GB2077207A - Stabiliser for Inflatable Marine Craft - Google Patents

Abstract

An inflatable marine craft, such as a liferaft (11), is provided with a stabilising means which comprises one or more duct means (16). A surface of the duct means is so inclined to the bottom (12) of the craft that the cross-section of the duct means decreases toward the periphery of the liferaft, whereby water flowing through the duct means exerts on the surface a downward stabilising force. <IMAGE>

Description

SPECIFICATION Stabiliser For Inflatable Marine Craft The present invention relates to a stabiliser for a marine craft which is at least partially inflatable, such as a liferaft. Hereinafter the term inflatable is used to indicate those craft which are of shallow draft and are buoyant by virtue of one or more gas-filled bags, generally in the form of tubes surrounding an occupying space.

It is well known that inflatable Iiferafts constructed with a floor surrounded by an inflatable buoyancy tube and provided with a closable canopy to protect occupants of the liferaft can readily become unstable in high winds and overturn. To provide for improved stability of such liferafts, it has hitherto been proposed to provide a plurality of water pockets extending in spaced relation around the periphery of the liferaft on the underside thereof below the waterline so that when the liferaft is floating on water the pockets fill with water through upper filling openings formed in the pockets. When the liferaft tilts in relation to the waterline the mass of entrapped water in the pockets is raised above the surrounding water level and exerts an effective restoring moment on the liferaft.

An enhancement of stability has also been achieved by the provision of a single large water pocket disposed below the floor of the liferaft, often in a central location.

While the use of stabilising water pockets as above described has been found to be generally satisfactory, there is the disadvantage that the pockets are essentially closed apart from the filling openings and need to be emptied through the filling openings when the liferaft is removed from the water and stowed.

It is furthermore weil known to provide a sea anchor for retarding the movement of a marine craft through the water and keeping its head to the wind. A sea anchor in common use may take the form of a collapsible frusto-conical shell made from flexible sheet material. The shell is attached by a line to one end of the craft so that relative movement between the craft and the water causes a flow of water into the larger of its two open ends and out of the smaller open end, thereby producing a pull on the line effective to retard the craft and keep it aligned to the wind.

Such sea anchors have again been found generally satisfactory, but suffer from the possible disadvantage that the shell or line can become snagged or tangled and on occasion may cause the sea anchor to become ineffective.

Furthermore it is well known that liferafts are most vulnerable to overturning before being boarded, and sea anchors have the further disadvantage that they are not commonly deployed until after boarding.

It is an object of the present invention to provide, on an inflatable marine craft, stabilising means which can be used both to replace or supplement the water pockets hereinbefore referred to and/or to replace or supplement a sea anchor such as hereinbefore referred to.

According to the present invention, there is provided an inflatable marine craft including stabilising means mounted to the underside of the bottom of the craft in a position below the waterline and comprising duct means, the cross section of which decreases from a first open end to a second open end disposed outwardly thereof, a surface of said duct means being so inclined to the bottom of the craft or so arranged as in use to take up an inclined position in relation to the bottom of the craft that during movement of the craft through the water the water exerts on the surface a downward stabilising force which opposes lifting of the craft in the water.

In a preferred embodiment of the invention, the stabilising means is such that the surface in use maintains a predetermined inclined position in relation to the bottom of the craft, whereby the downward stabilising force increases with increase in speed of the craft through the water.

It will be appreciated that where the inflatable craft takes the form of a lightweight liferaft of shallow draft and is provided with a protective canopy, an increase in wind velocity will produce a marked increase in the drift speed of the liferaft over the water and an increased tendency for the liferaft to overturn. With stabilising means in accordance with the preferred embodiment of the invention, the downward stabilising force produced thereby increases with drift speed and thus with wind speed and can be arranged to compensate for the increased tendency for the liferaft to overturn. Tests have indicated that a liferaft stabilised in accordance with the present invention may actually be more stable at a wind speed of 70 knots than it would be at lower speeds.By way of comparison, an unstabilised liferaft of otherwise similar design becomes liable to overturn at a wind speed in the region of 40 knots.

The inflatable craft may comprise a liferaft consisting of a floor surrounded by a peripheral inflatable buoyancy tube and the stabilising means may comprise a duct means formed by the floor and a flexible sheet secured at its longitudinal edges to the underneath surface of the floor. The duct is located in the region of the periphery of the floor, extends inwardly toward the centre of the liferaft and is provided with an inner open end which is larger than the outer open end, with the sheet between the inner and outer ends forming an open-ended half-shell of reducing cross-section and providing a surface which is inclined to the floor of the liferaft and against which water flowing through the duct impinges to produce the downward stabilising force on the liferaft.

For liferafts of substantially circular planform, a stabilising duct according to the invention may be provided at each of a plurality of positions spaced around the periphery of the floor of the liferaft and may be collapsible so that only those or that to windward distend and become effective while those at other positions round the liferaft collapse and become ineffective.

The stabilising ducts may be used alone to provide the necessary downward stabilising forces on the liferaft or may be used together with water pockets of the kind hereinbefore referred to, which may, for example, be interposed between the stabilising ducts, or may be embodied in close conjunction with the ducts.

The downward stabilising force produced by water pockets as hereinbefore referred to is obtained from the mass of water within the pockets under the action of either local upward acceleration while beneath the water surface or lifting above the waterline when the liferaft is subjected to ovcrturning moments under either wind or wave action. The stabilising duct according to the invention behaves in a similar way by providing a stabilising inertia force under the action of upward acceleration, though is generally less effective, size for size, under steady conditions above the waterline.

Embodiments of the invention will now be more particularly described by way of example and with reference to the accompanying drawings, in which: Fig. 1 is a schematic side elevation of an inflatable liferaft according to a first embodiment of the invention; Fig. 2 is a perspective view from below of stabilising means employed with the liferaft shown in Fig. 1; Fig. 3 is an end elevation of the stabilising means shown in Fig. 2; Fig. 4 is a plan view from below of the stabilising means shown in Fig. 2; Fig. 5 is a plan view from below of a liferaft according to a second embodiment of the invention, showing a plurality of stabilising duct means; Fig. 6 is a schematic side elevation of a stabilising duct distended and effective to produce a downward stabilising force; and Fig. 7 is a schematic side elevation of the stabilising duct shown in Fig. 6 in a collapsed condition and ineffective for stabilising purposes.

Referring now to the drawings, in particular to Fig. 1, an inflatable liferaft 11 of circular planform comprises a floor 12 secured at its periphery to a lower buoyancy tube 13 which has secured thereon a further buoyancy tube 14 which together with the tube 13 form the side of the liferaft. The upper buoyancy tube 14 has secured along its periphery a canopy 1 5 which is supported by inflatable arch tubes (not shown).

The floor 12 may be of multilayer construction to provide thermal insulation and strength and may if desired be of inflatable form. A single stabilising duct means 1 6 is formed on the underneath side of the floor 12 in the disposition shown.

As best seen in Figs. 2 to 4, the stabilising duct means 16 is formed by the floor 12 and a flexible sheet 1 7 secured at its longitudinal edges 1 8 and 1 9 to the underneath surface of the floor 12. The duct means 1 6 is located in the region of the periphery of the floor 12 as shown in the Figures, extends inwardly and is provided with an inner open end 20 which is larger than its outer open end 21. The sheet 1 7 between the inner and outer ends 20 and 21 forms a flattened half shell, which provides a surface 22 inclined to the floor 12 of the liferaft.

In the preferred embodiment, the duct means 1 6 comprises three ducts, separated by webs 23 and 24, with the additional space between the ducts and the longitudinal edges 18 and 19 occupied by fairly conventional water pockets 25 and 26. The surface 22 is held by webs 23 and 24 and by the water pockets 25 and 26 in a substantially planar configuration angled to the floor 1 2 and the force exerted on the surface 22 is transmitted to the liferaft 11 by the webs and by the surfaces bounding the water pockets 25 and 26. The water pockets, which are of course optional, operate conventionally, filling and emptying through apertures 27.

When the liferaft is drifting on the water under the force of a wind, there is caused a flow of water through the duct means 1 6. The water flowing through the duct means 16 impinges on the inclined surface 22 of the sheet 17, producing a downward stabilising force. This downwardly directed stabilising force is a function of the drift speed of the liferaft 11 and increases with it. The drift speed is moreover a function of wind velocity and the downward stabilising force therefore increases with increasing wind velocity. As a result, the increasing tendency for the liferaft 11 to overturn with increasing wind strength is opposed by an increasing downward stabilising force.

In addition to the downward stabilising force, the stabilising duct means 16 by virtue of its reducing cross section from the inlet end 20 to the outlet end 21 produces a retarding force on the liferaft in the same manner as that of the conventional sea anchor hereinbefore referred to.

However unlike conventional sea anchors, the stabilising duct means embodying the present invention is automatically and immediately self deploying and furthermore it can not suffer from any enagging problems such as may be associated with sea anchors connected to a liferaft by a line.

In addition to the above, it has been found that the stabilising duct means acts in the manner of a fixed rudder, and remains at the windward extremity of the liferaft and imparts directional stability thereto.

It can be shown that in addition to the downward stabilising force produced by the surface 22 and the retarding force produced as a result of the reducing cross-section of the duct 16, together with the above-mentioned rudder effect the duct 16 also opposes upward movement of the liferaft by virtue of an effectively entrapped water mass.

The entrapped water mass below the waterline provides a stabilising inertia force opposing upward acceleration of the liferaft. It furthermore momentarily opposes lifting of the liferaft above the waterline until the entrapped mass is lost by flowing out of one or both of the open ends 20 and 21.

The liferaft shown in Fig. 1 is of circular planform. On such a liferaft, a plurality of ducts 16 may be arranged either regularly or irregularly around the periphery of the floor 12. Such an arrangement with five equi-angularly spaced ducts 1 6 is shown in Fig. 5. With this arrangement, the ducts may be so constructed as to be collapsible, so that when- such a liferaft drifts, only the duct means 16 to windward of centre distend as shown in Fig. 6 and create the downward stabilising and retarding forces hereinbefore described. The duct means 16 to leeward of the centre, on the other hand, collapse under the flow of water over them as illustrated in Fig. 7 and are ineffective for stabilising and retarding the liferaft.

It would of course be possible to provide a craft of non-circular planform with one or more stabilising duct means in a manner similar to that described for a substantially circular craft.

A craft of circular or other planform may additionally be kept aligned with the wind and further retarded by a sea anchor attached by a line to the craft in the hitherto conventional manner.

In yet another embodiment of the invention, the duct means employed with the craft may be replaced by a single large stabilising duct having no webs nor any water pockets.

Of course, while the duct means have been described as having first and second open ends, the term open, particularly when applied to the second end, includes within its meaning an end or ends being partially closed or even completely closed by a porous or apertured membrane, provided that water can flow through the membrane sufficiently to exert a force on surface 22.

Claims (10)

Claims

1. An inflatable marine craft including stabilising means mounted to the underside of the bottom of the craft in a position below the waterline and comprising duct means, the cross section of which decreases from a first open end to a second open end disposed outwardly thereof, a surface of said duct means being so inclined to the bottom of the craft or so arranged as in use to take up an inclined position in relation to the bottom of the craft that during movement of the craft through the water the water exerts on the surface a downward stabilising force which opposes lifting of the craft in the water.

2. A craft as claimed in claim 1, wherein the stabilising means is such that the surface in use maintains a predetermined inclined position in relation to the bottom of the craft, whereby the downward stabilising force increases with increase in speed of the craft through the water.

3. A craft as claimed in either claim 1 or claim 2, wherein the stabilising means comprises a plurality of adjacently located stabilising ducts.

4. A craft as claimed in claim 3, wherein the plurality of adjacently located ducts are parallel one to another.

5. A craft as claimed in either claim 3 or claim 4, wherein the plurality of adjacently located ducts are bounded on at least one side by a water pocket.

6. A craft asclaimed in any one of the preceding claims, wherein the duct means is formed by the bottom of the craft and a flexible sheet which is secured at its longitudinal edges to the bottom of the craft in such an arrangement that the sheet between the first and second open ends forms an open-ended half-shell of reducing cross-section and provides said surface against which water flowing through the duct exerts a downward stabilising force.

7. A craft as claimed in any one of the preceding claims, wherein a plurality of said duct means are arranged in positions spaced around the periphery of the floor of the craft.

8. A craft as claimed in claim 7, wherein the duct means are collapsible and so arranged that only that or those to windward distend by passage of water through them and thereby become effective, while that or those to leeward collapse and become ineffective.

9. A craft as claimed in any one of the preceding claims, the craft being additionally provided with a sea anchor.

10. An inflatable marine craft substantially as hereinbefore described with reference to Figs. 1 to 4, 6 and 7, or 5 to 7 of the accompanying drawings.