Patent Description:
Traditional screw-propellers typically comprise a rotational hub with two or more blades which are fixed to the hub and protrude radially therefrom. The blades are arranged with a pitch such as to define a helical spiral resembling a helical screw line.

Such traditional screw-propellers typically present a comparatively high rotational resistance which results in high motor load and low energy efficiency. The traditional screw-propellers are also exposed to cavitation which in turn may damage the blades, cause undesirable vibrations, and give rise to noise. Additionally, such traditional screw-propellers typically exhibit a substantial slip which further reduces the energy efficiency.

<CIT> discloses a screw-propeller comprising a hub and a plurality of radiating supporting main blades fixed to the hub. Each main blade is connected a first auxiliary blade arranged in front of the main blade and to a second auxiliary blade arranged behind the main blade. The tips of the main blades and the auxiliary blades extend radially beyond the connections between the main and the auxiliary blades.

<CIT> discloses a propeller assembly with groups of three axially aligned blades attached to a central hub that are capped by an outer guide element.

An object of the present invention is to provide an enhanced propulsion device for generating thrust to a fluid.

Another object is to provide such a propulsion device which exhibits a high efficiency.

A further object is to provide such a propulsion device which generates an axial flow with a comparatively very small separation.

Yet another object is to provide such a propulsion device which exhibits comparatively low resistance to the fluid.

Still another objective is to provide such a propulsion device which prevents cavitation.

A further object is to provide such a propulsion device which provides a comparatively low slip.

According to a first aspect, the present disclosure provides a propulsion device as set out in the appended claim <NUM>. The propulsion device is arranged for exerting thrust to a fluid. The propulsion device comprises a central hub having a front end and a rear end and is rotational about a rotational axis extending longitudinally between the front and rear ends, and at least two propulsive arrangements which protrude radially from the hub and which are evenly distributed about the circumference of the hub. Each propulsive arrangement comprises a front propulsive element which extends radially from a front inner end to a front outer end, a rear propulsive element which extends radially from a rear inner end to a rear outer end. An outer guide element extends from the front outer end to the rear outer end at a radial outer guide element distance from the rotational axis and an inner guide element extends from the front inner end to the rear inner end at a radial inner guide element distance from the rotational axis. An elongate front distance member and an elongate rear distance member extend radially from the hub to the inner guide element. At least one intermediate propulsive element is arranged between the front and rear propulsive elements and extends radially from the inner guide element to the outer guide element. The longitudinal projections of the front propulsive element, the intermediate propulsive element and the rear propulsive element overlap at least partially. The inner guide element, the front distance member, the rear distance member and the periphery of the hub define an open space allowing free passage of the fluid.

By providing the propulsion device with an intermediate propulsive element which extends from an inner guide element arranged radially at a distance from the hub, the propulsion device provides additional thrust generating surface at a peripheral portion of the propulsion device. Thrust generating area arranged at the peripheral portion of the propulsion device provides higher trust and lower resistance in the axial and tangential direction than blade area at the central portion of traditional screw-propellers. The intermediate propulsive element thus increases the efficiency of the propulsion device by increasing the thrust while maintaining the resistance low. The inner guide element and outer guide element which extend longitudinally between all propulsive elements in one propulsive arrangement reduce radial flow and promote the generation of flows in the axial direction. Hereby, the axial component of the flow generated by the propulsion device is increased while keeping the lateral separation of the flow low. This also contributes to increase the efficiency of the propulsion device. The outer guide element and inner guide element also increase the rigidity of the propulsion device, thus allowing the propulsive elements to be comparatively thin and reducing vibrations. The outer guide element further reduces cavitation at the peripheral portion of the propulsive elements, which in turn reduces wear of the propulsive elements as well as noise and other vibrations.

The propulsion device has proven to exhibit high efficiency, low resistance, both in the axial and circumferential directions, a highly concentrated axial flow with small lateral separation, low degree of cavitation and low levels of noise and other vibrations.

At an embodiment of the propulsion device, the longitudinal projections of the front propulsive element, the intermediate propulsive element and the rear propulsive element fully overlaps. This results in a further reduction of axial resistance.

The lengths of the axial projection of the chord lines of the front, rear and intermediate propulsive element may be essentially constant. The axial projection of the propulsive elements thus exhibits a rectangular or rhomboid shape. By this means the active propulsive element area may be large while maintaining the chord lines short. This in turn increases the thrust while minimizing cavitation.

The maximal chord line of each of the front, rear and intermediate propulsive elements, , may be a short chord line. Such a short chord line is defined by NACA standard nomenclature. Thus, the propulsive elements exhibit, in the active region radially outside the inner guide element, a comparatively short extension in the circumferential direction, which reduces cavitation.

The inner guide element distance (IG) may be at least <NUM>% of the outer guide distance (OG). This means that the intermediate propulsive element is positioned at a sufficient distance from the rotational axis for providing great thrust in relation to the resistance.

The outer guide element may protrude rearwardly from the rear outer end. The protruding portion of the outer guide element then provides a so-called winglet which efficiently reduces cavitation.

The outer guide element and the inner guide element may exhibit an essentially constant longitudinal cross section which is outwardly convex. This reduces the resistance in the circumferential direction.

The outer guide element and the inner guide element may exhibit an essentially constant longitudinal cross section which is inwardly concave. This further reduces the resistance in the circumferential direction.

The radius of the convex or concave curvature may be essentially equal to the radial distance from the rotational axis to the respective outer guide element and inner guide element. This also reduces the resistance in the circumferential direction.

The distance between a leading edge and a trailing edge of each front, rear and intermediate propulsive element may be essentially constant. This also allows a large active propulsive elements area while keeping the chord line comparatively short.

The front distance member and the rear distance member may be longitudinally aligned.

The front distance member and the rear distance member may be arranged as radial extensions from the front inner end and the rear inner end respectively to the hub.

Each propulsive arrangement may comprise a plurality, preferably two or three intermediate propulsive elements. Hereby the advantages of arranging active propulsive elements area at the periphery of the propulsion device is further enhanced.

The propulsion device may comprise two propulsive arrangements mutually separated in the circumferential direction by <NUM>° or three propulsive arrangements mutually separated in the circumferential direction by <NUM>° or four propulsive arrangements mutually separated in the circumferential direction by <NUM>° or five propulsive arrangements mutually separated in the circumferential direction by <NUM>° or six propulsive arrangements mutually separated in the circumferential direction by <NUM>°.

Further objects and advantages of the propulsion device according to the first, second and third aspects will be apparent from the following description of exemplifying embodiments and from the appended claims.

The propulsion device shown in all <FIG> are intended for the propulsion of marine vessels. Other not shown embodiments within the scope of the present disclosure may be used at other applications such as for the propulsion of air crafts including drones or for generating forced fluid flows e.g. at fans, blowers and pumps.

The marine propulsion device according to the embodiment shown in <FIG> comprises a central hub <NUM> which is rotational about a central rotational axis A. In the shown example the hub <NUM> is arranged for passing exhaust gases from an outboard or inboard engine (not shown) through the interior of the hub. The hub is conical end tappers from a front end <NUM> to a rear end <NUM>. The hub <NUM> exhibits a central bore <NUM> extending longitudinally from the front end <NUM> to the rear end <NUM>. A cylindrical internally splined sleeve member <NUM> extends centrally through the bore <NUM>. The sleeve member <NUM> is fixed to the inner wall of the bore <NUM> by means of four radial struts <NUM> such that an annular channel for expelling exhaust gases is formed in the bore <NUM> around the sleeve member <NUM>.

The propulsion device further comprises two propulsive arrangements <NUM> which are fixed to the peripheral surface of the hub <NUM> and separated by <NUM>°. Both propulsive arrangements are identical and in the following, only one will be described for reasons of simplicity.

The propulsive arrangements <NUM> each comprise a front propulsive element <NUM>, a rear propulsive element <NUM> and an intermediate propulsive element <NUM>. The front propulsive element <NUM> exhibits a front inner end <NUM> and a front outer end <NUM>. Correspondingly, the rear propulsive element <NUM> exhibits a rear inner end <NUM> and a rear outer end <NUM>.

An outer guide element <NUM> extends in parallel with the rotational axis A, from the front outer end <NUM> to the rear outer end <NUM>. The outer guide <NUM> is arranged at a radial outer guide element distance OG from the rotational axis A. The outer guide element <NUM> protrudes axially a short distance rearwardly of the rear propulsive element <NUM>, forming a rearwardly protruding winglet <NUM>.

An inner guide element <NUM> extends in parallel with the rotational axis A from the front inner end <NUM> to the rear inner end <NUM>. The inner guide element <NUM> is fixed to the periphery of the hub by means of an elongate front distance member <NUM> and an elongate rear distance member <NUM>. The distance members <NUM>, <NUM> each extend radially from the hub to the inner guide element <NUM>. In the shown example, the front distance member <NUM> forms a radially inwardly directed extension of the front propulsive element <NUM> and the rear distance member forms a radially inwardly directed extension of the rear propulsive element <NUM>. However, at other not shown embodiments, the distance members may be arranged at other longitudinal positions of the inner guide element, as long as they hold the inner guide element <NUM> at a radial distance from the hub <NUM>.

The inner guide element <NUM> is arranged at a radial inner guide element distance IG from the rotational axis A. The inner guide element distance IG is shorter than the outer guide distance OG but larger than the maximum outer diameter of the hub <NUM>. the inner guide element <NUM> is arranged radially between the outer periphery of the hub and the outer guide element <NUM>. In the shown example the inner guide element distance IG is approx. <NUM> % of the outer guide element distance OG. It is preferred that the inner guide element distance IG is between <NUM>% and <NUM>% of the outer guide element distance OG.

The radially outer surface of the outer guide element <NUM> is convex with a radius of curvature which is essentially equal to the outer guide element distance OG. The radially inner surface of the outer guide element <NUM> is concave also with a radius of curvature which is essentially equal to the outer guide element distance OG.

Correspondingly, the inner guide element <NUM> has an outer convex surface and an inner concave surface, both of which exhibit a radius of curvature which is essentially equal to the inner guide element distance IG. By this means the circumferential resistance of the propulsion device is reduced.

The intermediate propulsive element <NUM> is arranged longitudinally at the centre between the front propulsive element <NUM> and the rear propulsive element <NUM> and extends radially from the inner guide element <NUM> to the outer guide element <NUM>. The intermediate propulsive element <NUM> has an intermediate inner end <NUM> which is fixed to the inner guide <NUM> and an intermediate outer end <NUM> which is fixed to the outer guide <NUM>.

From the above description it is understood that the front distance member <NUM>, the rear distance member <NUM>, the periphery of the hub <NUM> and the inner guide element <NUM> define an open space through which the fluid, which in this case is water, may flow freely and which does not present any axial or circumferential resistance as the propulsion device rotates.

The front <NUM>, rear <NUM> and intermediate propulsive elements <NUM> exhibit essentially identical geometries. At each propulsive element <NUM>, <NUM>, <NUM> the leading edge is arranged essentially in parallel with the trailing edge. Further, the axial projection of the propulsive elements is essentially rhombic. Additionally, the widths in the circumferential direction of the propulsive elements <NUM>, <NUM>, <NUM>, the inner guide element <NUM> and the outer guide element <NUM> are essentially equal.

The width in the circumferential direction of the propulsive elements <NUM>, <NUM>, <NUM> may also be expressed by the length of the chord line (or chord) extending through the respective propulsive element from the leading edge to the trailing edge and by the axial projection of this chord line. With a propulsion device according to this disclosure it is possible to keep the chord line and its axial projection short in comparison to traditional screw-propellers while still achieving satisfactory thrust. This is of great advantage since short or small chord lines reduces the risk of cavitation. It has shown that the chord line of the propulsive elements <NUM>, <NUM>, <NUM>, , preferably is a so called short or small chord line according to NACA standard terminology.

<FIG> illustrates a second embodiment of the propulsion device comprising a hub <NUM> and three propulsive arrangements <NUM> distributed along the periphery of the hub <NUM> and separated by <NUM>°.

<FIG> illustrates a third embodiment of the propulsion device comprising a hub <NUM> and four propulsive arrangements <NUM> distributed along the periphery of the hub <NUM> and separated by <NUM> °.

At the embodiments shown in <FIG> and <FIG> the hub <NUM>, <NUM> and each propulsive arrangement <NUM>, <NUM> are essentially equal to the hub <NUM> and the propulsive arrangement <NUM> shown in <FIG> and described above and the description is not repeated here.

At a not shown embodiment, each propulsive arrangement may comprise more than one intermediate propulsive element arranged between a front and a rear propulsive element. Also in such cases, the intermediate propulsive elements extend radially between an inner guide element which is arranged at a distance from the hub and an outer guide element, which outer guide element extends between the outer end of the front and rear propulsive elements. At such embodiments each propulsive arrangement comprises a total number of active propulsive elements arranged between the inner guide element and the outer guide element which is equal to the sum of intermediate propulsive element plus two.

At a further not show embodiment each propulsive arrangement may comprise more than two elongate distance members connecting the inner guide element to the hub.

At all propulsive arrangements shown and described above it is preferred that all propulsive elements in a propulsive arrangement, are aligned longitudinally and fully overlap each other in the longitudinal direction.

Claim 1:
A propulsion device for exerting thrust to a fluid, which comprises a central hub (<NUM>, <NUM>, <NUM>) having a front end (<NUM>) and a rear end (<NUM>) and being rotational about a rotational axis (A) extending longitudinally between the front (<NUM>) and rear (<NUM>) ends, and at least two propulsive arrangements (<NUM>, <NUM>, <NUM>) which protrude radially from the hub (<NUM>, <NUM>, <NUM>) and which are evenly distributed about the circumference of the hub (<NUM>, <NUM>, <NUM>), wherein each propulsive arrangement (<NUM>, <NUM>, <NUM>) comprises;
- a front propulsive element (<NUM>) which extends radially from a front inner end (<NUM>) to a front outer end (<NUM>),
- a rear propulsive element (<NUM>) which extends radially from a rear inner end (<NUM>) to a rear outer end (<NUM>),
- an outer guide element (<NUM>) which extends from the front outer end (<NUM>) to the rear outer end (<NUM>) at a radial outer guide element distance (OG) from the rotational axis (A),
- an inner guide element (<NUM>) which extends from the front inner end (<NUM>) to the rear inner end (<NUM>) at a radial inner guide element distance (IG) from the rotational axis (A),
- an elongate front distance member (<NUM>) and an elongate rear distance member (<NUM>), which distance members (<NUM>, <NUM>) extend radially from the hub (<NUM>) to the inner guide element (<NUM>), and
- at least one intermediate propulsive element (<NUM>) arranged between the front (<NUM>) and rear (<NUM>) propulsive elements and extending radially, from the inner guide element (<NUM>), to the outer guide element (<NUM>), wherein
- the longitudinal projections of the front propulsive element (<NUM>), the intermediate propulsive element (<NUM>) and the rear propulsive element (<NUM>) at least partially overlaps, and wherein
- the inner guide element (<NUM>), the front distance member (<NUM>), the rear distance member (<NUM>) and the periphery of the hub (<NUM>) define an open space allowing free passage of the fluid.