in turn connected to the propeller shaft 8, the propeller 10 fitted rotates simultaneously with the shaft 2.

The speed of the shaft 2 generated by the engine 4 is approximately 6,000 revolutions per minute (rpm) since this is a practical speed for use of the device as a brush cutter. However, when used for marine propulsion, it is sometimes desirable for the engine speed to be lowered in order to rotate the propeller 10 at a slower speed. This produces less cavitation and enables blades of greater pitch to be used. Thus a shaft speed of approximately 3,000 rpm is preferable to accommodate such propellers. This reduction in engine speed is achieved through the incorporation of a drive reduction gear box 20, the interior of which is illustrated in Fig. 6. The gear box 20 is preferably incorporated into the attachment 7 at the junction of the shafts 2 and 8.

As seen in Fig. 6, the gear box 20 includes a keyed output shaft 21 and a keyed input shaft 22. Mounted on shafts 21,22 are corresponding gear wheels 23,24. The shafts 21,22 are also supported by bearings 25,26 and 27,28 respectively. The output shaft 21 is also preferably provided with a substantially waterproof seal 29. Alternatively, as indicated in Fig. 7, the gear wheels 23,24 can be replaced by beveled gears 33,34.

A first embodiment of a marine craft is illustrated in Fig. 8, in which the tube 3 is mounted in a swivel 36 at the stern 37 of a dingy 38.

The swivel 36 is shown in Fig. 15 and includes a substantially U-shaped bracket 50 configured to mount to the stern 37. Although not illustrated, in use the U-portion is placed over the stern 37 with a sleeve 51 disposed on an outer arm of the bracket facing away from the stern 37. A threaded ram 52 is then wound to secure the bracket to the dingy 38 where an abutment surface 53 attached to the ram 52 clamps the stern 37 to the bracket 50.

The sleeve 51 receives a rotatable shaft 54. The shaft 54 includes a groove at one end for receiving a retaining clip 55 to (upwardly) restrain the shaft 54 within the sleeve 51.

The other end of shaft 54 includes a receiving bracket 56 having a pair of opposing spaced apart apertures 57. A tab 58 extending from a head member 59 includes an aperture 60, the tab 58 being configured to be rotatably and retainingly engaged in the space between the aperture 57 by an axle 61. Axle 61 is itself retained by a head 62 at one end and a retaining clip 63 at the other.

The head 59 further includes a pair of jaws 64 which clamp on a portion of the shaft 3 to thereby retain it. The jaws 64 are hingedly mounted so that one jaw is attached to the tab 58 and a screw 65 and wing nut 66 engage a tab 67 on each jaw to clamp the jaws to the shaft 3 or allow the jaws to open about their hinge to release the shaft 3.

When retained by the jaws, the shaft 3 is rotatable in the plane of the water by rotation about the shaft 54 and rotatable perpendicular to the plane of the water by rotation of the swivel 36 about axle 62.

No matter whether the user sits, kneels or stands, the swivel 36 allows the altitude angle of the tube 3 to be varied so as to allow manual positioning of the propeller 10 in the water by either raising or lowering the engine 4. This is indicated by arrows A in Fig. 8. The depth of engagement between the water and tube 3 is also adjustable by sliding the tube 3 longitudinally through the swivel 36 to alter the distance of the propeller 10 from the stern 37 of the dingy 38. This is indicated by arrows B in Fig. 8. As seen in Fig. 9, the swivel 36 also allows rotation of the tube 3 in azimuth relative to the swivel 36 to steer the dingy 38 left or right in the same way a rudder steers a marine craft. This is indicated by arrows C in Fig. 9.

A second embodiment of a canoe 40 is illustrated in Fig 10 where the tube 3 is mounted on a swivel 36 as before but offset from the side of the canoe 40. In this embodiment, the bracket 50 of the swivel 36 is configured to mount to the side wall of the canoe 40.

A third embodiment of a surf-ski 43 is illustrated in Figs. 11-14 where tube 3 and shaft 2 pass through the centre of the surf-ski 43. The engine 4 is above the supporting surface 44 and includes only one protective shroud 12 which is below the immersed surface 45. The surf-ski 43 is steerable by the user changing his weight distribution on the supporting surface 44.

The engine 4 is also encapsulated by a waterproof protective cover 47 that guards the engine 4 from being immersed when the surf-ski 43 moves through waves. The waterproof protective cover 47 is also fitted with a conventional snorkel breathing attachment (not illustrated) that allows air to pass through it and into the space defined by the interior of the waterproof protective cover 47 within which the engine 4 is contained. The snorkel contains a valve that allows the flow of air into the space to the engine 4 but prevents water from passing through the snorkel when the surf-ski 43 travels through waves. Such snorkels are conventionally found on many jet-skis.

It will be apparent that the above described embodiments provide many advantages. A low cost and light-weight propulsion unit is provided which is suitable for many applications in rivers, lakes, estuaries and the like.

The foregoing describes only three embodiments of the present invention and modifications, obvious to those skilled in the art, can be made thereto without departing from the scope of the invention. For example, in addition to the swivel 36, the engine end of the tube 3 can be supported by the free end of a tiller like arrangement. Similarly, an electric motor can replace the petrol engine 4 and the term'engine'is understood to include such a motor.

The term"comprising"as used herein in is used in the inclusive sense of meaning"having" or"including"and not in the exclusive sense of"consisting only of'.