METAL PLATE ASSEMBLY FABRICATION SYSTEM

Abstract: A metal plate assembly such as a vessel hull (10) is fabricated by securing longitudinal edges of aluminium hull plates (12, 14, 16) in position to prevent outward movement thereof. Adjacent longitudinal edges of the hull plates are welded together with sufficient heating to cause the welded hull plates to bow outwardly between the longitudinal edges. The convex outer surfaces (24) are placed in compression when the welded hull plates (12, 14, 16) cool to ambient temperature and ambient pressure on the inner and outer surfaces of the plates is equal. The resulting hull (10) can be curved into deeply bowed curvatures, and it provides the advantages of an outer surface in compression.

Description: METAL PLATE ASSEMBLY FABRICATION SYSTEM CROSS REFERENCE TO RELATED APPLICATION This application is a continuation in part of copending U.S. Patent Application Serial No. 07/632,346, filed December 21, 1990. BACKGROUND OF THE INVENTION This invention relates to a method for fabricating a light weight, high strength metal plate assembly such as a boat hull, and to the resulting assembly. My previous U.S. Patent 4,744,320 describes a method for fabricating a boat hull such as an aluminum plate boat hull. The disclosed method involves the steps of providing a frame, tack and stitch welding hull plates to the frame along the longitudinal edges of the hull plates, and then continuously welding the joints between adjacent hull plates. The hull plates bow outwardly, and the space between the hull plates and the frame between the edges of the plates is filled with a foam. The amount of hull plate curvature provided by the method described in my previous patent represents a considerable advance over the conventional method for fabricating aluminum boat hulls. However, the amount of curvature that is induced into the plates using the method of my previous patent is limited. SUBSTITUTESHEET It is a primary object of this invention to provide an improved fabrication method that further reduces weight and costs, increases strength, and allows increased curvature in metal plates such as hull plates, all as compared to my previous system. It is also an object of this invention to provide an improved metal plate assembly such as a vessel hull having the aforementioned advantages. SUMMARY OF THE INVENTION This invention provides a method for forming at least a portion of a metal plate assembly comprising the following steps: a) providing a plurality of metal plates, at least a first one of said plates having a pair of spaced longitudinal edges; b) securing the longitudinal edges of the first plate in position to prevent outward movement thereof; c) welding the longitudinal edges of the first plate in position with sufficient heating of the welded first plate to cause the welded first plate to bow outwardly between the longitudinal edges such that the welded first plate defines convex outer and concave inner surfaces, and such that the first plate defines a ratio H/S greater than 0.1, where S is the span between opposed longitudinal edges of the first plate and H is the maximum amount the first plate moves outwardly as a result of the welding step. Preferably, the welding step causes at least a . central portion of the convex outer surface of the first plate to be placed in compression along an axis extending along the span when the welded plates cool to ambient temperature and ambient pressure on the inner surface is equal to ambient pressure on the outer surface. SUBSTITUTESHEET The resulting metal plate assembly comprises a plurality of metal plates welded in position to form a portion of the assembly, wherein at least a first one of the plates is bowed outwardly between its longitudinal edges to define a convex outer surface and a concave inner surface. At least the first plate defines a ratio H/S which is greater than 0.1, wherein H and S are as defined above. The convex outer surface of the first plate is in compression along an axis extending along the span when ambient pressures on the inner and outer surfaces are substantially equal and the metal plate assembly is at ambient temperature. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a top, front, perspective view of a vessel hull which incorporates a first preferred embodiment of this invention. Fig. 2 is a side elevational view of the hull of Fig. 1. Fig. 3 is a perspective view of a frame included in the hull of Figs. 1 and 2. Fig. 4 is a cross-sectional view taken along line 4-4 of Fig. 2. Figs. 5, 6, and 7 are enlarged views of portions of Fig. 4. Fig. 8 is a cross-sectional view of a second preferred embodiment of the vessel hull of this invention. DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS Turning now to the drawings, Figs. 1, 2 and 4 show various views of a vessel hull 10 which incorporates a first preferred embodiment of this invention. The hull 10 is configured as a boat hull, SUBSTITUTE SHEET and in this example has an overall length of 80 feet and a beam measured at the deck of 20 feet. The outer surfaces of the hull 10 are formed by a pair of side plates 12, a pair of bottom plates 14, a pair of chine plates 15, and a top plate 16. A transom 18 extends between the opposed side plates 12 and bottom plates 14, and a superstructure 20 is positioned on the top plate 16, which forms the upper deck of the hull 10. As described in my previous U.S. Patent 4,744,320 (which is hereby incorporated by reference in the present specification) the plates 12, 14, 16 are preferably metal plates shaped to extend in uninterrupted fashion from the bow to the stern of the hull 10. Depending upon the size of the hull 10, the plates 12, 14, 16 may be either one piece, unitary plates, or they may alternately be fabricated from a plurality of component plates, as for example by welding. If the plates 12, 14, 16 are fabricated from component plates, the weld lines should preferably be ground and finished such that each of the plates 12, 14, 16 has many of the characteristics of a unitary plate. As described below, the plates 12, 14, 15, 16 are assembled around a frame 40, which is shown in perspective view in Fig. 3. The frame 40 is a welded assembly which includes a longitudinally extending keel 42 and an array of parallel bulkheads 44. As best shown in Fig. 4, each of the bulkheads 44 in this embodiment includes a pair of bottom braces 46 which are welded at their inner ends to the keel 42, a pair of side braces 50 that are welded to the outer ends of the bottom braces 46, and a top brace 52 that extends between opposed side braces 54. As shown in Fig. 3, adjacent bulkheads 44 are braced one with respect to the next by longitudinal braces 54 that are preferably SUBSTITUTESHEET welded in place to the bottom, side and top braces. In this embodiment the braces 46, 50, 52, 54 are all formed as I-beams. In alternate embodiments it may be preferable to use other sections, such as the T-shaped sections discussed in my previous U.S. Patent 4,744,320, for some or all of the braces. As pointed out below, the frame 40 is fabricated in stages, and the plates 12, 14, 16 are secured in place to the frame 40 as it is being fabricated. For this reason, Fig. 3 must be regarded as schematic, and the frame 40 is not completely assembled as shown in Fig. 3 before the plates 12, 14, 16 are secured in place. The hull 10 is preferably fabricated as follows. First, the two bottom braces 46 of each bulkhead 44 are securely welded in the desired geometry to the keel 42. Similarly, the side braces 50 are securely welded in place to the respective bottom braces 46. At this stage in the fabrication of the hull 10 the frame 40 is preferably inverted, with the keel 42 uppermost, and the ends of the side braces 50 that will be secured to the top brace 52 are positioned at or adjacent ground level. A suitable fixture (not shown) is used to hold the partially assembled frame 40 in position. The next step in the assembly of the hull 10 is to stitch weld the bottom plates 14, the chine plates 15, and the side plates 12 in place. The locations of the stitch welds are identified by the reference numerals 60 in Fig. 4, and they are positioned on the inside of the hull 10, between the bottom plates 14 and the keel 42, between adjacent edges of the bottom plates 14 and the chine plates 15, and between adjacent edges of the chine plates 15 and the side plates 12. Additionally, the stitch welding secures the plates 12, 14, 15 at their longitudinal edges securely to the bottom braces 46 and the side braces 50. The purpose of the stitch welds 60 is to immobilize the longitudinal edges of the side and bottom plates 12, 14 and to prevent the longitudinal edges from moving away from one another. In this preferred embodiment, the stitch welds are approximately 1/4 inch in width, and approximately 4 inches in length. Adjacent stitch welds are separated by an unwelded length of approximately 8 inches. Because the stitch welds 60 are confined to the longitudinal edges of the side and bottom plates 12, 14, the central portions of the side an bottom plates 12, 14 are free to move away from the braces 46, 50. Once the side, bottom and chine plates 12, 14, and 15 have been stitch welded in place, fillet welds 62 are used to secure the plates 12, 14 and 15 permanently together to form the lower portion of the hull 10. The fillet welds 62 should preferably have a penetration of greater than 50%, and a full penetration fillet weld will often be preferred. Fig. 5 shows an enlarged view of the portion of the bottom plates 14 adjacent to the keel 42. The included angle A between the longitudinal edges of the plates 14 is preferably between 45 and 90O. Fig. 6 shows an enlarged view of the welds at the chine plate 15. The angle between the bottom plate 14 and the chine plate 15 will be referred to as the inner chine angle B and is preferably about 30O. The outer longitudinal edge of the chine plate 15 is preferably bevelled as shown in Fig. 6 to define an outer chine angle C of approximately 30O between the chine plate 15 and the side plate 12. The fillet welds 62 are preferably formed in an uninterrupted fashion from one end of the hull 10 to the other. It has been discovered that, when welding conditions are chosen properly, the heat of welding will cause the plates 12, 14 to bow outwardly so as to define a convex outer surface and a concave inner surface. The plates 12, 14 are not secured to the bulkheads 44 intermediate their longitudinal edges, and they are therefore free to bow outwardly as the plates 12, 14 are heated in the welding operation. In general, it has been discovered that the more heat that is applied during welding the fillet welds 62, the greater the curvature that is induced into the plates 12, 14. Residual stress measurements on test samples indicate that when the plates 12, 14 cool after formation of the fillet welds 62, they retain an outwardly bowed configuration, with at least the central portion the outer skin 24 of the plates 12, 14 in compression along an axis extending along the span between the fillet welds 62 (i.e., in the plane of Figure 4) . Without intending to be bound by any theory of operation, it is believed that because the longitudinal edges of the plates 12, 14 are stitch welded in place, the thermal elongation of the plates 12, 14 during formation of the fillet welds 62 applies sufficient tensile forces to the outer skin 24 so as to increase the effective width of the plates 12, 14 at the outer skin 24. Then, when the plates 12, 14 cool to ambient temperature, this increased width of the plates 12, 14 causes the outer skin 24 to be in compression, even when the same ambient pressure is applied to both the convex outer surface and the concave inner surface of the plates 12, 14. As pointed out above, the degree of bowing of the plates 12, 14 has surprisingly been found to depend upon the amount of heat applied to the plates 12, 14 during formation of the fillet welds 62. In this preferred embodiment fillet welds are applied in a SUBSTITUTESHEET total of three passes at the keel angle A, while only two passes are used at the inner and outer chine angles B, C. Once the side and bottom plates 12, 14 have been fillet welded in place, the partially assembled hull is then inverted to place the keel 42 in the lower most position. The top braces 52 are then welded in place between opposed ones of the side braces 50, and the top plate 16 is then installed by first stitch welding it in place along its inner longitudinal edges, as described above. Then the top plate 16 is fillet welded in place using preferably only a single pass at the deck angle D. Thermal deformation associated with formation of this fillet weld causes the top plate 16 to bow away from the top brace 52. At this point, fabrication and assembly of the plates 12, 14, 15, 16 and the bulkheads 44 has been completed. The transom 18 and superstructure 20 are now added in the conventional manner, and the frame 40 is completed by welding the longitudinal braces 54 between adjacent bulkheads 44. The longitudinal braces 54 are preferably positioned approximately 3/4 inch from the inner surface of the plates 12, 14, 16 and securely welded in place to the bulkheads 44 only. The hull 10 is now allowed to stand for 24 hours to allow temperature differences to equilibrate. The next step is to apply a foam between the frame 40 and the plates 12, 14, 16. This foam provides many of the same functions as described in my previous U.S. Patent 4,744,320. In particular, the foam spreads loads such as the shock associated with water impact efficiently from the plates 12, 14, 16 to the frame 40. The following information is provided to define the presently preferred embodiment of this invention, and is only intended by way of illustration. In this embodiment the plates 12, 14, 15, 16, transom SUBSTITUTESHEET 18, and frame 40 are all formed of 5000 series aluminum or stronger, such as 5086H11G or 5086H32. The preferred dimensions for the elements 12 through 20 are defined in Table 1. Table 1 The bulkheads 44 are preferably spaced about 5 feet apart and. ave a preferred web depth of 8 inches and flange width of 4 inches. One quarter or 5/16 inch thick plate can be used. The longitudinal braces 54 are preferably spaced 2-1/2 feet apart and are fabricated from 1/4 inch thick plate aluminum with a web depth of 6 inches and a flange width of 4 inches. Of course prefabricated I beams may be used if desired. The presently preferred sealant foam is General Electric Silglaze N SCS2501 silicone sealant. The presently preferred welding parameters for formation of the fillet welds 62 include use of a MIG welding gun operating at 140 amps using 5356 filler rod having a diameter of 3/64 inch. The fillet weld is preferably formed slowly at a rate no less than 25 seconds per foot. As explained above, multiple passes are used until the desired degree of plate curvature is reached. In this example, the maximum bowing (measured at the center of the plate) is approximately 2-1/2 inches away from the bulkhead 44 at the side and bottom plates 12, 14, and approximately 1-1/2 inches away from the bulkhead 44 at the top plate 16. SUBS T ITUTESHEET The preferred embodiment described above uses the heat of welding the fillet welds 62 to induce an unusually large amount of curvature into the plates 12, 14. Figure 4 defines the dimensions H and S that will be used as a measure of the thermally induced curvature. As shown in Figure 4, S is the span between the fillet welds 62, and is a measure of the unsupported width of the plate 12, 14 between the stitch welds 60. H is the maximum height by which the plate 12, 14 moves outwardly as a result of the formation of the fillet welds 62, as measured at the center of the plate 12, 14 after the plate 12, 14 has returned to equilibrium temperature. The ratio H/S indicates the thermally induced height of curvature per unit span, and this ratio provides a measure of the thermally induced curvature of the plates. Table 1 shows the valves for H, S, and H/S for the plates 12 and 14: TABLE 1 Plate H (inch) S (inch) H/S 12 2.5 96 0.026 14 2.5 120 0.021 By way of comparison, the maximum thermally induced curvature I achieved with the process of my previous U.S. Patent 4,744,320 was much less than that of the plates 12, 14; with the process of the '320 patent I achieved H=0.25, S=48, H/S=0.0047. There is no suggestion in the '320 patent that it is possible to achieve thermally induced curvatures as large as with the present invention, in which the curvature is greater than H/S=0.1, preferably greater than H/S=0.15, and most preferably greater than H/S=0.2. The method of this invention produces a hull which is lighter and less expensive than that produced SUBSTITUTESHEET by the method of my previous U.S. Patent 4,744,320. The weight of the hull 10 is approximately 20% less than that obtainable with the method of the '320 patent and costs approximately 30% less to fabricate. This is in large part due to the greater spacing between adjacent bulkheads made possible with this invention. Furthermore, a wider range of hull shapes is possible in view of the increased plate bowing made possible by the present invention. This further reduces costs by reducing or eliminating conventional fairing steps. Because the outer skin 24 of the plates 12, 14, 16 is maintained in compression, the resulting hull is stiff in torsion and can be analyzed approximately as a compressively loaded cylinder. This allows the bulkheads to be placed farther apart, and it is believed to maintain the fillet welds in compression, thereby reducing weld stress. Particularly in the region of the bow, the present invention allows curves not previously feasible, and which are deeper, more rounded and more suitable to an optimum hull shape. Fig. 8 shows a cross-sectional view of a hull 110 which incorporates a second preferred embodiment of this invention. The hull 110 is for a rigid inflatable boat and includes a pair of bottom plates 112 and a top plate 114. The bottom plates 112 are secured together at a keel 120, and the hull 110 supports pontoon supports 116 which in turn support pontoons 118. The hull 110 is a shallow draft boat having an overall length of 30 feet, a maximum beam of 100 inches, and a hull depth of about 20 inches. In applications such as the hull 110, a frame of the type described above may be optional. That is, temporary fixtures may be used to secure the plates in place during fillet welding, eliminating the need for a permanent internal frame. The present invention is not limited to use in fabricating boat hulls, but can also be used to SUBSTITUTESHEET fabricate other types of hulls such as airplane or submarine hulls. The invention is not limited to use with aluminum plates, but can also be used with other types of metals, and it is not limited to relatively thick plate of the type described above. Thinner plates down to about 14 gauge or 0.125" in thickness can be used for applications such as aircraft hulls, and thicker plates can be used for applications such as submarine hulls. Furthermore, the present invention is not limited to use in fabricating vessel hulls, but can also be used to fabricate other types of metal plate assemblies, such as boat superstructures that require complexly curved surfaces. It is therefore intended that the foregoing detailed description be regarded as illustrative rather than limiting, and that it be understood that it is the following claims, including all equivalents, which are intended to define the scope of this invention. SUBSTITUTESHEET

Claims: I CLAIM: 1. A method of forming at least a portion of a metal plate assembly comprising the following steps: a) providing a plurality of metal plates, at least a first one of said plates having a pair of spaced longitudinal edges; b) securing the longitudinal edges of the first plate in position to prevent outward movement thereof; c) welding the longitudinal edges of the first plate in position with sufficient heating of the welded first plate to cause the welded first plate to bow outwardly between the longitudinal edges such that said welded first plate defines convex outer and concave inner surfaces, and such that the first plate defines a ratio H/S greater than 0.1, where S is the span between opposed longitudinal edges of the first plate and H is the maximum amount by which the first plate moves outwardly as a result of the welding step. 2. The method of Claim 1 wherein at least a central portion of the convex outer surface of the first plate is placed in compression along an axis extending along the span when the welded first plate cools to ambient temperature and ambient pressure on the inner surface is equal to ambient pressure on the outer surface. 3. The method of Claim 1 wherein the welding step c) comprises the step of forming a weld having a penetration of at least 50% along substantially the entire length of the welded longitudinal edges. 4. The method of Claim 3 wherein the securing step b) comprises the following steps: SUBSTITUTESHEET bl) providing an internal frame; and b2) welding the first plate to the frame adjacent the longitudinal edges to secure the longitudinal edges of the first plate in position. 5. The method of Claim 4 wherein step b2) comprises the step of avoiding any interconnection between the first plate and the frame intermediate the longitudinal edges prior to the completion of the welding step c) . 6. The method of Claim 5 further comprising the following step: d) introducing a foam between the frame and the concave inner surface of the bowed first plate. 7. The method of Claim 1 wherein the ratio H/S is greater than Q.15. 8. The method of Claim 1 wherein the ratio H/S is greater than 0.2. 9. The method of Claim 1 wherein the plates provided in step a) are formed of an aluminum alloy. 10. The method of Claim 1 or 2 or 3 or 4 or 5 or 6 or 7 or 8 or 9 wherein the plates are hull plates, and wherein the hull plates are secured in position in step b) in a configuration suitable for a boat hull. 11. The metal plate assembly produced by the process of Claim 10. 12. A metal plate assembly comprising: a) a plurality of metal plates welded in position to form a rigid assembly; SUBSTITUTESHEET b) at least a first one of said plates having a pair of longitudinal edges and said first plate being bowed outwardly between the longitudinal edges to define a convex outer surface and a concave inner surface; c) the first plate defining a ratio H/S which is greater than 0.1, wherein S is the span between opposed longitudinal edges of the first plate and H is the maximum amount of the first plate is thermally bowed; d) at least a central portion of said convex outer surface of the first plate being in compression along an axis extending along the span when ambient pressure on the inner surface is substantially equal to ambient pressure on the outer surface and the metal plate assembly is at ambient temperature. 13. The invention of Claim 12 wherein the plates are formed of an aluminum alloy. 14. The invention of Claim 12 wherein the metal plate assembly further comprises an internal frame, wherein the first plate is welded to the frame at the longitudinal edges, and wherein the first plate bows away from the frame between the longitudinal edges. 15. The invention of Claim 14 further comprising a foam bonded between the internal frame and the concave inner surface of the bowed first plate. 16. The invention of Claim 12 wherein each of the longitudinal edges is secured in place by a fillet weld having a penetration of at least about 50%. 17. The invention of Claim 12 wherein the ratio H/S is greater than 0.15. SUBSTITUTESHEET 18. The invention of Claim 12 wherein the ratio H/S is greater than 0.2. 19. The invention of Claim 12 or 13 or 14 or 15 or 16 or 17 or 18 wherein the metal plate assembly is configured as a portion of a boat hull, and wherein each of the plates is a hull plate. SUBSTITUTESHEET

Inventors: JOHNSTON DANIEL D, JOHNSTON, DANIEL, D.
Applicants: FALCON MARITIME VENTURES INC, FALCON MARITIME VENTURES, INC.
Application year: 1991
Main classification: B63B3
IPC codes: B63B3, B63B9