NO343796B1

NO343796B1 - Propulsion system with rim-integrated electric motor and methods for retrofitting a propeller

Info

Publication number: NO343796B1
Application number: NO20170369A
Authority: NO
Inventors: Alexey Matveev
Original assignee: Rolls Royce Marine As
Priority date: 2017-03-13
Filing date: 2017-03-13
Publication date: 2019-06-11
Also published as: NO20170369A1

Description

The present invention is related to a propulsion system for a water-borne vessel, comprising a first propeller and a second propeller, which propellers are arranged axially one after another on a central shaft. Wherein the first propeller is driven by the central shaft and the second propeller is driven by an electric motor integrated into the periphery of the second propeller.

BACKGROUND

The propulsion system of a marine vessel is normally located in the after part of the vessel. The propeller can be equipped with a hubcap or a cone at the end of the propeller shaft. The circulation of water around each propeller blade forms a vortex near the hub before they join to one hub vortex. This hub vortex cavitation can be harmful to the propulsion unit or the rudder behind the main propeller.

Propulsion of ships has traditionally been accomplished by using one or several rotating propellers and steering by making use of rudders in connection with a propellers. Contra-rotating propellers (CRP) are known to offer benefits over conventional single propeller as the load distributed into the blade area of two propellers instead of one and the rotational energy of the vortex of the first propeller may be more efficiently recovered.

It is also well known that by use of a nozzle in combination with a propeller, higher thrust force can be achieved. The thrust produced by the propeller is amplified by the nozzle, especially at low speed. Effect of the nozzle on thrust produced can be as high as 40%. Nozzles are used in e.g. dynamic positioning (DP) vessels, where high thrust is needed at low speed to keep the vessel in position in rough seas.

Another class of propulsion systems is based on so-called rim-drive arrangements. A rim-drive propulsion system comprises a rotor of electric machine positioned on the outer periphery of the propeller and a stator positioned within a nozzle surrounding the propeller. The benefit of the rim-drive propulsion is that the diameter of the rotor and the stator of the electric motor can be increased considerably, allowing an increased torque of the electric motor.

Also, well known to the prior art, are the PODed propulsors. PODed propulsors, which comprises an outboard propulsion body, offer flexibility in vessel design and machinery layout. They combine the functions of a propulsion motor, main propeller, rudders and stern thrusters in a single unit. The integrated electric motor drives the shaft, saving space on board the vessel and eliminates the need for a gearbox. Primary advantages are electrical efficiency, better use of hull space, and lower maintenance cost and good cooling to the motors.

Prior art discloses propulsors with multiple propellers. Such systems with two or more axially placed propellers are known for years. The publication RU 2128126 C by Tsentralnyj NI SKIJ I IM & ABB 1998, discloses a solution with three propellers all arranged on the same shaft. Another example is eSiPOD system by Siemens, and STP by Schottel, these systems discloses a system with two propellers rotating in the same direction.

Publication US 5289068 A discloses a two-stage submersible propulsion unit comprising a shroud having a water inlet and a water outlet, a shaft assembly centrally mounted within the shroud, an upstream and downstream propeller, each of which includes a separate hub rotatably mounted on the shaft assembly. A first and a second motor separately operating the upstream and downstream propellers. Each motor includes a rotor mounted around the outer periphery of one of the propellers and a stator mounted around the shroud.

The publication DE 102009030112 A1, discloses a propulsion system for a vessel comprising a first propeller mounted on a central shaft and a second propeller arranged outside the vessel and driven by an electric motor. The electric motor is configured as an external rotor motor with an annular stator and an annular rotor.

It is well known, and explained above, that two propellers arranged on the same shaft can generate propulsive force in the same axial direction while rotating in opposite directions. Propellers can both help each other, or one can collect energy from the other; the second propeller captures spiral motion of the flow after the first propeller and turn it into axial thrust. Contra-Rotating (CR) propellers are quite established area. There are over one hundred patents disclosing various CR concepts for propulsion of marine vessels. Probably, many of the patented CR concepts have been tested as prototypes never reaching the market as only a few CR products are offered today; counter-rotating Azipod and Rudderpod™ (both by ABB), Contaz by Rolls-Royce, counter rotating dual prop bow thruster by Wesmar, contra-rotating propeller system by Japan Marine United Corporation’s (JMU) and a few others.

There are also other ways of flow recovery. It is known from prior art, to use “vanes” or “flow directing fins” to capture helical/spiral motion of the flow after the propeller and turn it into axial thrust. One example is the patent US8435089 B2 by Alstom (called “Inovelis”).

ABB, discloses another example of a CR propeller in the publication EP2944560A1. This application discloses a solution with one rotating and one stationary propeller (“counter-propeller”) for streamlining the flow after the first one, where the counter-propeller has blade pitching capability. This feature is also disclosed in the publication RU 2115588 C1 by ATATSII flota Giprorybflot. This solution claims to provide 8-10% efficiency improvement compared to solutions without CR propeller.

The different CRP systems can be classified in the following categories:

Propellers driven through rim motor.

This system comprises similar size propellers integrated into a nozzle and, wherein each propeller is driven independently from own rim motor. Such arrangement is described in the publication US 2010279559 A1.

The drawbacks of such a system is that the propellers diameter is given/defined by the nozzle size, such that the propeller diameter has to equal the inner diameter of the nozzle. Hence, the arrangement cannot be fully optimized from a hydrodynamics point of view.

Propellers driven through cogwheels at their periphery.

Publication KR 101422239 B1, discloses propellers with nozzles having cogwheels in their periphery and are driven by the same cogwheel connected to shaft of an electric motor. These systems have the drawbacks that they have low reliability and efficiency.

Solutions with two propellers on separate shafts driven independently.

The publication JP 5524672 B2, discloses a solution with one propeller driven by a shaft from the hull and another propeller arranged as a rotatable thruster or POD, totally separated mechanically and independent from the first propeller. Samsung discloses another example where both units are thrusters, in publication KR101245735 B1. Yet another system where one of the propellers is smaller and can be turned in horizontal axis is disclosed in publication RU 2098316 C1. The drawbacks of such systems are that of an extra azimuth thruster which is complex, costly and requires additional space in the hull.

Solutions with concentric shafts.

(A) Solutions where the two shafts are connected by mechanical transmission.

Solution with gearbox integrated into the shaft and propeller arrangement, as disclosed in the publication US 2015030452 A1 by Samsung. “Shaftline” solutions with concentric shafts, where both shafts have cogwheels and there is a common (shared) cogwheel driving the two cogwheels in opposite direction. The common cogwheel is driven through mechanical transmission by a motor. Cogwheels and transmission are inside the hull, as disclosed in the publication JP 59084693 A2 by Mitsubishi. In the case of azimuth thruster, the cogwheels can be placed in a “bulb” outside the hull right next to the propellers such as disclosed in publication DE 19524325 A1 by Merz Josef.

(B) Solutions where the shafts are driven separately, independently from each other.

Solution with concentric shafts where both shafts are driven independently by own mechanical transmissions. The publication RU 2115589 C1, discloses a solution with concentric shafts, where each shaft is driven by a separate electric motor. The motors can be arranged in a POD.

(C) Solutions where the shafts are mechanically decoupled but driven by the same motor.

Solution with concentric shafts, where one shaft is driven by the first rotating part of an electric motor and the other shaft is driven by the second rotating part of the same motor. Each rotating part has slip rings for transferring power to it. The motors can be in a POD as disclosed in the publication RU2119875 C. The drawbacks of the solutions with concentric shafts is that the concentric shafts are complex (not reliable) and costly mechanical systems.

Solutions with two axially arranged propellers with a gearbox

Solution with one propeller driven directly by a shaft and the second propeller – via a gearbox as disclosed in the publication US 2015030452 A1, by Samsung. The drawbacks of this kind of solutions is that the system has low efficiency and that the gearboxes are known to be not reliable and require more service.

Thus, there is a need for a CRP propulsion system that overcomes some of the problems mentioned in prior art. More detailed, there is a need for a CRP propulsion system that are more reliable and having higher efficiency.

Further, there is a need for a system that are less complex than the systems used today and requires less space together with other benefits as reduced cost.

When the power of a propulsion system increases, the diameter of propeller will increase, leading to the increase of diameter of the rim-integrated machine. This leads to mechanical challenges and cost increase. The proposed system will allow achieving the same power with smaller diameter propeller, avoiding the challenges of the prior art.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a CRP propulsion system that is more efficient, reliable, compact and redundant and provides higher trust than existing systems.

It is another object of the present invention to provide a CRP propulsion system that can adapt to existing propulsion systems in form of retrofit.

It is yet another object of the present invention to provide a CRP propulsion system that are interchangeable and allowing access for service and repair.

It is yet another object of the present invention to provide a CRP propulsion system that are less complex by avoiding extra azimuth thruster, complex concentric shafts, sealing’s and mechanical gear.

The new propulsion solution claims higher efficiency, higher thrust, higher reliability and higher redundancy and smaller diameter. The propulsors employ two contra-rotating propellers for flow recovery and turning it into axial thrust, improving efficiency. The propulsors are characterized by use of a nozzle for further increase of the thrust force. The propulsors are further characterized in that they do not use complex mechanical arrangements like gearboxes, concentric shafts or sealing’s, which would reduce system reliability and increase cost. The proposed propulsor concepts allow hydrodynamic design optimization.

The present invention is related to a propulsion system for a water-borne vessel according to claim 1.

The propulsion system comprises a first propeller and a second propeller, which propellers are arranged axially one after another on a central shaft. The first propeller is driven by the central shaft, and the central shaft is driven by a first motor. Wherein the second propeller is rotatably mounted on the central shaft and driven by a second motor integrated into the periphery of said second propeller. The second motor is an electric motor.

In a preferred embodiment according to the present invention, the first propeller is a shaft-driven propeller and the second propeller is a rim-driven propeller.

The system according to the present invention can either be used on new built vessels or be used for replacing existing propulsion systems or be formed by upgrading/retrofitting existing propulsion systems.

In an embodiment according to the present invention, the at least one of the first propeller or the second propeller is arranged to be retrofit or added on the central shaft.

According to an embodiment of the present invention, the second propeller can be added to an existing propeller system in form of retrofit. The second propeller can be added to the central shaft at any side of the first propeller, such that they are arranged axially one after another. Likewise, the first propeller can be added to either side on a central shaft of a second propeller.

According to the invention, the second propeller is arranged on bearings mounted between the second propeller and the central shaft.

In another preferred embodiment of the present invention, the first propeller can be retrofitted to a second propeller on a common central shaft, such that the propellers are arranged axially one after another. The second propeller is driven by an electric second motor integrated into the periphery of said second propeller, and wherein the central shaft is driven by a first electric motor located outboard in a propulsion body, such as a POD or within the vessel hull.

Within the scope of the invention, the POD containing the first electric motor can be retrofitted together with the first propeller onto a central shaft of a second propeller (such as an azimuth thruster).

According to the present invention, the first propeller is driven by a central shaft, which again can be driven by a diesel engine or an electric motor. More preferably, the central shaft is driven by an electric motor. Most preferably, the central shaft is driven by an electric motor located outboard in a propulsion body, such as a POD, though shaft-line solution is also a viable option. In general, outboard propulsors have the advantage that they save space inside the hull and provide good cooling to the motors.

According to a preferred embodiment of the present invention, the second propeller comprises a nozzle comprising a rim-drive electric motor for operating the second propeller. Wherein the rim-drive electric motor comprising a rotor rim provided on the periphery of the second propeller, and a stator provided within the nozzle surrounding the rotor rim. With this arrangement, the diameter of the rotor and the stator of the electric motor can be increased considerably, hence, allowing an increased torque of the electric motor. That means, that a rim-drive motor produces more torque at low speed while using optimal amount of active materials such as steel, copper or magnets, due to the increase in diameter of the rotor and stator.

Sharing the power between the two propellers allows having smaller diameter of the propellers and the nozzle, which in certain cases is an advantageous for production of the propulsion system parts and for the hull design.

The thrust produced by the propeller is amplified by the nozzle at low speeds.

Effects of the nozzle on thrust produced can be as high as 40%. Nozzles are preferably used in Dynamic Positioning (DP) vessels, where big thrust is needed at low speed to keep the vessel in position in rough seas. Possible mutual location of the propellers and the nozzle is as follows:

- Both propellers inside the nozzle (in axial direction).

- One propeller (rim-driven) inside and the other propeller (shaft driven) outside the nozzle.

The propulsion system according to the present invention has the benefit that it can be optimized from a hydrodynamics point of view. The first propeller size does not depend on the nozzle diameter, thus, allowing no limits on choice of propeller diameters. Further, the propulsion system according to the present invention allows the distance between the propellers to be optimized. Powers delivered to the two propellers can be different according to the load taken by the propellers.

The motors of for driving the first and the second propeller can be driven by frequency converters connected to different parts of a switchboard or to different switchboards. The first frequency converter is arranged for supplying electric power to the electric first motor driving the first propeller and, wherein the second frequency converter is arranged for supplying power to the electric second motor driving the second propeller.

According to the invention, the first frequency converter is arranged on a first part of a switchboard and the second frequency converter is arranged on a second part of the switchboard. The first and second frequency converter can also be arranged on each individual switchboard. Wherein the first part and the second part of the switchboard is arranged such that if a failure occurs at one part the failure does not propagate to the other. This appropriate embodiment assures that if one propellers stops, the other propeller can still operate at elevated rpm. The operating propeller can run at elevated rpm so that it delivers more thrust to compensate at least partly for the loss of the stopped propeller. This provides redundancy for situations when a switchboard or a part of switchboard is out of operation. The propulsor maintains capability of providing partial thrust. This is important for DP operation.

According to the present invention, the first propeller can be a propeller with a smaller diameter than the second propeller and vice versa. The first and the second propeller are contra-rotating propellers (CRP).

The invention is also related to a method for retrofitting a rim-driven second propeller to a shaft-driven first propeller situated on a central shaft, such that the first propeller and second propeller are arranged axially one after another, wherein the first propeller is driven by the central shaft, which method comprises the steps of:

a) if present, removing hubcap or propeller cone

b) if required, removing the first propeller from the central shaft,

c) if required, replacing the first propeller by a smaller on-shaft first propeller, d) adding rim-drive integrated into a nozzle,

e) adding the second propeller situated on bearing on the central shaft, f) if b), adding the first propeller to the central shaft in line after the second propeller,

g) if a), adding a new hubcap or propeller cone design.

In the cases where a hubcap or propeller cone is situated at the first propeller, the said hubcap/cone has to be removed. The first propeller is removed in cases when it should be replaced by a smaller on shaft first propeller or in the case the second propeller should be mounted on the inside of the first propeller. However, if the second propeller is to be mounted aft of the first propeller and the first propeller need not be changed, the first propeller need not be removed.

The second propeller situated on bearings is added to the central shaft. A rim-drive integrated into a nozzle is added for driving the second propeller. Further, if the first propeller has been removed in case of b) and not replaced by another first propeller, then the first propeller will be added to the aft of the second propeller. Depending on the design of the propulsion system, a new designed hubcap can be placed to the aft propeller for streamlining the flow created by the propellers.

The invention is also related to a method for retrofitting a first propeller to a rimdriven second propeller situated on bearings on a central shaft, such that the first propeller and second propeller are arranged axially one after another, wherein the second propeller is driven by a rim-drive integrated into a nozzle,

which method comprises the steps of:

a) if present, removing hubcap,

b) adding the first propeller onto the central shaft,

c) adding a second electric motor integrated into a POD for driving the central shaft and the first propeller.

d) If a), adding a new hubcap.

The nozzle and the first and the second propellers can have following mutual locations;

- first and second propeller inside the nozzle, or

- one propeller inside and the other propeller outside the nozzle.

The first and the second propellers are contra-rotating propellers (CRP).

BRIEF DESCRIPTION OF THE DRAWINGS

The description above, as well as further objects, features and advantages of the present invention will be more fully appreciated by reference to the following detailed description of the preferred embodiment, which should be read in conjunction with the accompanying drawings in which:

Figure 1 shows an embodiment of the present invention where a second CR propeller is mounted on a central shaft in front of the first propeller.

Figure 2 shows another embodiment of the present invention where the second propeller is mounted to the end of the central shaft and aft of the first propeller.

Figure 3 shows the embodiment of figure 1 comprising a first electric motor for driving the central shaft.

Figure 4 shows another preferred embodiment of the present invention, where the first electric motor is situated inside a POD.

Figure 5 shows yet another preferred embodiment of a first electric motor situated inside a POD and wherein the second propeller is mounted at the aft of the first propeller.

Figure 6 shows a traditional propulsion system with an aft propeller mounted at the aft of a vessel.

Figure 7 shows a second propeller retrofitted to the traditional propulsion system in figure 6.

Figure 8 shows another embodiment of a second propeller retrofitted to the traditional propulsion system in figure 6.

Figure 9 shows an ordinary azimuth thruster with a second propeller with rim-drive.

Figure 10 shows a first propeller and a POD retrofitted to the azimuth thruster of figure 9.

Figure 11 shows a redundant system with frequency converters and switchboard.

DETAILED DESCRIPTION OF THE DRAWINGS

The figure 1 shows an embodiment of the CRP propulsion system according to the present invention. A first propeller 11 is driven by a central shaft 16, and wherein the central shaft 16 is driven by a first motor (not shown). A second propeller 21 is arranged on the central shaft 16 at the inside (or in front of the first propeller, viewed in a main direction of vessel movement) between the first propeller 11 and a vessel hull (not shown), such that the propellers 11, 21 are arranged axially one after another on the central shaft 16. The second propeller 21 is driven by a second motor 22 integrated into the periphery of the second propeller 21, which second motor 22 is an electric motor. The second propeller 21 is arranged with bearings 25 situated between the propeller 21 and the central shaft 16. Figure 1 also shows a hubcap 51 or propeller cone situated at the end of the central shaft, the said hubcap/cone 51can be removed.

In an embodiment of the present invention, the second motor 22 is a rim-drive motor, and wherein the second propeller 21 comprises a nozzle (not shown) comprising the rim-drive motor 22 for operating the second propeller 21. The rimdrive electric motor 22 further comprises a rotor 23 rim provided on the periphery of the second propeller 21.

Figure 2 shows another embodiment according to the present invention, where the second propeller 21 is mounted on the aft part of a central shaft 16. The propellers 11, 21 are situated such that they are arranged axially one after another. Likewise, the first propeller is driven by the central shaft 16 with a first motor or engine situated within a vessel hull 52 or inside a POD 15 (not shown), and the second propeller 21 is driven by an electric motor 22 integrated into the periphery of said second propeller 21. In accordance with figure 1, the electric motor 22 of the second propeller is a rim-drive motor with a rotor 23 rim provided on the periphery of the second propeller 21, and wherein the stator 24 is arranged radially outside the rotor 23. Preferably, the stator 24 is arranged within a nozzle (not shown) surrounding the second propeller 21.

Figure 3 shows the embodiment in figure 1 comprising a first electric motor 18 for driving the central shaft 16 of the first and second propellers 11, 12. Wherein the second propeller is driven by a rim-drive motor 22 and the first propeller 11 is driven by the central shaft 16.

Figure 4 shows another embodiment of the present invention disclosing a nozzle 26 with integrated rim-drive 22. The propulsion system comprises a first electric motor 18 situated inside an outboard mounted propulsion body 15, such as a POD. The POD 15 offers flexibility in vessel design and machinery layout. They combine the functions of a propulsion motor 18, first and second propellers 11, 21, rudders and stern thrusters in a single unit. The integrated electric motor 18 drives the central shaft 16, saving space on board and eliminates the need for a gearbox. The POD 15 combine both propulsive and steering functions in one device.

As seen in figure 4, the first propeller 11 is mounted on a central shaft 16 and driven by an electric motor 18 situated inside a POD 15. A second CR propeller 21 is mounted between the first propeller 11 and the POD 15 housing, such that the first propeller 11 and the second propeller 21 are arranged axially one after another on the central shaft 16. The first propeller 11 and the second propeller 21 has the approximate same sizes, though they can have different size ratio. The second propeller 21 is driven by a second electric motor 22 integrated into the periphery of said second propeller 21, such as a rim-drive 22. The rim-drive 22 propulsion system comprises a rotor 23 positioned on the outer periphery of the second propeller 21 and a stator 24 positioned within a nozzle 26 surrounding the second propeller 21, and possibly also the first propeller 11. The bearings 25 are situated between the second propeller 21 and the central shaft 16.

According to the present invention, the following mutual location of the first 11 and second 21 propeller and the nozzle 26 is possible.

- Both propellers 11, 21 inside the nozzle 26 (in axial direction).

- One propeller 21 inside and the other propeller 11 outside the nozzle 26.

As shown in figure 5, the system also allows the second propeller 21 to be mounted to the rear part of the central shaft 16, such that the second propeller 21 is situated aft of the first propeller 11 viewed in the main moving direction of the vessel. In this embodiment, the nozzle 26 is surrounding both the first propeller 11 and the second propeller 21. This embodiment allows a larger second propeller 21 to be added to the POD 15 propeller system.

Figure 6 shows a traditional aft propeller 11 located at the aft of a vessel hull 52. The aft propeller 11, which is the first propeller, is driven by a central shaft 16. According to the present invention, a second propeller 21 can be retrofitted to the traditional system as in figure 1. The second propeller 21, which is a CR propeller, can be mounted at the rear end or in front of the first propeller 11, when viewed in the main direction of vessel movement. In other words, according to the invention, the second propeller 21 or auxiliary propeller can be mounted on the central shaft at either side of the first propeller 11. As seen in figure 6, the central shaft 16 is arranged on bearing system 55 with seals 53.

Figure 7 shows a rim-driven second propeller 21, which is a contra-rotating propeller mounted to the aft part of the central shaft 16 of the propulsion system in figure 6. In this embodiment, the first propeller 11 is moved into the central shaft 16 allowing a second propeller 21 to be mounted to the aft part of the central shaft 16, and such that the first 11 and the second 21 propeller is arranged axially one after another. According to the invention, the first propeller 11 can also be replaced by a smaller on-shaft propeller. The second propeller 21 is driven by an electric motor 22 integrated into the periphery of said second propeller 21, such as a rim-drive 22. The rotor 23 of the rim motor is positioned on the outer periphery of the second propeller 21 and wherein a stator 24 positioned within a nozzle 26 surrounding the second propeller 21, and possibly also the first propeller 11. As seen in figure 7 the nozzle 26 is fixedly mounted to the vessel hull 52.

Equivalent to the embodiment of figure 7, the figure 8 shows the second propeller 21 mounted between the first propeller 11 and the vessel hull 52. The second propeller 21 is arranged on bearings 15 on the central shaft 16. The nozzle 26 is surrounding both the propeller 11, 21.

Figure 9 shows an ordinary azimuth thruster with a rim-driven second propeller 21. The rotor 23 of the rim motor is positioned on the outer periphery of the second propeller 21 and wherein a stator 24 positioned within a nozzle 26 surrounding the rim-driven second propeller 21

Figure 10 shows another preferred embodiment of the present invention where a first propeller 11 is added to the azimuth thruster from figure 9. The shaft-driven first propeller 11 is mounted at the end of the central shaft 16 such that the first propeller 11 and the second propeller 21 are arranged axially one after another. The first electric motor 18 for driving the central shaft 16 is integrated into a POD 15. Which POD 15 is mounted on the opposite side of the first propeller 11. Both the POD 15 and the first propeller 11 can be retrofitted to the azimuth thruster of figure 9.

Figure 11 shows a first frequency converter 32 arranged for supplying electric power to the first electric motor 18 driving the first propeller 11 and, wherein a second frequency converter 31 is arranged for supplying electric power to the rim electric motor 22 driving the second propeller 21.

Further, the first frequency converter 32 is arranged on a first part 34 of a switchboard and the second frequency converter 31 is arranged on a second part 33 of the switchboard, and wherein the first part 32 and the second part 33 is arranged with a bus-tie 35 (normally open) such that if a failure occurs in one of the sides 33, 34 it does not propagate to the other.

This appropriate embodiment assures that if one of the propellers 11, 21 stops, the other propeller 11,21 can still operate. The operating propeller 11, 21 can run at elevated rpm so that it delivers more thrust to compensate at least partly for the loss of the stopped propeller 11,21. This provides redundancy for situations when a part of switchboard is out of operation. The propulsor maintains capability of providing partial thrust. This is important for DP operation.

Although preferred embodiments of the invention have been illustrated in the accompanying drawings and in the foregoing detailed description, it will be understood that the invention is not limited to the embodiments disclosed, but is capable of numerous arrangements, modifications, and substitutions of parts and element.

Claims

1. A propulsion system for a water-borne vessel, comprising a first propeller (11) and a second propeller (21),

wherein the second propeller (21) is arranged with a nozzle (26) and driven by a rim-drive electric motor (22) comprising a rotor (23) rim provided on the periphery of the second propeller (21), and a stator (24) provided within the nozzle (26),

characterized in that the first propeller (11) and the second propeller (21) is arranged axially one after another on one central shaft (16) and the first propeller is driven by a first motor via the central shaft and the second propeller is rotatably mounted on said central shaft.

2. The system according to claim 1,

characterized in that the second propeller (21) is arranged with bearings (25) mounted between said second propeller (21) and the central shaft (16).

3. The system according to any one of the preceding claims,

characterized in that at least one of the first propeller (11) or the second propeller (21) is arranged to be retrofitted on the central shaft (16).

4. The system according to any one of the preceding claims,

characterized in that the first motor is an electric motor (18).

5. The system according to claim 4,

characterized in that a first frequency converter (32) is arranged for supplying electric power to the first motor (18) driving the first propeller (11) and, wherein a second frequency converter (31) is arranged for supplying electric power to the second motor (22) driving the second propeller (21).

6. The system according to claim 5,

characterized in that the first frequency converter (32) is arranged on a first part (34) of a switchboard and the second frequency converter (31) is arranged on a second part (33) of the switchboard, and wherein the first part (34) and the second part (33) is arranged such that if a failure occurs at one of the parts (33, 34) the failure does not propagate to the other.

7. The system according to any one of claims 4-6,

characterized in that the first motor (18) driving the central shaft (16) is located outboard in a propulsion body (15).

8. The system according to claim 7,

characterized in that the propulsion body (15) is arranged to be retrofitted to the central shaft (16).

9. The system according to any one of the preceding claims,

characterized in that the first propeller (11) and the second propeller (21) are arranged with different propeller diameter.

10. The system according to any one of the preceding claims,

characterized in that the first (11) and the second (21) propeller are contrarotating propellers (CRP).

11. A method for retrofitting a rim-driven second propeller (21) to a shaft-driven first propeller (11) situated on a central shaft (16), such that the first propeller (11) and second propeller (21) are arranged axially one after another, wherein the first propeller (11) is driven by the central shaft (16), which method comprises the steps of:

a) if present, removing a hubcap or a propeller cone,

b) if required, removing the first propeller (11) from the central shaft (16), c) if required, replacing the first propeller (11) with a smaller on-shaft propeller, d) adding rim-drive integrated into a nozzle (26),

e) adding the second propeller (21) situated on bearing on the central shaft (16), f) if b), adding the first propeller (11) to the central shaft (16) axially after the second propeller (21),

g) if a), adding a new hubcap or propeller cone design.

12. A method for retrofitting a shaft driven first propeller (11) to a rim-driven second propeller (21) situated on bearings on a central shaft (16), such that the first propeller (11) and second propeller (21) are arranged axially one after another, wherein the second propeller (21) is driven by a rim-drive integrated into a nozzle (26), which method comprises the steps of:

a) if present, removing hubcap or propeller cone,

b) adding the first propeller (11) on the central shaft (16),

c) adding a first electric motor (18) integrated into a POD (15) for driving the central shaft (16) and the first propeller (11),

d) if a), adding a new hubcap or propeller cone design.

13. The method according to any one of claims 11 or 12,

wherein the nozzle (26) and the first and the second propellers (11,21) can have following mutual locations;

- first and second propellers (11,21) inside the nozzle (26), or

- rim-driven second propeller (21) inside and the shaft-driven first propeller (11) outside the nozzle (26).

14. The method according to any one of claims 11-13,

wherein the first propeller (11) and the second propeller (21) are contrarotating propellers.