The present invention relates to a system for reducing wave- induced motions of a vessel, preferably a vessel having a displacement hull, of the kind disclosed in the preamble of claim 1.

10 Recent decades have seen the increasing use of catamarans as express passenger vessels. These offer many advantages, such as large passenger space in relation to the length of the vessel, reduced power requirement and better manoeuvring properties in relation to a more conventional vessel.

15

When a vessel travels through waves it will be subjected to forces which cause motions of the vessel. These motions can be divided into six components: a linear backwards and forwards motion in the longitudinal axis of the vessel

20 (surging) ; a linear motion across the longitudinal axis of the vessel (swaying) ; a linear motion along the vertical axis of the vessel (heaving) ; a rotation about the longitudinal axis of the vessel (rolling) ; a rotation about the transverse axis of the vessel (pitching) ; and a rotation about the vertical

25 axis of the vessel (yawing) .

Normally, a vessel will be subjected to all these motions simultaneously when it travels in a seaway, and these motions will to a greater or lesser extent cause the passengers to

50 experience seasickness after some time. It has become evident that it is especially accelerations in the vertical direction which cause this seasickness, cf. ISO 2631.

The vertical accelerations are due in the main to pitching,

35 rolling and heaving, and their effect on each other, and they become particularly troublesome when the frequency with which the waves strike the vessel corresponds to the vessel's

natural frequency in rolls or pitches.

To counteract these motions, it is known to mount so-called anti-roll fins, consisting of wing-shaped bodies which project out from the side of the vessel in the vicinity of the mid- section of the vessel, which, by virtue of the fact that they are rotatable and controlled by a system which registers the rolling motion of the vessel, can provide a moment of force to counterbalance the rolling motion. The use the rudder of the vessel to counteract this rolling motion has also been proposed.

However, in the case of catamarans it has become apparent that the rolling motions have on the whole been acceptable with a view to passenger comfort. On the other hand, the motion caused by pitching and heaving has resulted in unpleasantness for the passengers. This unpleasantness will to a great extent be linked to the speed at which the vessel travels through the water. Since this speed will very often result in a stress frequency which corresponds to the vessel's natural frequency, it may be advantageous either to reduce speed or to change course so that the waves have a different effect on the hull .

A solution to the problem of the kind mentioned above will, however, mean that it will not be possible to make full use of the boat with a view to speed and thus its competitive advantage in relation to other means of transport.

The objective of the present invention is to reduce the effect of the aforementioned forces on the vessel, also in poor weather, without having to reduce the speed of the vessel.

This is done by means of a system of the type mentioned by way of introduction, the characteristic features of which are made evident in claim 1. Additional features of the invention are disclosed in the other dependent claims.

The surfaces provide dynamic forces on the vessel and can vary the angle at which they meet the water flow, and are controlled in such a way that they counterbalance the vessel's tendency to move under the stress of the waves. Not only is the pitching motion reduced, but since the heaving motion is affec.ed by the pitching motion, the heaving motion will also be reduced considerably, whilst it will also be possible to reduce the rolling motion.

Said surfaces create a force which is proportional to the speed of the vessel to the second power, and by virtue of the fact that the surfaces are controlled by a system which registers the motion of the vessel and thereafter transmits signals to a hydraulic system which controls the lifting force, this will to a great extent reduce the undesired motions.

In order to create lift, the surfaces may either be rotatable through a transverse axis such that the angle of incidence is varied, or they may be equipped with so-called flaps placed either in the leading edge or the trailing edge of the surfaces. In order to provide lift, the flaps must be rotated through an angle in relation to the rest of the surface. It is essential that such flaps are able to create both a downwardly directed force to counteract an upwardly directed motion of the bow of the vessel, and an upwardly directed force to counteract a downwardly directed motion of the vessel's bow.

It should be pointed out that during lift, an actual lift of the vessel over time is not understood, but a lifting force on the vessel which is zero integrated over time.

The lifting surfaces may be located on downwardly directed struts located under the front part of the bottom of the hull, and in a catamaran or multiple-hull vessel, one on each of the

hulls

If the surfaces are provided on both sides of the bow of the vessel, this will result in the following adverse conditions:

The narrow sections which are normally found very far to the fore on, e.g., a catamaran hull, make access difficult for mounting and maintenance of the hydraulic or mechanical rotation system which must be capable of varying the lifting forces. Furthermore, the depth at which the surfaces may be located will be limited owing to the draught of the vessel, and their effect will be reduced by their being close to the surface of the water, especially when the vessel pitches upwards. In rough sea there is, moreover, a danger of the surfaces coming above the water, and during the downward motion which will follow thereafter there is a great danger that these surfaces will be exposed to such large forces that they may be ripped off.

If the surfaces are located as described above, these disadvantages are avoided.

If the hull is equipped with such lifting surfaces, these surfaces will be capable of counterbalancing pitching by providing forces which are directed opposite to the motion of the vessel, but by virtue of the fact that the surfaces are at such a great distance from one another thwartships, they will also be able to counteract rolling by providing a moment of force which counterbalances the rolling moment. It will also be possible to arrange these surfaces in pairs under each of the hulls on, e.g., a catamaran so that they are attached to a downwardly projecting strut placed in the front part of the bottom. The surfaces will then be capable of counteracting rolling in that either the two outer surfaces provide a moment of force which is directed opposite to the rolling moment, whilst the two inner surfaces provide forces

which counterbalance the pitching motion, or one pair of lifting surfaces provides an upwardly directed force whilst the other pair provides a downwardly directed force and this counterbalances the rolling motion, whilst the size of the forces relative to one another may be such that they also counterbalance the pitching motion, if this should be so desired.

It will also be possible to coordinate the forces from lifting surfaces of this kind with forces from trim flaps or similar dynamic surfaces placed in the stern of the vessel. This will give rise to an increased effect on both pitching and rolling compared with the use of lifting surfaces at the fore _>f the vessel alone. The coordination of these forces may take place, e.g., electronically.

Below, the invention wil oe described in more detail with reference to the drawings where:

Figure 1 is a schematic illustration of a circuit diagram of the control of the lifting surfaces with the control of a port and a starboard fin respectively.

Figure 2 illustrates a deta: I in the control of the lifting surfaces with accompanying fail safe system.

Under one or all the hulls of a vessel there is located a strut projecting from the hull and which at its other end is equipped with a lifting surface in the form of a fin or having flaps provided in the leading or trailing edge of the surface. The moveable part of the surface is rotatable around a transversely positioned axis . Inside the actual hull or the strut, there is provided a hydraulic actuator which moves the fin to the desired angle. In Fig. 2 the fin is indicated schematically by means of the reference numeral 3, and the accompanying actuator 4 in this case is in the form of a

piston device.

In order for the fins to be set at the right angle to attenuate the undesired motions, the parameters which give rise to these undesired motions must be registered and measured. This is done in that sensors are provided which measure the vessel's pitching angle 7 and rolling angle 16, and which emit signals which are dependent upon the angle of the vessel to the horizon. A speed sensor measures the vessel's angular velocity in pitch 9 and roll 18 respectively, i.e., they emit signals which vary with the change of roll and pitch per time unit. Furthermore, two vertical accelerometers are provided which sense the vertical acceleration 10, 19 of the vessel, and which emit signals that are dependent upon vertical acceleration. The pitch angle and the roll angle are subtracted from the desired pitch angle 14 and roll angle 15 in respective subtraction units 8, 17. The degree of influence on the control of the fins of these values may be regulated by means of respective amplifiers 11, 20 from the control panel . The influence of the pitch velocity 9 and the roll velocity 18 on the angular position of the fins may also be regulated via respective amplifiers 12, 21 from the control panel. The pitch acceleration 10 and the roll acceleration 19 provide an influence on fin control by means of permanently set, predetermined values provided by the amplifiers 13 and 22 respectively.

The respective values thus obtained for pitch, pitch velocity and pitch acceleration are added up in an adding means 23 in order to be fed to a fin control unit 28. However, as also the roll angle will have an influence on the motion, the respective values for roll, roll velocity and roll acceleration are also fed to the summation means 25 for pitch, pitch velocity and pitch acceleration prior to these being fed to the control of the port fin.

The difference between desired pitch and actual pitch is

amplified through amplifier 11. Furthermore, pitch velocity and pitch acceleration will be amplified through amplifiers 12 and 13. The summation of these three values together gives a fin angle command for pitch. The same is done for roll, and the summation of the three values gives a fin angle command for roll . A pitch command is divided equally between port and starboard (a positive pitch command results in the fins exerting an upward force, i.e., they press the bow up) . A positive roll angle command results in the vessel rolling over to starboard (i.e., the starboard fin -rovides a downward force whilst the port fin provides an upward force) . The roll commands for the starboard and port fins therefore have opposite signs. Control of the port fin is therefore the sum of a pitch command and a roll command, whilst control of the starboard fin is the difference between the pitch and roll command.

Fig. 2 is a schematic illustration of the hydraulic system for actuating a guide fin 30. The hydraulic system consists of a servo loop with an oil pressure unit 5 which provides the system with hydraulic oil under pressure. Furthermore, there is provided a valve block 1, 2 which controls the oil flow to and from the cylinder 4 so that the piston rod is moved up or down. Furthermore, a position indicator is provided which, in connection with the piston rod or in another manner, is coupled to the fin. The position indicator gives a signal to the servo amplifier card 6 re measured piston stroke/fin position and compares this with the desired position. If there is any discrepancy between these, the servo card gives a signal to the valve block 1, 2 which moves the fin towards the desired angle.

If any faults occur in the system, an oil accumulator 3 (oil reservoir) will ensure that the fin is pressed down so that the bow of the vessel is pressed up. It is arranged in this way to prevent the bow of the vessel from being pressed down, i.e., from coming deep in the water. If the fins draw the bow

down, this will result in great water pressure caused by the wave forces which result in considerable retardation and also a loss of directional stability, which in turn leads to major changes in course, especially in a rolling sea, with a subsequent danger of large rolling motions.

A valve 1 ensures that the oil flow is guided to the cylinder, either from the valve block or from an accumulator 3. In a normal situation the oil flow will come from the valve block, but on the discovery of a system fault or when a power supply system fails, the valve 1 will automatically connect the oil flow to the accumulator 3, and thus the piston/fin 4 is pressed down as shown in Fig. 2. Furthermore, a manual stop valve 2 makes possible the locking of the piston/fins in a desired position.