Field of the Invention This invention relates to improvements in shrouded or ringed propellers designed particularly for marine applications.

Background to the Invention Shroud or ring propellers are well known. They are formed with a ring or shroud of substantially circular configuration joined to the propeller blade tips. The propellers may be formed by welding or otherwise securing the ring to the blade tips, or by forming the ring and blades integrally. Such propellers are particularly desirable from a safety point of view when used with leisure water craft as the ring or shroud provides a barrier to an object engaging the blades through lateral relative movement. Many designs of ring propellers have been proposed to improve efficiency as compared to normal, unshrouded propellers and to ensure that the propellers are able to be manufactured relatively economically. However, it is relatively difficult to cast, mould or otherwise construct an efficient and effective ring propeller for a cost that is comparable to the cost of manufacture of an un-shrouded marine propeller, as the ring substantially complicates the manufacturing operation. A major design criterion of a ringed propeller is its strength and, when using materials and manufacturing techniques which can be susceptible to fatigue cracking, particularly fatigue strength. This is due both to the level of the stresses induced in the product during operation and the fact that these stresses oscillate, or cycle, at least once per revolution, which for a typical shaft speed in an outboard engine would be up to approximately 200,000 cycles per hour. A traditional approach, as applied to open bladed propellers, is to make the blade thick and stiff enough to carry the load. These designs generally have a thick blade root and taper towards the tip. Because these are "unconstrained" structures (essentially a simple cantilever, supported at one end), there is no difficulty in ensuring that the stresses are evenly spread. This makes it easier to design a product to avoid stress concentrations which may lead to fatigue cracks. A difficulty with ringed propellers, where the ends of the blades are constrained, is that the traditional approach to design, which would thicken up the blades until the maximum stress in the structure is within limit, would result in a blade thickness which would significantly reduce performance compared to the optimal blade thickness distribution..

Description of Prior Art Many forms of ringed propellers . have been proposed. United States patent number 4,370,096 discloses a propeller with a hub, three radially extending blades and a frusto-conical ring to which the outer ends of the blades are fixed such that the blades propel water through the space between the hub and the ring. The construction and shape of the ring is subject to various refinements to promote an efficient flow through the propeller. Thus, the inner surface of the ring is that of a frustum with a constant taper while the outer surface is convex. Such shapes are difficult to produce particularly if the propeller is formed by an injection moulding or die casting or die moulding process. In US patent number 4,836,748 to Church, there is described a ringed propeller in which the blades have parallel radial edges and are of constant cross-section. Reversing characteristics of the propeller are said to be improved by the provision of a plurality of apertures in the ring shroud. However, such apertures are found to diminish the hydrodynamic performance of the propeller. In US patent number 5,096,382 to Gratzer, a ring shrouded propeller is disclosed in which the shroud has an airfoil cross-section that varies between blade tips resulting in abrupt reversals in the shrouding section at each tip. It is said that this produces shroud transitional vortices that rotate oppositely to those formed by the blade tips. US patent numbers 5,405,243 and 6,102,661 disclose forms of propellers having the ring shaped to improve the hydrodynamic performance of the propeller. However, these propellers, while offering performance and efficiency characteristics comparable to an un-shrouded propeller, are complicated and difficult to economically manufacture, and do not necessarily have the fatigue strength required of a ringed propeller.

Summary of the Invention It is therefore desirable to provide a ring shrouded propeller which offers the desired characteristics in use and which is relatively economical to manufacture. It is also desirable to provide a ring shrouded propeller which is able to be manufactured with minimum manufacturing steps and machining processes. It is also desirable to provide an improved ring shrouded propeller which is of relatively simple, consistent design, but which offers the desired characteristics for a range of propeller sizes, engine .specifications and operating conditions. It is also desirable to provide an improved ring shrouded propeller which is hydrodynamically efficient in operation. It is also desirable to provide an improved ring shrouded propeller which is able to be manufactured by injection moulding, die casting or die moulding techniques. According to one aspect of the invention, there is provided a ring propeller comprising a hub, a plurality of outwardly extending propeller blades, and a shroud ring of substantially circular configuration connecting tips of the blades, said shroud ring having an outer surface which is either curved, or which has a forward section of substantially frusto-conical shape tapering inwardly in the forward direction with a rear section of the outer surface being curved or of frusto-conical shape, said shroud ring having an inner surface between blade tips that includes first and second parts each tapering outwardly away in opposite directions from a projected coaxial cylindrical surface that contacts said inner surface, each surface part extending around the ring in a circumferential direction, in the direction of blade rotation, from the area of inter¬ connection of the ring with the respective blade tip towards the trailing edge of the next blade, the intersection of the surface parts defining a line between the respective parts that runs from a position adjacent the leading edge of the ring adjacent one blade towards the trailing edge of the ring at or near the trailing edge of the next adjacent blade. Propellers formed in accordance with the invention may be of the type disclosed in the United States Patent Nos. 5,405,243 and 6,102,661 which disclose a shroud ring arrangement with relief points adjacent blade tips to enhance the efficiency of the propeller. However, the invention is also applicable to other propeller structures. The propeller of the invention is particularly suitable for manufacture by die casting or injection moulding with relatively simple dies or moulds. The triangular shaped, tapered sections of the inner surface of the ring adjacent each blade provides a draft angle axially outwardly from the blade surface in that area on the blade trailing surface where the ring is connected to the blade tip. Such a draft angle ensures that a moulded or cast propeller is able to be released from the die or mould. Still further, the provision of the tapered sections forming the increased draft angle adjacent the blades enables a propeller to be diecast or injection moulded with minimum post mould manufacturing processing. According to another aspect of the invention there is provided a ring propeller comprising a hub, a plurality of outwardly extending propeller blades, and a shroud ring of substantially generally circular configuration connecting tips of the blades, the blades each being formed with a blade tip portion that increases in thickness towards the ring, the tip portion also increasing in chord length distribution at the tip to form a forwardly extending, large radius fillet where the blade tip blends with an inner surface of the ring, and the leading edge of the ring being spaced forwardly thereof in the axial direction. In one embodiment, the propeller having the tip portion of increasing chord length combined with a blade skew distribution at the tip portion that forms the large radius blend onto the inner surface of the ring thereby provides structural qualities of fatigue strength with minimum blade thickness. Further, the high pressure surface of the blade is able to be conformed substantially to a helical surface for optimum hydrodynamic performance. According to another aspect of the invention there is provided a ring propeller including a hub, a plurality of outwardly extending propeller blades, and a shroud ring of substantially generally circular configuration connecting tips of the blades, the blades each having a root portion, connecting the blade to the hub, a mid-section and a tip portion, each blade having a generally rearwardly facing, high pressure surface and a forwardly facing, low pressure surface, the high pressure surface conforming generally to a substantially helical surface, the root portion having a blade thickness that decreases with increasing radius, the mid-section having a rate of decrease of blade thickness with increasing radius substantially less than that of the root portion, and the blade thickness of the tip portion increasing with radius to merge into an inner surface of the ring, said thickness changes being effected by changes to the low pressure surface away from a helical surface shape. Preferably, the blade root portion extends from 3% to 25% of the blade radius from the hub, more preferable from 5% to 15%, and most preferably about 10%. In preferred embodiments, the tip portion extends for between 75% and 98% of the blade radius, more preferably about 90%. Integrity of the high pressure surface is substantially maintained by ensuring that the thickness of the blade is designed such that it is supported during operation and that the stress levels and distribution are such that the structure of the propeller is sound. The high pressure surface is preferably designed to conform to a substantially helical surface which is designed for optimum hydrodynamic performance. The thickness distribution described above is therefore achieved by effecting changes to the low pressure surface only, apart for minor fillet radii at interfaces, thus diverging that surface away from the theoretical ideal, substantially helical surface, to thereby maintain the hydrodynamic integrity of the high pressure surface. The forwardly extending tip portion and fillet referred to above further assists in being able to maintain the substantially helical shape of the high pressure surface of the blade. In accordance with a still further aspect of the invention there is provided a ring propeller comprising a hub, a plurality of outwardly extending propeller blades, and a shroud ring of substantially generally circular configuration connecting tips of the blades, said shroud ring having an outer surface which is either curved or which has a forward section of substantially frusto-conical shape tapering inwardly in the forward direction with a rear section of the outer surface being curved or of frusto-conical shape tapering inwardly in the rearward direction, said blades each having a tip portion joining the ring, the tip portion including a blade/ring interface at which the blade tip portion curves forwardly from the line of a blade leading edge to merge into the ring. In preferred embodiments, the ring cross section is designed to maintain a consistent stiffness by maintaining a consistent second moment of inertia about its major axis. For rings with varying chord length, this will mean that the maximum width of the ring and the taper angle from that point (in the case of designs for injection moulding, die casting or die moulding, which require a draft angle) must be chosen to achieve this. Thus, at the points of the ring where the chord length is at its lowest, the ring must be at its widest (thickest) to maintain the same second moment of inertia as for regions of larger chord length. An additional feature of the blade / ring interface relates to the forward edge of the ring. This edge is positioned slightly ahead of the outer extremity of the leading edge of the blade (by approximately the radius of the fillet - between 1% and 3% mm of the outer ring diameter). This distance is sufficient to allow a blend radius to allow an even distribution of stress but is insufficient to significantly affect hydrodynamic performance. According to a further aspect of the invention there is provided a ring propeller comprising a hub, a plurality of outwardly extending propeller blades, and a shroud ring of substantially generally circular configuration connecting tips of the blades, said shroud ring having an outer surface which is either curved or which has a forward section of substantially frusto-conical shape tapering inwardly in the forward direction with a rear section of the outer surface being curved or of frusto-conical shape tapering inwardly in the rearward direction, said blades each having a tip portion joining the ring, the tip portion including a blade/ring interface defining a line between a first inner surface part of the ring and a second inner surface part of the ring, the first and second parts being angled relative to each other and to the axis of the propeller to define a mould release draft angle, said line extending in a circumferential direction for a short distance from the interface. 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 a view of a propeller blade and ring illustrating features of an embodiment of the invention; Figure 2 is an enlarged view of a detail of Figure 1; Figure 5 is an enlarged view showing a further detail of the propeller of Figure l; Figure 3 is an end view of the propeller of Figure 1 (excluding hub details); Figure 5 is a detailed view of one blade of the propeller of Figure 4; and Figure 6 is a cross-sectional view of a propeller blade and ring taken along the lines 6-6 of Figure 3, Figure 7 is a sectional view taken along the lines 7-7 of Figure 3, and Figure 8 is a view similar to that of Figure 7 but illustrating a further embodiment of the invention.

Description of Preferred Embodiments Referring to the drawings, Figures 1 to 7 show a ring or shroud propeller 10, with three blades 12, each blade 12 having a leading edge 14, and a trailing edge 16. The ring 17 is connected to the blade 12 at a tip portion 18 thereof, the ring 17 being concentric with the propeller hub 19. In this embodiment of propeller, as clearly seen in Figure 1, the chord length of the ring 17 varies around its circumference. The chord length is greatest at that point where the trailing edge 16 of the blade 12 merges with the ring 17 and is at its shortest part way along the length of the blade tip portion 18. The variation in chord length provides that a part 18a of each blade tip portion 18 has no annular ring portion radially outwardly of the blade on the high pressure surface, or trailing side, of the blade thereby defining a region permitting outward flow of liquid along the blades. To maximise fatigue strength, the ring cross section must be designed to maintain a consistent stiffness. The ring 17 of this embodiment, therefore, is designed to maintain a consistent second moment of inertia about its major axis. This means that the cross section of the ring 17 must vary so that, at the points of the ring 17 where the chord length is at its minimum, the ring must be at its widest (thickest) to maintain the same second moment of inertia as for regions of larger chord length. The varying cross sectional width of the ring 17 around its circumference is achieved, in the illustrated embodiment, by a first inner surface part 21 of the ring which is angled such that a line perpendicular to the surface at any point passes forwardly of the leading edge of the ring 17. A second inner surface part 22 which extends rearwardly from the first inner surface part 21, and in respect of which a line perpendicular thereto at any point will pass rearwardly of the trailing edge of the ring 17 defines a casting split line 23 around the inside of the ring. The casting split line 23 extends from a blade/ring interface 44, where the leading edge of the blade tip portion 18 merges with the inner surface of the ring 17 to a point 24 on or near to the ring trailing edgeadjacent the trailing edge of the next blade 12, and follows the trailing edge of the ring to the blade trailing edge. The split line 23 has an initial portion 23 a adjacent the blade ring interface which is substantially circumferential before sweeping outwardly to the trailing edge of the ring. This circumferential part 23 a of the split line 23 provides a land on the casting or moulding tool which, it is found, improves the performance of the tooling, minimises tooling cost and prolongs tool life. The land also facilitates an improved fillet radius at the blade/ring interface 44 in the propeller design thereby improving stress distribution and maximising fatigue strength. The outer surface of the ring 17 may take any one of a number of different geometric shapes and configurations. In one form of the invention, the outer surface of the ring 17 is curved. In another form, as illustrated, the outer surface of the ring 17 is formed of two intersecting surfaces 26 and 27. In this embodiment, the surface 26 is of substantially constant axial extent while the surface 27 has a varying axial extent which varies with the varying chord length of the ring 17 around its circumference. Both surfaces 26 and 27 may be of frustro-conical shape defining therebetween further casting split lines to facilitate the moulding or casting of the propeller. Referring to Figures 3 and 4, the leading edge 14 of each blade 12 is shaped so that the blade has a chord length distribution which decreases from a maximum, at a point radially spaced from the hub, to the tip portion 18, and then increases in the blade tip region. This increase in blade chord length at the tip portion forms a large radius blend from the blade leading edge 14 onto the ring 17, providing structural qualities of fatigue strength whilst maintaining the substantially helical nature of the high pressure surface and minimising the extent of the fillet 20 necessary to merge the blade tip portion 18 with the inner surface of the ring 17 and thereby optimising the hydrodynamic performance. A traditional blade design, which uses a chord length distribution which decreases with radius, at least for the outermost 20% of radius, would require a large connecting fillet 20 at the interface with the ring, which would not conform to a helical or hydrodynamically optimum blade surface, to provide the necessary fatigue strength. This represents a significant compromise to the hydrodynamic performance. With the present embodiment, the sweep forward of the blade leading edge 14 provided by an increase in chord length distribution over the last 10% of radius reduces the fillet size necessary at the interface 44 and further enhances the distribution of stress. As shown in Figure 5, to accommodate the sweep forward of the leading edge 14 of the blade 12, the leading edge 31 of the ring 17 is formed with an overhang 32 at the blade leading edge tip such that the ring leading edge 31 is forward of the tip leading edge. Thus, a large radius blend of the tip portion 18 leading edge onto the ring together with the ring overhang 32 of the tip portion leading edge provides the structural qualities required whilst enabling the high pressure surface 41 of the blade to maintain a substantially helical surface configuration which contributes positively to the hydrodynamic performance of the propeller The structure of the propeller ring 17 in accordance with this embodiment enables the propeller 10 to be formed by moulding or casting or other relatively simple and cheap manufacturing techniques, the structure permitting parts of the moulds or dies to be moved apart or away from the casting following the cast or moulding procedure. Thus, the reverse angled sections 22 adjacent each blade tip portion 18 provide a draft angle that enables die casting tools to release from the cast product. At the same time, the ring structure produces efficient hydro-dynamic effects minimising drag, minimising the creation of low pressure areas in the liquid flow, which might otherwise lead to cavitation and loss of thrust, and other undesirable effects such as tip vortices. This, therefore, optimises the thrust capacity of the propeller whilst ensuring that propellers of the invention are economically manufactured. The geometry of the ring also accords minimum negative interference with the flow of water over the blades, again to minimise detrimental performance effects such as inappropriate cavitation on the blades themselves, or loss of thrust via some other dynamic effect. Referring to Figure 6, the blades 12 of the propeller of the illustrated embodiment are formed with a varying thickness from the root, where the blade attaches to the hub 19, to the ring 17. Each blade 12 has a root portion 34, connecting the blade to the hub 19, a mid-section 36 and the tip portion 18. Each blade has a generally rearwardly facing, high pressure surface 41 and a generally forwardly facing, low pressure surface 42, the high pressure surface 41 substantially conforming generally to a helical surface. The root portion 34 has a blade thickness at the hub interface that is relatively great but that decreases sharply with increasing radius whereby the low pressure surface exhibits a curved fusing with the hub 19. The mid¬ section 36 of the blade 12 has a substantially reduced rate of decrease of blade thickness with increasing radius, but the blade thickness of the tip portion 18 increases again with radius such that the surfaces merge into the inner surface of the ring using fillets 20 and 20a (Fig. 5). It will be seen that the thickness changes along the length of the blade 12 are generally effected by changes to the low pressure surface 42, which changes the shape of that surface from the preferred helical surface shape. However, the high pressure surface is able to retain its preferred generally helical configuration. The blade root portion 34 may extend from about 3% to about 25% of the blade radius from the hub 19, more preferable from about 5% to about 15%, and most preferably about 10%. In preferred embodiments, the tip portion 18 extends for between about 75% and about 98% of the blade radius, more preferably about 90%. As indicated previously, integrity of the high pressure surface 41 is substantially maintained by ensuring that the thickness of the blade 12 is designed such that it is supported during operation and that the stress levels and distribution are such that the structure of the propeller is sound. The forwardly extending tip portion 18 and fillets 20 and 20a referred to above further assists in being able to maintain the substantially helical shape of the high pressure surface of the blade. Figures 7 and 8 show the cross-sectional shape of the ring in two embodiments of the invention. In Figure 7, the outer surface is formed by the two intersecting surfaces 26 and 27 referred to hereinabove. In the embodiment of Figure 8, the outer surface 43 is curved, in an airfoil shape. It will be appreciated by persons skilled in the art that numerous variations and/or modifications may be made to the invention as shown in the specific embodiments without departing from the spirit or scope of the invention as broadly described. The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive.