WO1993009026A1

WO1993009026A1 - An elastomeric propeller having a flexible elastomeric covering

Info

Publication number: WO1993009026A1
Application number: PCT/DK1992/000312
Authority: WO
Inventors: Nils Oluf EHRENSKJÖLD
Original assignee: Gori Af 1902 As
Priority date: 1991-10-30
Filing date: 1992-10-29
Publication date: 1993-05-13
Also published as: CA2122477A1; DK179791D0; AU2941392A; EP0609389A1; JPH07500549A

Abstract

A propeller for e.g. a ship has a central hub (34) with a plurality of propeller blades (12). Each blade (12) is composed of a flexible elastomer covering (14) of e.g. rubber which is vulcanized firmly on a core (13). The core (13) and the elastomer covering (14) are moulded and vulcanized to an integral unit which, without expensive after-treatment, forms a propeller having a finished smooth surface and a consistency capable of absorbing and damping the impact and shock pulses from the reaction forces of the water. The elastomer covering is not vulcanized firmly on the entire surface of the core, thereby forming on both sides of the core a compartment (16) which can be filled via a filling valve (18) with a fluid under pressure, e.g. air or water. The elastomer covering is adapted such that when deformed it passively adjusts the pitch of the blade optimally to the instantaneous state of operation. When selectively pumped-up, the propeller can moreover actively be given the geometrical shape best suitable for any purpose.

Description

An elastomeric propeller having a flexible elastomeric covering

The invention concerns a propeller for e.g. a ship and . 5 having a central hub with a plurality of propeller blades.

Propellers for e.g. ships are extensively moulded either integrally or with hub and blades separately of metals, such as aluminium alloys, bronze and steel. After moulding

10 the blades often have to undergo a relatively expensive subsequent working to obtain the close accuracy in terms of shape and the smooth surface necessary for the pro¬ peller to work with as low a friction as possible in the water. All the metals used have a great coefficient of

15 elasticity causing the blades to be stiff and maintain their shape unchanged under practically all conceivable . conditions of operation. The geometrical design of such metal propellers will therefore necessarily be the result of a compromise where it is attempted to achieve optimum

20. function of the propeller within a specific range of ope¬ ration, while under other conditions the propeller will work with reduced efficiency and in some cases even with harmful consequential effects in the form of e.g. cavita- tion or noise and vibrations which propagate from the pro-

25 peller into the hull to the inconvenience of those on board. If the blades are moreover subjected to impacts beyond the yield point of the metal by striking obstacles in the water or on the bottom during the rotation of the propeller, the blades will be deformed permanently. Then

30 the propeller can no longer work in a quiet and well- balanced manner, but will instead run untrue and transmit shakings and vibrations into the hull. Some of the em¬ ployed metals are moreover not particularly corrosion resistant to sea water. This applies e.g. to the aluminium

35 alloy used for a ship's propeller known from US Patent 3 744931, which is therefore protected against corroding attacks by the sea water by means of a thin vulcanized rubber layer.

Today there are also propellers which are made of fibre reinforced plastics with a relatively low coefficient of elasticity and consequently flexible blades. Thus, Norwe¬ gian Published Application 163 090 discloses a plastics propeller having fibre reinforcements which are applied in a manner such that the blades obtain some predetermined anisotropic properties. Furthermore, a rigid beam is mounted at the leading edge of each blade, and this beam together with the anisotropic properties of the blade causes the blade to be deformed elastically in response to the load. For example, the pitch may be changed in inverse ratio to the load, so that the propeller can. work with a high e ficiency within a much greater operating range than metal propellers. However, the structure is complicated and expensive, and the selected mounting of the fibre re- inforcements is an extremely critical operation which it is difficult to perform repeatedly with the required ac¬ curacy under practical conditions of production. If the fibre reinforcements are not completely identical from blade to blade, this will entail that the propeller will operate in a more or less unquiet manner so that it must be considered unuseful for most purposes. Another drawback of these fibre reinforced plastics propellers is that the damage to which the propeller is subjected if striking an object in the water or on the bottom during rotation, is frequently of such a destructive nature that the damage cannot, or only with difficulty, be repaired.

It is common to the metal or plastics materials used for the above-mentioned propeller types that they have a hard consistency which is incapable of damping or absorbing the shock pulses from the reaction forces of the water. The propellers therefore tend to transmit strong propeller noise in the water and convey the shock pulses from the turbulence of the water into the hull in the form of vi¬ brations and shakings.

US Patent 2 473 665 describes a propeller which, within certain limits, is capable of withstanding shocks and im¬ pacts from objects which the propeller might strike in the water during rotation. The blades of this propeller con- sist of rubber which merely yields elastically if the blade is struck, and immediately again assumes the origi¬ nal shape. To impart greater rigidity to the rubber than it inherently possesses, the blades are reinforced with some wires. However, this reinforcement is not sufficient to absorb the loads that occur in operation of the great majority of ships, and the reinforcement type cannot be used either for selectively controlling the deformation of the blades in response to the load.

This rubber propeller as well as the other propellers of metal or plastics, respectively, mentioned above are gene¬ rally manufactured using equipment which comprises rela¬ tively expensive moulds which, for economic reasons, are used again and again for the moulding of many propellers which all of them are completely identical. The geometri¬ cal design of such series-produced or mass-produced pro¬ pellers must therefore necessarily be based on a compro¬ mise entailing that a specific propeller cannot operate satisfactorily and with optimum efficiency for all pur- poses. A ship's propeller intended e.g. for a fast speed¬ boat will not simultaneously be the best suitable propel¬ ler for a sail-boat which under motor is merely to be cap¬ able of sailing and manoeuvering at a moderate speed, or for a cutter where the propeller is to supply a high driv- ing power while the cutter drags a trawl or a seine at a low speed. When these known propellers have been manufac- tured once and for all, they cannot be adapted to various purposes or at any rate only by means of inacceptably hugh operations and costly manual input or be adjusted if in operation the blades of the propeller should have changed their shape or have lost their balance because of e.g. release of internal stresses, shrinkage of material, changes in state which in particular materials of an orga¬ nic origin tend to undergo in the course of time, or di¬ rectly permanent deformations which might have been im- parted to the blades when these strike objects in the water.

The object of the invention is to provide a propeller of the type mentioned in the opening paragraph which can be moulded to finished size with a smooth and even surface that does not require expensive after-treatment.

Another object of the invention is to provide a propeller of the type mentioned in the opening paragraph which per se is capable of reducing the propeller noise and absorb¬ ing and damping shock pulses from the reaction forces of the water, and which can moreover withstand shocks and impacts from objects in the water or on the bottom to a considerable extent without being permanently deformed.

A third object of the invention is to provide a propeller of the type mentioned in the opening paragraph which is corrosion resistant to e.g. sea water.

A fourth object of the invention is to provide a propeller of the type mentioned in the opening paragraph which, by simple means, can safely and effectively adjust the pitch of the blades optimally to any state of operation in re¬ sponse to the instantaneous load on the propeller. A fifth object of the invention is to provide a propeller of the type mentioned in the opening paragraph which, by simple means, can be rapidly and easily adapted to various applications optimally and be adjusted if the blades should have changed their shape or have lost their ba¬ lance.

The novel and characteristic features according to the invention, by which the above-mentioned objects are ob- tained, are that at any rate the blades are composed of a flexible elastomer having a core which extends from the hub over a considerable portion of the area of each blade and consists of a material having a greater coefficient of elasticity than the elastomer, e.g. fibre reinforced plas- tics or metal. This core is capable of imparting the ne¬ cessary strength and stability to the blades, while the elastomer serves to flexibly control the pitch of the blades in response to the load and to dampen shock pulses as well as to absorb impacts without permanent deforma- tions. At the same time, the propeller obtains a consider¬ able corrosion resistance to e.g. sea water and a smooth surface already during the moulding which does not require any form of after-treatment.

The elastomer may advantageously be a natural or synthetic rubber which is vulcanized firmly on the core so as to provide an intimate and long-lasting connection between the core and the rubber. To strengthen the blade additio¬ nally the rubber may moreover be reinforced with fibres and/or a carcass of e.g. canvas, and the core may moreover be perforated so as to provide a direct bond via the per¬ forated holes between the rubber layers on both sides of the core. The rubber may also be composed of several layers, and when the outermost one of these consists of very soft rubber, the propeller obtains particularly good impact and shock absorbing properties. In a particularly advantageous embodiment the vulcaniza¬ tion or the joints between the core and the rubber and the optional rubber layers, respectively, are mutually inter¬ rupted by zones without joints, so that these joint-free zones may have the form of sections which are mutually se¬ parated by joints or by e.g. a string extending along the leading edge of the blade and having side ribs which sub¬ stantially extend in the same direction as the current along the blade in operation. This entails that a fluid, such as air or water, can be pressed via one or more fill¬ ing valves inwardly between the surfaces in the joint-free zones, thereby providing fluid-filled compartments at these locations which cooperate with the flexible elasto¬ mer at the changes in form which the blade undergoes in response to the instantaneous state of load.

By selectively controlling the .fluid pressure and the de¬ gree of filling in the various compartments it is moreover possible to actively change the geometrical design of the blades so that the propeller is optimally adapted to a de¬ sired application, and the propeller can also be adjusted if the blades have unintentionally changed their form or imbalance has been imparted to them. With a view to achieving these advantageous effects in the best possible manner the compartments may be connected individually or groupwise with their respective filling valves, so that the shape of the blade can be changed partially, i.e. in the area in which the respective compartments are located.

When the elastomer covering is composed of e.g. two layers of rubber, it will be an advantage if the fluid compart¬ ments in one blade is fluid-connected with the correspond¬ ing fluid compartment in the other blades, since the blades hereby automatically obtain the same outer geome- trical shape. The fluid connections may e.g. extend through channels in the hub. The applicant's Danish patent application No. 1392/91, "a folding propeller having at least three blades" and No. 1393/91 "a folding propeller having at least two blades", both of which are incorporated in the present patent application by reference, describe pivotal propeller blades provided with a rubber covering on the innermost end portion which carries the engagement means for syn¬ chronization of the pivotal movements of the blades. According to the invention, this rubber covering may ad- vantageously be integral with the flexible elastomer of the rest of the blade, so that fluid compartments may also be provided in the innermost end portion of the blade for controlling the shape of the engagement means at this lo¬ cation and their pressure against the engagement means of the other blades.

The invention will be explained more fully by the follow¬ ing description of embodiments which solely serve as exam¬ ples, and with reference to the drawing, in which

fig. 1 is a partially sectional end view of a fraction of a first embodiment of a propeller according to the inven¬ tion,

fig. 2 shows a section along the line II-II in fig. 1,

figs. 3a-3e show a cross-section through a propeller blade seen in various situations of operation,

figs. 4a-4c show a cross-section through various embodi¬ ments of a propeller blade for a propeller during forward rotation,

fig. 5 is a partially sectional end view of a fraction of a second embodiment of a propeller according to the inven¬ tion, fig. 6 shows a section along the line VI-VI in fig. 1,

fig. 7 is a partially sectional end view of a fraction of a third embodiment of a propeller according to the inven- tion,

fig. 8 shows a section along the line VIII-VIII in fig. 7,

fig. 9 is a partially sectional end view of a fraction of a fourth embodiment of a propeller according to the inven¬ tion,

fig. 10 shows a section along the line X-X in fig. 9,

fig. 11 is a partially sectional end view of a folding propeller blade according to the invention,

fig. 12 shows a section along the line XII-XII in fig. 11,

fig. 13 is a partially sectional end view of a fraction of a fifth embodiment of a propeller according to the inven¬ tion,

fig. 14 shows a section along the line XIV-XIV in fig. 13,

fig. 15 shows a section along the line XV-XV in fig. 13, and

fig. 16 schematically shows the self-adjusting fluid con- nections between identical fluid compartments in propeller blades having two elastomer layers.

Figs. 1-2 show a typical structure of an elastomer pro¬ peller according to the invention for a ship. In this case the propeller, which is generally designated 1, has three propeller blades 3 which are firmly arranged on a hub 2 for the mounting of the propeller on the drive shaft of the ship. The blades are composed of a core 4 and a flex¬ ible rubber covering 5, which is fixedly adhered or vulca¬ nized on the core. As is the normal case with propellers of this type, the blades are screw-shaped with a radially outwardly decreasing pitch, as appears from fig. 2, which moreover shows that the blades have an airfoil profile. However, the selection of blade profile constitutes no part of the present invention, and the blades may there- fore equally well have e.g. an ogival profile. The core 4 is made of e.g. metal of fibre reinforced plastics having a sufficient strength to impart the required rigidity to the blades 3. In the shown case, the core is perforated with holes 7 through which a bond is achieved between the rubber coverings on both sides of the core, and the blade will hereby be better capable of permanently withstanding the quickly varying changes in form which the blade under¬ goes in operation.

Figs. 3a-e show various situations of operation for a pro¬ peller blade having a flexible rubber covering 8 which is vulcanized on a firm core 9. The arrows at the blade illu¬ strate the pressure or negative pressure which occurs when the blade moves through the water at various speeds. These speeds and their orientation are moreover illustrated by the arrows to the right of the profiles. In fig. 3a the propeller stands still, and its blades are solely loaded by the static pressure of the water. In fig. 3b the pro¬ peller has now begun to rotate, thereby generating a posi- tive pressure on the top side of the blade and a negative pressure on its underside. However, the speed of rotation is still so small that the changes in pressure are not capable of deforming the flexible rubber covering 8 consi¬ derably. This, however, is the case in fig. 3c, in which the propeller is in rapid forward rotation, and the pres¬ sure changes are so considerable that the rubber covering on the top side of the blade is compressed, while it ex¬ pands on the underside of the blade so that the profile and pitch of the blade change. The flexible trailing edge of the blade is simultaneously bent in the direction of flow and thereby reduces the turbulence and the flow re¬ sistance in the water. In fig. 3d the engines have just been reversed. The pressure load on the rubber covering has changed its direction, as shown, but is still so small that it has not been able to deform the rubber covering significantly. In fig. 3e the propeller rotates full speed astern, so that the rubber covering is compressed on the underside of the blade and expands on the top side of the blade. The flexible trailing edge of the blade is simul¬ taneously turned on the top side to form a profile which, seen in relation to the direction of rotation, has a rounded leading edge which is hydrodynamically more suit¬ able for penetrating through the water than a sharp edge. The propeller blades can be arranged in this manner to work with optimum efficiency within a much greater range of operation than is the case with the known propellers.

It is often intended to design the coverings such that the pitch decreases with increasing load, i.e. normally with increased speed of rotation, and increases when the en¬ gines are reversed.

Figs. 5 and 6 show a second embodiment of a propeller 10 according to the invention. Like the propeller described above, the propeller has three blades 12 which are secured on a hub 11. In this case the rubber covering is only vul- canized firmly on the core 13 along a narrow rim area which is provided with holes 15 to effectively ensure attachment of the covering to the core. The rest of the core is not connected with the rubber covering, thereby forming a joint-free area or compartment which is con- nected via a channel 17 with a filling valve 18 at the side of the hub 11. The compartment 16 can now be filled with air or liquid as desired for affecting the flexible properties of the blade in coaction with the rubber cover¬ ing, such that the shape of the blade changes optimally in response to the instantaneous load.

Figs. 4a-c illustrate the advantages which can be obtained in this manner by means of a fluid filling in the blade over a blade with a rubber covering vulcanized firmly on the core along its overall surface. The latter case is shown in fig. 4a, in which a solid, soft rubber covering 20 is vulcanized firmly on a core 21 to the full extent. In operation the blade changes its shape as desired in re¬ sponse to the instantaneous load, as described before with reference to figs. 3a-e. The change in shape that can be achieved in this manner, however, is limited, since the elastic deformation ability of the rubber only permits relatively small changes in the thickness of the rubber coverings. In fig. 4b the rubber covering 20 is now no longer vulcanized firmly on the core 21 along its overall surface. Therefore, in operation the pressure load will make the rubber covering lift from the core on one or the other side thereof to form air compartments 22. Thus, without being overloaded, the rubber covering can now move considerably more outwardly with respect to the core than in the structure with fully vulcanized rubber shown in fig. 4a. In the structure shown in fig. 4b it will be ex¬ pedient to provide the core with through holes 23 for con¬ sistently conveying the air from the pressure side to the negative pressure side to ensure that the rubber covering optimally changes its shape as desired in response to the load. In fig. 4c the compartment 22 is now filled with li¬ quid, e.g. water, instead. Since a liquid is incompres¬ sible, the rubber covering of this structure cannot be deformed to the same extent as if the compartment 22 were filled with air, as shown in fig. 4b, but the deformation ability is greater than in the structure shown in fig. 4a in which the rubber covering is vulcanized firmly on the core along its overall surface. The use of liquid, which has a considerably greater viscosity than air, moreover provides the advantage of damping the liquid flow that may occur between the compartments on the two sides of the blade by sudden changes in the orientation of the pressure loads, thereby preventing the propeller from being sub¬ jected to impact stresses, because the rubber covering suddenly flings over from one side to the other side.

The fluid compartment does not have to be a single large compartment like in the embodiment shown in figs. 5 and 6, but may also have a more complicated form according to the purpose, where zones with vulcanization between the rubber covering and the core alternate with vulcanization-free zones. An example of such a structure is sho n in figs. 7 and 8, which show a propeller having three blades 25 on a hub 26. The rubber covering 30 is removed almost right down to the core 31 on the top portion of the blades of the propeller, and, as will be seen, the liquid compart¬ ment in this case consists of a string 28 extending along the leading edge of the blade and a plurality of ribs 29 which emanate from the string 28 and extend toward the trailing edge of the blade in the same direction as the water substantially flows over the blade when the propel¬ ler works. Furthermore, the hub accommodates a filling valve 32 which is connected with the string 28 and serves to fill the compartment 28, 29 with a fluid, such as air or water under pressure. When the compartment 28, 29 has been filled in this manner with a fluid under pressure, the rubber covering expands in the area at the ribs 29, as shown in fig. 8, whereby these will act as guides which guide the water flow over the blade and thus serve to in¬ crease the efficiency of the propeller. In the embodiment shown in figs. 7 and 8 the rubber covering 30 is vulca¬ nized firmly on the surface of the core 31 at short inter- vals, which gives the propeller blade a considerably more stable shape than in the embodiment shown in figs. 5 and 6 with just a single large compartment.

Figs. 9 and 10 likewise show a propeller having three blades 33, a hub 34, a core 25 and a rubber covering 36. However, in this case the blade 33 has three separate fluid compartments 37, 38 and 39, each having its own filling valve 40, 41 and 42, respectively. The advantage of this arrangement is that the compartments can be pumped independently of each other, so that one and the same pro¬ peller can be given a shape which is suited for various purposes. It is noted in this context that the number and the arrangement of the compartments in the structure shown in figs. 9 and 10 are just given by way of example, and that the numbers and the shapes of the compartments can be adapted in any expedient manner as required.

As mentioned before, the propeller blades on the propel- lers described in the applicant's Danish patent applica¬ tions No. 1392/91 and No. 1393/91 are provided with a rub¬ ber covering on the innermost end part of the blades which is located in the hub and carries the engagement means for synchronization of the pivotal movements of the blades. Figs. 11 and 12 show such a folding propeller blade which is generally designated 43. The innermost end part 44 of the blade is provided with a rubber covering 45 which is vulcanized firmly on the core 52 integrally with the rub¬ ber covering 46 on the actual blade. In this case, the blade, like in the embodiment shown in figs. 5 and 6, includes a single large fluid compartment 47, which is filled with e.g. air or water 51. The compartment 47 com¬ municates via a fluid channel 50 with a filling valve 49 arranged at the end of the swing axle 48 of the blade. In the shown case the rubber covering 45 on the end part 44 of the blade is solid, but it is readily possible also to provide fluid compartments in the rubber covering 45 so that the engagement means on it can be adjusted by pumping when worn. Of course, the fluid compartment can be pro¬ vided in any other manner, e.g. as shown in figs. 7, 8 and 9, 10.

All the previously described embodiments of an elastomer propeller according to the invention have been provided with a rubber covering which consists of a single layer of rubber. However, in many cases it may be an advantage to provide the blades of the propeller with several layers of rubber. By way of example of such a structure figs. 13, 14 and 15 show a propeller which is generally designated 53 and which has three blades 54, a hub 55 and a core 56. In this case the blade is coated with two layers of rubber, an inner layer 57 and an outer layer 58. The inner layer 57 and the core 56 define between them a single inner fluid compartment 56 extending over most of the area of the core, and the outer layer 58 and the inner layer 57 define between them an outer fluid compartment 60 which likewise extends over most of the area of the core. How¬ ever, both compartments may be adapted in any other suit¬ able manner, e.g. like in the embodiments shown in figs. 7, 8 and figs. 9, 10, respectively. The inner fluid com- partments 59 in each blade are interconnected via an inner annular groove 51 in the hub 55, and the outer fluid com¬ partments 60 are correspondingly interconnected via an outer annular groove 62 in the hub 55. The hub moreover includes a valve 53 for filling the inner fluid compart- ments 59 and a valve 64 for filling the outer fluid com¬ partments 60. Thus, the inner fluid compartments in the three blades are in fluid communication with each other and are pumped at the same time via the same filling valve. The same applies to the outer fluid compartments. The principle of this structure is shown schematically in fig. 16, which symbolically illustrates the three blades A, B and C with inner fluid compartments 65a, b, c and outer fluid compartments 66a, b, c. The inner fluid com- partments are interconnected with fluid conduits 68 in which a filling valve 70 is provided, and the outer fluid compartments are interconnected with fluid conduits 67 in which a filling valve 69 is provided. Elastomeric mate¬ rials, such as rubber, have anisotropic properties to a certain extent and therefore tend to stretch disuniformly when loaded. Since the inner fluid compartments are inter¬ connected, the pressure in them will always be the same, but since the inner rubber layer does not stretch comple¬ tely uniformly in the three compartments owing to the ani- sotropic properties of the material, the first compartment 65a is larger than the two other compartments 65b, c in the shown example. However, the inner compartment 65a dis¬ places fluid from the outer compartment 66a via the con¬ duits 67 into the outer compartments 66b, c, resulting in the same fluid amount being maintained in each blade, and these will therefore always have the same outer geometri¬ cal shape. This structure is thus self-adjusting.

The above-mentioned self-adjusting properties can be pro- moted by using even more layers of rubber than two, while ensuring the working order of the propeller even if one or more of the layers should puncture. This working order is also ensured by dividing the fluid compartments into seve¬ ral mutually independent compartments, as mentioned ear- lier.

As appears from the foregoing, the elastomer propeller of the invention is passively capable of changing the geome¬ trical shape of the blades, such that it is optimally adapted to the instantaneous state of operation. The pro¬ peller can moreover actively be given the shape best suit- able for any purpose and be adjusted if this should be ne¬ cessary. This takes place merely by pumping up the blades again in the same simple manner as e.g. a car tyre is pumped up. The rubber covering and a possible air filling moreover diminish the propeller noise and dampen the water noise which penetrates into the hull via the propeller. Furthermore, the propeller can withstand shocks and im¬ pacts from objects in the water or on the sea bottom con¬ siderably better than the conventional structures, without receiving permanent deformations. To this should be added that the propeller can readily be moulded with an even and smooth surface which does not require expensive after- treatment, and of elastomers, such as rubber, which per se have an extremely good corrosion resistance against attacks from e.g. sea water.

Further advantageous properties, and effects are obtained according to the invention by arranging the elastomer covering in the same manner as stated in the applicant's patent application DK xxxx/91, "elastomer propeller having a flexible core", which has the same filing date as the present patent application and is incorporated in the pre¬ sent patent application by reference.

The elastomer propeller has been described above by way of example in embodiments with three blades. However, the propeller can have any number of blades within the scope of the invention. In some cases, e.g. small propellers, propellers without or with just a small core are moreover conceivable. The propeller, which is then frequently re¬ inforced with a carcass for defining and retaining the geometrical shape of the propeller, then obtains its strength and rigidity by means of the inner positive air pressure like a car tyre. The application of the propeller is moreover not restricted to the propulsion of ships, but may equally well be used for many other purposes where the advantageous properties and effects of the propeller can be utilized, e.g. turbines and ventilators.

Claims

P a t e n t C l a i m s :

1. A propeller for e.g. a ship, having a central hub with a plurality of propeller blades, c h a r a c t e r i z e d in that at any rate the blades are composed of a flexible elastomer having a core which extends from the hub over a considerable portion of the area of each blade and con¬ sists of a material having a greater coefficient of elas- ticity than the elastomer, e.g. fibre reinforced plastics or metal.

2. A propeller according to claim 1, c h a r a c t e r ¬ i z e d in that the elastomer and the core are joined by means of adhesion, .vulcanization or a similar connection having a greater strength than the elastomer.

3. A propeller according to claim 1 or 2, c h a r a c ¬ t e r i z e d in that the elastomer consists of at least two elastomer layers which are joined by means of adhe¬ sion, vulcanization or a similar connection having a greater strength than the elastomer.

4. A propeller according to claim 1, 2 or 3, c h a - r a c t e r i z e d in that the joints are interrupted by zones without a joint.

5. A propeller according to one or more of claims 1-4, c h a r a c t e r i z e d in that the joint-free zones have the form of sections which are mutually separated by the joints.

6. A propeller according to one or more of claims 1-4, c h a r a c t e r i z e d in that the joint-free zones have the form of a string extending e.g. along the leading edge of the blade and having side ribs which extend sub- stantially in the same direction as the current along the blade in operation.

7. A propeller according to one or more of claims 1-6, c h a r a c t e r i z e d in that a fluid is provided between the surfaces of the joint-free zones.

8. A propeller according to one or more of claims 1-7, c h a r a c t e r i z e d in that at least one joint-free zone in each blade is in fluid communication with a fill¬ ing valve.

9. A propeller according to one or more of claims 1-8, c h a r a c t e r i z e d in that at least one joint-free zone in each blade is in fluid communication with the cor¬ responding zone in the other blades.

10. A propeller according to one or more of claims 1-9, c h a r a c t e r i z e d in that at least some of the joint-free zones in each blade are in groupwise fluid commmunication with the same filling valve.

11. A propeller according to one ore more of claims 1-10, c h a r a c t e r i z e d in that at least some of the joint-free zones in each blade are in groupwise fluid communication with the corresponding zone groups in the other blades groupwise.

12. A propeller according to one or more of claims 1-11, c h a r a c t e r i z e d in that the fluid connections between the zones or groups of zones, which correspond to each other blade by blade, are provided as channels in the propeller hub.

13. A propeller according to one or more of claims 1-12, c h a r a c t e r i z e d in that the core is perforated.

14. A propeller according to one or more of claims 1-13, c h a r a c t e r i z e d in that the elastomer is re¬ inforced with fibres and/or a carcass of e.g. canvas.

15. A propeller according to one or more of claims 1-14 and of the type where the propeller blades are arranged pivotally on the hub, and the end of each blade located in the hub is composed of an elastomer at any rate in an area which is provided with engagement means for synchronizing the pivotal movements of the blades by engagement with complementary engagement means on the adjacent blades, c h a r a c t e r i z e d in that the elastomer of the blade end located in the hub is integral with the ela¬ stomer on the rest of the blade.