GB2284577A - System for augmenting ship stability using inflatable buoyancy bags - Google Patents

Abstract

A system and method for augmenting the stability of a ship comprises providing inflatable buoyancy bags 8 along both sides of the ship, maintaining the bags in a partially inflated state during normal operation of the ship so that leakage can be detected during normal non-emergency conditions, and inflating the bags fully on sensing an emergency situation likely to give rise to reduction in the ships' stability, e.g. the presence of flood water. If leakage is detected in a bag, inflation thereof is prevented as well as that of a corresponding bag on the other side of the ship. The bags are stowed within fairing's 73 which are pivoted outwardly on inflation of the stowed bag. <IMAGE>

Description

Control of Pneumatic Devices This invention relates to control of pneumatic devices and in particular to inflatable bags or sponsons for imparting enhanced stability to a vessel.

In order to facilitate the transport of vehicles by sea, some large vessels, in particular Ro-Ro ferries, have extensive open deck spaces situated near the waterline level. Ro-Ro ferries are vessels which incorporate ramps usually at the bow and/or stern to enable vehicles to be driven directly from the quayside onto a deck of the vessel. It is a characteristic of Ro-Ro ferries that they have one or more substantially unobstructed deck spaces which often extend from bow to stern. If these deck spaces (e.g. a car deck) become flooded, due to their flat nature, the flood water is unrestrained by compartmentation, leading to the creation of a large Free Surface Area (F.S.A.). As a result of this flooding, severe roll instability can rapidly develop.

One approach to remedy this type of instability is to provide a number of separate, large diameter gas-filled flexible buoyancy bags, for example, of cylindrical shape, which may be stowed in individual metal fairings, mounted on each side of the vessel down the greater part of each side. The individual bags may each be inflated with an inert gas from their own separate gas cylinder.

In an emergency leading to instability of the vessel due to F.S.A., the bags may be inflated on both sides of the vessel to provide additional and almost instantaneous buoyancy and stability augmentation.

Adoption of this type of stability augmentation has so far been restricted by doubts about reliability of inflation, both by manual or automatic initiation and integrity of inflatable flexible bags. These doubts arise from a fear that the bags might, in practice, develop an undetected leak during their stowed life in the fairings, and thus in an emergency, deploy unevenly on the two sides of the vessel which, rather than augmenting stability, might even result in a further decrease in the vessel's stability.

The necessity of frequent and repeated inspection of the flexible bags by inflating and re-stowing them in their fairings would result in lost revenue to the shipping company. Concern over accidental deployment by either manual or automatic means has also inhibited the adoption of inflatable buoyancy for stability augmentation purposes.

The present invention is therefore directed to finding a solution to the above problems and, in particular, to providing a system for augmenting the stability of a ship which utilizes flexible buoyancy bags in a reliable manner.

According to one aspect of the present invention there is provided a method of augmenting the stability of a ship which comprises providing inflatable buoyancy along both sides of the ship in the region of the water-line, maintaining the bags in a partially inflated state during normal operation of the ship so that leakage can be detected during normal non-emergency conditions and inflating said bags fully on sensing an emergency situation likely to give rise to reduction in the ship's stability.

A gas delivery conduit for inflating an associated buoyancy bag may be located down-stream of a gas inflation control valve. The buoyancy bag itself is pre-charged with gas, e.g. by means of a one way valve, advantageously, but not necessarily, situated in the gas delivery conduit, thus permitting a pre-charge supply to be delivered to the conduit and flexible bag, either by a small amount of gas bled from the main supply cylinder or from an outside supply source via the one way valve.

A pressure gauge mounted on the gas delivery conduit or connected directly to the buoyancy bag enables the precharge pressure to be verified. The exact pressure can be adjusted, if necessary, by bleeding off excess gas by depressing a central spigot on the one-way valve. This may be desirable, for example, in cases where the ship is subjected to high ambient temperatures causing the gas within the expand.

The gas delivery conduit, e.g. a gas pipe, is also equipped with a pressure operated switch. If the pressure in the gas delivery pipe, and therefore the flexible buoyancy bag, reduces below a pre-set value, the pressure switch circuit closes, isolating the current to an electrically controlled remotely-operated inflation valve, not only of the faulty flexible bag but also the servo-valve controlling inflation of the bag on the other side of the vessel which corresponds with the faulty, flexible buoyancy bag, thus maintaining balanced buoyancy on the two sides of the vessel.

Additionally, closure of the switch illuminates a warning light on a remotely situated display panel, typically, but not necessarily, positioned on the bridge of the vessel.

The display panel can take the form of separate annotated warning lights or individually colour-coded warning lights illuminating a point on a diagrammatic representation of the vessel, to show the exact location of the faulty flexible buoyancy bag in relation to the whole vessel.

Typically, the location of the buoyancy bags would be marked around the outside of the line indicating the extremity of the hull. Inside this line a further row of warning lights show flood alarm warnings and the location of inflation valves where power to the inflation servo has been isolated by a signal from a pressure switch indicating that pressure in a flexible buoyancy bag has dropped below the preset value.

Simultaneously, with the warning lights from the flood alarm an audio/visual warning starts and the time delay switch starts a count down to automatic deployment.

When the time delay switch closes the circuit, all the servos are automatically energised to fully open the inflation valves attached to the gas cylinders. The automatic inflation sequence can also be initiated immediately by means of a manual override switch. A further switch will be capable of cancelling deployment but can still be reinstated to the active state by means of a re-set switch.

The only servo controlled valves not activated on completion of the circuit are those isolated by the pressure switches connected to faulty bags and their opposite counterparts on the other side of the vessel.

Advantageously, but not necessarily, a computer will monitor electrical signal outputs from all sensors and servos to give a serviceability print-out for maintenance and record purposes. A TV and/or Thermal Imager Camera combined with suitable lighting arrangements may be provided to monitor the sides of the vessel to oversee deployment of flexible bags and assess collision damage or disembarkation.

Typically, when in their inflated state, the bags will be about 10 to 20 feet in diameter, e.g. about 12 to 16 feet in diameter. Use of very large diameter bags results in high hoop stresses. While high strength fabric materials such as 'Kevlar' (Du Pont Trade Mark) can be stitched or otherwise attached to the bag fabric as a series of circumferential bands to resist such stresses, bags of diameter greater than about 25 feet should be avoided. The bags may be up to about 50 feet in length or more. However, it may be convenient to employ a larger number of shorter bags so as to minimise loss of buoyancy if one or more should fail. Typical lengths may be 25, 30, 35 or 40 feet. A very large buoyancy may be developed. For example, 20 bags each of a length of 50 feet and a diameter of 15 feet would hold approximately 140,000 cubic feet of gas, which would provide about 3,500 tons of positive buoyancy.

Several design features can be adopted to ensure the retention of the bags when fully inflated. First, the partially inflated bags are stored in housings incorporating an outer fairing pivotally attached to the hull in such a way that when the bag is deployed, the fairing forms an outwardly extending upward abutment for the bag. The pivot or hinge between the fairing and the hull may include a locking device which restricts upward pivoting of the fairing beyond the point where it extends substantially at right angles to the hull. Further restraint on upward pivoting of the fairing can be provided by linkages, e.g. rods or wire ropes, between the hull and the remote end of the fairing. Such linkages are preferably located at the longitudinal ends of each fairing. Fairings are preferably manufactured from metal, e.g. steel, about 5 to 10 mms, e.g. 6 mms thick.

Strengthening ribs can be formed in or attached to the fairings. Other possible materials include glass reinforced plastics.

In addition to an abutment provided by a projection extending outwardly from the hull above the buoyancy bag, such as the fairing, the bags may be restrained by circumferential bands which allow the bags to inflate but restrain further movement outwardly from the hull.

As explained above, there are considerable advantages in maintaining the bags in a partially inflated condition.

However, the internal pressure necessary to maintain this condition is less than 1 psi, preferably less than about 0.5 psi, e.g. about 0.2 to 0.3 psi. In the case of a fairing which is say 30' and 6' wide, a pressure of this order will exert a force of about 3 tons on straps holding the housing closed. When gas is introduced into the bags from gas cylinders, e.g. of compressed nitrogen, the instantaneous pressure will be high and will break frangible straps or shear bolts holding the housing closed. An inflated pressure of 1 to 2 psi is usually sufficient to achieve an inflated cylinder whose surface is 'drum-tight' and is capable of being controlled and restrained. Preferred materials for constructing the bags include heavy duty materials traditionally used for the manufacture of inflatable lift rafts.These materials include neoprene/polyurethane reinforced with woven and non-woven nylon, 'Dacron' and 'Kevlar' fabrics. Fabric weights may range from 10 to 50 ozs/sq foot, preferably 15 to 25 oz/sq foot, e.g. 17 to 22 ozs/sq foot.

Preferably, the housing for the bags should be located close to the water line so that a soon as inflation commences, the lower surface of the bags touch the water and contribute to stabilising the ship immediately.

The size and number of bags will depend upon the length and tonnage of the vessel. Bags may vary in length, e.g. from 10' to 50' and from 3 to 12, say 5 to 8 or 10 may be positioned for deployment along each side of the vessel. A cylindrical buoyancy bag 40' in length and 15' in diameter will provide positive buoyancy of 200 tons. For a Ro-Ro ferry of 15,000 tons displacement, ten such bags on each side will provide 8,000 tons of positive buoyancy, which should be sufficient as a minimum to maintain the vessel afloat for an adequate time to evacuate all the passengers and, probably, would prevent the vessel sinking even after a major collision.

The gas supply will normally include pressure relief valves to guard against the bags being subjected to excessively high pressures, e.g. where the local temperature is particularly high.

Any suitable gas may be used to inflate the bags.

The preferred gases are inert, e.g. nitrogen, helium or carbon dioxide. Nitrogen is preferred for reasons of economy and safety. Helium may be too expensive for most applications. Carbon dioxide may be used but has the disadvantage that freezing of the gas and also surrounding moisture could occur because of the rapid pressure drop on releasing the gas. To overcome this heating outlets could be directed onto the pipe for delivering gas to the bags and also onto the associated valves. As an alternative, air may be used or a mixture of air and other gases.

It is generally satisfactory to feed gas directly from one or more gas cylinders to inflate a bag. For reasons of reliability, individual gas bottles are provided for each bag. In order to increase the rate of inflation, an aspirated system may be employed in which gas from the gas bottle is fed into a restrictor having an open end in the upstream direction and, thus, drawing air in with the gas as a result of a venturi effect.

The buoyancy bags are preferably stowed in the housing in a folded condition in which the folds extend longitudinally of the vessel, e.g. in a 'concertina' arrangement. If the vessel is likely to operate in very cold waters, it may be desirable to provide heaters to melt any ice which may form between the folds of the bags.

Such heaters may inject warm air or water into the housings which, in any event, are fitted with drainage holes in their base. The bags are preferably triangular when viewed in end cross-section in their uninflated condition. The arrangement should be such that an apex of the triangle is upward and one side is aligned with the hull of the ship.

A specific embodiment of the invention applied to a Ro-Ro vessel will now be described by way of example with reference to the accompanying drawing in which: Figure 1 shows in perspective the vessel fitted with buoyancy bags, showing most of the bags deployed, and additionally shows the closed fairing of one bag which has not deployed and in an inset details of one method of attachment of the bag to the hull; Figure 1A is a block diagram showing in simplified terms the control and sensing circuits; Figure 2A is a section through the inflated bag of Figure 1A and through part of the hull; Figure 2B is a plan view of the system with the bags deflated, both Figures 2A and 2B showing diagrammatically the connections to gas cylinders and to the monitoring, control and display units on the vessel's bridge; and Figures 3A, 3B and 3C show details of methods of attaching the bags to the hull and their connection to a gas supply from a gas cylinder; Figure 3D illustrates one method of fitting restraining straps to the bags; Figure 4 shows a typical display panel and one manner of indicating the condition of the buoyancy bags at various stations along the vessel.

In Figures 1 and 1A and 2A and 2B, the reference numerals indicated identify the following items arranged to control the inflation of the buoyancy bags, i.e. (1) the gas cylinder, (2) the gas discharge valves, (3) gas delivery pipe, (4) the pressure switch, (5) servo to open (2), (6) the pressure gauge, (7) a fairing (open), (8) inflated flexible buoyancy bag, (9) flood alarm (one of 20 approx.), circuits (18) for transmitting signals from flood alarm, (9) to signal processor/relays (14) and signals from the flood alarm via the signal processor/relays to servo (5), (12) is a display unit, (13) an audio/visual alarm, (15) computer/printer, (16) TV/Thermal imager, (19) time delay switch, manual override and reset, sensor information circuit from signal processor (14).

Referring to the drawings, the automatic inflation control of the buoyancy bags with combined automatic fault analysis and information display comprises a pressure switch (4) on the gas delivery pipe (3) to the flexible bags (8) which isolates the servos (5) which control the gas discharge valves (2). The pressure switch (4) closes the circuit when the pressure in the gas delivery pipe (3) and therefore the buoyancy bags (8) drops below a preset value. This prevents the gas discharge valves (2) from releasing gas into a damaged buoyancy bag (8) or into an undamaged bag immediately opposite on the other side of the vessel, by preventing the opening of either valve (2).

Circuit (20) links pressure sensors on the gas delivery pipes to the signal processor (14) so as to convey information about the pressure existing in each bag.

Additionally, the pressure switch (4) transmits a signal via circuit (18) to the signal processor (14) and to the bridge display unit (12) which illuminates the position of the fault on the bridge display unit. The bridge display unit is shown in detail in Figure 4.

Referring to Figure 1A, this shows the control and sensing circuits which have been simplified by showing the circuits to one pair of oppositely disposed bags (8) and two flood alarms (9).

The flood alarm (9) on sensing presence of water starts the running of the time delay switch (19) via a circuit to the signal processor/relay unit (14). The location of the activated flood alarm (9) is also displayed on the bridge display unit (12), via the sensor information circuit from the signal processor (14).

Additionally, but not necessarily, all items of information passing through the signal processor (12) are recorded by the computer/printer unit (15) to form a permanent printed serviceability record. Advantageously, a flood alarm signal from these units (9) can start TV cameras or Thermal Imaging pictures of the deployment of the buoyancy bags (8) and instantaneously set off the audio/visual alarm (13). Additional information can be displayed on the VDU (16). Each flood alarm station (9) can advantageously, but not necessarily, be made up of three flood alarms grouped together, any two of which will activate the audio/visual alarm (13) but not necessarily start the time delay sequence.This stratagem can also apply to separate flood alarm stations (9) where two flood alarm stations would need to be activated by water before initiation of the automatic inflation sequence started by the time delay switch (19).

As best shown in Figures 1 and 2A, the buoyancy bags are housed in a partially inflated state within housings, which are shown in their closed position at (71). The housings are protected from damage from quayside impact or abrasion by a longitudinal strake (72). The housings are located a short distance above the rubbing strake 72 and include a base plate (70) hingedly attached at one side to the hull. Frangible straps (74) are provided to hold closed the doors or fairings (73) and the base plates (70) of the housings during normal use. However, under the pressure generated on full inflation, the straps break and allow the fairings to be pivoted upwardly and the base plates downwardly as shown in Figure 1. As can be seen in Figures 1 and 2A, in their closed position, the fairings are sloped upwardly towards the hull so as to minimise risk of damage by quayside impact.

It will be appreciated that the fairings (73) preferably slope downwardly in their uninflated condition of the bags so that water is shed from the housings as fast as possible. The base plates should also be apertured so that water does not collect in the housings but drains away. Preferably, the fairings are hinged to the hull by a series of individually separated and spacedout hinges (rather than the 'piano' type hinges), to discourage jamming in the event of distortion.

The buoyancy bags may be manufactured from laminated, fabric-reinforced neoprene rubber of a similar gauge to that used for the skirts of hovercraft. At its open side adjacent to the hull, the buoyancy bags are bolted by means of a frame and gasket in a gas-tight manner to the hull of the vessel. This is shown in Figures 2A, 3D and 3E. The bags (8) when in their partially inflated condition in readiness for deployment are stored in a folded or concertina arrangement. Although several longitudinal folds are shown in Figures 3D and 3E, it is usually sufficient to provide two or three pleats or folds (302) above and below, as shown in dotted lines in Figure 3E. In this way, the bags have a triangular cross-section. This shape avoids the need to introduce a further longitudinal hinge in the fairing as shown in Figure 2A.Straps or bands 101 can be used to return the bags to the deflated condition of after deployment, by pulling on the ends (102). The straps (101) have another function in restraining pulling away from beneath the extended fairings (73), in the inflated condition of the bags. While the straps can pass through the rollers (103), once the T stop (104) at each end reaches the roller, the bags can move no further away from the hull under the force of the inflation gas.

An arrangement for attaching the bags to the hull and connecting the bags in gas-tight manner to the gas delivery pipe is shown in Figures 3A to 3E. A gasket E, e.g. of stainless steel or other non-corrosive metal, is sewn into a sleeve in the face of the bag adjacent the hull. A back plate B is fixed to the hull so as to stand away from the face of the hull A and provide a gap to insulate the bags in the event of a ship's fire. An additional function of the distance pieces is to prevent distortion from the pressure in the inflated bag. Also, for maintenance purposes, the entire bag and back plate can be removed for servicing without removing the fairing.

Sandwiched between the gasket E and the back plate B, a flexible gasket, e.g. of neoprene, can be located to provide a seal when the bolts H are passed through the gasket E and holes in the back plate B and the parts bolted together. The gas pipe J can be conducted through the hull and connected at a collar F in the back plate B.

A preset pressure-limiting valve on the gas delivery pipe inside the hull between the gas cylinder control valve and the point of discharge into the bag would prevent over inflation of the bag. Also, each gas pipe line to the individual bags is preferably equipped with a pressure gauge and a pressure sensitive switch. The purpose of the gauge (which would be sited downstream of the solenoid-controlled inflation valve) would give an individual and independent verification that pressure remained in the pipe and therefore, more importantly, in the bag. The pressure switch (which would monitor the pressure in the bags) would have two functions: 1. To inform the signal processor (by closure of the switch) when the pressure in the bag falls below a predetermined minimum pressure, e.g. 0-25 psi. The gas pipe leading to the bag folded within its housing would normally be precharged with gas at about 0.2 to 1 psi.

If pressure falls below the predetermined minimum, the pressure switch will signal the processor which, in turn, will send information to the display panel on the bridge indicating a fault requiring investigation.

2. The signal processor/logic unit would thus already be informed of a malfunction and would (in the case of emergency deployment) instantly cancel the inflation command of the opposite and/or equivalent bag on the other side of the ship. The inflation valves could be operated by solenoid, electrical actuator, electrohydraulic actuator or hydraulic actuator (any of which would be controlled by a signal from the signal processor/logic unit).

As can be seen best from Figure 2A, the bags are in contact with the water and are restrained above by the abutment surface of the extended fairing (73). Bracing to the fairing is provided by stainless steel strops (105), one end of which is anchored to the hull and the other connected to the fairing. The effect of a line of inflated sponsons on each side of the vessel is to exert a substantial stabilising action to the vessel.

It will be appreciated that with the bags deployed, the ' fairings would form a horizontal, substantially continuous flat platform giving a walkway along the side of the ship which could also provide a vantage point for crew members assisting and supervising lifeboats and life rafts.

In the event that it is desired to test the operation of individual bags without activating the entire system, access to the bags from outside the hull may be provided through inspection plates (106). These give access to external inflation valves (108) and deflation valves (107). Using these valves, individual bags can be tested for proper deployment and gas tightness.

To check a faulty bag in port, an outside gas source would be connected to the inflation valve on the bag itself. After repairs the bag could be deflated and restowed in its fairing. If the problem lay with the servo/valve, it could be replaced either at sea or in port.

If a failure related to remotely operated valves, a light would be illuminated at the relevant bag station (just inside the hull). Its opposite number would also be illuminated to show that its operation had been cancelled. Verification of valve servos can be achieved by running a continuous micro-current through the servo to ensure electrical continuity.

It will be appreciated that the housings and their contents are protected from impact, e.g. with docks and jetties, during normal manoeuvres by the ships rubbing strake (72) which can be extended, if necessary (see Figure 1). Damage to the hull from normal wear and tear, e.g. during berthing, is generally confined to a line running lengthwise of the ship at about 1/2 to 2/3 of the ship's height. Thus, provided the buoyancy bags are not fixed to the extreme stern or bow areas, damage to the housings is unlikely to occur. It may, however, be desirable to fit a deflector to the end housing closest to the bow, particularly where the bow has a flared shape.

In the event of a collision serious enough to rupture the hull at the level of the lower car deck, an impacted bag might fail to rupture, being merely crushed and thus retain its precharge pressure but would, because of the impact, be incapable of deployment. In such a case, it would be important to ensure that there would be a pressure loss so that the equal and opposite bags would receive a signal not to deploy. This would be achieved by placing a small explosive charge (e.g. 12 bore cartridge size) in a sealed pocket on the inside of the bag, which would blow a hole large enough to ensure a rapid loss of pressure. Detonation of the charge could be brought about by the fracture of a wire stretched under tension across the impact point inside the hull. This fracture would be the colliding vessel.This frangible wire would be enclosed in a plastic or metal tube to render it tamper-proof. The electrically forced charge would normally be shorted out by the frangible wire and only when the wire was broken would the charge explode.

The flood alarms could be mechanical float switches or probes which are sensitive to a change in electrical resistance or capacity.

Information from the flood alarms would be transmitted to the signal processor and displayed on the display panel shown in Figure 4, together with information about the condition and status of all the bags. Any indication of flooding on the car deck would be brought to the attention of the Captain by insistent audio and visual alarms. On actuation of a second group of flood alarms, the words " system armed" would appear on the display panel. The system would then be armed and ready to deploy and would set a time delay which could be preset to start inflating the bags after, say, 10 to 60 seconds, say 25 to 30 seconds. The time set would appear on the display panel. If no action were taken within the delay period the system would then automatically deploy. it could, however, be overridden by the Captain/First Officer by starting the delay once more or deploying the bags immediately.

Claims (15)

CLAIMS:

1. A system for augmenting the stability of a ship wherein a plurality of inflatable buoyancy bags are located on both sides of the ship and arranged sequentially and lengthwise of the ship, means for maintaining the bags in a partially inflated state during normal operation of the ship, detection means which detect abnormal leakage from the bags during normal, nonemergency conditions, and operating means for inflating said bags fully on sensing an emergency situation likely to give rise to reduction in the ship's stability.

2. A system as claimed in claim 1 wherein said detecting means are linked to said operating means in such a way so as to prevent inflation of a bag in which a leak is detected as well as a corresponding bag on the other side of the ship, whereby the inflated bags provide for evenly balanced additional buoyancy.

3. A system according to claim 1 or 2 wherein sensing means are arranged to sense the presence of flood water within a designated part of the ship and to deploy the inflatable bags automatically.

4. A system according to claim 3 wherein a time delay device is interposed between the sensing and operating means and the system includes a manual override control so that deployment of the inflatable bags can be aborted in the event that the emergency situation is contained.

5. A system according to claim 3 or 4 wherein the sensing means is linked to a display device on the ship's bridge or other control position so as to maintain an indication of the condition of the bags.

6. A system as claimed in claim 5 wherein the display device comprises a diagrammatic representation of the vessel having visual indicators of the condition of the bags.

7. A system for augmenting the stability of a ship wherein a plurality of elongated bags are stowed lengthwise of the ship within fairings, which are hingedly attached along one edge to the ship's hull, each fairing being pivotable outwardly on inflation of the bag housed within it but being restrained against further movement in its extended position so as to provide an overhead abutment against which the inflated bag can exert a righting movement to the ship.

8. A system as claimed in claim 7 wherein the fairings are restrained against pivoting movement beyond a condition in which the plane in which they lie is substantially at right angles to the ship's hull.

9. A system as claimed in claim 7 or 8 wherein in the deflated condition of the bags, the outer surface of the fairings slope downwardly.

10. A system as claimed in any one of the preceding claims wherein the bags are stored in housings attached to the ship's hull above a rubbing strake located in the region of the water line, the bags being thereby protected from impact damage by said rubbing strake.

11. A system as claimed in claim 10 wherein the rubbing strake is extended outwardly by attachment of an additional member to give greater protection for the bags and the housings.

12. A system as claimed in any one of the preceding claims in which the bags are folded in their non-inflated or partially inflated condition in generally 'concertina' fashion, the folds thereof extending in a direction generally longitudinal of the ship.

13. A system for augmenting the stability of a ship wherein a plurality of inflatable buoyancy bags are located on both sides of the ship and arranged sequentially and lengthwise of the ship, operating means for inflating bags on each side of the ship simultaneously, either manually or automatically, in order to provide enhanced stability in an emergency and detecting means for sensing the operating condition of the bags, said detecting means being linked to said operating means and being arranged to prevent inflation of a bag in which a leak is detected, as well as a corresponding bag on the other side of the ship, whereby the inflated bags provide for evenly balanced additional buoyancy.

14. A method of augmenting the stability of a ship which comprises providing inflatable buoyancy bags along both sides of the ship in the region of the water-line, maintaining the bags in a partially inflated state during normal operation of the ship so that leakage can be detected during normal, non-emergency conditions, and inflating said bags fully on sensing an emergency situation likely to give rise to reduction in the ship's stability.

15. A method according to claim 14 wherein the bags are stowed in their normal, partially inflated state in housings extending lengthwise of the ship, full inflation of the bags causing the housings to open to deploy the inflated bags.