Boat hull comprising two composite materials

The invention relates to a boat or ship's hull comprising two composite mate ^¬ rials with a transition between one of the constituent composites to the other constituent composite.

The Swedish corvette Visby is built of a conventional composite sandwich material consisting of a carbon fibre reinforced polymer laminate. This mate ^¬ rial gives the ship considerable strength and shock resistance despite its low weight. The composite material enables the use of stealth technology, which minimizes the magnetic signature of the corvette and makes radar detection more difficult.

Laminated composite materials of the conventional sandwich type consist of two outer sides made of a fibre reinforced polymer, where the fibre can be of a kind which is known for such use in the art, for example glass fibre, aramid fibre or carbon fibre, with an intermediate cell core of a foam-plastic material, such as Divinycell®. The foam-plastic material has a low density and thus reduces the weight of the hull material while still providing sufficient strength for it to be used in a ship's hull.

US 4,719,871 relates to a ship with a unitary-construction single-shell hull made of a composite material, which consists of a glass fibre reinforced polymer. The thickness of the hull increases towards the keel so as to im ^¬ prove its ability to withstand the shocks caused by mines being detonated in the vicinity of the ship.

Smaller boats need armour that protects against small-bore projectiles, which hit the sides and deck of the ship. The traditional way of protecting ships against small-bore projectiles is to reinforce the inside of the hull, often by attaching a thick metal plate or a liner of, for example, aramid fibre to the in ^¬ side of the hull. This makes the boat heavy and reduces its remaining payload capacity. To lower the weight of such boats, composite materials are used in their hulls too. A lower weight of the hull means better possibilities of using temporary armour to protect against hostile fire depending on the current level of threat.

RECORD COPY-TRANSLATION (Rule 12,4)

Modular supplementary armour is used to reinforce the external armour of a vehicle. Such armour may be in the form of ceramic, aluminium or composite plates, which are attached to the outside of the vehicle. US 6,082,240 dis- closes a mounting system for modular supplementary armour. The mounting system consists of two resilient strips, which secure the supplementary armour panels by clamping them to the vehicle. The system enables easy replacement or removal of the supplementary armour of the vehicle depending on the conditions of threat.

Vacuum injection technique is used for the construction of fibre reinforced polymer structures. US 4,902,215 and US 5,958,325 disclose a technique for vacuum injection (the SCRIMP® methods, where a mould is coated with a fibre (glass fibre, aramid fibre, carbon fibre, etc.). The mould is then covered with a plastic sheet, such that a tight seal is obtained between the plastic sheet and the mould, whereupon a vacuum pump is connected to the sheet- covered mould. All the air is pumped out of the mould and a polymer mixture, a matrix, is then allowed to be sucked into the mould by means of the vacuum formed. The polymer matrix fills up all the cavities of the mould, even the space around added fibres. The mould is then cured in an oven for a number of hours. After the mould and the plastic sheet have been removed from the fibre reinforced polymer structure, said structure is ready for use.

US 6,048,488 discloses a method of manufacturing a composite material by using a Co-injection Resin Transfer Molding (the CIRTM method). The manufactured composite material consists of two or more different matrices, without the formation of separable layers, and the method is a variation of the SCRIMP® method. The different selected fibres are prepared in the mould and a thin separation layer is placed between the fibres. The matrices are simultaneously injected into the respective selected fibre and the mould is then cured in an oven in the manner of the SCRIMP® method.

The object of the invention is to provide an improved boat hull, where the hull is made up of two different composite materials, and the transition between the two different composite materials occurring in the form of an overlap between end portions. The combination of composite materials in the boat hull

according to the invention provides excellent ballistic protection, high strength and a reduced total weight of the boat.

This object is achieved by a hull having a composite material combination as defined by the appended independent claim.

According to the invention, a boat hull is suggested which has a transition between a first and a second composite material. The first composite material consists of a conventional sandwich material having two outer sides that en- close an inner part and forms the part of the hull that, preferably, is located below the waterline of the boat; the second composite material consists of a single-shell oversized with respect to strength and forms the sides of the hull, which preferably are located above the waterline of the boat.

The waterline of the boat is defined as the borderline on the hull between the part that is located above and the part that is located below the water surface in the unloaded state of the boat.

The single-shell is manufactured with a thickness from three to nine times that which is normally needed for a hull which does not require excellent ballistic protection. By increasing the strength of the hull in this way, in addition to excellent ballistic protection, fewer frame ribs are also needed in the boat, which in turn increases the inner volume of the hull. Moreover, the oversized single- shell forming the sides of the hull helps to increase the shock resistance of the hull, thus allowing the boat to withstand reckless handling, for example when engaging the edge of a quay, without being damaged.

The transition between the first and the second composite of the boat hull according to the invention occurs by the first composite material terminating in a wedge shape, the second composite material extending a certain distance onto the termination of the first composite material on the outer sides thereof. In the second composite material fasteners may be fixed by casting on the side adapted to form the outside, so as to enable mounting of modular supplementary armour.

The conventional sandwich material is made up of a first and a second side of a fibre-reinforced polymer which enclose a cell core of foam-plastic material.

The oversized single-shell is made of a fibre-reinforced polymer, the fibre being preferably glass fibre and/or aramid fibre. These fibres give the single- shell high strength and excellent ballistic protection against bullets, shrapnel and shaped charges. The polymer of the oversized single-shell is preferably a matrix of polyvinyl ester. A phenol-based matrix can be used on the side of the single-shell that forms the inside of the hull in order to improve the fire resistance. By applying a thin layer of a semiconductive layer, preferably of carbon fibre, on the side of the single-shell that forms the outside of the hull, the radar signature of the boat will be improved, for example making it more difficult to detect.

In the boat hull according to the invention, the oversized single-shell preferably has a thickness from 15 mm to 50 mm. The conventional sandwich material preferably has a thickness from 30 mm to 60 mm.

The invention will now be described in more detail with reference to the accompanying figures.

Fig. 1 shows the boat hull with the first composite material forming the part of the hull that is located below the waterline and the second composite material forming the sides of the hull that are located above the waterline;

Fig. 2 is an enlarged view of the transition between the first composite material and the second composite material of the boat hull according to the invention.

According to a preferred embodiment, a boat hull is provided which has a structure as shown in Fig. 1 and which is manufactured according to the SCRIMP® method. The sandwich material (1 , 2, 3) forms the hull below the waterline (7) and consists of two outer sides (1 , 2) of carbon fibre reinforced polymer with a thickness of 3 mm, which enclose an intermediate cell core (3) of Divinycell® with a thickness of 50 mm. The single-shell forms the sides of the hull above the waterline (7) and consists of glass fibre reinforced polyvinyl ester with a thickness of 40 mm. The transition (5) between the sandwich materials is achieved by the first sandwich material (1 , 2, 3) terminating in a wedge shape, the single-shell (4) extending a certain distance onto the termi-

nation of the sandwich material on the outer sides (1 , 2) thereof . Aluminium fasteners (6) have been fixed by casting in the outer sides of the single-shell (4) so as to enable the mounting of modular supplementary armour. An enlargement of the transition (5) between the materials can be seen in Fig. 2.

According to a further preferred embodiment, a boat hull is suggested having a single-shell which comprises a phenol-based matrix oriented towards the interior of the boat and which is manufactured according to the CIRTM method, adapted for the SCRIMP® method. The phenol-based matrix consti- tutes a third of the thickness of the single-shell and improves the fire resistance of the boat. On the side of the single-shell that forms the outside, a 0.8 mm layer of semiconductive carbon fibre is applied during manufacture. The semiconductive carbon fibre material changes the signature of the boat, which reduces the risk of the boat being detected by radar.