GB2059305A - Forming hollow bodies - Google Patents

Abstract

A method of forming a hollow body, for example a boat hull, comprises sealing together sheet members to define a chamber, pressurising the chamber to cause the sheet members to deform into a predetermined shape and removing the fluid from the deformed chamber. The method can be used to form boat hulls of, for example, steel or aluminium, in which case two of the sheet members form the hull. <IMAGE>

Description

SPECIFICATION Method of fabrication This invention relates to a method of fabrication.

Fabrication of large shaped structure of metal, for example metal boat hulls, has previously been performed by preshaping metal plates and welding or rivetting them individually into place to build up the structure in a gradual manner. The amount of work involved on this has been considerable, and a large number of plates has been used.

According to the present invention there is provided a method of forming a hollow body, comprising sealing sheet members together to define a chamber having an inlet for pressurised fluid, supplying pressurised fluid to the chamber through the inlet to deform the chamber into a desired shape, and removing the fluid from the deformed chamber, the sheet members being such as to retain their deformed shape.

The general method of the invention involves cutting the sheet material to shapes which are flat developments of the required shapes and, when deforming, providing controlled feed of sheet material into the areas requiring maximum fullness. The areas required to stay relatively flat are constrained.

Differential fullness across a section is achieved during deformation by feeding to one side without provision for flow across the sheet and by using longitudinal stresses to contain expansion.

The use of longitudinal stress may also be used to produce saddle sections.

The sheet members preferably are sufficiently rigid to retain their shape in the absence of the pressurisation. The method is of especial benefit in producing bodies of rigid "flowable" material such as metal, so that the cold deformation causes the metal to "flow" into the desired shape. Steel and aluminium are particularly effective.

The method of the invention can be used in forming boat hulls, in whih case the sheet members may be provided one to form each side of the hull and one to form a deck which may, following expansion of the chamber, be removed to leave the open hull. The hull can then receive a permanent deck and fittings to form the finished boat, as required. Internal or external frame or reinforcement members may be provided to control the expansion of the chamber and thereby to restrict the deformation of the sheet members to a predetermined shape. In boat hull fabrication such members may be provided at the bow and stern to ensure that the desired profile is attained; furthermore, a keel may be provided for the hull sufficiently strong as not to be deformed from its original shape during the pressurisation.For the best practical effect to be obtained in the fabrication of boat hulls by the method of this invention it is preferably that a central reinforcement member which does not deform during the pressurisation be provided to run from bow to stern thereby to ensure correct alignment of the hull. This reinforcement member is preferably of triangular prismatic profile.

It has been found that boat hulls can best be formed by the present method by providing a temporary deck which extends across the hull side members and is secured and sealed thereto. The side edge portions of the deck can be fixed to incline upwardlyfromthe main longitudinal axis of the hull at an angle of about 14.5". Alternatively a hinged flap arrangement can be located at the side edge portion of the deck, the hingeing movement being control led, for example, by hydraulic rams.

Where the required shape is oblong then the flaps may feed into the long side or sides so that the longitudinal expansion is entirely deformation or involves only a relatively small feed provision.

If the hydraulic pressure acting on the flaps is not enough to keep the sheet in tension then this can be corrected by the hydraulic rams. This will particular ly be the case in an area where the flaps are narrow.

The structure may be wholly or partly assembled before forming. In the case of steel or aluminium alloy, welding will be the most convenient method of sealing sections together. The weld must be sound in order to withstand the deforming forces, but, since the weld fillet is normally larger than the sheet thickness and the periphery of the sheet itself does not deform, weld failure is not a serious problem.

The sheet member itself should preferably be in a soft annealed state before forming.

The material used for performing the method of this invention can be selected according to the intended function of the fabrication, but generally metals such as steel and aluminum lend themselves best to the reqirements of the method. The specification of the material depends on the required final strength of the body; boat hulls can be fabricated from sheet members of 2mm mild steel plate.

This invention is also a hollow body whenever fabricated by the present method.

Embodiments of this invention will now be described by way of example with reference to the accompanying drawings, in which: Figure 1 is a schematic end section through an arrangement of interconnected members prior to supply of pressurised fluid to form a double ended boat hull by the method ofthe invention; Figure 2 corresponds to Figure 1, after supply of pressurised fluid; Figure 3 is a schematic side elevation corresponding to Figure 2; Figure 4 is a schematic plan view corresponding to Figure 3; and Figures 5 to 8 correspond to Figures 1 to 4 respectively, for a transome stern boat hull, with the hydraulic rams and reinforcement members omitted.

Referring now to Figures 1 to 4, a 15 foot long boat hull is fabricated as follows. A frame is constructed of a triangular prismatic steel backbone 1, a temporary steel deck 2 and four fore and aft generally triangular steel plates 3. The backbone can be made from 1 inch thick steel plate, although it may alternatively be of box section, and extends along the length of the frame. The deck 2 is of 1 inch thick steel plate and has a pair of flaps 5 hinged to it at 5A along its sides. Hydraulic rams 6 extend between the flaps 5 and the backbone 1. The deck 2 has a high-pressure valve (not shown) located in an aperture at the aft end.

The backbone 1, deck 2 and plates 3 are welded together.

Side hull members 7 are cut out of 2mm thick steel plates, being shaped according to a preformed template, and similarly a 1 inch thick steel keel 8 is provided, although the keel may be made from box section steel or may be fabricated from two or more parts; for example, a stern, main keel, stern post and stern pipe. The keel 8 fits into retaining recesses on the backbone 1 and is clamped along its length to the frame by a hydraulic clamp (not shown) on top of the frame and clamped in position. The side members 7 were originally planar but during assembly they are bentfore-and-aft in single curvature into rounded form to meet the end portions of the keel 8.

The side members 7 are welded permanentlyto the keel 1 with a fillet at least double sheet thickness, and welded or clamped to the flaps 5 in preset position. This forms an enclosed fluid-tight chamber bounded by the keel 8, side members 7 and deck 2 and flaps 5 communicating with the outside through the high-pressure valve.

Initial pressure is now taken up on the flap rams 6.

These rams 6 are commoned transversally but not longitudinally, i.e. both forward rams 6A have common pump and bleed systems and similarly the aft rams 6B are commoned. The pump and bleed systems are disposed outside the structure. Water is then pumped into the chamber by hydraulic pump and the air is bled from the top by means of a tube 9 through the stern tube of the keel 8.

The air bleed tube 9 is plugged when all air has been driven out and pressurisation begins.

The pressurisation is preferably quite slow; during this time (15 - 30 minutes) the ram 6 pressures are controlled to allow raising of the flaps 5 without forming buckles or creases. After the first hull formation the ram 6 pressures may be automatically regulated for later hulls to a fixed pressure or in proportion to chamber pressure as necessary.

Chamber pressure for full formation depends on the hull size and shape but the double ended hull of Figures 1 to 4 in 16 feet size needs about 4 bar.

Afterfull formation of the hull (Figures 2 to 4) the pressure is released, the air bleed tube 9 opened and the water drained. The clamp or well between the side members 7 and the flaps 5 is then released, the keel 8 clamp is released and the hull lifted from the frame. Fitting out can then be carried out, the fitting of any frame/stringers being done carefully to avoid distorting the fairness of the hull by weld distortion.

The pressurisation of the chamber causes the side members 7 to deform into the profile shown in Figure 2, the flaps 5 pivoting into the position shown underthe control of the rams 6thereby allowing the side members 5 to bulge into the desired shape. The plates 3 assist in keeping the desired shape of the hull as the chamber inflates and prevent the side members 7 from creasing.

Figures 5 to 8 show a method of forming a shallow hulled transome stern hull. In this case a saucer bow is shown but a deep bow could be formed as in the double ender if required. The same reference-numer- als have been used as in Figures 1 to 4to identify equivalent items.

By use of these general techniques most of the round bilge boat hull variations can be produced.

Where extreme shapes are required, or flat sections needed, extra internal ties or external partial moulds or plate can be used; or for example the lower part of a hull could be formed using this technique and then conically developed topsides added.

The method of these embodiments produces "natural" shapes from planar form, producing a smooth natural form which has excellent natural hydrodynamic and aerodynamic properties.

The method of this invention can also be used to form, for example, aerodynamic body panels for vehicles, furniture, lighting structures, fluid containers and equipment enclosures.

Various modifications can be made to the embodiments described above depending on the final desired form of the hull; for example further frames or moulds can be positioned to control the rate and extent of deformation during pressurisation at selected locations of the hull; for lifeboat construction the backbone 1 can be omitted and a one-piece deck (without flaps) left in place after inflation to provide a permanent deck sealed to the hull; and the side members may be made from rigid plastics material.

In the above embodiments the deformation of the chamber causes an increase of from 1 to 3 % in the extent of the sheet members, preferably 2%.

Other modifications and improvements can be made without departing from the scope of the invention.

Claims (18)

1. A method of forming a hollow body, comprising sealing sheet members together to define a chamber having an inlet for pressurisied fluid, supplying pressurised fluid to the chamber through the inlet to deform the chamber into a desired shape, and removing the fluid from the deformed chamber, the sheet members being such as to retain their deformed shape.

2. A method according to Claim 1, wherein the sheet members are of metal.

3. A method according to Claim 2, wherein the sheet members are of steel or aluminium.

4. A method according to Claim 1, 2 or 3, wherein the sheet members are secured to a non-deformable frame for determining the extent of deformation of the chamber.

5. A method according to Claim 4, wherein at least part of the frame is disposed within the chamber.

6. A method according to any one of Claims 1 to 5, wherein part of the chamber wall is formed by a jig which is removed after deformation of the chamber to leave an open structure.

7. A method according to Claim 6, wherein the jig has pivotal side flaps which are secured to the sheet members in forming the chamber, and during pressurisaton of the chamber the side flaps pivot to control the deformation.

8. A method according to Claim 7, wherein the side flaps are controlled in their pivotal movement by hydraulic rams bearing against the nondeformable frame.

9. A method according to any one of Claims 1 to 8, wherein the sheet members are selected so that when sealed together the chamber approximates the shape of a boat hull.

10. A method according to Claim 9, wherein the sheet members comprise a pair of hull members and a deck member so that the chamber is generally triangular in cross-section prior to deformation.

11. A method according to Claim 10, wherein the deck member is in the form of a reusable jig.

12. A method according to Claim 9, 10 or 11, wherein a reinforcement member is provided within the chamber and extending from the bow to the stern.

13. A method according to Claim 12, wherein the reinforcement member is of triangular prismatic profile.

14. A method according to any one of the preceding Claims, wherein the deformation of the chamber causes an increase of from 1 to 3 % in the extent of the sheet members.

15. A method according to any one of the preceding Claims, wherein each sheet member is held under tension in all planar directions during deformation of the chamber.

16. A method of forming a hollow body substantially as hereinbefore described with reference to Figures 1 to 4 or Figures 5 to 8 of the accompanying drawngs.

17. Ahollowbodywheneverformed by the method according to any one of the preceding Claims.

18. A boat hull whenever formed by the method according to any one of Claims 1 to 16.