CA1127468A

CA1127468A - Marine propellers

Info

Publication number: CA1127468A
Application number: CA356,924A
Authority: CA
Inventors: John R. Coxon
Original assignee: WOODCOXON ENGINEERING (INTERNATIONAL) Ltd
Current assignee: WOODCOXON ENGINEERING (INTERNATIONAL) Ltd
Priority date: 1979-09-07
Filing date: 1980-07-24
Publication date: 1982-07-13
Also published as: KR830003331A; EP0025260A1; SG45083G; NO802572L; IN152435B; DK378080A; EG15036A; IE801869L; JPS5639992A; IE50118B1; DK160005C; ATE3394T1; KR840000541B1; GB2058231A; NZ194232A; GB2058231B; MX150877A; DD153786A5; DE3063309D1; AU6012880A

Abstract

Improvements in marine propellers ABSTRACT

A variable-pitch marine propeller comprises two or more helicoidal blades 6 which are mounted on a hub 1 so that each blade is free to pivot about a pivot axis 25 which extends radially outwards from the hub 1.
The pivot axis 25 of each blade 6 is spaced in front, in the direction of rotation 24 of the propeller, of the centre of pressure of the blade 6 so that in operation, the water pressure acting on the blade 6 exerts a torque which tends to turn the blade 6 about its pivot axis 25 in a direction to bring the surfaces of the blade into line with the flow of water over the blade. The pivot axis 25 is also spaced behind with respect to the direction of movement of the axis of rotation of the propeller through the water, at least a major portion of the pressure surface of the blade so that, in operation, the resultant of the drag of the water on the blade exerts a torque which tends to turn the blade about its pivot axis 25 in an opposite direction. Additionally, and most importantly, the shape and mass distribution of the blades 6 relative to their pivot axes 25 is such that centrifugal effects tend to move the blades, in the absence of hydrodynamic forces, into a pitch equal to that of the helicoid. The various torques acting on the blades are such that, in operation, over a range of rotational and axial speeds, each blade 6 adopts a stable equilibrium position in which its pitch is optimally suited to the speed of rotation and the linear axial speed of the propeller.

Description

~3LiZ7~

A problem which arises with propeller-driven marine craft, and especi-ally with small high-speed planing motor boats is that a fixed-bladed propell-er is very inefficient over some part of the speed range of the craft. If a propeller of coarse pitch is used which operates efficiently when the craft is moving at a speed at or near its maximum, a great deal of cavitation is pro-duced when the craft is starting from rest or moving at a slow speed. In con-sequence the fuel consumption o-f the engine of the craft is higher than it need be at low speeds and the acceleration of the craft to higher speeds is also much less than it could be if the propeller were able to operate effici-ently over a wider range of speeds. Indeed, the problem is so pronounced thatwith some very high speed racing boats, the cavitation is such that no thrust at all is produced when the boat is stationary and it is necessary for the boat to be towed up to a certain minimum speed before it can be propelled by its own engine and propeller.
This problem can be overcome entirely by the use of a variable pitch propeller. Most existing variable pitch propellers are hydraulically operated and are heavy, complex and consequently expensive.
It has previously been proposed, in German Specification No. 410401, published February 26, 1925, to make a marine propeller which comprises two or more blades which are mounted on a hub so that the blades are free to pivot about a pivot axis which extends outwards from the hub with a radial compon-ent. Each blade is provided ~' ~z~

at its trailing edge with a trim tab which is so inclined to the remainder of the blade that "~hen ~he propeller is in operation, the tab exerts a torque on the blade which turns the blade about its pivot axis and holds it at a substan-tially constant angle of at~ack to the stream of water passing over the surfaces of the blade.
As far as is known, however, propellers as described in German Specif-ication No. ~10401 have never been made commercially and it is thought that this is because the provision of the trim tabs increases the drag of the water on the blades to such an extent that the advantage gained from the free pivot-ing of the blades to maintain a substantially constant angle of attack islargely nullified.
It has also been proposed in British specification No. 1,414,362 published November 19, 1975, to make a marine propeller with blades which are freely pivoted on a hub so that they can turn about radial axes which are off-set rearwardly, considered in relation to the direction to which the propeller moves axially through the water, from the pressure faces of the blades. The pivot axes are also in predetermined positions with respect to the leading edges of the blades and the location of the pivot axes in this way causes the resultant of the hydrodynamic forces acting on the blades to cause them to be ~0 self-adjusting in pitch.
Whilst this propeller may to some extent operate in the manner intend-ed, it is believed that it has never been exploited on a commercial scale. It is thought that this may be because the blades are not self-adjusting in a stable manner over a sufficiently wide range of speeds and also because the blades will not remain stable at the optimum pitch when the craft to which the propeller is fitted is moving at its designed cruising speed. The maintenance of an optimum pItch at cruising speed is an essential requirement of any viable variable-pitch propeller because if the propeller does not have a sufficiently high efficiency at cruising speed, any other advantages 5. which may accrue are of no avail.
The ef~ect of centrifugal forces acting on the blades is mentioned in British Specification No. 1,414,362, but it is said that this effect is of secondary importance.
10. It is also stated in ~ritish Specification No. 1,414,362 that self-adjusting variable-pitch marine propellers have been proposed for many years, but no viable construction has been produced '~itherto.
This is believed to be true and indeed is still true 15. upto the time of the making of the present invention.
We have now produced a marine propeller of the kind comprising two or more blades which are pivotally mounted on a hub so that they are free to pivot about a~es extending radially outwards from the 20. hub, the blades being arranged so that, in operation, they reliably adopt a pitch which is suited to the speed of rotation of the propeller and to the speed through the water of the craft to which the propeller is fitted, the pitch being both stable and substantially 25. optimum over a wide range of speeds and especially at the designed cruising speed of the propeller.
The invention is based on the discovery that amongst othèr criteria, far from being secondary, the centrifugal effects acting on the blades are of 30. paramount importance and must be specifically related to the hydrodynamic forces which also act on the blades.
The rake of the blades relative to their pivot axes and the shape of the blades, especially the location of the trailing edge portions of the blades, in relation 35. to their pivot axes have also been found to be critical.
Thus, according to the present invention, in a marine propeller comprising two or more blades which are pivotally mounted on a hub so that they are ~ ~7 ~ ~

free to pivot about axes extending rad~ly outwards from the hub, the pivot axes being displaced rearwardly, considered in relatio~ to the direction in which, in operation, the propeller moves axially through the water, 5. of the pressure faces of the blades, the blades and their pivot axes have the following features:-a) The blades are helicoidal;
b) The mass distribution of each blade relative to its pivot axis is such that the 10. centre of mass of the blade is spaced behind the pivot axis of the blade considered in relation to the direction of rotation of the blade and such that, when the propeller is rotated, in the absence 15. o~ hydrodynamic ~orces, centrifugal effects cause the blade to adopt a pitch substantially equal to the pitch of the helicoid;
c) Each blade is raked rearwardly relative to 20. the propeller plane with a mean angle of rake of at least 10 multiplied by the Pitch Ratio of the propeller and divided by the Aspect Ratio of the blade; and, d~ Each blade has a skewed-back shape ~ --25. with the trailing tip of the blade spaced behind the pivot axis of the blade, considered in relation to the direction of rotation of the blade, by a distance equal to atleast 60% of the maximum width of 30. the blade, and the position of the pîvot axis in relation to the shape and the rake angle of the blade is such that, in operation, hydrodynamic lift and drag on the blade acting in combination with 35. the centrifugal effects cause the blade to adopt, over a range of rotational and axial speeds, a position such that it has 6f~

an angle of incidence to the stream of water passing over it which produces a substantially optimum thrust.
Since the propeller has a variable pitch, the Pitch Ratio is defined as the pitch of the helicoid to which the blades are formed divided by the diameter of the propeller. The Aspect Ratio of the blade is defined as the ma~imum radius of the blade measured from the axis of rotation of the propeller divided by the maximum width of ~he blade and is thus inversely proportional to the Blade Width Ratio. The pressure face of the blade may be substantially straight as seen in section on the propeller reference line and in this case the rake angle of the blade is constant. Alternatively the pressure face may be curved as seen in this section and in this case the rake angle will vary from the root to the tip of the blade. The mean angle of rake is the mean angle between a plane perpendicular to the axis of rotation of the propeller and the pressure face of the blade in section on the propeller reference line.
l~hilst the pivot axes of the blades may extend outwards in planes which are exactly radial to the axis of rotation of the propeller, they may alternatively be inclined to some extent to radial planes and the term "ex~end-ing radially outwards" is intended to be construed as covering both of these O arrangements provided that the axes extend outwards from the axis of rotation of the propeller with major radial components. Further, the pivot axes may lie in a plane normal to the axis of rotation of the propeller and for most purposes this is preferred. In some cases, however, the pivot axes may be raked either forwards or rearwards from this plane.
With a propeller having all the characteristics just described, the blades will adopt a stable pitch which is suited to the rotational and axial speeds of the propeller over a wide range of ~Z7~6~

both of these speeds. It is believed that such stability has not previously been achieved.
Preferabl~ the pivot axis of each blade is so located that, when the blade is pivoted into 5. a p~sition of minimum pitch, a plane containing the pivot axis and the axis of rotation of the propeller divides the blade area in a ratio of substantially 3:1, subst~ntially one quarter of the area being in front of the pivot axis and substantially three 10. quarters of the area being behind the pivot axis in the direction of rotation of the propeller.
Each blade may be pivoted so that it can only turn about its pivot axis within predetermined limits, which are set by stops 3 to provide a variation in 15. pitch between a minimum and a maximum. In this case, if the propeller is driven in an astern direction, it will always adoptits maximum pitch and there will be no self-adjustment. Preferably therefore, the blade are pivotally mounted so that they can rotate 20. freely in all directions. ~ith this arrangement, if the propeller shaft is rotated in an ahead direction, the blades will turn to produce an angle of attack to provide forward thrust and when the propeller shaft is rotated in an opposite direction, the blades will 25. turn about their pivot axes through almost 180 de~rees to give the same angle of attack in an astern direction and hence a reverse thrust. Owing to this rotation of the blades throughalmost 180 degrees, the pivot axes of the blades are still spaced behind the pressure faces 30. of the blades since the blades are now travelling through the water in an opposite axial direction.
Each of the blades may be pivotally mounted on - the hub entirely independently of the other blades and this, for most purposes, is the preferred arrangement.
35. Alternatively, however~ the blades may be mechanically interconnected within the hub so that they are constrained to turn about -their pivot axes in unison and all the blades adopt the same instantaneous pitch.
The blades are preferably, as is usual, of aerofoil cross-section and then the pressure acting . on the blade as the blade is rotated is increased by the hydrodynamic lift of the blade. The total drag on the blade is also increased insofar that the drag then consists of the frictional drag of the water on the blade together with a drag component of the 10. hydrodynamic forces acting on -the aerofoil section.
Two examples of propellers in accordance with the invention will now be described with reference to the accompanying drawings, in which:-Figure 1 is an exploded perspective view of 15. one example;
Figure 2 is an axial section through the first example showing one of the blades of the propeller in plan, that is as seen in a direction in which the blade presents a maximum projected area;
20. Figure 3 is an elevation of one of the blades of the first example as seen looking radially inwards towards the axis of rotation of the propeller;
Figure 4 is a section as seen in the direction of the arrows on the line IV-IV of ~igure 3; and, 25. Figure 5 is an axial section through a second example showing a part only of one of the blades.
The first example illustrated in Figures 1 to 4 has helicoidal blades, the pitch of the helix being 200mm. The diameter of the propeller is also 30- 200mm so that the Pitch Ratio of the propeller is 1.
The blade width is 124mm and the Aspect Ratio is accordingly approximately 0.8.
The propeller shown in Figures 1 to 4 has a hub 1 formed in two parts la and lb. The parts 35- la and lb mate on a central plane which is normal to the axis of rotation of the propeller and are fixed together by three screws 2 which pass freely 13L'~7~

through bores 3 in the part la and are screwed into tapped bores 4 in the part lb. The parts la and lb also have a central bore 5 in which, in use, a propeller shaft fits.
5. The propeller has three blades 6 which are identical to each other and the blades are all pivotally mounted on the hub 1 in the same way as each other. Accordingly only one of the blades and its attachment to the hub 1 will be described.
10. The blade 6 is cast integrally with a circular boss 7 which has a cylindrical recess 8 in its underside and has a central countersunk bore 9 which is coaxial with the pivot axis about which the blade 6 is freely rotatable relative to the hub 1.
15- A radial and thrust ball bearing comprises a rotatable bearing ring 10 with a projecting collar 11 and two fixed bearing rings 12 and 13. A first ring of balls 14 is interposed between the rings 10 and 12 and a second ring of balls 15 is interposed between 20. the rings 10 and 13. The bearing is assembled and it is then inserted in a cylindrical socket 16 in the hub 1. The socket 16 is formed as the hub parts la and lb are mated with each other, and as will be seen, the the bearing assembly can only be inserted before the 25- hub parts la and lb are mated with each other and then-fixed together and once the hub parts have been fixed together, the bearing assembly is held in position in the hub by an inwardly directed flange 17.
The boss 7 of the blade is then fltted over 30- the socket 16 containing the bearing assembly and over the flange 17 with the rim of the boss 7 fitting within an annular groove 18. The assembled posi~ion is shown most clearly in Figure 2.
To hold the blade 6 with its boss 7 in 35- position, firstly a pin 19 is inserted through a small aperture 20 in the boss 7 and then into a registering aperture 21 in the collar 11. This prevents the bearing ring 10 from rotating relative to the boss 7 and then a screw 22 is inserted through the bore 9 and is screwed into a tapped bore 23 in the collar 11. This clamps the underside of the bOss 7 tightly against the upper surface ol the collar 11 as shown most clearly 5. in Figure 2 so that the boss 7 is able to rotate with the bearing ring 10 which is itself freely rotatable within the socket 16.
As isshown most clearly in Figure 2 the ring of balls 14 wi-thstands radial loads on the bearing 10. assembly and also axial loads radially outwards along the pivot axis of the blade. The ring of balls 15 withstands inward axial thrust.
In this example the pivot axes of all three blades lie in a plane which is normal to the axis of 15. rotation of the propeller9 that is the æis of the bore 5. The blades move through the water in the direction of an arrow 24 shown in Figure 2. The centre of pressure of the blade is spaced behind the pivot axis 25 of the blade, that is nearer the trailing edge of the 20. blade, but this distance varies in dependence upon the angle of incidence of the blade and upon other factors.
Thus the resultant P of the pressure acting upon the blade acts at a variable dis~nce ~ from the axis 25 as isshown in Figure 3. ~s is also shown in Figure 3, 25. the resultant D of the drag on the blade acts at a distance d ~rom the pivot axis 25 and this distance also varies to some extent. ~owever the torques on blade produced by the resultant pressure and drag act in opposite directions.
30. As is shown in Figure 4, the blade has a rake angle 27 of 15 degrees. In this example the pressure face of the blade is straight in the section shown in Figure 4 and therefore the rake angle is constant. The blade may however be radially curved 35- so tha-t the rake angle varies radially. It is the mean rake angle which is then of importance.
The pivot axis 25, as seen in Figure 2 9 divides the blade into an area 28 in fron-t of the pivot axis and an area 29 behind the pivot axis. The area 29 is substantially three times the area 28.
The skewed-ba`ck shape of the blades together 5. with their ra~e relative to their pivot axes and the location of the pivot axes causes the mass distri~ution of the blades relative to the pivot axes and to the axis of rotation of the propeller to be such that centrifugal effects move the blades until their pressure 10. faces lie substantially on a common helicoidal surface of 200mm pitch when the propeller is rotated in a vacuum and at a speed such that gravitational forces become negligible.
The second example shown in Figure 5 of the 15. drawings is the same in all respects as the first example except that the blades are interconnected within the hub 1 by meshing gearwheels so that the blades are all constrained to turn about their pivot aY~es in unison with each other.
20. For this purpose, the hub 1 has a socket 16' of somewhat greater radial extent than the socket 16 of the first example. Also, in place of the bearing ring 10 of the first example, there is a bearing ring 10', which has a greater radial extent than the bearing ring 25. 10 and is provided with bevel gear teeth 30. The hub 1 comprises a part 1~ similar to the part la of the first example and a part llb which is similar to the part lb of the first example except tha-t it is provided with an axially extending annular groove 31 ~hich is concentric 30. with the bore 5 and intersects the soc~ets 16'. The annular groove 31 contains a bevel gear wheel 32 which is supported by a ball bearing 33 and has bevel gear teeth 34 which mesh with the teeth 30 of the bearing rings 10' of all three blades.
35. Propellers in accordance with the invention have very great advantages which vary in dependence upon the purpose of the craft to which the propellers ~1 2 7 ~ ~ ~

are fitted. Thus in small outboard motor boats, such as are used for towing water skiers, acceleration of the boat may be greatly improved and is greatly helped in pulling the skier quickly throu~h the 5. critical speed at which the skier's ski or skis start to plane. Further, and this is of the grea-test importance in the present days of fuel shortage, with displacement hulls and other hulls which are intended to be operated over a quite a wide range of lO. speeds, owing to the ability of the propeller to adapt its pitch to the speed of the boat, the efficiency of the propeller is maintained at a maximum value over the whole speed range of the boat. This gives rise to a very great drop in overall fuel consumption when 15. the boat is being driven at any speed below the maximum which can be produced by the engine with which it is fitted. Not only does the drop in fuel consumption give rise to considerably increased economy, but it also produces a greatly increased range for a boat 20. with a given fuel tank volume. This can be of considerable importance particularly ~or fishing boats.
Propellers in accordance with the invention can also be used to advantage on trawlers. Trawlers are required to sailto their fishing grounds at 25. a speed which is as high as possible subject to the requirement of reasonable fuel economy, but when fishing they are required to sail very much more slowly and yet their propellers must produce su~ficient thrust to drag the trawl. A fixed bladed propeller cannot be 30. efficient under both these circumstances and it is not unusual therefore for trawlers to be fitted with propellers the blades of which can be adjusted to either one of two different pitche-s. This adjustment is, however, carried out hydraulically or by a complex 35. mechanical arrangement and such propellers are therefore very expensive. Propellers in accordance with the invention will achieve the same desirable effects as these variable pitch propellers, but at a much smaller 6~3 cost.
Propellers in accordance with the invention can produce an astern thrust on a boat moving forwards very much more quickly than can a conventional fixed-5. bladed propeller. This enables the boat to be stoppedvery much more quickly and greatly improves saf~ty.
The reason for this is that with a fixed-bladed propeller, the direction of flow of the water over the surfaces of the blades is such that when the propeller is first 10. rotated in an astern direction as opposed to moving ahead, the cavitation produced by the propeller is very great indeed and in consequence the astern thrust is minimal. With propellers in accordance with the present invention, however, even though the propeller 15. shaft may be rotated at full speed in an ahead direction and then be at once reversed and rotated at full speed in an astern direction, the blades will at once assume their c~rrect angle of attack relative to the direction of thestream of water passing over their surfaces.
20. Accordingly considerable astern thrust is at once developed.
Finally, propellers in accordance with the invention have great advantages when used on steeply inclined propeller shafts. The ef~iciency of fixed-25. bladed propellers falls rapidly with an increase ofinclination of the shaft on which the propeller is mounted because the inclination causes the angle of incidence of the blades to vary in each revolution as the propeller rotates. The blades of propellers in 30. accordance with the invention, however, oscillate about their pivot axes when fitted to inclined shafts and the pitch of the blades thus varies cyclically as the propeller rotates. This gives rise to a remarkable increase in efficiency. This advantage is of particular 35. significance with hydrofoil craft where very steeply inclined shafts cannot be avoided.

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

l. A marine propeller comprising two or more blades which are pivotally mounted on a hub so that they are free to pivot about axes extending radially outwards from the hub, the pivot axes being displaced rearwardly, considered in relation to the direction in which, in operation, the propeller moves axially through the water, of the pressure faces of the blades, wherein the blades and their pivot axes have the following features:
a) the blades are helicoidal;
b) the mass distribution of each blade relative to its pivot axis is such that the centre of mass of the blade is spaced behind the pivot axis of the blade considered in relation to the direction of rotation of the blade and such that, when the propeller is rotated, in the absence of hydrodynamic forces, centrifugal effects cause the blade to adopt a pitch substantially equal to the pitch of the helicoid;
c) each blade is raked rearwardly relative to the propeller plane with a mean angle of rake of at least 10° multiplied by the Pitch Ratio of the propeller and divided by the Aspect Ratio of the blade; and d) each blade has a skewed-back shape with the trailing tip of the blade spaced behind the pivot axis of the blade, considered in relation to the direction of rotation of the blade, by a distance equal to at least 60% of the maximum width of the blade, and the position of the pivot axis in relation to the shape and the rake angle of the blade is such that, in operation, hydrody-namic lift and drag on the blade acting in combination with the centrifugal effects causes the blade to adopt, over a range of rotational and axial speeds, a position such that it has an angle of incidence to the stream of water passing over it which produces a substantially optimum thrust.
2. A propeller according to Claim 1, in which the pivot axis of each blade is so located that, when the blade is pivoted into a position of minimum pitch, a plane containing the pivot axis and the axis of rotation of the pro-peller divides the blade area in a ratio of substantially 3:1, substantially one quarter of the area being in front of the pivot axis and substantially three quarters of the area being behind the pivot axis in the direction of rotation of the propeller.
3. A propeller according to Claim l, in which the pivot axes of the blades lie in a plane normal to the axis of rotation of the propeller.
4. A propeller according to Claim l or 2 or 3, in which the blades are freely rotatable in all directions about their pivot axes.
5. A propeller according to Claim l, in which the blades are mechanically interconnected within the hub so that they are constrained to turn about their pivot axes in unison and all the blades adopt the same instantaneous pitch.
6. A propeller according to Claim 5, in which the blades are mechanically interconnected by meshing gear wheels in the hub.