Background of the Invention Ringed propellers are known and are intended to provide a compromise between a completely open bladed propeller and a fully caged propeller. Open bladed propellers suffer from the drawback that in operation the propeller is obviously dangerous if people need to be in the water in proximity to the propeller. In order to protect people from a rotating propeller, fully caged propellers have been proposed. However, fully caged propellers do not deliver the performance required by many users. Thus, ringed propellers have been proposed which increase safety by decreasing the probability that a person will come into contact with propeller blades of the ringed propeller because the ring of the ringed propeller will prevent ingress of body appendages into the path of the rotating blades. However, since ringed propellers are a compromise between a completely open bladed propeller and a fully caged propeller, they still do not offer the performance of a completely open bladed propeller.

Another problem with ringed propellers is their difficulty of manufacture. This is particularly the case if the ringed propeller is intended to be cast in a mould.

Because of the design of ringed propellers, casting in a two piece mould can be difficult because of the possibility of undercuts in the propeller design which makes separation of the mould parts difficult. Thus, when designing a ringed propeller for manufacture in a mould, it is often necessary to compromise the design so as to form a propeller shape which can be moulded in a two part mould, and in which the two parts of the mould can be separated to enable the propeller to be released from the

mould.

Summary of the Invention The object of a first aspect of the invention is to provide a ringed propeller which offers better performance than conventional ringed propellers.

The invention may therefore be said to reside in a ringed propeller comprising: a central hub for connection to a drive shaft to rotate the propeller ; a plurality of propeller blades connected to the central hub and extending outwardly of the central hub ; each blade having an outer tip ; a ring surrounding the blades and connected to the outer tips of the propeller blades ; the blades being arranged at a pitch angle ; and the ring having a leading edge and a trailing edge which are substantially parallel with respect to one another.

The provision of the parallel leading and trailing edges of the ring, together with the blades being arranged at a pitch angle, increases performance of the propeller.

Preferably the pitch angle is between 2.5% to 11. 3% less than the design pitch of an open bladed propeller for the same application.

In one embodiment the pitch angle is 3. 9% to 11. 3% less.

In another embodiment the pitch angle is 2. 5% to 8. 6% less.

Preferably the diameter of the propeller is 2% to 9. 2% less than the diameter of the design diameter of a comparable open bladed propeller.

In one embodiment the diameter is 3. 6% to 9. 2% less.

In another embodiment the diameter is 2% to 8% less.

Preferably the ring has an inner surface which is flat and an outer surface which is a hydrofoil section.

Most preferably the inner surface is parallel with the axis of the propeller.

Preferably the blade surface area of each blade is increased with respect to the design blade surface area of a comparable open bladed propeller.

The invention may also be said to reside in a ringed propeller comprising : a central hub for connection to a drive shaft to rotate the propeller ; a plurality of propeller blades connected to the central hub and extending outwardly of the central hub ; each blade having an outer tip; a ring surrounding the blades and connected to the outer tips of the propeller blades ; the propeller blades being arranged at a pitch angle which is between 2. 5% to 11. 3% less than the design pitch of an open blade propeller for the same application ; and the diameter of the propeller is 2% to 9. 2% less than the diameter of the design diameter of a comparable open bladed propeller.

In one embodiment the pitch angle is 3. 9% to 11. 3% less.

In another embodiment the pitch angle is 2.5% to 8. 6% less.

In one embodiment the diameter is 3. 8% to 9. 2% less.

In another embodiment the diameter is 2% to 8% less.

Preferably the ring has an inner surface which is flat and an outer surface which is a hydrofoil section.

Most preferably the inner surface is parallel with the axis of the propeller.

Preferably the blade surface area of each blade is increased with respect to the design blade surface area of a comparable open bladed propeller.

A second aspect of the invention is concerned with enabling manufacture of the propeller in a two part mould without the need to compromise the propeller design to facilitate manufacture in such a mould.

The invention may also be said to reside in a ringed propeller comprising: a central hub ; a plurality of blades connected to the central hub and extending outwardly of the central hub ; each blade having a blade tip ; a ring surrounding the blades and connected to each of the blade tips ; and the ring having an outer surface and an inner surface, the inner surface being substantially flat and the outer surface being a hydrofoil section.

The provision of an inner flat surface on the ring and the outer hydrofoil section facilitates manufacture of the blade, because it enables the blade to be formed in a two part mould, and for the two parts of the mould to be separated without interference from the ring shape, whilst at the same time not reducing the performance of the

propeller.

Preferably the pitch angle is between 2. 5% to 11.3% less than the design pitch of an open bladed propeller for the same application.

In one embodiment the pitch angle is 3. 9% to 11.3% less.

In another embodiment the pitch angle is 2.5% to 8. 6% less.

Preferably the diameter of the propeller is 2% to 9. 2% less than the diameter of the design diameter of a comparable open bladed propeller.

In one embodiment the diameter is 3. 6% to 9. 2% less.

In another embodiment the diameter is 2% to 8% less.

Most preferably the inner surface is parallel with the axis of the propeller.

Preferably the blade surface area of each blade is increased with respect to the design blade surface area of a comparable open bladed propeller.

Preferably each blade has a hydrofoil shape in cross section.

The invention may also therefore be said to reside in a ringed propeller comprising : a central hub ; a plurality of propeller blades connected to the central hub ; each propeller blade having a blade tip ; a ring surrounding the blades and connected to the blade tips ;

the blade having a fillet between the blade tip and the ring to prevent an undercut which would prevent separation of a two part mould used to mould the propeller.

Thus, the blade can therefore be more easily moulded in a two part mould without the establishment of undercuts due to the blade profile and shape which would otherwise lead to difficulty in separation of the two mould halves of the mould which is used to form the blade.

Preferably the radial thickness of the blade increases towards the tip of the blade to facilitate a smooth parting line for the two part mould, and also to lead into the fillet.

Preferably the blades are arranged at a pitch angle which is between 2.5% to 11. 3% less than the design pitch of an open bladed propeller for the same application.

In one embodiment the pitch angle is 3. 9% to 11. 3% less.

In another embodiment the pitch angle is 2. 5% to 8. 6% less.

Preferably the diameter of the propeller is 2% to 9.2% less than the diameter of the design diameter of a comparable open bladed propeller.

In one embodiment the diameter is 3.6% to 9. 2% less.

In another embodiment the diameter is 2% to 8% less.

Preferably the ring has an inner surface which is flat and an outer surface which is a hydrofoil section.

Most preferably the inside surface is parallel with the

axis of the propeller.

Preferably the blade surface area of each blade is increased with respect to the design blade surface area of a comparable open bladed propeller.

Brief Description of the Drawings A preferred embodiment of the invention will be described, by way of example, with reference to the accompanying drawings in which: Figure 1 is a rear view of a propeller according to the preferred embodiment of the invention ; Figure 2 is a cross sectional view along the line A-A of Figure 1 ; Figure 3 is a view of the detail B shown in Figure 2 ; Figure 4 is a cross sectional view along the line B-B of Figure 1 and Figure 5 is a side view of the propeller.

Detailed Description of the Preferred Embodiments With refrence to Figures 1 and 2, a ringed propeller 10 is shown which has a central hub 12. As is best shown in Figure 2, the central hub 12 has a central bore 14 defined by an inner wall 16. The inner wall 16 has an in-turned peripheral flange 18 at the leading edge of the hub 12.

An outer peripheral wall 20 surrounds the inner wall 16 and is connected to the wall 16 by three struts 22 which are best shown in Figure 1.

An annular passage 24 is therefore provided between the peripheral wall 20 and the inner wall 16 to provide an exhaust path for exhaust gases produced by the engine which drives the propeller.

As is best shown in Figure 2, the outer wall 20 has an outwardly flared section 26 at its trailing edge which is

attached to the wall 20 after moulding of the propeller 20. The outwardly flared wall 26 causes water to flow over the flared wall 26 and for a low pressure region to be established immediately behind the trailing edge of the hub 12. This low pressure region acts to draw exhaust gases through the annular chamber 24 and therefore assist extraction of exhaust gases from the engine which drives the propeller 12.

A plurality of propeller blades 30 are connected to the hub 12 and, in particular, to peripheral wall 20, and extend radially outwardly of the hub 12. The blades 30 each have a leading edge 32, a trailing edge 34 and a tip 36. The blades are preferably of helix shape and are provided at a predetermined pitch angle having regard to the use of the propeller and the motor with which the propeller is to be used.

The bore 14 receives an insert (not shown) formed partly of rubber and brass into which a drive shaft (not shown) is located so as to attach the propeller to the drive shaft so the drive shaft can rotate the propeller 10.

A ring 40 surrounds the propeller blades 30 and is connected to the tips 36 of the propeller blades 30.

As best shown in the detailed view of Figure 3, the ring 40 has an inner surface 42 and an outer surface 44. The inner surface 42 is flat and substantially parallel with the axis of the propeller. The outer surface 44 is of hydrofoil shape. The ring 40 has a leading edge 46 and a trailing edge 48. The leading edge 46 and trailing edge 48 are parallel with respect to one another. As previously noted, the blade 30 is of generally a helix shape which curves so that the blade has its leading edge which commences outwardly of the ring 40 towards the leading end 50 of the hub 12 (as is best shown in Figure

5). The tip 36 of the blade extends across the ring 40 from leading edge 46 to trailing edge 48, and with the trailing edge 34 extending back to the hub 12 at a location generally flush with the trailing edge 48 of the ring 40.

As can be best seen in Figure 3, the thickness of the blades 30 increase as the blades extend outwardly towards the ring 40. At the tip 36, the blade is provided with a fillet 52 (which is best seen in Figure 4) which provides a transition of the tip 36 to the ring 40 so as to prevent an undercut being provided by the profile of the blade 30, which would otherwise prevent separation of two mould parts which are used to form a mould to make the propeller 10 in a casting process.

The ring 40 has a small taper 71 at the trailing edge 48 for improved hydrodynamic characteristics, and a rounded bluff 72 at the leading edge 46 to also improve hydrodynamic performance. The rounding of the leading edge 46 is such that it does not produce an undercut when the moulding tool is manufactured. The outer surface 44 of the ring 40 is a short geometrical symmetrical version of the actual blade section with zero camber. This section shape imparts low drag at high speed. The ring 40 is preferably as thin as possible whilst still maintaining the shape of the ring and structural integrity.

The pitch of the blades 30 is slightly less than a comparable open bladed propeller for the same application.

Typically, it is about 2. 5% to 11. 3% less. In one embodiment the pitch angle is about 3. 9% to 11. 3% less, and in another embodiment the pitch angle could be about 2. 5% to 8.6% less. The pitch will obviously depend on the application and the motor with which the propeller is to be used, and as an example, for a 50 horsepower motor the pitch will be in the order of 13 to 14 inches, and for a

200 horsepower it would be in the order of 19 to 21 inches.

The diameter of the propeller is also less than the design diameter of a comparable open bladed propeller, and most preferably about 2% to 9.2% less. In one embodiment the diameter is 3. 8% to 9. 2% less, and in another embodiment is 2% to 8% less. To compensate for the smaller diameter, the cord length of the propeller blades 30 have been increased to maintain the correct blade area required for power transfer. Thus, the blade area of the propeller blades 30 is slightly greater than the design blade area of a comparable open bladed propeller.

The reduction of pitch is important to the preferred embodiment so as to extract maximum performance from the propeller, whilst not overstressing the engine which drives the propeller. The preferred embodiment of the <BR> <BR> <BR> invention enables the sectional shape of the components of the propeller to be extremely thin, which thereby reduces parasitic drag. For conventional open bladed propellers, such thin sections would result in structurally unsound blades that would be easily damaged. The propellers of the preferred embodiment can utilise thin sections because the ring acts as a supporting beam for the blade tips.

This greatly increases the structural moments of inertia about any direction.

Since modifications within the spirit and scope of the invention may readily be effected by persons skilled within the art, it is to be understood that this invention is not limited to the particular embodiment described by way of example hereinabove.