Propulsion arrangements for large seagoing vessels have traditionally been implemented using a main propeller, the shaft of which is fitted to the hull with bearings. Separate rudder gear has normally been used to steer the vessel. The efficiency of propulsion power has been increased by fitting another propeller to the same axis, rotating in the opposite direction; this is called a CRP propeller (Contra Rotating Propulsion). A prior art solution for improved manoeuvrability includes a turning, so-called azimuth-type, propeller gear fitted behind the main propeller; the propeller rotates in the opposite direction to the main propeller, and the propeller gear can be turned around the vertical axis of its shank to steer the vessel.

Even though prior art solutions provide sufficient propulsion power and manoeuvrability for many circumstances, they are associated with limitations that prevent or hamper their usability. Increased shaft power output from the main engine requires a larger main engine, and particularly as the power increases, a coaxial propeller arrangement requires special measures with regard to bearings and sealing.

Naturally, any increase in the propulsion power of the main propeller will necessitate a larger power engine, which is a substantial cost factor. In plans for large vessels, the full driving power of the engine that operates the main propeller, such as a diesel engine, is completely dedicated to driving the propeller, and the propeller may be coupled to the engine shaft directly or through a gear. Other electrical power requirements of the vessel, such as pumping power required on board, must be produced using separate power engines that have lower ratings than the one driving the main propeller. Thus the total power required by the vessel is the aggregate of the aforementioned power engines.

The purpose of the invention is to eliminate the disadvantages of prior art and create a new preferred solution that provides good overall propulsion power and sufficient steering power, as well as overall economy. In order to achieve this, the invention is characterised by the features specified in the characteristics section of Claim 1. The dependent claims define a number of preferred embodiments of the invention.

The solution according to the invention provides a powerful vessel propulsion effect thanks to the CRP effect. The power of the vessel's main propeller can be limited to a level substantially lower than the required propulsion power, while steering can be implemented using a rudder that has sufficient area and is compliant with regulations.

Some embodiments of the invention are particularly preferred for the implementation of a vessel propulsion device. The second propeller that rotates in the opposite direction, as well as the motor driving the propeller, are supported by the suspension arm of the rudder gear. Propulsion power can be increased further by shaping the rudder suspension arm in a streamlined fashion. Furthermore, manufacture can be expedited by using a module- type motor and propeller combination in the pod, such as the one described in the publication WO 02/26558. In a similar fashion, the other propeller can be fitted into its own suspension arm that protrudes downwards from the hull. In this case, the rudder gear can be designed and implemented separately.

The preferred method of operating the other motor is an electric motor supplied and controlled by a frequency converter using electrical power from the vessel's electrical power system. Electric motor technology used in azimuth pods, such as the AZIPODO devices, is extremely suitable for this purpose.

According to a preferred embodiment, the rated power of the second propeller is approximately one half of the rated power of the main propeller. This provides good propulsion power in relation to the vessel's total power. The rated power of the engine that operates the main propeller, such as a diesel engine, can be decreased in comparison to the conventional alternative where the main propeller is responsible for the entire propulsion power. During normal operation of the vessel, most of the total electrical power on board is used for propulsion.

Both the main propeller and the second propeller can preferably be operated independently, regardless of the operating state of the other propeller. When the approximate propulsion power of each propeller is at least one third of the total propulsion power of the vessel, the vessel can be driven on a single propeller in circumstances where the other propeller is temporarily out of operation, for example, due to a failure.

According to another embodiment, on vessels where pumping power is required the rated power of the second propeller is essentially in the same order of magnitude as the vessel's rated pumping power. In the case of large ocean-going vessels, the required pumping power can be approximately one third of the rated propulsion power. When this pumping power is produced using separate power engines that are independent of the engine driving the main propeller, the power of these engines can easily be utilised to drive the second propeller. Preferably, pumping power is produced using one or more diesel generators, the output of which is connected to the vessel's electrical power system. The solution provides flexibility not only to the production of electrical power but also to the placement of equipment.

In the following, the invention will be described in detail by referring to the enclosed drawings, where - Figure 1 illustrates a set of equipment according to the invention, and - Figure 2 illustrates a power supply arrangement according to the invention.

The shaft 6 of the main propeller 4 is fitted with bearings to the aft section of the vessel's hull 2 and driven using a well-known method using either the electric motor 10 or the main engine of the vessel, such as a diesel engine (not shown). The electric motor 10 is controlled using the frequency converter 11, which is supplied from the vessel's electrical power system 18. Also using a well-known method, the main propeller 4 is equipped either with fixed propeller blades or adjustable propeller blades 8. The blades and/or the engine driving the main propeller are regulated in order to produce the desired propulsive force. Another propeller, the aft propeller 12, is fitted on essentially the same axial line as the main propeller shaft 6, rotates in the opposite direction compared to the main propeller and is located behind the main propeller. The aft propeller 12 is fitted to the shaft 13, which is supported by the stator of the electric motor 14 using a rotating arrangement. Preferably, the electric motor 14 is a permanent magnet synchronous motor controlled by the frequency converter 16 using power supplied from the vessel's electrical power system 18. One of the preferred embodiments of the electric motor is the permanent magnet synchronous motor used in the Compact Azipod0 system, cooled directly through the motor housing by the surrounding water.

The electric motor 14 is mounted so that it cannot tam or move in relation to the hull. The hull of the vessel is fitted with a rudder suspension arm 20 protruding downwards from the aft section of the hull, on which the rudder plate 22 is attached with a turning arrangement controlled by the steering device 24. The suspension arm 20 is streamlined in order to prevent harmful turbulence. The suspension arm is fitted with attachment devices or flanges on which the motor module containing the electric motor 14 is attached using bolts or similar.

The aft propeller 12 is installed at a case-specific distance from the main propeller that provides the optimal propulsion power for the vessel. During operation, the propeller rotation speeds and the positions of adjustable propeller blades are adjusted in accordance with predetermined instructions. These dimensioning and control measures are implemented according to a well-known method used in CRP drives. The preferred ratio of power ratings between the main propeller 4 and the aft propeller 12 is in the range of 50/50-70/30, where the figures indicate percentages of aggregate propeller power. For example, when the power range of the main propeller is approximately 4 MW, the power of the other motor can be approximately 2.5 MW.

Figure 2 is a schematic diagram of the framework of the vessel's electrical power system according to the invention. It depicts a case where the main propeller is driven by the vessel's power engine, such as a diesel engine, and the other energy requirements of the vessel are catered for using lower-power engines driving generators whose outputs are connected to the vessel's electrical power system. The diesel engine 26 driving the main propeller 32 is coupled directly to the propeller shaft 34 or through a gear system (not shown). Through its shaft 36, the aft propeller 30 is coupled to the drive motor 28 which receives its power supply from the vessel's electrical power system 46 through the cable 38 and control gear such as the frequency converter 39. The vessel's electrical power system 46 is supplied by other power engines, such as the diesel generators 40,42 and 44, the rated power of which is clearly lower than that of the main power engine 26; the difference may be as much as one order of magnitude. For example, if the power rating of the main power engine 26 is approximately 5 MW, the power rating of each diesel generator 40,42 and 44 can be in the order of 0.7 MW. This makes it possible to achieve a propulsion power output as high as 7 MW. The power ratings and the number of diesel generators can naturally vary in accordance with the rated power of the vessel. The

electrical power system 46 supplies the vessel's other systems through the cables 48. If the power requirement of these systems is high, for example, if the pumping power is high, the amount of power available to the aft propeller will be correspondingly lower and the vessel cannot cruise at full power.

The distribution of propulsion power sources into several parts provides significant advantages to the design of vessel machinery, as well as degrees of freedom and flexibility to the placement of the corresponding equipment. When the size of the main power engine, in proportion to the vessel's total propulsion power decreases, the amount of undivided space required for the power engine is substantially smaller than in a case where the main power engine would produce the entire propulsion power. Smaller diesel generators can be flexibly placed in different locations.

Even though the example described above specifies that the motor driving the aft propeller is a permanent magnet synchronous motor, more specifically adapted to an AZIPOD-type unit, the scope of the invention does not by any means exclude any other motor solutions. The same is true for rudder solutions and propeller control in general.

Preferably, the propellers are regulated using a control system where a single control command controls both propellers; the control signals for both propeller drives are derived from the control command. The electric motor driving the main propeller can also be implemented using an AZIPOD-type solution.

It should be understood that the above description of the invention by reference to preferred embodiments must not be considered restrictive ; different implementations within the limits specified by the following Claims are possible.