A propeller operates by generating a thrust to drive a body, such as an aircraft or boat, by applying a force to the fluid in which it operates in order to change the momentum of the fluid in the direction opposite to that in which it is desired to drive the body. Each blade of the propeller, screw etc. is inclined to the desired direction of thrust, so that, as it rotates through the fluid, the normal reaction force has a component parallel to the direction of drive, the orientation being set to ensure that this force tends to push the fluid in a direction opposite to the desired direction of travel. Under Newton's first law, action and reaction are equal and opposite, and hence the fluid applies an equal and opposite force to the screw, propeller, etc. which reaction force is transmitted to the body and in turn drives the body through the fluid.

Conventional screw design is based on the works of Archimedes (c. 250BC) and was later modified by Francis Petit Smith, and a typical such marine screw comprises a number of blades, normally 2 to 4, which are fixed to a hub either with their longitudinal axis perpendicular to the axis of the hub or inclined thereto towards the back of the craft on which the propeller is attached so as to produce aft rake. The blades are attached to the hub in a symmetrical pattern and each blade typically has a curved outer profile, normally formed by ogival or aerofoil sections, tapers outwardly from root to tip and twist from root to tip so that the tip of the blade has a greater angle of attack to the incident fluid than the root.

This conventional configuration has, however, a number of disadvantages. The shape of the blades tends to generate lift, and the resulting pressure differences can lead to early cavitation. Also, the twist or skew on a typical blade, which results in a variation in pitch along the blade, makes the propeller more complicated and hence more expensive to produce. Whilst attempts have been made over the years to reduce the impact of these drawbacks and also to improve efficiency, reduce cavitation etc of propellers, any improvements that have been achieved have been relatively small, and involved minor variations in blade shape, twist, size and taper without changing the underlying design of the marine screw.

According to a first aspect of the present invention there is provided a propeller comprising a hub having a plurality of blades attached to an outer surface thereof with the longitudinal axis of each blade is being inclined to the longitudinal axis of the hub towards the front of the hub so as to produce a forward rake angle.

A propeller according to the first aspect of the invention has the advantage that the forward rake angle contributes to water flow into the blade, thereby increasing efficiency.

Preferably, the hub is formed as a frustro-conical body with each blade being fastened to the hub with its longitudinal axis perpendicular to the tapering outer surface thereof. In this way, the rake angle of the blades is set in a reliable way by the taper angle of the external surface of the hub. The hub may alternatively be formed with a cylindrical outer surface, in which case each blade is attached thereto with its longitudinal axis inclined to the surface at the required rake angle.

According to a second aspect of the invention, there is provided a propeller blade having a root and a tip and at least a first blade portion extending between said root and said tip, said first blade portion having an arcuate cross section and being in the form of a longitudinal segment of a hollow truncated cone defined between two planes which extend longitudinally of the cone, are inclined to each other and which each intersect each other along the longitudinal axis of the cone.

A propeller blade in accordance with the second aspect of the invention has the advantage that it is effective across the maximum blade area, thereby improving efficiency.

Furthermore, in the case of a marine screw, lift at the bow of the craft is reduced and straight-line propulsion is maximised. This reduces cavitation and aeration under the stern, which means that the classic aft trough is greatly reduced and the hull therefore runs much flatter in the water than with a conventional screw. The result is a stable and longer waterline length that means less interaction between bow and stern wave systems, thereby reducing wake.

The blade section is preferably formed as the segment of a truncated cone which is of circular cross section, so as to provide a propeller blade having a root and tip and at least a first blade portion extending between said root and said tip, said first blade portion having an arcuate cross section whose radius of curvature varies continuously from the root to the tip of the blade, in particular increases continuously from root to tip. The blade section may, however, also be defined by an annular segment of a cone of elliptical or other cross- sectional shape.

Preferably, the blade has a constant angle along it length, that is the blade does not skew or twist between root and tip. Furthermore, the first portion of the blade is preferably symmetrical, and advantageously has a constant thickness both across the blade section and along the length of the blade, although it has been found to be advantageous for the leading and trailing edge to be chamfered or rounded to provide a smooth transition between the upper and lower surfaces.

In a particularly advantageous embodiment, the blade includes a second blade section which extends between said root and said tip of the blade, said second blade portion having an arcuate cross section and being in the form of a longitudinal segment of a hollow truncated cone, said segment being defined between two planes which extend longitudinally of the cone, are inclined to each other and which intersect the longitudinal axis of the cone, said first and second sections being arranged side by side facing in opposite directions so as to form a sigmoid cross section. In this way, a blade is formed which has a cupped leading edge and a reversed pressure face. This arrangement has the advantage that it enables the blade to operate effectively for both providing both forward and reverse drive.

The first and second blade portions may be of identical shape, being fastened together with one portion rotated through 180 degrees relative to the other. This has the advantage of producing a symmetrical blade cross-section. In an alternative arrangement, however, which has been found to be particularly advantageous, the second or trailing blade portion is formed with a smaller radius or curvature than the first, in particular from a cone which is of smaller radius than that used to form said first blade portion, although the longitudinal tapering characteristics of the two portions should preferably be the same, so the ratio of the curvatures of the two portions at each point along the profile of the blade is constant.

The two planes which define the shape of the or each blade portion are preferably inclined at 45 degrees to each other. In this way, if the portions are produced from a cone having a circular cross-section, eight blade portions may be produced from each cone.

According to a third aspect of the present invention, there is provided a propeller comprising a hub having a plurality of blades each in accordance with the second aspect of the invention extending substantially radially therefrom. In particular, the third aspect of the invention provides a propeller according to the first aspect of the invention having a plurality of blades each according to the second aspect of the invention.

In order that the invention may be well understood, there will now be described an embodiment thereof, given by way of example, reference being made to the accompanying drawings, in which: Figure 1 is a perspective view of the propeller of the invention ; Figure 2 is an end view of the propeller of Figure 1 ; Figure 3 is a front view of the propeller of Figure 1; Figure 4 is an end view of the propeller of Figure 1; and Figure 5 is a perspective view of a truncated cylindrical cone used to manufacture the blades of the propeller of the invention.

Referring first to Figure 1, there is shown a propeller of the invention comprising a hub 1 having a pair of blades 3 connected to its outer surface 5. The hub 1 has an axial opening 2 extending therethrough which is configured for non-rotatably mounting the propeller on a suitable shaft such as the propeller shaft of a boat. The non-rotatable coupling may be achieved by forming the surface of the through opening as a conical friction surface which frictionally engages a corresponding surface formed on the propeller shaft, by forming teeth on the surface of the through opening which mate with complementary teeth formed on the propeller shaft, or by other drive coupling means well known to the skilled person.

As best shown in Figure 1, the hub 1 takes the form of a hollow frusto-conical boss so as to provide a tapering outer surface 5, with the blades 3 being mounted with their axis perpendicular to the outer surface 5 (not to the axis of the hub 1) such that the axis of each blade is inclined to the longitudinal axis of the hub 1. The hub 1 is then mounted, in use, on a propeller shaft or the like so that it points in the forward direct of the vessel (that is with its narrow end pointing towards the front of the vessel) and the blades 3 are tilted forwards, producing a forward rake angle equal to the cone angle of the outer surface 5 of the hub 1.

In this way, in operation, the propeller produces a divergent thrust pattern and hence a diverging wake. It is, of course, also possible, within the scope of the invention, use a cylindrical hub, i. e. with a non tapering outer surface, and in attached the attached the blades 3 to the outer surface thereof at an acute angle to as to achieve the required rake angle.

Each blade 3 may be attached to the outer surface 5 of the hub 1 in any well known fashion such as by screw thread, welding, casting as a unitary assembly with the hub etc, and may either be rigidly mounted to as to have a fixed pitch or may be rotatably mounted thereon with conventional means being provided to control the pitch angle of the blades.

The blades 3 themselves have a symmetrical sigmoidal cross-section 7 which is of constant thickness both from leading 9 to trailing edge 8 and also from root 10 to tip 11 as can best be seen in Figure 4. Each blade 3 is also straight, i. e. include no root to tip twist, but tapers inwardly from tip 11 to root 10 as best seen in Figure 2. The blade section is formed by two identical segments each cut from a hollow frusto-conical body as shown in Figure 5. More particularly, each segment is formed by making two longitudinal cuts 21,22 along the cone 20, each cut being along a plane 21a, 22a which includes a radius to the cone and said two planes being inclined to each other and intersecting along the longitudinal axis 23 of the cone. The two segments are fastened together along longitudinal edges with one segment being rotated 180 degrees about its longitudinal axis, and hence being reversed relative to the other. The two oppositely facing sections 3a, 3b typical of the sigmoid shape are thereby produced, giving each blade a cupped leading face 13 and reversed pressure face 12.

This results in a symmetrical blade shape which operates well in both forward and reverse modes. The two sections 3a, 3b of the blade 3 are preferably fastened together by welding, but other fastening means may also be used.

In the preferred embodiment, each section 3a, 3b of the blade 3 describes an arc of 45 degrees, the sections being produces by cutting the frusto-conical body radially into eight equal segments. However, both the radius of curvature of the sections 3a, 3b, and the angle of the arc described by each may be varied to suit the particular application.

The pitch of each blade is controlled by varying its angular orientation on the bub 1 and hence the angle of attack. This is set in degrees measured from a zero point perpendicular to the shaft line, and adjustment of this parameter moves the whole blade, including the root, as opposed simply to varying the twist of a conventional propeller during which the angle of attack of the root of the blade is not varied.