WITH MAIN AND SECONDARY PROPULSION DEVICES

TECHNICAL FIELD The invention relates to a ship presenting a hull having a bow and a stern, the ship comprising a plurality of propulsion devices. The invention also relates to a method for manoeuvring a ship of this kind.

BACKGROUND

The propulsion system of a ship can be divided into internal propulsion devices, e.g. including one or more piston engines or turbines, and external propulsion devices adapted to be in contact with the water on which the ship floats, e.g. including one or more propellers. The propulsion devices could also be water jet outlet nozzles fed by pumps. The internal and external propulsion devices are connect by some kind of power transferring arrangement, such as one or more drive shafts.

Improving the efficiency of a ship's external propulsion devices has been the object of many ideas. For example, traditional single screw ship propulsion has been

supplemented with ideas regarding multiple external propulsion devices and examples include suggestions in US1628837A, US3996877A, US8028636B2,

WO2012047753A2 and GB1467758.

Apart from traditional propeller rudder combinations, other types of external propulsion devices have emerged, such as pod propulsion. Pod propulsion units have a propeller mounted on a pod including an electric engine, which pod is fixed to a distal end of a strut which is formed as a rudder. The unit including the strut and the pod can be rotated to change the direction of the propeller thrust force, as well as to provide a sideways force by the strut acting as a rudder.

EP1329379A1 and JP200967213 disclose vessels with a main propeller, a rudder astern thereof, and two additional propulsion units. However, there is still a desire to further increase the efficiency of a ship's external propulsion devices. SUMMARY

It is an object of the invention to increase the efficiency of a ship's propulsion. It is a further object of the invention to increase the efficiency of the propulsion of wide ships.

These object are reached with a ship presenting a hull having a bow and a stern, the ship comprising

- at least one main propulsion device located towards the stern, and

at least two secondary propulsion devices located towards the stern, the thrust capacity of each secondary propulsion device being less than 30% of the thrust capacity of any of the at least one main propulsion device, - wherein the propulsion devices are arranged, as projected on a plane which is perpendicular to a hull centreline, so that the horizontal extension of each secondary propulsion device does not overlap the horizontal extension of any of the at least one main propulsion device,

wherein the at least two secondary propulsion devices are arranged so that regions where the prop washes from the secondary propulsion devices leave the secondary propulsion devices are at least partly located, when the ship is moving straight ahead at full speed or flank speed, within a boundary layer of the hull.

Herein, the hull is understood to not include parts of the ship's propulsion system, such as propellers and water jet outlet nozzles, or devices for steering or stabilising the ship, such as rudders or ship stabiliser wings.

As understood, at least one main propulsion device and the at least two secondary propulsion devices being located towards the stern means that they are closer to the stern than to the bow.

Thrust capacity means the maximum propulsion force that can be provided by the propulsion device during static conditions, i.e. when the ship is not moving. The thrust capacity of each secondary propulsion device is less than 30%>, and can be than 20%>, or even less than 10% of the thrust capacity of the main propulsion device. Preferably, where the first and second propulsion devices are main and secondary propellers, respectively, the swept area of each secondary propeller is less than 30% of the swept area of the main propeller.

A prop wash is the water pushed aft by a secondary propulsion device. As exemplified below, the secondary propulsion devices can be rotating units in the form of secondary propellers, or they can be outlet nozzles fed by water jet pumps. For example, where the secondary propulsion device is a secondary propeller, the prop wash is the water pushed aft by the secondary propeller, and the region where the prop wash from the secondary propulsion device leaves the secondary propulsion device is at the secondary propeller. More specifically, for this presentation, in the case of a secondary propulsion devices being a propeller, (secondary propeller), the region where the prop wash from the secondary propulsion device leaves the secondary propulsion device is understood to be identical with the region delimited by the propeller disc, i.e. it is delimited by the swept area of the propeller and the extension of the propeller blades in a direction that is perpendicular to the rotational axis of the propeller. Where the secondary propulsion device is an outlet nozzle of a water jet, as exemplified below, the prop wash is the water pushed aft by the pump of the water jet, and the region where the prop wash from the secondary propulsion device leaves the secondary propulsion device is at an outlet nozzle of the water jet. More specifically, for this presentation, in the case of a secondary propulsion device being an outlet nozzle of a water jet, the region where the prop wash from the secondary propulsion device leaves the secondary propulsion device is understood to be a flat, two-dimensional region extending perpendicularly to the flow of the water jet, and being delimited by the internal, water jet contacting surfaces of the nozzle at a distal end thereof, where the water jet leaves the nozzle.

It is understood that by arranging the propulsion devices, as projected on a plane which is perpendicular to a hull centreline, so that the horizontal extension of each secondary propulsion device does not overlap the horizontal extension of any of the at least one the main propulsion device, each secondary propulsion device is completely separated in a horizontal direction from the main propulsion device. I.e. the horizontal extension of any of the secondary propulsion devices is completely offset in the lateral direction of the ship from the horizontal extension of the main propulsion device. Herein, the horizontal extension of a propulsion device is understood as the maximum horizontal extension of the region where the prop wash from the propulsion device leaves the propulsion device. In the case of a propeller, the horizontal extension would be the horizontal diameter of the propeller swept area. Preferably there is, in the lateral direction of the ship, a horizontal distance between the horizontal extension of each secondary propulsion device and the horizontal extension of the main propulsion device.

As is known in fluid mechanics, a boundary layer is the layer of fluid in a vicinity of a solid body where the effects of viscosity are significant. The thickness of the boundary layer is normally defined as the distance from the body at which the flow velocity is 99% of the freestream velocity. This boundary layer thickness will normally depend on the speed of the ship. Also, for a given ship speed, the thickness of the boundary layer will normally depend on the location on the hull. The thickness of the boundary layer at the locations of the secondary propulsion devices can be determined, e.g. by means of CFD (Computational Fluid Dynamics).

As is known per se the ship might present a design waterline. As is well known in the art, the design waterline (DWL), also known as the load waterline (LWL), or the summer load line, is the line where, for a specific water type and temperature, the hull meets the surface of the water, when the ship is floating freely at rest in still water and loaded to its designed capacity. The design waterline is indicated on the hull with a so called PlimsoU line. The PlimsoU line is a reference mark with a horizontal line through a circle. The horizontal line of the PlimsoU mark is at the same level as the design waterline, and indicates the maximum depth to which the ship may be safely immersed when loaded, i.e. the legal limit to which the ship may be loaded, for a specific water type and temperature in order to safely maintain buoyancy.

Preferably, the secondary propulsion devices are completely below the design waterline. The main propulsion device could be completely below the design waterline.

Preferably, the at least two secondary propulsion devices are arranged so that at least 50%, preferably all, of each region where a prop wash from a secondary propulsion device leaves the secondary propulsion device is

- within 0.35 metres from the hull where the length at waterline (L.W.L.) of the hull is below 100 metres,

within 0.60 metres from the hull where the length at waterline (L.W.L.) of the hull is at least 100 metres and below 150 metres,

within 0.83 metres from the hull where the length at waterline (L.W.L.) of the hull is at least 150 metres and below 200 metres,

within 1.05 metres from the hull where the length at waterline (L.W.L.) of the hull is at least 200 metres and below 250 metres, within 1.25 metres from the hull where the length at waterline (L.W.L.) of the hull is at least 250 metres and below 300 metres,

within 1.45 metres from the hull where the length at waterline (L.W.L.) of the hull is at least 300 metres and below 350 metres,

- within 1.64 metres from the hull where the length at waterline (L.W.L.) of the hull is at least 350 metres.

Thereby, it will be ensured that the secondary propulsion devices are arranged so that regions where the prop washes from the secondary propulsion devices leave the secondary propulsion devices are at least partly located, when the ship is moving straight ahead at full speed or flank speed, within a boundary layer of the hull. This can be verified by the fact that the boundary layer thickness can be calculated using the following expression for the thickness of turbulent boundary layers over a flat plate: δ « 0.382x/Re _x ^/5 where δ is the overall thickness of the boundary layer,

x is the distance downstream from the start of the boundary layer,

Re _x is the Reynolds Number, Re _x = ριι ₀ χ/μ ,

p is the density,

u _Q is the freestream velocity, and

μ is the dynamic viscosity

The advantageous locations given above of the regions where the prop washes from the secondary propulsion devices leave the secondary propulsion devices were derived from assuming that the secondary propulsion devices are located at the stern so that x is the length at waterline of the hull, and assuming conservatively that the water temperature is 20 degrees Celcius. Also, a ship's full speed of 20 knots was assumed.

As is known in the art the boundary layer separates from the hull, usually in a region close to the stern. Preferably, the secondary propulsion devices are arranged so that the regions where the prop washes from the secondary propulsion devices leave the secondary propulsion devices are located, when the ship is moving straight ahead at full speed or flank speed, forward, i.e. in the direction towards the bow, of the boundary layer separation region. Although the invention entails pushing water backwards, which can locally increase the friction between the hull and the water, the invention makes it possible to provide a ship with improved propulsive efficiency since the water accelerated by the secondary propulsion devices can be used to energise the boundary layer towards the stern of the ship. At the same time the larger main propulsion device, which can be embodied as a propeller with a large swept area diameter, contributes to maintaining a high efficiency of the propulsion system itself. Basically, the invention provides a balance between an increased hull efficiency and a high efficiency of the propulsion system. The increased hull efficiency, provided by the secondary propulsion devices energising the boundary layer, combined with the high propulsion efficiency provided by the main propulsion device, increases the total efficiency of the ship's propulsion, in particular on wide ships, e.g. with a high beam to draft ratio. The reason is that one or more large main propulsion devices can be provided, with a number of secondary propulsion devices distributed along the lateral direction of the ship, providing the boundary layer energising effect throughout a major portion of the beam of the ship.

Since the propulsion devices are arranged, as projected on a plane which is

perpendicular to a hull centreline, so that the horizontal extension of each secondary propulsion device does not overlap the horizontal extension of any of the at least one main propulsion device, it is secured that this advantageous lateral distribution of the propulsion devices is provided. Preferably each secondary propulsion device is arranged, as projected on a plane which is perpendicular to the hull centreline, so that its horizontal extension is not overlapping the horizontal extension of any of the other secondary propulsion devices.

In some embodiments, the main propulsion device is centred with respect to the hull centreline. Thereby, in a horizontal, lateral direction of the ship, a single main propulsion device can be located in the middle of the hull.

Preferably, at least two of the secondary propulsion devices are distributed on both sides of a hull centreline, and are located outboard of the at least one main propulsion device. The hull is, below the design waterline, normally substantially symmetric in relation to a vertical symmetry plane intersecting the bow and the stern. The hull centreline is a horizontal line within the vertical symmetry plane. The secondary propulsion devices are preferably equally distributed on both sides of the hull centreline. There could be only one secondary propulsion device on each side of the hull centreline, or there could be more than one secondary propulsion device on each side of the hull centreline. Thus, in some embodiments, at least two secondary propulsion devices are located on the port side of the hull centreline, outboard of the at least one main propulsion device, and at least two secondary propulsion devices are located on the starboard side of the hull centreline, outboard of the at least one main propulsion device.

In some embodiments, the ship comprises two main propulsion devices, each located on a respective side of the hull centreline, wherein at least one of the secondary propulsion devices is located laterally between the main propulsion devices.

The distribution of the secondary propulsion devices on both sides of the hull centreline can provide for the boundary layer to be energized across the beam of the hull towards the stern, and this is especially advantageous in wide ships, such as many cruise ships. Also, the distribution of the secondary propulsion devices on both sides of the hull centreline decreases differences in velocities in the lateral direction of the hull, providing an increased efficiency.

Preferably, the regions where the prop washes from the secondary propulsion devices leave the secondary propulsion devices, in particular centres of such regions, are closer to the design waterline than a lowest part of the hull. The lowest part of the hull can be a point, a line or, e.g. in case the hull presents a bottom surface which is substantially planar, a region.

As suggested, at least one, preferably all, of the secondary propulsion devices can be a secondary propeller. Preferably, at least two of the secondary propulsion devices are secondary propellers, equally distributed on both sides of the hull centreline. Preferably, all of the secondary propulsion devices are secondary propellers, equally distributed on both sides of the hull centreline. A secondary propeller can be shaft driven. Also within the scope of the invention, at least one of secondary propellers can be a rim driven secondary propeller. In some embodiments, at least one of secondary propellers is a pod or an azipull thruster. In some embodiments, at least one of secondary propellers is an azimuth thruster.

As also suggested, in some embodiments, at least one, preferably all, of the secondary propulsion devices is an outlet nozzle of a water jet. Thereby, the main propulsion device may be a main propeller. Preferably, at least two of the secondary propulsion devices are outlet nozzles of water jets, equally distributed on both sides of the hull centreline. Preferably, all of the secondary propulsion devices are outlet nozzles of water jets, equally distributed on both sides of the hull centreline. Preferably, inlets of the water jets are wide; in particular they have a greater extension in the hull's lateral direction than in its longitudinal direction; this will advantageously reduce local flow deceleration at the inlet.

It should be noted that the set of second propulsion devices can be a combination of shaft driven secondary propellers, rim driven secondary propellers, pods, azipull thrusters and/or water jet outlet nozzles.

In some embodiments, where the main propulsion device is a main propeller, at least one, preferably all, of the secondary propulsion devices is a secondary propeller presenting a swept area diameter which is smaller than a swept area diameter of the main propeller. E.g. as suggested above, the swept area of each secondary propeller can be less than 30% of the swept area of the main propeller. The centre of the secondary propeller can be located within an upper third of a distance from the design waterline to the lowest part of the hull. The swept area diameter of the secondary propeller can be less than the distance from the centre of the secondary propeller to the design waterline. Preferably, where at least one or all of the secondary propulsion devices is a secondary propeller, the centre of the secondary propeller can be located, when the ship is moving straight ahead at full speed or flank speed, within a boundary layer of the hull. Further, the entire swept area of the secondary propeller can be located, when the ship is moving straight ahead at full speed or flank speed, within a boundary layer of the hull.

Where at least one or all of the secondary propulsion devices is an outlet nozzle of a water jet, the nozzle can be located, when the ship is moving straight ahead at full speed or flank speed, at least partly within a boundary layer of the hull. Preferably, where the main propulsion device is a main propeller, at least one of the regions, preferably all regions, where a prop wash from a secondary propulsion device leaves the secondary propulsion device is astern of the main propeller. This provides for said regions to be as far back as possible, which is advantageous. Preferably, the ship has a beam to draft ratio of at least 2.50, at least 3.0, at least 3.5, or at least 4.0. By arranging the secondary propulsion devices distributed in the beam direction, this embodiment results in a particularly beneficial arrangement of energising the boundary layer across the beam extension of the hull combined with the high hull efficiency provided by the relatively large beam to draft ratio.

Where at least one, more than one, or all of the secondary propulsion devices is a secondary propeller, the hull can present at least one groove, oriented substantially parallel to the hull centreline, and at least partially enclosing the respective secondary propeller. A portion of the groove can be located above the waterline. Preferably, the entire groove is located below the waterline. Preferably, at least a part of the groove presents a part-circular cross-section, with a radius that is 5% - 20% larger than half the diameter of the swept area of the secondary propeller.

Preferably, the diameter of the swept area of the main propeller is at least 50% of the distance from design waterline to the lowest part of the hull. Advantageously, the main propulsion device comprises a rudder astern of the main propeller. The rudder can be e.g. of a semi-balanced type or a spade type rudder.

The rudder can have a bulb which presents a hubcap which is in close proximity to a hub of the main propeller. Thereby, the propeller and the rudder of the main propulsion device form an integrated propeller rudder system, such as the one known as a Promas system. This offers increased propulsive efficiency without any loss in manoeuvrability.

The objects are also reached with a ship according to claim 23. Such a ship can for example be embodied according to any one of claims 3-22. Preferably, the propulsion devices of the ship according to claim 23 are arranged, as projected on a plane which is perpendicular to a hull centreline, so that the horizontal extension of each secondary propulsion device does not overlap the horizontal extension of any of the at least one main propulsion device. Preferably, the secondary propulsion devices are completely below a design waterline of the ship. The at least one main propulsion device can be completely below the design waterline.

The objects are also reached with a method for manoeuvring a ship according to any one of claims 1-23. The method comprises controlling the secondary propulsion devices so that the collective thrust of at least one secondary propulsion device on one side of the hull centreline is higher than the collective thrust of at least one secondary propulsion device on the other side of the hull centreline. Thereby the ship can be steered with different thrusts on the secondary propulsion devices.

The objects are also reached with a ship presenting a hull having a bow and a stern, the ship comprising at least one main propulsion device located towards the stern, and at least two secondary propulsion devices located towards the stern, the thrust capacity of each secondary propulsion device being less than 30% of the thrust capacity of any of the at least one main propulsion device, the at least two secondary propulsion devices being arranged so that regions where the prop washes from the secondary propulsion devices leave the secondary propulsion devices are at least partly located, when the ship is moving straight ahead at full speed or flank speed, within a boundary layer of the hull. Such a ship can for example be embodied according to any one of claims 2-22.

The objects are also reached with a ship presenting a hull having a bow and a stern, the ship comprising two main propulsion devices located towards the stern, each of said two main propulsion devices being located on a respective side of the hull centreline, a plurality of secondary propulsion devices located towards the stern, the thrust capacity of each secondary propulsion device being smaller than the thrust capacity of the main propulsion device, wherein at least one of the secondary propulsion devices is located laterally between said main propulsion devices, wherein the at least two secondary propulsion devices are arranged so that regions where the prop washes from the secondary propulsion devices leave the secondary propulsion devices are at least partly located, when the ship is moving straight ahead at full speed or flank speed, within a boundary layer of the hull. Such a ship can for example be embodied according to any one of claims 2, 4-5 and 7-22. Preferably, the propulsion devices are arranged, as projected on a plane which is perpendicular to a hull centreline, so that the horizontal extension of each secondary propulsion device does not overlap the horizontal extension of any of the at least one main propulsion device. Preferably, the secondary propulsion devices are completely below a design waterline of the ship. The main propulsion devices could be completely below the design waterline.

The objects are also reached with a ship presenting a hull having a bow and a stern, the ship comprising at least one main propulsion device being a main propeller located towards the stern, and at least two secondary propulsion devices each being an outlet nozzle of a water jet located towards the stern, wherein the at least two secondary propulsion devices are arranged so that regions where the prop washes from the secondary propulsion devices leave the secondary propulsion devices are at least partly located, when the ship is moving straight ahead at full speed or flank speed, within a boundary layer of the hull. Such a ship can for example be embodied according to any one of claims 2-7, 15-17 and 20-22. Preferably, the thrust capacity of each secondary propulsion device is smaller than the thrust capacity of the main propulsion device. Preferably, the propulsion devices are arranged, as projected on a plane which is perpendicular to a hull centreline, so that the horizontal extension of each secondary propulsion device does not overlap the horizontal extension of any of the at least one main propulsion device. Preferably, the secondary propulsion devices are completely below a design waterline of the ship. The main propulsion device could be completely below the design waterline.

DESCRIPTION OF THE DRAWINGS

Below embodiments of the invention will be described with reference to the drawings in which,

fig. 1 shows a side view of a ship,

fig. 2 shows a view from behind of the ship in fig. 1 ,

fig. 3 shows a part of the view on fig. 1 ,

fig. 4 - fig. 6, fig. 9, fig. 11 and fig. 13 show views from behind of hulls of ships according to alternative embodiments of the invention,

fig. 7 shows a side view of a portion of the hull in fig. 6,

fig. 8 shows a part of a cross-section oriented along the line VIII-VIII in fig.

7,

fig. 10 shows a side view of a portion of the hull in fig. 9,

- fig. 12 shows a side view of a portion of the hull in fig. 11 ,

fig. 14 shows a side view of a portion of the hull in fig. 13, and

fig. 15 shows a view from below of the ship in fig. 1.

DETAILED DESCRIPTION

Fig. 1 shows a side view of a ship 1. A hull 2 of the ship 1 has a bow 201, a stern 202 and a design waterline 203, as defined above. As suggested in fig. 2, the ship has a beam to draft ratio B/D of 3.00. The ship 1 comprises a main propulsion device 3 located towards the stern 202. The main propulsion device 3 is a main propeller 301, and a rudder 302 mounted on a skeg 303. The entire the main propeller 301 is below the design waterline 203. The swept diameter of the main propeller 301 is 60% of the distance D from design waterline 203 to the lowest part of the hull 2. As can be seen in fig. 1 and fig. 2, two secondary propulsion devices 4 are located towards the stern 202. Each of the secondary propulsion devices 4 is a secondary propeller 401. The secondary propellers are distributed on both sides of a hull centreline CL, and located outboard of the main propulsion device 3. The secondary propellers 401 each present a swept area diameter which is smaller than the swept area diameter of the main propeller 301.

As can be seen in fig. 2, each secondary propeller 401 is arranged, as projected on a plane which is perpendicular to a hull centreline CL, so that its horizontal extension HES is not overlapping the horizontal extension HEM of the main propeller 301.

As can be seen in fig. 3, a bottom surface 204 of the hull 2, which is planar along a major portion of the hull 2, is towards the stern 202, replaced by a rise 205 with continuous smooth shape as it gets gradually closer to the waterline 203 as the distance to the stern 202 gets shorter. The main and secondary propulsion devices 3, 4 are located within the extension of the rise 205 along the longitudinal direction of the hull 2, (parallel with the hull centreline CL). In fig. 3 a boundary layer, as appearing when the ship is moving straight ahead at full speed or flank speed, is indicated with a line 6. The boundary layer 6 is generally thicker at the rise 205 than at the bottom surface 204.

The secondary propellers 401 are located within the boundary layer 6 of the hull 2. Thereby they are arranged so that regions 7 where the prop washes 8 from the secondary propellers 401 leave the secondary propellers 401 are also located within the boundary layer 6 of the hull. Further, the regions 7 where the prop wash 8 from the secondary propellers 401 leave the secondary propellers 401 are astern of the main propeller 301.

It should be noted that the regions 7 where the prop washes 8 from the secondary propellers 401 leave the secondary propellers 401 are below the design waterline 203, and closer to the design waterline than the lowest part of the hull 2, i.e. the bottom surface 204. More specifically, as can be seen in fig. 2, the centre of the secondary propellers 401 are located within an upper third of the distance D from the design waterline 204 to the bottom surface 204. Fig. 4 shows an embodiment where two secondary propellers 401 are located on the port side of the hull centreline CL, outboard of the main propeller 301. Two further secondary propellers 401 are located on the starboard side of the hull centreline (CL), outboard of the main propeller 301.

Fig. 5 shows an embodiment where two main propellers 301 are distributed on both sides of the hull centreline CL. A secondary propeller 401 is located on the port side of the hull centreline CL, outboard of the main propeller 301 which is located on the port side of the hull centreline CL. Another secondary propeller 401 is located on the starboard side of the hull centreline CL, outboard of the main propeller 301 which is located on the starboard side of the hull centreline CL. A further secondary propeller 401 is located between the main propellers 301.

Fig. 6 - fig. 8 show an embodiment where the hull 2 presents four grooves 206, oriented substantially parallel to the hull centreline CL. Each grove 206 partially encloses a respective of the secondary propellers 401. As suggested in fig. 8, a part of each groove 206 presents a part-circular cross-section with a radius that is 5% - 20% larger than half the diameter of the swept area of the secondary propeller 401. As can be seen in fig. 7, the secondary propellers are located within the boundary layer 6 at full speed or flank speed of the ship. Thereby they are arranged so that regions 7 where the prop washes 8 from the secondary propellers 401 leave the secondary propellers 401 are also located within the boundary layer 6 of the hull. As can be seen in fig. 7, this embodiment presents a spade rudder 302 astern of the main propeller 301.

In the embodiments described above, the secondary propellers 401 are shaft driven. Fig. 9 and fig. 10 show an alternative embodiment with rim driven secondary propellers 401. As can be seen in fig. 10, the rim driven secondary propellers 401 are located partly within the boundary layer 6 at full speed or flank speed of the ship. Thereby they are arranged so that regions 7 where the prop washes 8 from rim driven secondary propellers 401 leave the propellers 401 are also partly located within the boundary layer 6 of the hull.

In this embodiment, the rudder 302 astern of the main propeller 301 presents a bulb 304 having a hubcap 305 which is in close proximity to a hub 306 of the main propeller 301. Thus, the embodiment presents an integrated propeller rudder system known as a Promas system.

Fig. 11 and fig. 12 show an embodiment where each of two secondary propulsion devices is an outlet nozzle 402 of a water jet 401. As can be seen in fig. 12, the outlet nozzles 402 are located, when the ship is moving straight ahead at full speed or flank speed, within the boundary layer 6 of the hull. Thereby they are arranged so that regions 7 where the prop washes 8 from the outlet nozzles 402 leave the outlet nozzles 402 are also located within the boundary layer 6 of the hull.

Fig. 13 and fig. 14 show an embodiment where each of three secondary propulsion devices 401 as well as each of two main propulsion devices 301 is an outlet nozzle of a water jet. The secondary propulsion device water jet outlet nozzles 402 are located, when the ship is moving straight ahead at full speed or flank speed, within the boundary layer 6 of the hull.

As can be seen in fig. 15, depicts a manoeuver in an embodiment of a method according to the invention. The secondary propulsion devices 4 are controlled so that the thrust Ts of the secondary propulsion device 4 on the starboard side of the hull centreline CL is higher than the thrust Tp of the secondary propulsion device 4 on the port side of the hull centreline CL. Thereby the ship 1 will be steered with the different thrusts Ts, Tp to the port.