US3580205A - Ship stabilizers - Google Patents

Abstract

A tank stabilizer for marine use has a high-Q tuned U-tank system in which the water levels in the two side tanks are boosted, when the system is operating at resonance, by a booster arrangement. The booster arrangement is insufficiently powerful to operate the stabilizer as an active stabilizer but improves the Q factor of the tank system to enhance its stabilizing power. The booster arrangement is controlled by roll-sensing means.

Description

United States Patent Bell, John, deceased, late of Beckenham,

inventors [56] References Cited Kent, England, by Barclays Bank Limited FOREIGN PATENTS and Bella Bell, Executors, Beckenham, 451 923 93 Great B i i 1 14/ 125 Kent England Y OTHER RapEReii Appl. No. 796,649 Filed Jam, 9 9 German Printed Applrcation No. 1,262,812 dated 8-1966 Patented May 25, 1971 classified in class 114- 125 (Halden) Asslgnee Mull'head Limited Primary ExaminerTrygve M. Blix Kent, England AttorneyHolman, Glascock, Downing and Seebold Priority Feb. 2, 1968 v Great Britain 5374/68 v ABSTRACT: A tank stabilizer for marine use has a high-Q tuned U-tank system in which the water levels in the two side 22 .2 3 tanks are boosted, when the system is operating at resonance, wing by a booster arrangement. The booster arrangement is insuffi- U.S. Cl 114/125 ciently powerful to operate the stabilizer as an active stabilizer Int. Cl B63b 43/06 but improves the Q factor of the tank system to enhance its Field of Search 114/122, stabilizing power. The booster arrangement is controlled by 125 roll-sensing means.

I 1 l 4 l X l l I I I l l SHIP STABILIZERS BACKGROUND OF THE INVENTION This invention relates to ship stabilizers, more particularly to tank stabilizers in which the motion of the mass of liquid is employed to produce a torque for stabilizing the oscillatory motion of the ship.

Tank stabilizers for ships have long been known. In the case of roll stabilization such a stabilizer comprises two tanks respectively disposed in the port and starboard sides of the ship, with the tanks being partly filled with a liquid and connected at their bottom portions by a liquid channel. The tanks may be connected at their upper portions by a gas channel so that the gas above the liquid in the tanks is able to flow between the tanks during rolling motion of the ship. In some arrangements the upper portions of the tanks may communicate with atmosphere. In practice, the liquid used in the stabilizer is water and the gas is air.

Tank stabilizers can be considered as falling generally into two classes, active stabilizers and passive stabilizers.

The passive stabilizers were the first to be developed. They have the advantage that they require no external power to be applied for their operation as they rely entirely on the rolling movement of the ship to alternatively fill the tanks of the stabilizer so that they provide a countering torque to suppress the rolling motion.

One such passive stabilizer was developed by Frahm and is the tuned U-tube stabilizer. The two limbs of the U are provided by the tanks at the sides of the ship and the intermediate portion of the U provides an interconnecting passage between the lower ends of the tanks. The whole system is tuned to a resonant frequency f which is normally the resonant frequency f of the ship most likely to be encountered in practice. When the ship is rolling at the resonant frequency the liquid in the U-tube surges back and forth between the tanks with a phase relationship such that the synchronous rolling of the ship at the frequency f is suppressed by the 90 phase lag of the movement of liquid between the two tanks of the stabilizer.'

Although the turned passive stabilizer has theoretically certain advantages it does have a practical disadvantage in that it sometimes is difficult to achieve a desirably high Q-factor together with the necessary stabilizing power, without an unacceptably large loss of the metacentric height GM of the ship due to the free surface effect of the liquid in the stabilizer tanks.

The active stabilizer utilizes an external source of power to control the movement of liquid between the two tanks of the stabilizer. The active stabilizer has the advantage that it may be used to suppress a very much wider range of rolling frequencies than can be suppressed by the tuned passive stabilizer. As the control of the liquid flow between the tanks is exercised entirely by an external source of power, the active tank system normally has a natural resonant frequency lying beyond the range of rolling frequencies likely to be experienced by the ship in practice. This ensures that there is no likelihood of the natural resonant frequency of the tank SUMMARY OF THE INVENTION In accordance with the broadest aspect of this invention a method of stabilizing a vessel carrying a high-Q U-tube tank stabilizer tuned to the resonant frequency of oscillation of the vessel, comprises applying a force to the tank liquid at a frequency substantially equal to the resonant frequency and in such a manner as to stabilize rolling motion of the vessel. I

In accordance with a second aspect of this invention a tank stabilizer has a tank system which, when filled with liquid, has a resonant frequency lying within the range of rolling frequencies likely to be experienced by a ship into which the stabilizer is to be fitted, the system comprising two tanks having an interconnecting liquid passage between their lower end portions and having the liquid flow between them at the resonant frequency influenced predominantly by the rolling movement of the ship and also by a booster arrangement controlled to assist the resonant liquid flow by imparting to the liquid a force which varies with rolling and which is controlled by a control system actuated by roll-sensing means which respond to the rolling movement of the ship.

The booster arrangement enables the stabilizing power of the system to be increased without a corresponding decrease in the metacentric height.

The tank stabilizer of the invention cannot be classified as an active stabilizer, as the booster arrangement is incapable of providing a sufficient force to produce the necessary flow of liquid between the tanks in order to stabilize rolling motion of the shipl Neitherhowever is the stabilizer a passive system as it does consume power during its operation. In practice the booster arrangement requires only a relatively small amount of power to operate it as the operation of the stabilizer relies primarily on the resonance of the system. The booster arrangement merely provides a small force to make the operation of the stabilizer more effective.

The force applied by the booster arrangement preferably varies continuously with the roll. However, the booster arrangement may be arranged to provide a continuous constant force during rolling, with the force being reversed in direction at opposite ends of the roll. Alternatively, the booster arrangement may be arranged to provide a force for a short duration at the ends of the roll so that the advantage of the invention is obtained when the force is integrated over a prolonged period of time.

The invention may also be used to enable the volume of liquid used in the stabilizer and thus its free surface area to be reduced. A consequential increase of the metacentric height GM is then achieved and the ship's stability is enhanced. v

The resonant'frequency of the tank system may be tuned to nearly the desired resonant frequency of the vessel f by varying the effective area of the liquid channel interconnecting the tanks. Such variation is useful to allow forthe change in the vessel's rolling frequency as a result of loading or other variations of the vessel's condition.

Preferably the booster arrangement utilizes an air pump connected in a. gas channel extending between the upper portions of the tanks. A pump of comparatively low power is all that is required and this can discharge in controlled manner into respective tanks by way of a suitable valve forming'the control system and whose position is actuated by the rollsensing means.

Preferably the control system responds primarily to the roll acceleration of the ship.

Although the invention relates primarily to high-Q and therefore low loss tank systems, in practice'there may be difficulties in getting the Q-factor of the tank which determines the sharpness of the response curve of the tank when plotted against frequency, as large as is desired for optimum stabilization. The Q of the tank must not be made too high as'this would have an adverse effect on the stabilization as the level of the oscillation of the tank liquid would take too long to build up. It is therefore preferred to design the stabilizer so that O has its optimum value when it is equal to Q The quantity O the quality factor of the ship and-characterizing the damping of the ship in the sea with the tank stabilizer inoperative, maybe defined as'Q =fi/2a, where-Bis the peak-to-peak roll angle caused by waves impinging on the ship at the resonant frequency of the ship f and a is'the median angle of slope of such waves. In a ship with bilge keels Q,,,,,, is typically around while if there are no bilge keels Q may be as high as 10. The factor Q may be increased to some extent by rounding off edges inside the tank and by streamlining the flow channels interconnecting them.

By constructing the stabilizer as two two-tank systems arranged one within the other it is possible to obtain certain ad vantages. For example, the two systems may be tuned to different frequencies lying in the range of rolling frequencies experienced by the vessel, so that the overall frequency response of the stabilizer has a humped band-pass characteristic. The booster arrangement enhances the stabilizing power of the two two-tank systems at the different rolling frequencies in the passband. In some circumstances the use of two two-tank systems may enable a better utilization of the available space to be achieved so that an improved Q-factor is obtained for the combination.

In the case where the force applied by the control system is transmitted to the gas space above the liquid in the tanks by means of a pump arranged in a gas flow channel interconnecting the upper end portions of the tanks, a single pump working into highand low-pressure reservoirs connected to its output and input sides, respectively, may be employed to drive a number of independent stabilizer tanks. Alternatively each pair of tanks of each stabilizer may have its own pump with or without such reservoirs. The tops of the tanks may be indepen-.

dently connected together or/and to the atmosphere via a leak or leaks for the flow of gas therethrough to prevent the air pressure above respective tanks providing a constant bias to the stabilizer.

The invention will now be described in more detail, by way of examples, with reference to the accompanying drawings, in which:

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 illustrates diagrammatically one form of tank stabilizer; and

FIG. 2 represents diagrammatically a second form of tank stabilizer.

, Referring to FIG. 1 a tank stabilizer 1 is mounted within a ships hull diagrammatically shown in athwartships section at 2 and provided with bilge keels 3.

The stabilizer is of tuned U-tube type and comprises a pair of side tanks 4 and 5 which are interconnected at their lower portions by a number of parallel channels 6. The upper end portions of the tanks 4, 5 are joined by a duct 7 provided with a lead valve 8 in the form of an adjustable constriction for equalizing the air pressure in the two tanks 4, 5 when the stabilizer is not in use. The duct 7 is provided with a branch pipe 9 terminating in a vent valve 10 formed by an orifice of small size through which excess pressure in the stabilizer can exhaust to atmosphere. The vent 10 also ensures that the air pressure in the stabilizer is the same as the outside atmosphere during times that the stabilizer is not operating. The stabilizer is filled with liquid to a level indicated by the chain line X-X.

The tops of the tanks 4 and 5 are respectively connected by piping 12, 13 to a booster arrangement shown generally at 14.

The booster arrangement 14 includes a pair of reservoirs 15, 16. The reservoir 15 is connected to high-pressure output side 17 of an air pump 18 while the reservoir 16 is connected to low-pressure input side of the air pump 18. The reservoirs l5 and 16 are also connected to outlets 20, respectively, through which airflow is controlled by a valve member 21 having a pair of interconnected piston heads 22 disposed opposite the outlets 20. The piston heads 22 are arranged on opposite sides of a diaphragm 23 through which the valve member 21 passes and which spans across a working chamber 24 for the member 21. The ends of the working chamber 24 are closed by diaphragms 25 and it is provided with three laterally opening ports, 26, 27 and 28. The port 27 is connected to one end of the pipe 12 while the other pipe 13 is connected to both of the ports 26 and 28.

connected to the passages 12 and 13, respectively, or vice versa. The instantaneous position of the member 21 also determines the size of the orifice through which the reservoirs 15, 16 are connected to the pipes 12, 13 so that the pressure of air supplied to the stabilizer 1 is continuously controlled by the roll-sensing means.

The parallel passages or channels 6 are of adjustable cross section to allow the response frequency of the stabilizer to be altered. The upper wall of each of the channels 6 is formed by a horizontal plate which is vertically movable by screwjacks 31. At opposite ends of the plate 130 are respective flaps 32 spring urged upwardly to provide a smooth entry to each end of the channel. By raising and lowering the plate 130 the resonant frequency of the stabilizer system may be adjusted to a value best suited to the operating conditions of the ship, as the cross-sectional area of the channel is altered.

The tank stabilizer described operates in the following manner.

When the ship is not rolling, the output of the roll-sensing means moves the member 21 to the position shown in FIG. 1 at which the outlets 20 are both closed by the pistons 22. The reservoirs l5 and 16 are held at the output high pressure and the inlet low pressure, respectively, of the pump 18 which is a continuously running constant volume pump. Alternatively, the pump 18 may be arranged, by suitable electrical connections shown by broken lines, to be controlled in accordance with the pressures within the reservoirs l5 and 16 and it may then be of constant displacement type. The leak valve 8 ensures that the air spaces above the liquid in both of the tanks are at the same pressure which, because of the vent 10, is atmospheric pressure.

If the vessel begins to roll at a resonant frequency f which corresponds to the tuning frequency flank of the stabilizer, the roll-sensing means starts to move the member 21 in the working chamber 24 in sympathy with the roll. The member 21 is reciprocated back and forth in harmonic fashion by the rollsensing means 30 and the amplitude of its movement is determined by the acceleration of the roll. This is a minimum when the ship is on an even keel and is a maximum when the roll amplitude is a maximum.

The movement of the member 21 controls the extend of opening of the outlets 20 so that air under pressure is fed in a continuously controlled manner to one or other of the tanks 4 and 5. Simultaneously, the other tank is connected to the reservoir 16 which is held at a pressure beneath atmospheric pressure so that a differential pressure is established having a continuous relationship with the roll of the vessel.

The differential air pressure acts to assist the operation of the stabilizer by building up the water levels at resonance so increasing the stabilizing power of the tank. The bulk of the energy required to operate the stabilizer is obtained form the rolling movement of the ship and only a relatively small amount of additional energy is required to operate the pump 18.

If it is desired to alter the response frequency of the stabilizer with a change in loading of the ship, the screwjacks 31 are operated in the appropriate direction to raise or lower the plate 130.

A second embodiment of the invention will now be described with reference to FIG. 2. Parts of FIG. 2 identical with corresponding parts of FIG. 1 have the same reference numerals and will not be again described. It will be understood that FIG. 2, like FIG. 1 is wholly diagrammatic.

The tank stabilizer shown in FIG. 2 comprises a pair of tank systems arranged one within the other. Both tank systems are of high-Q factor, tuned U-tube type and have similar connections leading to a booster arrangement 14 to allow the pressure above the liquid in the individual tanks of both systems to be varied. To prevent overcomplicating the drawing, the connections associated with the outer tank system only are shown but it is to be understood that the connections associated with the inner tank system and partly shown at 54, are similar. The two tank systems are tuned, respectively, to different frequencies in the range of roll frequencies likely to be encountered by the vessel.

The first tank system comprises a pair of tanks 50, 51 disposed at opposite sides of the centerline of the ship and interconnected at their lower ends by a passage 53.

The second tank system is formed by a pair of tanks 55, 56 disposed outwardly of the tanks 50, 51, respectively, and interconnected at their lower end portions by a number of parallel passages 57.

The tanks 55 is connected at its upper end portion by a pipe 60 to poppet valves 61, 62. The second tank 56 is connected by a pipe 63 to another pair of poppet valves 64, 65. The poppet valves 61, 64 have stems 66 which rest on a reciprocable shaft 67 positionally controlled by roll-sensing means 68 similar in function and operation to the roll-sensing means 30 of FIG. 1. The shaft 67 is provided with two camming flanges 70, 71. The pair of poppet valves 62, 65 have their stems 72 also resting on the shaft 67.

Individual opening of the poppet valves 61, 65 allows air to be drawn past them through a conduit 172 which extends to a reservoir 73 and to the low-pressure inlet of a pump 74.

Individual opening of the other two poppet valves 64 and 62 allows compressed air to flow through them from a conduit 76 connected to high-pressure outlet side of the pump 74 and also to an air reservoir 77. Electrical connections shown by dotted line 78 control the booster arrangement 14 so that the air pressures of the reservoirs 73 and 77 are maintained at desired values.

The two tanks 55, 56 are interconnected at their upper ends by a channel 7 having a branch pipe 9 and containing a leak valve 8. The branch pipe 9 terminates in a vent valve 10. The parts referenced 7, 8, 9 and perform the same functions as correspondingly referenced parts in FIG. 1 and will not therefore be again described.

The stabilizer shown in FIG. 2 operates in the following manner.

When the ship is on an even keel and not rolling, the rollsensing means 68 shifts the shaft 67 to the position shown in FIG. 2 at which the four poppet valves are closed. The liquid or water levels in the two tank stabilizer systems assume equilibrium positions and the air pressures above all of the tanks are at atmospheric pressure. The pump 74 is then controlled solely by the reservoirs 73 and 77 to maintain the air pressures within them within desired limits.

If the vessel commences to roll at the frequency lying in the I band to which the tank stabilizer is tuned, the roll-sensing means 68 responds by reciprocating the shaft 67 to and fro. When the shaft 67 reciprocates to the left, the two poppet valves 64 and 61 are opened, and when the shaft reciprocates to the right, the two poppet valves 62 and 65 are opened. The degree of opening of the poppet valves for any position of the shaft 67 is determined by the camming surface of the shaft and the camming flange with which the stems of the poppet valves come into contact. The stems of the poppet valves are resiliently biased towards the shaft 67 as is customary with the poppet-type valve.

When the pair of poppet valves 64, 61 is opened, the other pair of valves 62, 65 remains closed. The open valves allow air pressure to flow through the pipe 63 to the tank 56 and air to exhaust from the tank 55 by way of the pipe 60 and the conduit 172 to the inlet side of the pipe 74. The differential pressure acting on the two tanks 56, 55 causes the water level in the tank 56 to be depressed and the water level in the tank 55 to be increased. The roll-sensing means so controls the shaft 67 that the differential pressure acting on the tanks 55 and 56 assists the natural movement of the water or liquid flow between the tanks 55, 56 at the tuning frequency of the tank system. The other tank system formed by the two tanks 50, 51 is operated in the same way.

As the two tank systems are tuned to different resonant frequencies their band-pass characteristic acts to suppress the roll of the ship over a range of frequencies. The force exerted by the differential gas pressure on the tanks 55, 56 varies with the acceleration of the roll so that the tank system is able to provide an adequate torque for suppressing the roll.

The pump 74 may be a centrifugal pump or a fan-type pump. Although a displacement pump could be used it would tend to be wasteful of power and would require special safety devices to avoid pressures building up inside the tanks.

The Q-factor of the tanks maybe increased to some extent by rounding off edges inside the tanks and by stream lining the flow channels and by the use of other techniques known in the art for tuning tank stabilizer systems.

Various modifications of the above-described arrangements are possible.

For example, the ship may be provided with several tank stabilizers of the type shown in FIG. 1 or the type shown in FIG. 2. Each of the tank stabilizers could be provided with its own booster arrangement formed by the pump and associated valve system. If desired each tank stabilizer could be provided with its own booster arrangement and the two tanks of the stabilizer which are to have the differential air pressure exerted across them could be connected to the same air pump directly without the use of reservoirs. On the other hand, a number of tank stabilizers may be used which receive supplies of compressed air and low pressure from two reservoirs in which the desired air pressures are maintained by a single pump.

The roll-sensing means may employ a continuous electrical control such as would utilize gyros and electrical roll-sensing devices, a hydraulic or pneumatic control, or ON/OFF impulse or any other control.

It is claimed:

1. In a tuned tank stabilizer including two tanks, means defining a liquid passage interconnecting the lower regions of said tanks, liquid contained in said tanks and said passage imparting to said stabilizer a resonant frequency lying within a range of rolling frequencies likely to be experienced by a ship into which the stabilizer is fitted, the improvements comprising a power-consuming booster arrangement, means for imparting a force exerted by operation of the booster arrangement to the liquid in the tanks, a control system operably related to said means for adjusting said force, and roll-sensing means operably related to said control system for operating said control system in dependence upon the rolling movement of the ship at said resonant frequency, whereby the power to operate the stabilizer at said resonant frequency is derived predominantly from the rolling motion of the ship and is assisted only to a minor extent by the booster arrangement which is therefore designed to provide a limited power output insufficient to operate the stabilizer as an untuned active stabilizer.

2. In the tuned tank stabilizer as set forth in claim 1, the provision of a booster arrangement defined by an air pump having a low-pressure inlet and a high-pressure outlet, a control system defined by a valve having an operating member moved rectilinearly by the roll-sensing means, and means defining airflow passages leading from the control system to both tanks whereby the connection of one tank to the pump inlet is accompanied by simultaneous connection of the other tank to the pump outlet.

3. A tuned tank stabilizer having two tanks each having upper and lower portions, a liquid passage interconnecting said lower portions of said tanks, a gas channel interconnecting said upper portions of said tanks, leak valve means in said gas passage, a branch pipe extending from said passage, vent means in said branch passage and opening to atmosphere, liquid in said tanks and said passage and imparting to said stabilizer a resonant frequency lying in a range of rolling frequencies likely to be experienced by a ship into which the stabilizer is fitted, a booster arrangement, pipe means extending from said upper portions of said tanks to said booster arrangement, an air pump in said booster arrangement and having a lowpressure inlet and a high-pressure outlet, a control system in said booster arrangement and having a valve member rectilinearly movable from an intermediate position to two extreme positions, roll-sensing means responsive predominately to the roll acceleration and positionally controlling said valve member, and conduits providing separate gas flow paths between said control system and said high-pressure inlet and low-pressure outlet of said pump, whereby said valve member is displaced by said roll-sening means during rolling of the ship form said intermediate position at which it disconnects said pump from said tanks and the extent of such displacement regulates the force applied to the tank liquid by the differential air pressure exerted in the upper portions of said two tanks.

4. The tuned tank stabilizer as set forth in claim 3, wherein said booster arrangement includes a low-pressure gas reservoir connected to said pump inlet, a high-pressure gas reservoir connected to said pump outlet, and means controlling operation of said pump in accordance with the pressures in said reservoirs.

5. A tuned tank stabilizer having a first tank system arranged within a second tank system, each of said tank systems comprising two tanks each having upper and lower portions, a liquid passage interconnecting said lower portions of said tanks, a gas channel interconnecting said upper portions of said tanks, leak valve means in said gas passage, a branch pipe extending from said passage, vent means in said branch passage and opening to atmosphere, liquid in said tanks and said passage and imparting to said stabilizer a resonant frequency different from the resonant frequency to the other system and lying in a range of rolling frequencies likely to be experienced by a ship into which the stabilizer is fitted, a booster arrangement pipe means extending from said upper portions of said tanks to said booster arrangement, an air pump in said booster arrangement and having a low-pressure inlet and a high-pressure outlet, a control system in said booster arrangement and having a valve member rectilinearly movable from an intermediate position to two extreme positions, roll-sensing means responsive predominantly to the roll acceleration and positionally controlling said valve member, and conduits providing separate gas flow paths between said control system and said high-pressure inlet and low-pressure outlet of said pump whereby said'valve member is displaced by said roll-sensing means during rolling of the ship from said intermediate position at which it disconnects said pump from said tanks and the extent of such displacement regulates the force applied to the tank liquid in the two systems which, being tuned to different frequencies, exhibit a band-pass characteristic offering a range of frequencies over which the stabilizer is effective.

6. The tuned tank stabilizer as set forth in claim 5, in which said booster arrangement includes a low-pressure gas reservoir connected to said pump inlet, a high-pressure gas reservoir connected to said pump outlet, and means controlling operation of said pump in accordance with the pressures in said reservoirs.

Claims (6)

1. In a tuned tank stabilizer including two tanks, means defining a liquid passage intErconnecting the lower regions of said tanks, liquid contained in said tanks and said passage imparting to said stabilizer a resonant frequency lying within a range of rolling frequencies likely to be experienced by a ship into which the stabilizer is fitted, the improvements comprising a power-consuming booster arrangement, means for imparting a force exerted by operation of the booster arrangement to the liquid in the tanks, a control system operably related to said means for adjusting said force, and roll-sensing means operably related to said control system for operating said control system in dependence upon the rolling movement of the ship at said resonant frequency, whereby the power to operate the stabilizer at said resonant frequency is derived predominantly from the rolling motion of the ship and is assisted only to a minor extent by the booster arrangement which is therefore designed to provide a limited power output insufficient to operate the stabilizer as an untuned active stabilizer.

2. In the tuned tank stabilizer as set forth in claim 1, the provision of a booster arrangement defined by an air pump having a low-pressure inlet and a high-pressure outlet, a control system defined by a valve having an operating member moved rectilinearly by the roll-sensing means, and means defining airflow passages leading from the control system to both tanks whereby the connection of one tank to the pump inlet is accompanied by simultaneous connection of the other tank to the pump outlet.

3. A tuned tank stabilizer having two tanks each having upper and lower portions, a liquid passage interconnecting said lower portions of said tanks, a gas channel interconnecting said upper portions of said tanks, leak valve means in said gas passage, a branch pipe extending from said passage, vent means in said branch passage and opening to atmosphere, liquid in said tanks and said passage and imparting to said stabilizer a resonant frequency lying in a range of rolling frequencies likely to be experienced by a ship into which the stabilizer is fitted, a booster arrangement, pipe means extending from said upper portions of said tanks to said booster arrangement, an air pump in said booster arrangement and having a low-pressure inlet and a high-pressure outlet, a control system in said booster arrangement and having a valve member rectilinearly movable from an intermediate position to two extreme positions, roll-sensing means responsive predominately to the roll acceleration and positionally controlling said valve member, and conduits providing separate gas flow paths between said control system and said high-pressure inlet and low-pressure outlet of said pump, whereby said valve member is displaced by said roll-sening means during rolling of the ship form said intermediate position at which it disconnects said pump from said tanks and the extent of such displacement regulates the force applied to the tank liquid by the differential air pressure exerted in the upper portions of said two tanks.

4. The tuned tank stabilizer as set forth in claim 3, wherein said booster arrangement includes a low-pressure gas reservoir connected to said pump inlet, a high-pressure gas reservoir connected to said pump outlet, and means controlling operation of said pump in accordance with the pressures in said reservoirs.

5. A tuned tank stabilizer having a first tank system arranged within a second tank system, each of said tank systems comprising two tanks each having upper and lower portions, a liquid passage interconnecting said lower portions of said tanks, a gas channel interconnecting said upper portions of said tanks, leak valve means in said gas passage, a branch pipe extending from said passage, vent means in said branch passage and opening to atmosphere, liquid in said tanks and said passage and imparting to said stabilizer a resonant frequency different from the resonant frequency to the other system and lying in a range of rolling frequencies likely to be experienced by a ship into which the Stabilizer is fitted, a booster arrangement pipe means extending from said upper portions of said tanks to said booster arrangement, an air pump in said booster arrangement and having a low-pressure inlet and a high-pressure outlet, a control system in said booster arrangement and having a valve member rectilinearly movable from an intermediate position to two extreme positions, roll-sensing means responsive predominantly to the roll acceleration and positionally controlling said valve member, and conduits providing separate gas flow paths between said control system and said high-pressure inlet and low-pressure outlet of said pump whereby said valve member is displaced by said roll-sensing means during rolling of the ship from said intermediate position at which it disconnects said pump from said tanks and the extent of such displacement regulates the force applied to the tank liquid in the two systems which, being tuned to different frequencies, exhibit a band-pass characteristic offering a range of frequencies over which the stabilizer is effective.

6. The tuned tank stabilizer as set forth in claim 5, in which said booster arrangement includes a low-pressure gas reservoir connected to said pump inlet, a high-pressure gas reservoir connected to said pump outlet, and means controlling operation of said pump in accordance with the pressures in said reservoirs.

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