Field of the Invention This invention relates to improvements in propellers and relates particularly to an improved marine propeller that may be used with a variety of inboard or outboard engines. Background to the Invention Many forms of propeller are available or have been proposed to propel a marine craft. The prime function is to translate the rotating output shaft from the marine craft engine to forward (or reverse) propulsion. Consequently, the simplest forms of propeller have a hub from which propeller blades extend, the hub being adapted to directly engage the motor shaft. While such direct engagement may be satisfactory in certain applications, it is desirable that a propeller hub has additional functions, particularly on recreational marine craft. Such additional functions may include the need to absorb vibration and shock loadings that may be applied either from the engine shaft to the propeller hub or from the hub to the shaft. Such a shock loading may occur if the propeller strikes an object, for example. In the event of a large shock load such as might be sustained in a collision, it is preferable that the hub is constructed in such a way as to provide some form of protection to the drive train, including shafts, gears and drive engine. Many different forms of propeller hub and drive components have been proposed. A common feature of most propeller drives is that the output shaft from the marine engine is formed with parallel, axially extending splines to which the hub is to be fitted either directly or with some form of adaptor. Mould and Die tools used to manufacture propellers, however, are relatively expensive and the cost increases if different designs of hub and fittings need to be produced for the large variety of propellers used. Further, it is often necessary to change propellers on a marine craft when the nature of use of the craft changes. Thus, the propeller needed for towing a skier will probably be different to one for use in high speed pursuits, and may also be different to one to be used for a fishing expedition. It is therefore important that the propeller hub and drive components facilitate propeller change or replacement notwithstanding the harsh, corrosive environment to which propellers are subjected. Description of Prior Art United States Patent No 5,244,348 discloses a shock absorbing drive sleeve of moulded plastic that directly mounts the propeller to the drive shaft. The sleeve has splines at a rearward end portion to connect to the shaft and a forward outer diameter portion to connect to the propeller hub. The sleeve absorbs shock loadings by torsional twisting along its length. United States Patent No 6,478,543 discloses a relatively complicated torque ' transmitting device for use with a marine propulsion system wherein an adaptor is mounted on the drive shaft and has a second portion connected to the hub, the adaptor parts being connected by a plurality of titanium rods. Such an adaptor is relatively complex and is generally unsuited for low cost, low maintenance torque transmitting inserts. United States Patent No 5,201,679 discloses a propeller hub having a generally polygonal cross sectional shaped aperture and an insert having an outer shape corresponding to the aperture. The insert is of resilient material so that, in the even of slippage occurring between the propeller and the drive shaft, the bush will slip within the propeller hub. Other forms of shock absorbing drive sleeve have also been proposed. United States Patent No 5,630,704 discloses a sleeve having an asymmetric spring rate such that, when driven in one direction, the sleeve has a greater torque bearing capability than when driven in the other direction to thereby protect the weaker, reverse drive components of the driving gear train. None of the previously proposed hub designs and insert designs has achieved the object of providing a versatile, relatively economic, relatively safe but reliable drive system connecting a propeller to the drive shaft of a marine craft. It is therefore desirable to provide such a drive system which obviates at least some of the disadvantages of existing systems. It is also desirable to provide an improved structure for transferring torque from an engine shaft to the propeller blades through the hub. It is also desirable to provide an improved structure for transferring axial thrust from the propeller to the marine craft. It is also desirable to provide an improved structure for connecting a propeller hub to a drive shaft which absorbs vibration and shock loads. It is also desirable to provide an improved drive system which may be designed to enable the drive shaft to rotate without corresponding rotation of the propeller blades in the event of the blades contacting an object, but which, in other designs, greatly restricts independent rotation. It is also desirable to provide a propeller drive system which is adaptable to fit a plurality of different output drive shafts without the need for special installation equipment. It is also desirable to provide a drive system for a propeller which minimises shock loading to the propeller drive train due to sudden speed or gear changes. Summary of the Invention In accordance with one aspect of the invention there is provided a propeller drive for a propeller hub having an axially extending, non-circular, tapering aperture therethrough, the drive including a hollow drive bush formed of a resilient material, the bush having an external surface corresponding to that of the tapering aperture surface of the hub so that the bush closely engages within the hub aperture, the inner surface of the drive bush having a generally frusto-conical shape and having a plurality of spaced, generally axially extending grooves, a drive sleeve having a generally frusto-conical external surface that corresponds to the inner frusto-conical surface of the drive bush, with generally radially extending drive ribs to engage with the drive bush grooves, the sleeve having an internal surface to drivingly engage with a drive shaft. Preferably, the propeller hub has a conical aperture therethrough, the conical surface having a plurality of spaced ribs extending in the axial direction., and the hollow drive bush has an external surface complementary to the ribbed conical surface of the hub, including grooves to receive the ribs when the bush is engaged within the hub aperture. In one form of the invention the ribs have a radial extent defining a conical surface that is radially spaced from a conical surface defined by the drive ribs on the drive sleeve. In embodiments of the invention, the conical aperture is oriented so that its diameter increases from the rear to the front. Preferably, the conical surfaces of the hub, the drive bush and the sleeve all have substantially the same or similar cone angle. Preferably, the conical surfaces of the hub and the sleeve create, when assembled, a substantially enclosed volume in which sits the resilient drive bush. Thus, in one form of the invention, the resilient drive bush is in a substantially uncompressed condition when assembled but prior to use. In another form of the invention, the resilient bush is able to be compressed to a predetermined compression within the hub aperture. In some preferred embodiments, hub aperture has six ribs of substantially semi¬ circular cross-section. In other forms of the invention, the tapering aperture surface of the hub has a generally polygonal cross-sectional shape. The generally polygonal cross-sectional shape may have non-planar or concave surfaces bowed inwardly towards the hub axis with rounded corners where the surfaces merge with each other. In one form, the non-planar surfaces have a wave configuration. The hub may be formed with an annular shoulder at the rear end of the aperture. The shoulder may be abutted by the rear end of the resilient drive bush when the bush is engaged within the hub aperture. Alternatively, a space may provided between the rear end of the resilient drive bush and the shoulder and into which the bush may be compressed or deformed, either on assembly, if the bush is in a compressed state, or during use. An aft bush may be fitted to the drive shaft, the aft bush being held in position on the shaft by fastening means, such as a threaded nut and lock washer or other suitable securement. In one form, the aft bush has a forward end formed to engage a rear end of the sleeve in a manner to prevent relative rotation therebetween. Thus, lugs, shoulders, dogs or the like may be formed on the corresponding ends to restrict relative rotation. The forward end of the hub may be formed with one or more annular shoulders having a depth and diameter to accommodate different shapes of thrust washers commonly used with drive shafts of different marine engines. Thus, the hub is designed to be used with engines of several different engine manufacturers, and sleeves may be formed with internal drive structures corresponding with the drive shafts of the different manufacturers but with identical external shape and dimension to engage a common drive bush structure. Thus, the structure of the present invention enables the propeller hub to be used with several different drive shafts. The structure also separates the transmission of driving torque from the drive shaft, to the propeller hub from the axial thrust transmitted by the hub to the thrust washer. Thus, the functions of torque transmission and axial thrust transmission are independent and are performed by different design features. In one form of the invention, the drive keys of the drive sleeve extend at an angle to the corresponding drive grooves formed in the inner surface of the drive bush such that, on engagement, only a portion of a drive side of the drive splines engage a side of the respective drive grooves. Preferably, the resilience of the drive bush results in distortion thereof on application of increasing torque such that the greater part of the length of the drive grooves is brought into contact with the respective splines thereby providing varying torsional stiffness of the hub assembly minimising shock loading on components. The relative angles between the splines and corresponding grooves may lie between from 1° to 20° more preferably between 5° and 15°. In order that the invention is more readily understood, embodiments thereof will now be described with reference to the accompanying drawings. Description of the Drawings Figure 1 is an exploded view of a propeller drive in accordance with one embodiment of the present invention; Figure 2 is a sectional elevational view of engaged sleeve, drive bush and propeller hub on a drive shaft, in an embodiment of the invention; Figure 3 is a detailed perspective view of the drive sleeve, Figure 4 is a sectional detailed of assembled sleeve and drive bush of another embodiment. Figure 5 is a view similar to that of Fig. 4 showing a further embodiment, Figure 6 is a cross-sectional view from the rearward end of a hub showing a modified form of hub, drive bush and drive sleeve, Figure 7 is a view similar to that of Figure 6 but showing a further modified form of hub, drive bush and drive sleeve, Figure 8 is a view similar to that of Figure 6 showing another modified form of hub, drive bush and drive sleeve, and Figure 9 is a view similar to that of Figure 6 showing a further modified form of hub, drive bush and drive sleeve, Description of the preferred embodiments Referring to the drawings, a propeller drive in accordance with a first embodiment of the invention comprises a propeller structure which, in the illustrated embodiment of Figures 1 to 3, is a ring or shroud propeller 16 having a hub 17 from which extends three propeller blades 18 connected at their outer ends or tips by the ring or shroud 19. It will be understood that the invention, however, is not restricted to a ring or shroud propeller but may be used with any form of marine propeller. The hub 17 has an axial, substantially conical opening 21 therethrough which increases in diameter from the rear end 22 of the hub 17 to the forward end 23. The rear end has an annular, inwardly extending projection defining a shoulder 24 at the rearward-most end of the opening 21, and a plurality of ribs 26 are formed in the conical surface and extend axially forwardly from the annular shoulder 24. The ribs 26 are generally equally spaced circumferentially around the conical surface and have a height corresponding generally with the height of the annular shoulder 24. A hollow, drive bush 27 is adapted to engage within the conical opening 21 of the hub 17. The drive bush 27 is formed of a resilient material, preferably a synthetic plastics material such as synthetic rubber, Hytrel (Trade Mark), or the like elastomer. The outer surface of the drive bush 27 corresponds with and is closely received by the conical surface of the hub 17, and is formed with a plurality of grooves 28 to receive the ribs 26. Preferably, the ribs are of a substantially semi-circular sectional shape and the grooves 28 have a corresponding shape. As shown, the grooves extend from the rear end of the drive bush 27 but terminate rearwardly of the forward end of the drive bush 27. The length of the grooves corresponds closely with the length of the ribs 26. The inner surface of the drive bush 27 is similarly conically shaped, the respective cone angles being substantially the same or similar. The inner surface of the drive bush is formed with a plurality of circumferentially spaced drive grooves 29 extending generally axially rearwardly from the forward end of the drive bush 27 but terminating forwardly of the rearward end of the bush. Preferably, the drive grooves 29 have a cross sectional shape which is substantially quadrilateral, preferably rectangular. A drive sleeve 31, which may be formed of metal or substantially rigid synthetic plastics material, is adapted to engage within the drive bush 27 the drive sleeve 31 has an external conical surface 32 to closely engage the internal conical surface of the drive bush 27. The sleeve 31 is formed with a plurality of axially extending, circumferentially spaced drive splines or keys 33 which are shaped to engage within the drive grooves 29 of the bush 27. In one form of the invention, the keys 33 closely engage within the drive grooves 29. The drive sleeve 31 has a flange 34 on its forward end, and an axial opening therethrough of a shape corresponding to that of a drive shaft 38 to which the drive sleeve 31 and propeller hub 17 is to be fitted. In the illustrated embodiment, the drive shaft 38 is splined and the drive sleeve 31 is formed with corresponding internal splines 37. The embodiment illustrated includes an aft bush 41 adapted to engage the drive shaft 38 at the rear end 22 of the hub 17. The aft bush 41 may be formed with splines to engage those of the drive shaft 38. More preferably, however, the aft bush has no direct connection to the drive shaft 38 but has recesses 42 which engage corresponding rearwardly extending lugs 43 formed in the rear end of the drive sleeve 31. A lock washer 44 and securing nut 46 engage the end of the drive shaft 38 to lock the aft bush against the drive sleeve 31. The aft bush 41 has an outwardly extending flange 47 which engages behind the annular shoulder 24 at the rear end 22 of the hub 17. In the assembled state, the drive bush 27 fits within the space defined by the outer surface of the drive sleeve 31 and the conical surface of the hub 17. The forward end of the hub 17 is formed with one or more shoulders 51 having axial and radial dimensions to receive any one of a number of different sizes of thrust washers 52 that are used with the different drive shafts 38 of various manufacturers of marine engines and drive trains. With the hub 17 engaged on the drive shaft 38, the forward end 23 engages the relevant thrust washer 52 to transfer axial thrust from the propeller and hub 17 to the drive shaft 38 whilst rotational torque transmission is effected through the engagement of the drive shaft splines 39, the drive sleeve 31, the drive bush 27 driving through the ribs 26 to the hub 17. The various conical surfaces of the components are of substantially the same or similar cone angles so that they are substantially parallel to one another or closely aligned thereby allowing components to be accommodated within relatively small spaces without loss of functionality. This allows the propeller drive of the invention to be designed to suit small diameter hubs, and therefore smaller propellers, than has been previously accomplished. The propeller drive of the present invention is able to be fitted without special tooling or equipment. The structure of the invention therefore allows the propeller drive components to be supplied as a kit with one or more different drive sleeves 31 designed for different drive shafts of the various marine engine manufacturers. The propeller drive of the invention is, therefore, extremely flexible in its application and economical to manufacture and supply. Thus, for any given size, one propeller with various drive sleeves will suit similar sized marine engines from a number of different manufacturers. As the drive sleeves 31 may be formed by injection moulding, casting, die-casting or other relatively simple and cheap manufacturing techniques, the costs of the sleeves is extremely small by comparison to costs of other components such that it may be economic to include three or four sleeves with each propeller kit. The ribs 26 on the inside of the hub and the corresponding grooves 28 on the drive bush are the principal interface through which torque is transferred to the hub 17 in the embodiment shown in Figures 1 to 3. A large frictional force is not the principal means of torque transfer. Referring to Figures 4 and 5, there is shown a modification of the invention in which the drive keys 33 on the outer conical surface 32 of the drive sleeve 31 extend at an angle to the corresponding drive grooves 29 formed in the inner conical surface of the drive bush 27. Thus, when the drive bush 27 is engaged with the drive sleeve 31, only a portion of the drive side 61 of the drive spline 33 engages the side 62 of the groove 29. However, as torque is applied to the drive sleeve 31, the resilience of the drive bush 27 causes a greater extent of engagement between the keys 33 and the grooves 29. At large torques, the resilience of the drive bush 27 results in distortion thereof such that the greater part of the length of the drive groove 29 will be brought into contact with the spline 33. This has the effect of providing varying torsional stiffness of the hub assembly as a whole thereby minimising shock loading on drive components and engine components. The progressive engagement of the keys 33 with the sides of the respective grooves 29 may be achieved by a number of different structures. Thus, the relative angles of the keys 33 and corresponding grooves 29, or parts thereof, may be varied from 0° up to or even greater than 20°, with the initial contact occurring at either the rearward edges or the forward edges. In other arrangements, the progressive engagement may be effected by appropriately shaping either or both the keys 33 and grooves 29, as shown in Figure 5. The torsional stiffness of the hub assembly as a whole and the inherent damping characteristics of the material of the drive bush 27 is such that shock loads and vibration will be dampened rather than being transmitted un-attenuated through the drive train to the engine and/or marine craft. Of great advantage, however, is that the structure of preferred embodiments of the invention enables the hub, drive bush and drive sleeve to be designed so as to enable a predetermined amount of slippage to occur between the propeller hub 17 and the drive shaft 38 above a critical torque level for given applications. Thus, the design of the hub 17, the resilient drive bush 27, the drive sleeve 31, and the keys 33 and grooves 29 may be such that slippage will occur between the outer conical surface of the drive sleeve 31 and the inner conical surface of the drive bush 27 when the torque levels reach a predetermined amount. This slippage occurs by compression and distortion of the material of the resilient drive bush adjacent the grooves 29 caused by the keys 33 such that the keys 33 move out of their corresponding grooves 29. However, the slippage may occur without destruction or damage to the drive bush 27 so that, when the torque level is reduced, the keys 33 reengage within the grooves 29 and the driving force will be reapplied to the hub 17 and the propeller 16. By facilitating slippage between the drive bush 27 and the drive sleeve 31, damage to the hub 17 is obviated. Of course, in other embodiments of the invention, it may not be keys but other rotationally symmetrical features that allow this disengagement and re- engagement feature to operate. Even in the unlikely event that a drive bush 27 is damaged or destroyed, such as due to the introduction of foreign matter or the like, or heat, the radial clearances are such that the respective ribs 26 and keys 33 will not clash against each other but, due to the substantially enclosed nature of the cavity in which the drive bush 27 is located, little of the material thereof will escape thereby allowing some transmission of torque between the keys 33 and the ribs 26 and hub 17 for a user to "get home". However, the design of the hub 17, the resilient drive bush 27, the drive sleeve 31, and the keys 33 and grooves 29 may also be such that slippage will not occur between the outer conical surface of the drive sleeve 31 and the inner conical surface of the drive bush 27 when the torque levels reach a predetermined amount. Thus, in some applications, while the structure will minimise shock loadings on drive components, the design and materials used will ensure the drive shaft will not rotate independently of the propeller for a significant angular displacement (i.e. as great as a whole revolution). While the drive bush 27 need not be compressed within the hub 17, it may be desirable in some installations to compress the bush 27. In this case, the drive bush 27 is compressed within the hub 17 by the aft bush 41. The conical design of the propeller drive of the invention, acting as a wedge, allows a substantial pressure to be produced with relatively little force applied to the aft bush by the securing nut 46. The relative compression of the sleeve may be used to vary the slippage torque of the system so that a predetermined amount of slippage can be controlled by controlled tightening of the nut 46. Thus, if the propeller drive of the invention is to be used in an application where slippage is undesirable, the nut 46 can be tightened thereby compressing the drive bush 27 and reducing the possibility of slip. Conversely, in an application of the drive system where the propeller 16 is likely to strike objects or the like, increased slippage can be effected by reducing compression of the drive bush 27 by loosening the nut 46. Although the invention is described with the orientation of the nested cones such that they increase in diameter from rear to front, the invention is equally applicable if the orientation is reversed and the cones face rearwardly, increasing in diameter from front to rear. Different thrust interfaces may be provided. For example, in one arrangement, the aft bush would become a forward bush, preferably incorporating a thrust interface for the thrust washer of the drive system. In another arrangement, a forward bush could be omitted, and the forward end of the hub would be the thrust interface, as previously described. With the orientation of the frusto- conical surfaces reversed, the resilient drive bush may be designed to be axially compressible by tightening the securing nut, the wedge action of the drive sleeve creating a significant pressure in the resilient drive bush. In the absence of a forward bush, the compression would be achieved without a positive location being achieved. This would allow the pressure in the resilient drive bush to be adjustable thereby altering the release or slippage torque of the system. Such an arrangement would also allow for variations in dimensions of the hub system of the invention due, for instance, to manufacturing tolerances, while still ensuring that the system performs to its optimum potential. Alternatively, with a forward bush in position as a thrust interface, the compression may be limited to a predetermined amount by a positive location of the forward movement of the rear bush contacting the forward bush. It will be understood that if axial thrust were to be transmitted through the resilient sleeve, which is possible in the preferred orientation shown in the drawings due to its conical shape, then the release or slippage torque would be affected by the amount of axial thrust present at the time. Therefore, it is advantageous that the axial thrust transmission be independent of the torque transmission. Referring to Figures 6 to 9, there is illustrated different forms of cross-sectional shapes of the internal surface of the hub 17, the drive bush 27, and the drive sleeve 31. With the cross-sectional shape shown in Figure 6, the hub 17 is formed with generally axially extending grooves 128 to receive cooperating ribs or projections 126 extending from the drive bush 27. The drive bush 27 retains the drive grooves 29 in its inner surface but these are aligned with the ribs 126. The drive sleeve 31 is substantially the same structure as shown and described with reference to Figures 1 to 5. Figure 7 illustrates a modified drive bush 127, drive sleeve 131 and hub 117. In this arrangement, an axially tapered internal surface of the hub 117 is of a generally polygonal shape with, in the embodiment illustrated, six sides 141. Each tapered side 141 is of a non-planar, wave configuration that merges with adjacent sides into radially, outwardly extending, rounded corners 142 that approach the configuration of the ribs 126 of the previous embodiment. The drive sleeve 131 is also formed with an outer surface configuration which is similar to that of the drive bush 127, with the rounded corner portions 142 of the drive bush aligned with similar rounded corner portions 143 of the drive sleeve 131. The embodiment of Figure 8 is similar to that of Figure 7 but the tapered internal surfaces of the hub 217 and corresponding sides 241 of the drive bush 227 are concave in cross-sectional shape with less pronounced rounded corner portions 242. In the embodiment of Figure 9, the tapered internal surfaces of the hub 317 and corresponding sides 341 of the drive bush 327 are substantially planar and merge smoothly into rounded corners 342. The outer surface of the sleeve 331 is similarly shaped These configurations provide varying characteristics of transmission of drive torque from a drive shaft to a propeller with appropriate slip functions for different applications, drive sizes and desired outcomes.