Such an object is known as a marine vessel modified according to US patent 4,193, 367. In that patent specification, the outside skin of the vessel is covered with pre-stressed membranes, such as fiber glass plates, destined to prevent or at least curb transmission of a shock wave in the water, resulting from an underwater explosion, to the heart of a vessel.

The membranes have as a disadvantage that the construction is rather vulnerable in use, that it is complicated and can easily spring a leak as a result of which a water mass can penetrate between the membranes and the vessel, which adversely affects the boating properties of the vessel. Moreover, due to the leakage, the protection against shock may be reduced or be undone.

In addition, adjacent the edges of the membranes, a relatively large surface will remain that continues to be vulnerable to impact of the shock wave. At this location, the shock will indeed be partially transmitted to the vessel.

The object of the invention is to provide an improvement of such a structure, which is relatively simple and inexpensive in manufacture and maintenance, and which does not adversely affect the boating properties.

This object is achieved by providing an object with a structure as mentioned in the opening paragraph, while the members have been manufactured from a layer of elastic material in which, at least in a side remote from the object, gas-filled spaces are present, such that the members are compressible over a distance of at least one water displacement amplitude resulting from an underwater shock.

Due to such a structure, a protective layer is formed around the vessel, so that a pressure wave resulting from an underwater explosion cannot

reach the vessel. As the protective layer has a very low acoustic impedance, the pressure wave is reflected back into the water as a reflection wave. The energy of the shock wave is then absorbed in the water, in a manner similar to that near the water surface, in that, as a result of this reflection, bulk-cavitation occurs. Consequently, there is hardly any transmission of the energy of the underwater shock wave, so that in the vessel hardly any additional shock resisting measures need be taken as long as the path of displacement of the pressure wave does not exceed the maximum displacement distance of the compressible material. This has as an advantage that the inside construction of the vessel needs to be far less shock resistant than in conventional designs for protection against underwater shock. Further, the invention can be utilized without making use of complex mechanical constructions, which are expensive in maintenance and can easily become defective.

This reflection phenomenon, for that matter, is of a totally different nature than curbing a collision between a vessel and water waves slamming against the vessel. Constructions are known, as, for instance, from US patent 3,960, 100, wherein the impact energy is stored as resilient energy in gas chambers, and is transmitted to the vessel in a retarded manner, i. e. with lower peak forces. As the maximum water displacement amplitude resulting from underwater shock is relatively small (as a rule, this is approximately 6-10 cm), relatively limited thickness dimensions of the material will suffice, so that a very robust and easily applicable structure is obtained.

In a preferred embodiment, the spaces are connected to gas pressure regulators. In particular in submarines, which operate at greater depths, due to the prevailing water pressure, the volume of the chambers would become too small if not adjusted, so that the pressure wave has a path of displacement which will be greater than the chamber diameter or the chamber height.

Regulating the amount (=mass) and the pressure of the gas in the chamber can prevent the occurrence of"bottoming", i. e. resilient material colliding internally, whereby the shock wave can no longer reflect but slams into the

vessel Further, the regulators have as an advantage that small leakages, due to, for instance, local leakages or diffusion of the gas from the chambers, can be eliminated.

In a further preferred embodiment, the spaces are connected to a gas buffer member. Such a member prevents gas pressures from rising too high in the chambers, in that, during compression, a portion of the gas can flow away into a buffer. As a result, the chambers can be designed to be relatively small.

Further, it is advantageous when the specific gravity of the compressible material has a gradient having, at an outside remote from the object, a relatively low specific gravity, and at the inside, a relatively higher specific gravity. Due to a low specific gravity at the outside, the impedance difference between the surrounding water and the shock protection structure is increased, as a result of which a better reflection of the shock wave occurs.

Due to an increased specific gravity towards the inside, the structure can be of robust design, while shock absorbing properties of the compressible material itself can be optimally utilized.

In order to improve the boating properties and the mechanical strength, a relatively thin, rigid plate can be provided on an outside of the compressible material. This plate can be made of steel, aluminum, fiber glass or a different, relatively light material which, as long as its thickness is not too great, remains relatively transparent to the shock wave.

In a preferred embodiment, the gas-filled compressible material is manufactured from a substantially homogenous, impermeable foam of a thickness of approximately 10-20 cm.

An alternative preferred embodiment consists of the gas-filled compressible material comprising flexible tubes welded together over a longitudinal side, which tubes have been brought to a predetermined gas pressure, which tubes have a diameter of a thickness of approximately 10-20 cm. Preferably, the tubes are disposed continuously next to and along

each other, thereby covering the surface of the vessel to be protected, preferably completely.

The invention also relates to a structure for protecting an object against impact of an underwater shock according to one of the above- mentioned aspects.

The invention will be further elucidated with reference to the Figure. In the Figure: Fig. 1 shows a schematic representation in cross section of a marine vessel, such as, for instance, a frigate, provided with a protective structure according to the invention; Fig. 2 shows a schematic representation in cross section of a submarine, provided with a protective structure according to the invention; Fig. 3 shows a detail of a protective structure according to a first embodiment; Fig. 4 shows a detail of a protective structure according to a second embodiment; and Fig. 5 shows a schematic representation of a protective structure according to a third embodiment.

In the Figure, the same or corresponding parts are indicated with the same reference numerals.

In Fig. 1, a schematic representation is shown of a marine vessel 1 which is exposed to an underwater explosion 2. Due to the explosion 2, a shock wave 3 (pressure wave) is generated moving radially outwards as a spherical front at the speed of sound in water. When. the shock wave arrives at the water surface 4, the shock is negatively reflected and returns as a reflection wave 5.

"Bulk cavitation"occurs, as a result of which cavitation bubbles 6 can form.

The shock wave 3 is characterized by a step-wise displacement of the water over a distance s. The size of the displacement depends on the intensity of the explosion and the distance to the explosion and is in the order of, at

most, 6 cm. During the occurrence of this shock displacement, momentaneous peak pressures can occur in the order of 100 bars or more.

When the shock wave arrives at the marine vessel 1, without the use of the protective measures according to the invention, a part of the shock wave 3 is introduced into the structure of the ship. Shock transmission takes place and the ship is shock-loaded, which leads to extremely large local accelerations. As a rule, the displacement s in the ship is then in the same order as the water displacement and is therefore also at most 6 cm. As a result of the shock, equipment on board of the vessel can become defective.

Furthermore, the outside skin of the vessel may become damaged. Especially with less ductile materials, this is an actual threat (fiber glass reinforced polyester mine combating vessels, aluminum vessels).

According to the invention, on the outside skin of the marine vessel a resilient, gas-filled plastic or rubber-like material 7 has been provided which is compressible over a distance of at least twice a water displacement amplitude resulting from underwater shock. What is achieved by using gas- filled plastic or rubber-like material is that the compressible properties are improved due to the presence of gas in the material, so that a displacement of a magnitude of twice the shock displacement can easily be performed, without entailing the risk of"bottoming". In this last case, the material impacts internally, giving rise to a high internal pressure in the gas and material.

On the other hand, as the structure has a relatively low density, the acoustic impedance of the surrounding water towards the plastic or rubber-like material exhibits a sharp discontinuity. The fact is that the acoustic impedance of this gas damper, defined as the product of the density p and the speed of sound c of the gas (nitrogen or air etc. ) is considerably less (pc=1. 25*330=412.5) than that of water (oc=1000*1500=1, 500,00).

As a result, the damper behaves as air, so that at the buffer, bulk cavitation occurs. At the location of the damper, the shock is negatively

reflected and returns into the water as a reflection wave, so that the shock is (virtually) not introduced into the vessel.

A displacement shock of water only occurs over approximately 6-12 cm. This displacement is absorbed without problems by the shock damper (foam or air bed).

Through the use of a shock damper with sufficient resilience path (more than 12 cm), and a sufficiently low rigidity, the force transmitted by the damper to the ship's skin is very strongly reduced.

In Fig. 2, a schematic representation of a submarine 8 is shown.

Around the submarine, air chambers 9 have been provided, the amount of gas (or mass of gas) of which in the chamber can be regulated. By adding or blowing off gas during change of the diving depth, the height of the air chamber can remain virtually constant, while the pressure of the chambers remains virtually equal to the actual ambient pressure of the water at the respective diving depth. The height of the chambers 9 remains almost constant, so that also at a greater depth under water, the shock resistance remains at the proper level and the buoyancy is not adversely affected.

By means of a duct system 11, a gas-buffer member 10 is connected to the air chambers 9. The gas-buffer member is connected to a compressor and/or a gas bottle with gas at high pressure 12, for regulating the gas pressure and/or the gas volume of the chamber. In addition thereto or as an alternative, use can be made of a blow-off valve, which opens at a maximally acceptable pressure load.

In Fig. 3 it is represented how, by means of flexible plastic hoses or an air mattress 13, a vessel skin 14 can be covered. The hoses have been welded together over a longitudinal side, and have been brought at a predetermined gas-pressure. The hoses can have a diameter of approximately 12-20 cm. The hoses 13 are closed at one end, and, at another end, can also be closed or connected to a compressor/gas bottle (not shown).

In Fig. 4 it is schematically represented how in a preferred embodiment the structure for protection against underwater shock has been built up. The gas-filled compressible material is manufactured from a substantially homogeneous, impermeable foam 15 with a thickness of approximately 10-20 cm. On the foam 15, for protection or for a lower boating resistance, on an outer side of the compressible material, a relatively thin, rigid plate 16 has been provided from, for instance, steel, aluminum, plastic.

The thickness of the protective layer must be as thin as possible; preferably less than 10 mm.

The specific gravity of the compressible material has a gradient with a relatively very low specific gravity on the outside 17 and a relatively higher specific gravity on the inside 18. The whole is attached to a steel vessel skin 14, for instance by means of gluing or vulcanization.

In Fig. 5, schematically, it is represented how a vessel skin 14, such as for instance a skin of a frigate or a submarine, can be covered with a structure for protection against underwater shock according to the invention.

The structure consists of laminated elements 19 which can have a standard size of, for instance,'1 by 1 meter. The elements are provided close together on a vessel skin 14, by means of bolt connections, gluing/welding and/or magnetic coupling (not shown). This last variant offers the possibility of temporarily protecting a vessel against underwater explosions, by magnetically fixing the elements on the vessel skin.

The elements consist of a layer of foam rubber 15 with a thickness of approximately 25 cm, which is covered on both sides with relatively thin, rigid plates 16 of steel, aluminum and/or fiber glass.

The invention is not limited to the preferred embodiment represented in the drawing but may contain all sorts of variations thereon. For instance, in a protective structure, use can be made of a combination of inflatable elements with foam rubber elements, allowing the impact resistance of a vessel to be temporarily increased. Also, in the protecting elements, reinforced sections can be present for increasing the self supporting capacity of the elements. Such variants are all understood to fall within the scope of the invention as defined in the following claims.