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Title:
ROLL DAMPENING APPARATUS
Document Type and Number:
WIPO Patent Application WO/2011/016730
Kind Code:
A1
Abstract:
A roll dampening apparatus (200) for reducing rolling motion in an ocean vessel (100) includes an elongate tank (20) disposed transversely across the vessel (100) partially filled with a liquid (10). A control arrangement (210, 220) monitors in operation a roll angle (or) of the vessel (100) and controls dynamically an effective depth (d) of the liquid (10) so that a wave (30) propagating on a surface of the liquid (10) is at least partially in antiphase to the rolling motion of the vessel (100) for reducing a magnitude of the rolling motion.

Inventors:
HELLESVIK, Eirik (Dalveien 16, Myre, N-8430, NO)
Application Number:
NO2010/000297
Publication Date:
February 10, 2011
Filing Date:
August 05, 2010
Export Citation:
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Assignee:
HELLESVIK, Eirik (Dalveien 16, Myre, N-8430, NO)
International Classes:
B63B39/03
Foreign References:
US3968353A
US3521593A
DE1916587A1
US3580205A
US3915109A
US3422781A
JP2006219114A
JP2005291830A
Attorney, Agent or Firm:
ACAPO AS (P.O. Box 1880 Nordnes, Bergen, N-5817, NO)
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Claims:
CLAIMS 1. A roll dampening apparatus (200) for reducing rolling motion in an ocean vessel (100), characterized in that the apparatus (200) includes an elongate tank (20) disposed transversely across said vessel (100) partially filled with a liquid (10), a control arrangement (210, 220) for monitoring a roll angle (a) of the vessel (100) and for controlling dynamically an effective depth (d) of the liquid (10) so that a wave (30) propagating on a surface of the liquid (10) is at least partially in antiphase to the rolling motion of the vessel (100) for reducing a magnitude of said rolling motion.

2. A roll dampening apparatus (200) as claimed in claim 1 , wherein the apparatus (200) is adapted to control the effective depth (d) dynamically in operation by removing liquid (10) from the tank (20) and/or filling liquid (10) into the tank (20).

3. A roll dampening apparatus (200) as claimed in claim 1 or.2, wherein the apparatus (200) is adapted to control the effective depth (d) dynamically in operation by way of modulating an effective bottom surface of the tank (20) by using an actuated baffle arrangement (500, 510).

4. A roll dampening apparatus (200) as claimed in any one of claims 1 , 2 or 3, wherein the apparatus (200) includes at least one of following sensors coupled to said control arrangement (200, 210) for measuring dynamic propagation of the wave (30) on the surface of the liquid (10):

(a) a camera (240) disposed to image the surface of the liquid (10) for monitoring an instantaneous position of the wave (30) within the tank (20);

(b) a liquid surface level sensing arrangement (250) disposed at a plurality of locations within the tank (20) for monitoring an instantaneous position of the wave (30) within the tank (20); and

(c) a radar arrangement for monitoring the surface of the liquid (20) for monitoring an instantaneous position of the wave (30) within the tank (20).

5. A roll dampening apparatus (200) as claimed in claim 2, wherein the effective depth (cf) of the liquid (10) is controlled dynamically in operation by selectively removing liquid at a first end of the tank (20) and selectively injecting liquid (10) at a second end of the tank (20).

6. A roll dampening apparatus (200) as claimed in claim 2, wherein said apparatus (200) includes a reservoir (300) into which liquid (10) removed from the tank (20) is pumped and from which liquid (10) is pumped for injecting into the tank (20), such that the tank (20) and the reservoir (300) form a closed system for the liquid (10).

7. A method of controlling a roll dampening apparatus (200) as claimed in any one of the preceding claims, characterized in that said method includes:

(a) monitoring a roll angle (a) of an ocean vessel (100) equipped with the apparatus

(200);

(b) monitoring propagation characteristics of a surface wave (30) propagating in a liquid

(10) in a tank (20) of the apparatus (200); and

(c) adjusting an effective depth of the liquid (10) in the tank (20) so that the propagation of the surface wave (30) in the tank (20) is controlled so as to oppose, at least partially, a rolling motion of the vessel (100) as evident in the roll angle (a) of the vessel (100).

8. A method as claimed in claim 7, wherein the method includes controlling the effective depth (d) dynamically in operation by removing liquid (10) from the tank (10) and/or filling liquid into the tank (20).

9. A method as claimed in claim 7 or 8, wherein the method includes controlling the effective depth (cf) dynamically in operation by way of modulating an effective bottom surface of the tank (20) by using an actuated baffle arrangement (500, 510). 10. A method as claimed in claim 7, 8 or 9, wherein the method includes using at least one of following sensors coupled to a control arrangement (200, 210) of said apparatus (200) for measuring dynamic propagation of the wave (30) on the surface of the liquid (10):

(a) a camera (240) disposed to image the surface of the liquid (10) for monitoring an instantaneous position of the wave (30) within the tank (20);

(b) a liquid surface level sensing arrangement (250) disposed at a plurality of locations within the tank (20) for monitoring an instantaneous position of the wave (30) within the tank (20); and

(c) a radar arrangement for monitoring the surface of the liquid (20) for monitoring an instantaneous position of the wave (30) within the tank (20).

11. An ocean vessel (100) including at least one apparatus (200) as claimed in any one of claims 1 to 6 for reducing a rolling motion of the vessel (100) in operation.

12. A software product recorded in a machine readable data storage medium, wherein said product is executable upon computing hardware (220) for implementing a method as claimed in any one of claims 7 to 10.

Description:
ROLL DAMPENING APPARATUS

Field of the invention

The present invention concerns roll dampening apparatus for reducing rolling motion in ocean vessels. Moreover, the present invention relates to methods of controlling roll dampening apparatus for reducing rolling motion in ocean vessels. Furthermore, the present invention concerns software products recorded on machine-readable data storage media, wherein the software products are executable on computing hardware for implementing the methods.

Background of the invention

It is well known that ocean vessels are affected by ocean waves acting thereupon, causing the ocean vessels to experience both angular rotation (roll, pitch, yaw) as well as linear displacement (sway, heave, surge). To personnel on ocean vessels, roll and heave motions are usually most noticeable. Roll motions concern oscillatory motions of ocean vessels about their substantially horizontal elongate axis, and heave motions concerns linear oscillatory motions of ocean vessels in a vertical direction. It is known that angular motion can be resisted by Coreolis gyroscopic devices, for example as employed in contemporary water jet propelled boats which include large internal turbines for generating the propulsion water jets; the turbines are synergistically operable to function as large rotating flywheels. However, such gyroscopic devices are not able to resist linear displacements (sway, heave, surge) on account of the principal of conservation of linear momentum. An Australian company ShipDynamics Ltd. (Australia) has recently reported a gyroscopic roll stabilizer for a 50-metre yacht which is capable of completely preventing rolling motion of the yacht. However, such a gyroscopic device requires approximately 20 kW input power to function and is therefore a potentially costly luxury refinement in a World in a context of rapidly dwindling World fossil fuel reserves.

In a published Japanese patent application no. JP 2006219114, there is described a pitch- and-roll reducing device for an ocean vessel. The device employs a liquid movement monitoring sensor. Moreover, in a published Japanese patent application no. JP 2005291830 (Shinko Engineering & Maintenance Co. Ltd.), there is described an image measuring apparatus and a corresponding image measuring method for sensing liquid level. Contemporary passive apparatus for reducing rolling motion in ocean vessels utilizing tanks of liquid often function inadequately because dynamic characteristics of the tanks are not adequately monitored. In certain circumstances, these contemporary apparatus can even render ocean vessel rolling motions more severe than would be case for the vessels devoid of the contemporary apparatus. In certain adverse weather conditions, aforementioned contemporary apparatus for reducing rolling motion can cause considerable personnel and/or passenger discomfort, and even bring stability of ocean vessels in momentary danger.

Summary of the invention

The present invention seeks to provide a roll dampening apparatus which is capable of functioning more reliably and effectively than known types of passive roll dampening apparatus.

According to a first aspect of the present invention, there is provided a roll dampening apparatus as claimed in appended claim 1 ; there is provided a roll dampening apparatus for reducing rolling motion in an ocean vessel, characterized in that the apparatus includes an elongate tank disposed transversely across the vessel partially filled with a liquid, a control arrangement for monitoring a roll angle (a) of the vessel and for controlling dynamically an effective depth (d) of the liquid so that a wave propagating on a surface of the liquid is at least partially in antiphase to the rolling motion of the vessel for reducing a magnitude of the rolling motion.

The invention is of advantage in that dynamically controlling propagation of the surface wave by way of dynamically controlling the effective depth of the liquid within the tank enables rolling motion of the vessel to be better compensated by the apparatus.

Optionally, the apparatus is adapted to control the effective depth (d) dynamically in operation by removing liquid from the tank and/or filling liquid into the tank. Optionally, the apparatus is adapted to control the effective depth (d) dynamically in operation by way of modulating an effective bottom surface of the tank by using an actuated baffle arrangement.

Optionally, the apparatus includes at least one of following sensors coupled to the control arrangement for measuring dynamic propagation of the wave on the surface of the liquid:

(a) a camera disposed to image the surface of the liquid for monitoring an instantaneous position of the wave within the tank; (b) a liquid surface level sensing arrangement disposed at a plurality of locations within the tank for monitoring an instantaneous position of the wave within the tank; and

(c) a radar arrangement for monitoring the surface of the liquid for monitoring an instantaneous position of the wave within the tank.

More optionally, the roll dampening apparatus is implemented so that the effective depth (d) of the liquid is controlled dynamically in operation by selectively removing liquid at a first end of the tank and selectively injecting liquid at a second end of the tank. More optionally, the roll dampening apparatus is implemented to include a reservoir into which liquid removed from the tank is pumped and from which liquid is pumped for injecting into the tank, such that the tank and the reservoir form a closed system for the liquid.

According to a second aspect of the invention, there is provided a method of controlling a roll dampening apparatus pursuant to the first aspect of the invention, characterized in that the method includes:

(a) monitoring a roll angle (a) of an ocean vessel equipped with the apparatus;

(b) monitoring propagation characteristics of a surface wave propagating in a liquid in a tank of the apparatus; and

(c) adjusting an effective depth of the liquid in the tank so that the propagation of the surface wave in the tank is controlled so as to oppose, at least partially, a rolling motion of the vessel as evident in the roll angle (a) of the vessel.

Optionally, the method includes controlling the effective depth (d) dynamically in operation by removing liquid from the tank and/or filling liquid into the tank.

Optionally, the method includes controlling the effective depth (d) dynamically in operation by way of modulating an effective bottom surface of the tank by using an actuated baffle arrangement.

Optionally, the method includes using at least one of following sensors coupled to a control arrangement of the apparatus for measuring dynamic propagation of the wave on the surface of the liquid:

(a) a camera disposed to image the surface of the liquid for monitoring an instantaneous position of the wave within the tank;

(b) a liquid surface level sensing arrangement disposed at a plurality of locations within the tank for monitoring an instantaneous position of the wave within the tank; and (c) a radar arrangement for monitoring .the surface of the liquid for monitoring an instantaneous position of the wave within the tank.

According to a third aspect of the invention, there is provided an ocean vessel including at least one apparatus pursuant to the first aspect of the invention for reducing a rolling motion of the vessel in operation.

According to a fourth aspect of the invention, there is provided a software product recorded in a machine readable data storage medium, wherein the product is executable upon computing hardware for implementing a method pursuant to the second aspect of the invention.

It will be appreciated that features of the invention are susceptible to being combined in any combination without departing from the scope of the invention as defined by, the appended claims.

Description of the diagrams

Embodiments of the present invention will now be described, by way of example only, with reference to the following diagrams wherein:

FIG. 1 is an illustration of an elongate tank which is partially filled with a liquid, and which is operable to support propagation of one or more surface waves within the tank;

FIG. 2 is an illustration of an ocean vessel equipped with a tank as illustrated in FIG.

1 disposed in a transverse manner within the vessel;

FIG. 3 is an illustration of an embodiment of a roll dampening apparatus pursuant to the present invention;

FIG. 4 is an illustration of a liquid level sensor for use in the apparatus of FIG. 3;

FIG. 5 is an illustration of a submerged actuated baffle arrangement for modulating an effective depth of the tank of FIG. 1 as employed in the apparatus of FIG. 3; and

FIG. 6 is an illustration of a large ocean vessel equipped with two apparatus as illustrated in FIG. 3.

In the accompanying diagrams, an underlined number is employed to represent an item over which the underlined number is positioned or an item to which the underlined number is adjacent. A non-underlined number relates to an item identified by a line linking the non-

, underlined number to the item. When a number is non-underlined and accompanied by an associated arrow, the non-underlined number is used to identify a general item at which the arrow is pointing.

Description of embodiments of the invention

Referring to FIG. 1 , it is well known that a quantity of liquid 10 in an elongate tank 20 can provide a liquid waveguide in which one or more surface waves 30 are able to propagate. The one or more surface waves 30 have motional energy fields which fall exponentially with depth z into the liquid 10, for example water or oil. When the liquid 10 in the tank 20 has a depth which is less than the extent of the motional energy field of one or more surface waves 30, the depth d of liquid in the tank 20 becomes a parameter which influences propagation characteristics of the one or more surface waves 20, for example their propagation velocity v and also cusp-to-trough amplitude p. Various mechanisms can be used to stimulate formation of surface waves in the elongate tank 20. One approach, as employed in experimental wave tanks, is to use actuated baffles within the tank 20. Another approach is to vary periodically an inclination angle a of the tank 20, thereby stimulating surface wave propagation within the tank 20. Such a situation would arise if the tank 20 were disposed transversely across an ocean vessel.

The tank 20 is beneficially considered to behave as a one-dimensional waveguide. Moreover, the end faces 40 of the tank 20 can be considered to be wave reflectors, such that the tank 20 then behaves as a single dimensional etalon, namely a wave cavity. If the tank 20 is made sufficiently short in length relative to wavelengths λ of the one or more surface waves 30, only a few wavelength modes are susceptible to being maintained within the tank 20. For example, it is possible to design physical dimensions of the tank 20, namely its size and shape, such that it is only capable of accommodating a single wave cusp propagating within the tank 20. The present invention is concerned with utilizing such a single cusp wave 30 propagating within the tank 20. As aforementioned, the velocity of the single wave 30 propagating within the tank 20 is determined by the energy field of the single wave 30 interacting with a bottom plane of the tank 20; varying the depth d thus influences the propagating velocity of the single wave 30 propagating back and forth between the end faces 40.

When the tank 20 is disposed transversely across a hull 110 of an ocean vessel 100, for example as depicted in FIG. 2, and the tank 20 is proportioned so that only a single cusp 30 is capable in operation of propagating between the end faces 40 of the tank 20, a rolling motion of the vessel 100 by an angle a about a principal elongate axis 120 of the hull 110 causes the wave 30 to transfer a centre of gravity of the vessel 100 from one side of the side vessel 100 to the other side thereof. When the wave 30 in FIG. 2 propagates totally in phase with the natural rolling motion of the vessel 100, such propagation of the wave 30 has a tendency to enhance a rolling motion of the vessel 100, namely to "amplify" such a rolling motion. Conversely, when the propagation of the wav 30 is substantially in antiphase to a rolling motion of the vessel 100, a rolling motion of the vessel 100 is potentially significantly reduced.

It is found when observing wave motion in an open ocean environment that ocean waves have a predominant wave propagation direction associated with a prevailing wind direction, together with a degree of random wave motion in other directions. It has been appreciated by the inventor of the present invention that wave propagation in an ocean environment is a complex motion with a random element, namely not a simplistic regular oscillatory motion as often hitherto assumed. Moreover, the inventor has also appreciated that contemporary attempts to reduce rolling motion of ocean vessels by using passive transverse wave tanks have hitherto provided unsatisfactory performance results because the depth d of water in such transverse tanks has not been adequately controlled in respect of instantaneous ocean wave motion. In reality, the depth d of water in the traverse tanks needs to be dynamically regulated so that the wave 30 propagating sideways from right to left and back again is to a major extent in antiphase to rolling motion of ocean vessels to which the tanks are installed. The present invention is concerned with methods of controlling tanks partially filled with liquid disposed transversely across ocean vessels for providing an improved anti-roll compensation for these vessels.

Referring to FIG. 3 and FIG. 4, a roll dampening apparatus is indicated generally by 200. The apparatus 200 is adapted for its associated tank 20 to be disposed transversely across an ocean vessel, for example as illustrated in FIG. 2. The apparatus 200 comprises the tank 20 operably partially filled with a liquid; for example, the liquid is beneficially water, although a low viscosity oil could alternatively be used which also lubricates pumps for pumping the liquid. Optionally, the tank 20 is filled with fuel oil for propelling the ocean vessel. The tank 20 is elongate and includes end faces 40 which are operable to function as wave reflectors;

, these wave reflectors may be implemented as flat end faces of the tank 20 or specially shaped surfaces which are adapted to preserve wave energy and momentum upon wave reflection from the end faces 40. The apparatus 200 further includes a control unit 210 coupled to a computer 220 for coordinating operation of the apparatus 200. The ocean vessel has installed thereon a roll angle sensor 230 for determining an instantaneous roll angle αof the vessel and providing a corresponding roll-angle indicative signal to the control unit 210. Moreover, the apparatus 200 includes a sensor arrangement for monitoring surface wave formation and propagation within the tank 20; the sensor arrangement is beneficially implemented as one or more of: an optical camera 240, a configuration of float sensors 250 disposed at various positions within the tank 20. The sensor arrangement is employed by the apparatus 200 for measuring an amplitude of the wave 30, its propagation direction within the tank 20 from a series of instantaneous position measurements of the wave 30 at various time instances. Operation of the sensor arrangement will be described in greater detail below.

The apparatus 200 includes an outlet arrangement for the tank 20 disposed to remove liquid from a first end of the tank 20. The outlet arrangement includes a valve 270 in series with a pump 260 which is operable to pump liquid from the tank 20 to a reservoir 300. The valve

270 and the pump 260 are controlled in operation from the control unit 210. Additionally, the apparatus 200 includes an inlet arrangement for the tank 20 disposed to add liquid to a second end of the tank 20. The inlet arrangement includes a valve 290 in series with a pump 280 which is operable to pump liquid from the reservoir 300 to the tank 20. The valve 290 and the pump 300 are controlled in operation from the control unit 210. By coordinated use of the pumps 260, 280 and the valves 270, 290, the control unit 210 is thereby capable of dynamically adjusting an overall depth (d) of the liquid 10 within the tank 20, as well as being capable of stimulating formation of the wave 30 by removing a portion of the liquid 10 from - the first end of the tank 20 and adding the portion of the liquid 10 at the second end of the tank 20.

The reservoir 300 is susceptible to being implemented in several different potential manners. Optionally, in a first manner, the reservoir 300 is a configuration of several small tanks which are individually too small to support significant wave motion therein. Beneficially, these several small tanks are disposed along an elongate axis of the vessel 100, namely near its centre of gravity. Such an arrangement is known as a "close system" because the liquid 10 is recirculated in operation. Alternatively, in a second manner, the reservoir 300 is the ocean environment itself and one or more of the pumps 260, 280 and/or valves 270, 290 are furnished with filters for preventing build up of marine debris in the tank 20, the valves 270, 290 and the pumps 260, 280. Such an approach avoids a need for the vessel 100 to bear a weight of the reservoir 300. Optionally, when implemented in such a second manner, an inlet for ocean water to be injected into the tank 20 is beneficially implemented on an underside of the vessel 100; for example, a forward motion of the vessel 100 can assist to force ocean water via a submerged scoop rapidly into the tank 20, thereby potentially avoiding a need for the pump 260, whereas gravity can be used to assist to remove water from the tank 20, thereby avoiding a need for the pump 280. Such a second manner of implementing the apparatus 200 is known as an "open system". The camera 240 in FIG. 3 is beneficially implemented as a standard charge-coupled-device (CCD) or complementary-metal-oxide-semiconductor (CMOS) optical camera operable to provide pixel stream data to the control unit 210. The control unit 210 in combination with its computer 220 is operable to image a surface of the liquid 10 at an oblique angle, and then process a series of images of the surface of the liquid 10 as a function of time for determining therefrom an amplitude of the wave 30, a propagation velocity of the wave 30, a position of the wave 30 as a function of time, and a direction of propagation of the wave 30 within the tank 20. The control 210 beneficially derives such data by presenting a series of images of the surface of the liquid 10 in the tank 20 to a neural network of the control unit 210 and its associated computer 220 which has been pre-programmed with images of the tank 20 with various known wave positions and wave heights so that the neural network is capable of providing an immediate indication of amplitude and position of the wave 30. By repeating such neural network computation at various known time intervals enables a direction of propagation and a velocity of propagation of the wave 30 to be computed. The neural network can either be implemented using software in combination with a parallel-processing engine or by way of a hardware-implemented neural network. As an alternative or supplement to use of the camera 240, the apparatus 200 can be provided with a microwave and/or ultrasound Doppler radar for determining propagation characteristic of the wave 30 within the tank 20. Referring to FIG. 4, the float sensor 250 provides a simple and robust manner for measuring propagation characteristic of the wave 30 within the tank 20 without a need for performing sophisticated data processing within the control unit 210 and its associated computer 220. The float sensor 250 is beneficially designed to be small and compact in relation to a size of the tank 20 and its associated wave 30 so as not to influence propagation of the wave 30 within the tank 20 when the apparatus 200 is in operation. The sensor 250 includes a float 360 which is buoyant in the liquid 10 and which is operable to slide along an elongate sensor member 350. The float 360 is beneficially of an annular form and surrounds a portion of the member 350 in operation. The float 260 includes one or more gas-filled buoyancy cavities 400 and one or more permanent magnets 410; the float 260 is beneficially manufactured from plastics material with the one or more cavities 400 and the one or more permanent magnets 410 moulded therein. The elongate sensor member 350 is beneficially a hollow tube, for example fabricated from non-magnetic stainless steel or Aluminium, in which a series of glass-encapsulated vacuum reed switches 420 are included along a length of the member 350. The reed switches 420 are normally open, unless the one or more magnets 410 are in close vicinity thereto, for example within a range of approximately 3 cm, which cause the switches 420 to close. Optionally, the member 350 includes digital integrated circuits therealong for encoding opening and closing of the reed switches 420 into a stream of data for communication to the control unit 210. For each sensor 250, the control unit 210 and its associated computer 220 are thus operable to receive rapidly in real time, via a data link 370, an indication of an instantaneous depth of the liquid 10 in a vicinity of the float sensor 250, without a need for the control unit 210 and its computer 220 to perform any computationally demanding computations. The float sensors 250 are beneficially disposed in a row centrally along the tank 20 from its first end face 40 to its second end face 40. From the measured instantaneous depth of the liquid 20 at a given spatially disposed series of point along the tank 20, amplitude, propagation velocity and propagation direction characteristics of the wave 30 can be determined rapidly with modest computing resources. As aforementioned, the float arrangement can optionally be employed in substitution for, or in addition to, the camera 240, and/or aforesaid radar.

The valves 270, 290 are beneficially implemented as rotary butterfly valves with rotating vanes for providing a very rapid open/close response as well as exhibiting a low flow resistance when in an open state. The pumps 260, 270 are beneficially implemented as multi-stage rotary turbine pumps of low inertia so that they can come quickly brought into and out of pumping operation. It will be appreciated for a large ocean vessel that the tank 20 can be of a very considerable size and include a quantity of liquid 10 amounting to over approximately 1000 tonnes. In consequence, the pumps 260, 270 then are relatively powerful items of equipment. Optionally, for obtaining quickest dynamic response, the pumps 260, 270 are each implemented as a parallel configuration of a plurality of smaller pumps.

The present invention is concerned with a method of operating the apparatus 200 as shown in FIG. 3. The method includes:

(a) monitoring the roll angle a of the vessel 100 equipped with the apparatus 200;

(b) monitoring propagation characteristics of the surface wave 30 propagating in the tank 20 of the apparatus 200; and

(c) adjusting an effective depth of the liquid 10 in the tank 20 so that the propagation of the wave 30 in the tank 20 is controlled so as to oppose, at least partially, a rolling motion of the vessel 100 as evident in the roll angle a of the vessel 100. The effective depth of the liquid 10 in the tank 20 can be influenced by removing liquid 10 or adding liquid to the tank 20 via the pumps 260, 280 and their associated valves 270, 290 respectively. Additionally, or alternatively, the effective depth can be modulated by use of rotatable vanes or baffles, for example as illustrated in FIG. 5. In FIG. 5, a lower region of the tank 20 is equipped with a series of rotatable "L"-shape baffles 500 which are pivotable about their respective axes 510. In a first position of the baffles 500 as illustrated in solid, the effective depth of the tank 20 is shallower and orthogonal parts of the baffles 500 hinder a flow of liquid beneath the baffles 500. In a second position of the baffles 500 as illustrated dotted, a major portion of the baffles 500 are in a vertical orientation and have relatively little influence upon propagation of the wave 30 such that the liquid 10 has an effective depth corresponding to a true depth of the liquid 10 within the tank 20. An advantage of the baffles 500 is that they can be rotated with great rapidity, for example by 90° within a second, with relatively little energy expenditure to modulate the effective depth of liquid 10 within the tank 20 and hence a velocity of propagation of the wave 30 within the tank 20. By selectively adding and/or removing liquid 10 from the tank 20, and optionally employing the baffles 500 to modulate an effective depth of the tank 20, it is feasible to maintain the wave 30 propagating back and forth within the tank 20 such as to oppose, at least partially, a rolling motion of the vessel 100 as detected by the roll sensor 230. Referring to FIG. 6, there is shown a large ocean vessel 100 equipped with two apparatus 200 pursuant to present invention with their respective tanks 20 disposed transversely across the vessel 100 at approximately front and rear positions within the vessel 100. It is feasible to include one or more examples of the apparatus 200 on each ocean vessel 100. Optionally, an example of the apparatus 200 is also disposed along a length of an ocean vessel 100 to reducing pitching of the vessel 100. The present invention provides an advantage when reducing rolling motions in ocean vessels by requiring vastly reduced energy input in comparison to corresponding gyroscopic stabilization systems, thereby improving vessel fuel efficiency. Moreover, the present invention provides an improved roll motion reduction in comparison to that provided by known passive liquid tanks for reducing rolling motion.

Modifications to embodiments of the invention described in the foregoing are possible without departing from the scope of the invention as defined by the accompanying claims. Expressions such as "including", "comprising", "incorporating", "consisting of, "have", "is" used to describe and claim the present invention are intended to be construed in a nonexclusive manner, namely allowing for items, components or elements not explicitly described also to be present. Reference to the singular is also to be construed to relate to the plural. Numerals included within parentheses in the accompanying claims are intended to assist understanding of the claims and should not be construed in any way to limit subject matter claimed by these claims.




 
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