TECHNICAL FIELD

The present invention relates to a counter rotating propeller propulsion system for a vessel, comprising a first propeller mounted at a front end of a shaft that extends essentially horizontally within a POD/thruster housing, wherein said housing is attached to a column extending downwardly from the bottom of the vessel, via a shaft enabling rotation of the POD/thruster unit.

BACKGROUND ART

Propulsion of various ships or similar vessels (such as passenger ships and ferries, cargo, barges, oil tankers, icebreakers, off-shore vessels, military vessels, or other floating body most usually is accomplished by a pushing or pulling (see e.g.

W09746445) force provided by a rotatable propeller or a plurality of propellers. Known applications also include CRP (Contra Rotating Propellers), which is beneficial since the contra rotating arrangement leads to that rotative losses behind a single propeller are regained in the second propeller. This can be achieved either by having two propellers on a single shaft line (see e.g. US 6,966,804) or by arranging a thruster (being a mechanical thruster or electrical pod) behind the shaft line propeller, e.g. as known from WO 0154971. Both or any of the propellers can be of either fixed or controllable pitch type. The benefits of having a thruster behind the shaft line propeller are several. The shaft lines for the two propellers are separated and a complex arrangement with double tube propeller shafts may be avoided. Also, by using separate shaft lines complex gear-, bearing- and sealing arrangements can be avoided. Another benefit is the advanced steering capability provided by the thruster.

The drawback in this arrangement is that when arranging the propellers behind each other, large dynamic loads will appear due to the interaction between the propellers when steering with the thruster, especially at high speeds. There are different arrangements to avoid this. One is to use a reduced thrust on one of the propellers when steering, which may not be an optimal way of operating. Another one is to arrange the thruster in a pushing arrangement, like in the patent EP1270404. The drawback of EP1270404 is that the regenerative propeller is situated far behind the first propeller and thus reducing the efficiency of the regeneration, and also in that the arrangement occupies much space. It is also possible to fit a separate rudder behind the thruster unit as suggested in patent EP1466826 and EP1329379. The drawback of that is that when steering with the thruster there will be interference between the strut of the thruster and the rudder, reducing the benefits of the thruster. There is also a loss of efficiency when using the thruster for manoeuvering at low speeds. Obviously it is also a negative cost implication to have an extra rudder including its steering machinery taking up space inboard the vessel. The rudder also give extra resistance of the vessel.

In one specific example the problem to be solved relates to a marine propulsion and steering unit of the kind described in EP-B-0 394 320, comprising a pod having front and rear ends, a driving machine accommodated in the pod, a substantially horizontal propeller shaft drivingly connected to the driving machine and provided with a propeller externally of the front end of the pod, and an upright pod supporting strut rigidly attached to the pod and having at the upper end thereof swivel bearing means supporting the pod supporting strut and the pod below a buoyant body for angular motion about a substantially vertical axis, wherein the buoyant body may be a ship, a work platform, a pontoon, or a similar floating body. Because the unit is angularly movable about a vertical axis, it may be used not only for the propulsion, but also for the steering of the ship or other buoyant body equipped with the unit, and at the same time the rudder can also be used for the steering. Angular adjustment of the entire unit may also be combined with deflection of the rudder. If the buoyant body equipped with the unit is a ship adapted to be run at a high speed, 20 knots or more, for example, the unit will be subjected to very great forces by the water if it is turned while the ship is running at such high speed. The swivel bearing and the actuators and other components used for the turning of the unit will therefore be heavily stressed during steering manoeuvres. When the ship is running at a high speed, steering by means of the rudder is therefore preferred. Steering by turning the entire unit, possibly combined with deflection of the rudder, is resorted to when steering at a lower speed, such as when the ship is manoeuvred in harbours or narrow waterways. In the known unit, the pivotal axis of the rudder coincides with the vertical turning axis of the unit. In certain operating conditions, this arrangement of the turning axis causes hydrodynamical problems which are related to the position of the rudder and the inhomogeneous flow of the water impinging on the unit when the unit is angularly offset from the fore and aft vertical centre-line plane of the ship. Further, from JP2005239083 there are known a plurality of different solutions aiming at, at least partly, solve some of the problems mentioned above. SUMMARY OF THE INVENTION

It is an object of the present invention to provide an improved solution to the above described problems. This is achieved by means of a propulsion system for a vessel in accordance with claim 1, which accomplishes a compact unit, allowing close positioning of the propellers in the CRP arrangement and good manoeuvrability.

Thanks to the invention there is achieved a very compact solution with optimal hydrodynamic performance as well as excellent manoeuverability. With the flap fitted on the thruster, no additional space will be required inboard the vessel.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following detailed description, reference will be made to the accompanying drawings, in which:

Fig. 1 is a side view of a propulsion system according to a schematically presented embodiment of the present invention.

Fig. 2 is the same view as above showing a more realistic embodiment and added with lines indicating space requirements.

DETAILED DESCRIPTION OF THE DRAWINGS

In Fig. 1 there is shown a schematic side view of an arrangement according to the invention. There is shown a fixed (position) propeller 6 mounted on a fixed shaft 8 protruding from the hull 7 of a vessel. Rearward of the fixed propeller 6, at a distance X (from centre to centre of the propellers 1, 6), and coaxial with the first propeller 6 there is positioned a second propeller 1, which forms a pulling propeller 1 of a thruster unit 2, 4, 5 (which may be an electrical POD or a mechanical thruster). In the shown embodiments both propellers have the same diameter Dl . The thruster unit, in a traditional manner, comprises a lower housing 5, containing the electrical motor and/or a transmission, to drive a shaft 50 having the propeller 1 at it's front end. Also in a traditional manner the thruster unit 2, 4, 5 is turnable attached to the hull 7, by means of a steering shaft 4 (or any other structure) attached to a steering machinery (not shown) that enables positioning of the thruster unit 5 in any desired deflected position, in relation to its substantially vertically extending turning axis C. Between the thruster housing 5 and the steering shaft 4 there is a column 2 acting as a kind of rudder device. Further, in the preferred embodiment the downwardly facing surface 52 of the pod body 5 is unobstructed to provide minimum drag of the unit. Moreover, in accordance with the invention there is arranged a rudder flap 3 in the rear portion of the column 2. This rudder flap 3 has a form that corresponds to the design of the column 2 and is hingedly attached to the intermediate body 2 by means of pivotal arrangement (not shown) that allows positioning of the rudder flap 3 in any desired deflected position by means of pivoting about its pivoting axis P. The pivoting axis P is positioned adjacent the front edge 30 of the rudder flap 3 and extends substantially vertically but preferably forming a sharp angle a in relation to the turning axis C. The area A ₃ of the rudder flap 3 is preferably in the range of 10 - 40 %, in relation to the total area A ₂ of the whole intermediate portion 2, i.e. also including A ₃ . The lower most edge 31 of the rudder flap 3 is positioned at a distance Si above the propeller axis PC, wherein Si is shorter than Di/2. (Preferably Si<0,7*Di/2) to have a substantial amount of flow from the POD propeller 1 interacting efficiently with the rudder flap 3.

One aspect that enables a more compact unit is the beneficial effect provided by the positioning of the rudder flap 3 in accordance with the invention. As a consequence of its positioning and dimensioning the total azimuthal torque exerted on the pod unit, when it is turned, will be lower in comparison with a traditional concept. Furthermore, there is normally no need for any turning movement of the thruster unit 2 at high speed of the vessel (very high forces would otherwise occur) but mostly turning movements are merely needed at lower speeds, when increased manoeuvrability is needed. Thanks to the decreased need of azimuthal torque exerted by the column 2, it is feasible to position the turning axis C further back in relation to a conventional concept. In accordance with the invention it is preferred to position the turning centre C adjacent a mid position in relation to the length extension Li of the unit including the length L of the housing and also the protruding hub part 51 of the propeller 1. In a preferred embodiment the distance Yi between the front end 51 A of the hub 51 and the turning centre C is in a range of 0.45 - 0.65 x Li, preferably 0.45 Li < Yi < 0,55 L _1; with the thruster unit 2, 4, 5 preferably rotatable 360° around its turning axis C. Another preferable parameter according to the invention is that Yi is about equal to Di facilitating the propeller blades to not interfere with the hub shaft 8 of the front propeller 6. In some applications there may not be a need of having the thruster unit 2, 4, 5 turnable 360°, but merely a limit degree, e.g. ± 30. However, also in such an application the relation should preferably exceed at least 0,6, preferably at least 0,7. Thanks to the arrangement according to the invention, the two propellers 1, 6 may be positioned very close to each other (as mentioned above). It may be possible to position the propellers 1, 6 even closer to each other by eliminating the use of any traditional hub portion, i.e. not having a distinct stream lined spherical upstream end. In accordance with a preferred embodiment of to the invention the hub portions 51, 80 have a combined form that has as its main object to provide a streamlined transition between the two propellers 1, 6, and closely positioned opposing hub portions 51 A, 80 of the propeller 1 on the fixed shaft 8 and the propeller 1 on the shaft 50 of the thruster propeller 1. Thanks to the fact that the propellers 1, 6 are coaxially positioned, in combination with the fact that the most frequent positioning of the propellers are in a coaxial position (during high speed) the normal, streamlined hub cap may be eliminated and replaced with a hub 51 with optimized surface transition between the two hubs 8, 51, with increase of efficiency, and thereby obtaining an even closer positioning of the propellers 1, 6. According to the invention the maximum gap Z between the opposing hub portions 51 A, 80 may be arranged to be less than 3% of Dl, even more preferred less than 2% of Dl . Moreover the opposing end surfaces 51 A, 80 are shaped to be essentially flat, i.e. having an average radius that is substantially larger than traditional hub surfaces, implying an average radius that is at least larger than 1 m.

In Fig. 2 there is shown another (more realistic) embodiment in accordance with the invention wherein dotted lines represent the swept action volume of the concept, i.e. positioning the turning centre C adjacent the middle of length extension LI of the unit. Further it is shown that in some applications it may be preferred to have the fixed position propeller 6 with a larger diameter D ₂ than the diameter Di of the propeller 1.

Thanks to the use of a rudder flap 3 positioned and designed in accordance with the invention there is achieved a compact unit, wherein considerably smaller deflections need to be used about the steering shaft 4 to achieve a desired turn compared to known kinds of CRP systems. Further the distance X between the pulling propeller 1 and the fixed propeller 6 may be considerably smaller than has been possible with known, traditional solutions.

According to the preferred embodiment the distance X between the propellers centers may be in the range of 0, 1 <^- <0,5, preferably 0,2 <^- <0,4, wherein D _m is the mean diameter of the two propellers 1, 6, i.e. D _m = ^¾ ^ ^¾J . Thanks to this solution it is feasible to regain considerably much more by the CRP concept, i.e. since the aft propeller 1 may be positioned relatively close to the front propeller 6. According to preferred aspects in accordance with the invention, the thruster unit may have an output within the range of 0,5 till 30 MW, and a propeller having a diameter within the range of 1 till 10 m, enabling the vessel to have a maximum speed of within the range 10 till 40 kn. The deflection range of the rudder flap 3 may be in the range -90 till +90 degrees, preferably -50 till -50, and more preferred -30 till +30 degrees.

The invention is not to be seen as limited by the preferred embodiments described above, but can be varied within the scope of the appended claims. For instance, it is evident for the skilled person that the rudder flap 3 may be combined with an interceptor arrangement (not shown) or indeed in some applications even exchanged for an interceptor arrangement. Further it is evident that the propellers 1, 6 may be varied in many ways without departing from the scope of invention, e.g. by varying the number of blades, for instance using a different number of blades on the two propellers 1, 6.