The present invention relates to an arrangement for the reduction of wave-induced motions of a vessel having at least in part water jet propulsion, wherein each water jet engine comprises a nozzle being rotatable around a substantially vertical axis, said nozzle having an inlet end for the water flow and an outlet opening which forms the water jet that creates the reaction force for the propulsion of the vessel.

In recent decades increasing use of high-speed vessels with water jet propulsion for passenger and car transport, as ambulances and feeder ships, and in marine activities in general have been seen. These high-speed vessels offer many advantages over a more conventional vessel, such as large passenger capacity, reduced operating costs and improved manoeuvring performance.

When a vessel travels at speed through waves, it will be subjected to forces which cause motions of the vessel, i.e., wave-induced motions. An analysis of these motions shows that they are composed of six freedoms of movement: surging in the longitudinal axis of the vessel; swaying transverse to the longitudinal axis of the vessel; heaving along the vertical axis of the vessel; rolling as rotation about the longitudinal axis; pitching about the transverse axis of the vessel; and yawing as rotation about the vertical axis of the vessel.

Normally a vessel will be subjected to all these motions simultaneously when it moves in waves, and these motions will to varying degrees of severity cause the passengers to suffer from seasickness, the crew from fatigue, and the vessel experiences considerable speed loss or other operational limitations. It has become evident that it is in particular vertical accelerations and large pitching and rolling motions which cause seasickness.

The vertical accelerations are due in the main to pitching, rolling and heaving, and also their combined effect, and become particularly bothersome when the frequency of the waves encountering the vessel is almost equal to the resonance frequency of the vessel in pitching or rolling.

To counteract these motions, it is known to mount anti-roll fins and anti-pitch fins, consisting of wing-shaped bodies which protrude from each side of the vessel amidships or underneath the bow or stern of the vessel. These systems provide considerable drag

at cruising speed and thereby generally reduce the speed of the vessel. Said fins may be passive, i.e., rigidly mounted and non-moveable, or they may be rotatable, equipped with an electronic system which monitors the rolling and pitching motions of the vessel, and regulates the fins in positions that counteract these undesirable motions.

For high-speed vessels it appears that both rolling motions and pitching motions, including the resultant vertical accelerations, result in severe discomfort for both passengers and crew, depending on the speed and heading of the vessel relative to the waves, so that speed and heading often result in a wave induced frequency almost equal to the resonance frequency of the vessel. In such situations, it may be advantageous either to reduce speed or to change heading so that the encountered waves provide a different effect on the vessel.

A solution to the above described problem will, however, mean that to make full use of the boat with regard to speed and thus the primary competitive advantage of the vessel over other means of transport, will not be possible.

Fins or lifting surfaces which are normally positioned in the foremost part of, e.g., a catamaran hull, will be exposed to severe forces in the vertical direction, in particular if the surface of the hull leaves the water and subsequently with high velocity, vigorously hits and breaks through the water surface, an effect known as slamming. The forces generated by slamming will be so excessive that the structural strength of faces, braces and mounts in the vessel will give rise to increases in weight of the vessel, with an attendant reduction of the speed potential or load capacity of the vessel.

The object of the present invention is to reduce the negative effects of the aforementioned forces on vessels propelled by water jet propulsion without reducing the speed of the vessel as a result of the system, in either calm or rough waters.

This is achieved with an arrangement for the reduction of wave-induced motions of a vessel of the aforesaid type, which is characterised in that the nozzle in addition is pivotable about a substantially horizontal axis, said rotatability in the horizontal plane about the vertical axis being for the purposes of navigation, whereas the pivotability in the vertical plane about the horizontal axis is to counteract or reduce the said wave- induced motions of the vessel. Thus vertically acting reaction forces are established by the water jets counteracting the wave-induced vertical forces and moment arms acting on the vessel.

Suitably the system is manoeuvrable with the aid of a hydraulic cylinder which causes the rotational movement in the horizontal plane and a second hydraulic cylinder which causes the pivotal movement in the vertical plane.

The nozzle outlet portion in the upstream end thereof may be designed as a first hemispherical surface which embraces a second hemispherical surface on the downstream end of the nozzle inlet portion, said first hemispherical surface tightly embracing and complementary fitting the second hemispherical surface. The nozzle inlet portion may be stationary, whereas the nozzle outlet portion may be moveable and in principle has freedom of movement similar to a ball joint.

As an alternative, the nozzle outlet may be mounted as an extension of the nozzle capable of being tilted in both the horizontal plane and the vertical plane.

The wave-induced motions can be recorded by sensors which transmit signals to a control unit, which in turn transmits signals that activate the hydraulic cylinder which causes the pivotal movement in the vertical plane. The sensors can record pitching motion, pitch angular speed and pitch angular acceleration, rolling motion, roll angular speed and roll angular acceleration, and vertical and horizontal accelerations, the respective values being a measure of the angular position in the vertical plane of the respective water jet nozzles relative to the motions of the vessel.

It will be understood that with the water jet flow which is steerable in the vertical plane imposes dynamic forces on the vessel which can be controlled in such a way that they counteract the tendency of the vessel to move under the influence of the waves. Not only are the pitching and rolling motions reduced, but as the heaving motion is affected by these motions, this motion will also be reduced significantly. As the motions are reduced so can the motion-induced speed loss in waves also be reduced.

By virtue of the fact that the water flow is controlled by a system which records the motions of the vessel and then transmits signals to a hydraulic/mechanical apparatus which controls the vertically directed force, the undesired motions will be substantially reduced in a controllable manner. Furthermore, it will be understood that it is essential that the movement of the water flow momentum is able to generate both a downwardly directed force by directing the water flow upward, thereby counteracting downwardly directed motion of the bow of the vessel, and an upwardly directed force by directing

the water flow downward to counteract a downwardly directed motion of the bow of the vessel. By controlling the flow of the water jet in the vertical plane, the forces that occur will be capable of counteracting pitching by establishing forces in a direction opposite to the motions of the vessel. Water jet engines are often installed in pairs, and often widely spaced athwartships, e.g., one in each hull of a catamaran. In this way they will also be able to counteract rolling by establishing a moment arm which counteracts the rolling action.

It will also be possible to coordinate described forces in the vertical plane with normal steering forces in the horizontal plane. The coordination of these forces must take place by means of an integrated, electronic steering system wherein in particular all safety aspects have been carefully considered and implemented.

Other and additional objects, features and advantages will be made apparent in the following description of currently preferred embodiments of the invention, given for the purposes of description, without thereby limiting the scope of the invention, and given in connection with the enclosed drawings where:

Figure 1 is a schematic side view of an arrangement for the reduction of wave- induced motions according to the invention;

Figure 2 is a schematic rear view of the arrangement according to Figure 1 ;

Figure 3 is a schematic top plan view of the arrangement according to Figure 1 ;

Figure 4 is a schematic side view of a second embodiment of the arrangement for the reduction of wave-induced motions according to the invention; and

Figure 5 is a schematic top plan view of the arrangement according to Figure 4.

Reference is now made to Figures 1 to 3, which show a first embodiment of the arrangement 10 for the reduction of wave-induced motions of a vessel. The arrangement 10 can be attached to the stern transom 7 of the vessel by means of securing devices 8 which are capable of rigidly securing the arrangement 10 whilst the vessel is in operation. The arrangement 10, which functions like a water jet engine, comprises a nozzle 1 capable of being rotated. The nozzle 1 comprises a suction inlet portion lb and a discharge outlet portion la. In the illustrated embodiment the inlet portion lb is stationary, whereas the outlet portion la is moveable, both in the horizontal plane and in the vertical plane. The outlet portion la is capable of being pivoted in the vertical plane about a horizontal axis 5. A hydraulic cylinder 6 will,

when activated in the direction of the arrows, cause this movement in the vertical plane. A second hydraulic cylinder 3 is capable of causing the outlet portion 1 a to rotate about a vertical axis 2. This function is for normal navigation of the vessel and will, when rotated, change the heading of the vessel. The ability to pivot in the vertical plane about the horizontal axis 5 is to counteract or reduce the previously discussed wave-induced motions of the vessel. In the illustrated embodiment, the outlet portion la of the nozzle in the upstream end thereof is designed as a first hemispherical surface 4a which embraces a second hemispherical surface 4b on the upstream end of the inlet portion lb of the nozzle. The first hemispherical surface 4a tightly embraces and is complementary fitted to the second hemispherical surface 4b. The inlet portion lb of the nozzle is stationary, whereas the outlet portion la of the nozzle is moveable and in principle has freedom of motion like a ball joint. The movements are , however, controlled by the hydraulic cylinders 3 and 6 to function only in the horizontal plane and the vertical plane. It will be possible to obtain an angular deviation of up to ± 35° of the nozzle part la.

The water flows as shown by the arrows in Fig. 1, into the inlet portion lb and is passed out through an outlet opening 12 in the outlet portion la which forms the actual water jet, and when the jet is discharged into the surrounding water a reaction force is generated which functions as a propulsion force. The way in which the water is conducted to the inlet portion lb is not shown in detail as this is considered to be generally known in the art.

Figures 4 and 5 illustrate a second embodiment of the arrangement for the reduction of wave-induced motions of a vessel. In this embodiment, the inlet portion 10b of the nozzle and the outlet portion 10a of the nozzle are rigidly connected to one another and form an integral unit. On the outlet portion 10a, there is provided a nozzle extension 9 which by means of a hydraulic cylinder 16 is tiltable about a substantially horizontal axis 5'. The nozzle extension 9 is capable of an angular deviation of about ±15° relative to the horizontal line. The nozzle extension 9 can also be tilted about a substantially vertical axis 2 ^" by means of the hydraulic cylinders 13. It is conceivable that this solution may serve as accessory on existing water jet structures where only the steering nozzle has been capable of being rotated in the horizontal plane.

In case of malfunctions in the navigation system, the water jet with considerable momentum will rapidly position itself in the horizontal plane, thereby avoiding uncontrollable vertical forces in the case of steering failure.

Sensors are installed on the vessel for monitoring the motions to which the vessel is subjected in the form of pitching, pitch angular speed, pitch angular acceleration, rolling, roll angular speed and roll angular acceleration and horizontal accelerations. Furthermore, it includes a control unit for processing the registered sensor values or signals, the respective values being a measure of the angular position in the vertical plane of the respective water jet nozzles relative to the motions of the vessel. The control unit transmits signals which activate the hydraulic cylinder 6 which causes the pivotal movement in the vertical plane. The electronic processing of sensor values, regulating algorithms and steering signals includes a comparison with the desired motion of the vessel where differential values produced in the electronic system are supplied to the hydraulic cylinders of the water jet engine to adjust the water jet flow to the correct angle in the vertical plane.