TECHNICAL FIELD

The present invention relates to a method of reducing the azimuthal torque acting on a pulling pod unit or azimuth thruster having a rotary pod housing with a substantially vertical slewing axis and a downwardly directed fin means carried by the pod housing abaft the slewing axis.

It also relates to a device for reducing the azimuthal torque acting on a pulling pod unit or azimuth thruster having a rotary pod housing with a substantially vertical slewing axis and a downwardly directed fin means carried by the pod housing abaft the slewing axis.

BACKGROUND ART

The azimuthal torque around the slewing axis of a pod unit or azimuthal thruster has to be handled by an installed steering machine for all combinations of steering angles, propeller speeds and ship speeds.

The main causes of the azimuthal torque are:

• The lateral force that is created due to an oblique flow toward the propeller together with the distance between the propeller and the vertical slewing axis. This distance forms one arm of a lever that is pivotal around the slewing axis. · At turning an oblique flow to the propeller blades will give a varying angle of attack over a complete turn at a given radius. This variation causes a torque that affects the total azimuthal torque.

• The distance between the load center, due to hydrodynamic forces, of the pod housing and the vertical slewing axis will together with the resulting lateral force give a torque that affects the total steering torque.

A common way of reducing the azimuthal torque for pod units and azimuth thrusters is to place a fin with a wing profile abaft the slewing axis. The fin creates a lateral force due to the angle of attack, especially at turning of the pod unit. The lateral force gives rise to a torque that acts in a direction opposite to the sum of other torque contributions and therefore it reduces the maximum azimuthal torque. At certain operating conditions, a fin with a wing profile placed in the slip stream of a propeller may generate a forward directed force, which is greater than the total drag on the fin that acts in the opposite direction. Thereby, this regain of the rotational energy in the slip stream will give a positive thrust contribution that increases the efficiency of the pod unit. The distance between the slewing axis and a center of the lateral forces acting on the fin forms a second arm of the lever.

The use of such a fin is disclosed in WO 2005/012075 Al (Rolls-Royce Marine AS) and JP 2004090841 (A) (Kawasaki Heavy Ind. Ltd.), for example.

JP 2009214650 (A) (Universal Shipbuilding Corp.) discloses a pod type propulsion unit capable of reducing propulsion resistance, wherein rectangular-plate vanes (current plates) are fixed to the side surface of the pod body so as to be disposed parallel to the axial direction of the pod body and in the direction normal to (the same as the radial direction of) the side surface of the pod body. The amount of projection of the vane is 40 % or smaller of the radius of the propeller, i.e. the projection is extremely small compared to conventional known fins and therefore may not provide sufficient capability. Further, from WO 01/54973 and JP2010221975 there are known POD arrangements having a plurality of fins, but not for the purpose of reducing torque or reducing resistance, but to improve cooling and recover swirl, respectively.

SUMMARY OF THE INVENTION

The object of the present invention is to improve in comparison to that for a pod unit or azimuth thruster having a conventional downward extending fin.

This object is achieved in accordance with the present invention by means of the novel concept as defined in the independent claims.

Thanks to the use of a two fins configuration according to the invention significant performance advantages may be gained compared to a single fin configuration. Inter alia a better cavitation performance may be achieved, resulting in the advantage that a larger cavitation free steering angle is easily obtained. Moreover, it has shown that it is possible to recover more of the rotational propeller flow losses, compared to a single fin configuration, i.e. to increase the hydrodynamic unit efficiency. According to an additional aspect a part of the superior cavitation performance may also be traded to further increase the gain in hydrodynamic unit efficiency by making the fins relatively thinner, i.e. designing the fins having a thickness t that is relatively smaller in relation to the flow length C compared to a conventional single fin, wherein preferably the total thickness of the two fins is smaller than the thickness of one fin. tt,f /

A further advantage that may be achieved with the invention is that the risk of grounding may be made lower than for a design using a single fin, since the two fins may be positioned to protrude a smaller distance below the POD

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, the invention will be described in more detail with reference to preferred embodiments and the appended drawings.

Fig. 1 is a side view of a preferred embodiment of a pod unit or azimuth thruster

according to the present invention, and,

Fig. 2 is a side view and a view from behind respectively of a mounted, two fins design according to the invention,

Fig. 3 is a side view and a view from behind respectively of a mounted, conventional single fin design according to prior art,

Fig. 4 is a side view presenting pressure characteristics appearing on the front part of a mounted, two fins design according to the invention and a conventional single fin design according to prior art, respectively,

Fig. 5 presents a diagram showing test results regarding steering torque at different steering angles for a POD unit without any fin, equipped with a conventional single fin and equipped with a two fins design according to the invention, respectively,

Fig. 6 presents a diagram showing test results regarding efficiency at different fin axis intersection positions for a POD unit equipped with a two fins design according to the invention.

DETAILED DESCRIPTION

Fig. 1 shows a pulling pod unit or azimuth thruster 1 having a rotary neck 2. At the lower end of the neck 2 there is arranged a pod housing 4 having a central axis 3B extending generally horizontal. The rotary neck 2 provides a substantially vertical slewing axis 3A, around which the pod unit 1 or azimuth thruster may be rotated. At the front of the pod housing 4 there is arranged a pulling propeller 7, having a radius R. The slewing axis 3A may be positioned at different locations, as exemplified according to this embodiment, it may be positioned a distance x in front of the vertical center line 5 of the pod housing 4. Abaft the slewing axis 3 A, the pod housing carries two fixed, generally downwardly directed fins 6. The fins 6 are positioned on the aft part 40 of the POD housing 4, at a first axial distance r (to a front attachment part of the fin) from the slewing axis 3 A, which distance r may vary, but that in some applications preferably is greater than the radius R of the propeller 7. The end surface 41 of the aft part 40 of the POD housing 4 is positioned at second axial distance L _a from the slewing axis 3 A which second axial distance is larger than the first axial distance r. The aft end 65 of the fins 6 preferably terminate in line with or at a distance Y frontwardly from the aft part end 41. The first axial distance r, i.e. between the POD azimuth rotation axis 3A, and the fins 6 should preferably be 10% to 85%, even more preferred 50%> to 70%>, of the second axial distance L _a between the slewing axis 3 A and the aft part end 41. As shown in Fig 2 the fins 6 are positioned and extend on one side each in relation to a vertical longitudinal plane 30 of the pod housing 4. The fins 6 comprise one starboard fin 6A, positioned on the starboard side of the aft part 40 in relation to the slewing axis 3, and one port fin 6B, positioned on the port side of it in relation to the slewing axis 3A.

In accordance with the invention, the two fins 6 are preferably identically shaped and symmetrically positioned in relation to the vertical longitudinal plane 30 of the pod housing 4 and extend downwardly below the lower edge 42 of the pod housing, such that both fin tips 63 are positioned below the lower edge 42 of the pod housing. The fins 6 extend substantially radially to present an angle β them between, which preferably is in the range of 0° < β < 90°, more preferred β < 70°, even more preferred β < 50°. As is evident for the skilled person the fins 6 may extend in different angles β, to fulfil the purpose of the invention, e.g. even in parallel β = 0°, with each other. Test indicate that it is an advantage to have β > 0°, and to have the fins 6 relatively close to each other to form a kind of flow tunnel in the space between the two fins 6, which will cause a synergetic effect regarding the flow of water passing around the fins 6. Hence, the distance δ between the inner portions of the centre plane 65 of the fins 6 shall advantagely be smaller than the diameter Dp of the POD housing 4, preferably 0,1 Dp < δ < 0,7 Dp, more preferred 0,2 Dp < δ < 0,4 Dp and preferably β is such that the crossing point of the center lines 65 of the fins 6 will cross at a point above the horizontal center plane of the pod housing 4. Both fins are preferably in form of streamlined wing profiles having a largest thickness t and presenting a side area A, projected along the vertical longitudinal plane 30. The fin span Sf should preferably be about 40% to 100%, even more preferred 60%> to 95%, of the radius R of the propeller 7. The fins 6 will protrude a vertical distance Ld from the lowest surface 42 of the POD housing 4, wherein preferably 0,1 Sf < Ld < 0,7 Sf, more preferred 0,3 Sf < Ld < 0,6 Sf.

According to the preferred shown embodiment in Figs. 1 and 2 the fins 6 have a front portion 61 that protrude downwards and abaft presenting an angle a in relation to a vertical axis, which angle a preferably is in the range of 0-80°, more preferred 30-60°. Also, the aft portions 62 protrude downwards and abaft presenting an angle γ in relation to the vertical axis, which preferably is smaller than the angle a (in any direction in relation to the vertical) of the front portion 61, preferably in the range of 0-70°, more preferred 20-50°. Thanks to this design, a larger fin area may be exposed (compared to non-angled) within a vertical space limited by a desired depth level, e.g. Ld.

Thanks to the invention there is an advantageous influence on the POD steering torque, i.e. by having a twin fin arrangement in the aft part of the POD unit. A larger total fin area Atot, is projected along the longitudinal vertical plane 30, compared to a

conventional single fin configuration (see fig. 3). In general terms Atot = 2 x A, implying that a much larger surface Atot can be active in comparison with a single fin.

Furthermore, that larger area Atot may be achieved at the same time as they 6 protrude a vertical distance Ld that is significantly smaller than with a single fin.

In Fig. 4 there are shown schematic views of fins 6 presenting results from analysis of low pressure zones at 7,5° steering angle for an arrangement according to the invention in the left hand side of Fig. 4 and for a single fin in the right hand side. There are shown five cavitation zones 61a-61e (from stronger to weaker). It is clear from this figure that the cavitation performance is better with a design according to the invention. The width 61 of the cavitating part of a double fin 6 is significantly smaller than that 61 ' of a single fin. Thanks to this result a larger cavitation free steering angle is easily achieved.

A part of the superior cavitation performance can also be traded to further increase the gain in hydrodynamic unit efficiency by making the fins relatively thinner, i.e.

designing the fins 6 having a thickness t that is relatively smaller in relation to the flow length C compared to a conventional single fin, wherein preferably t/C is in the range of 0,05- 0,3, more preferred 0,1-0,2. In fig. 5 there is presented a diagram showing test results regarding steering torque at different steering angles for a POD unit without any fin (curve A), equipped with a conventional single fin (curve B) and equipped with a two fins design according to the invention (curve C), respectively. As is evident a much better effect is achieved by means of the invention. It is shown significant differences, e.g. it should be noted that using a POD having no fin as reference for torque, i.e. a maximum of 1, a POD with one fin presents a maximum of 0,5, whereas according to the invention it merely is 0,3.

The diameter D of the propeller is preferably in the range of 1 meter - 10 meter, most preferred 1,5 meters to 8 meters.

The horizontal span C is preferably in the range such that C/R is from 0,4 to 0,8.

The diameter Dp of the pod housing 4 according to a preferred embodiment is in the range of 0,4 R - 1,2 R, more preferred 0,5 R - R.

Further, the pod neck 2 itself and/or the transition zone between the pod housing 4 and the neck 2, may preferably be designed to achieve a further reduction of the azimuthal torque by presenting further areas and/or a kind of vane located along the upper part of the pod housing 4, whereby the unit efficiency will be slightly further improved.

Fig. 6 presents a diagram showing test results regarding efficiency at different fin axis intersection positions for a POD unit equipped with the two fins design according to the invention. As is evident from the diagram it is an advantage to have the fins 6 intersecting at a point above the horizontal center plane (including the centre line 3B) of the pod housing 4 and to use fins that are thin. Three different arrangements have been tested, including two different fin designs, a first fin design, graphs a) and c), having a first fin thickness tl and a second fin design b) having a second fin thickness t2, wherein t2 is larger than tl . In other aspects, the fin designs are the same. Further, graph c), presents an arrangement where the POD housing has irregularities (e.g. screw heads) near by the fins, which causes extra turbulence. All three arrangements, a), b) and c), were tested having their intersecting point A (see Fig. 2) of the center lines 65 at said centre plane, and at a point B that intersects 0,3 Dp above said centre plane,

respectively. Further two of the fins sets, a) and b), were tested having their intersecting point C of the center lines 65 at 0,6 Dp above said centre plane. It is evident from the tests that the efficiency is better when the intersecting point is above said centre plane, and best when the intersecting point is at about 0,3 Dp above said centre plane. Further, it is evident from the tests (cf. graph a) and b)) that the efficiency is better when there is used thinner fins. Finally, the tests indicate (see graph c)) that the efficiency is even more improved when the POD housing has irregularities to use a positioning of the fins, such that their intersecting point is above said centre plane (cf. point A and B).

Preferably, the intersecting point A, B, C of the center lines 65 of the fins 6 is positioned between 0,1 to 0,7 times the diameter Dp of the POD housing 4 above the horizontal center plane of the pod housing 4, preferably 0,2-0,5 times the diameter Dp.

The invention is not limited by what that is described above but may be varied within the scope of the claims. For instance, it is evident that the skilled person knows that there is a big variety of different materials that can be used to fulfill the function of the fin 6, but that metal, e.g. steel, will often be preferred. Furthermore, in some

applications, the fin may be bent or twisted to meet the flow in a way to improve efficiency at low steering angles.