Abstract:
A method for preparing and assembling parts of a skin for a curved structure, and apparatus for use in connection therewith. A gantry may be used to carry cutting torches and welder heads along predetermined paths to cut skin plates accurately into skin parts and to weld the skin parts together again after they have been fastened to internal frame elements.

Description:
BACKGROUND 
     The present disclosure relates to assembly of structures with smoothly curved surfaces, and relates particularly to construction of hydrofoil shapes such as those of skegs for oceangoing barges. 
     Oceangoing barges can be towed more economically when equipped with hydrofoil skegs such as those disclosed in Gruzling, U.S. Pat. Nos. 4,217,844 and 4,569,302, and in Heyrman, et al., U.S. Pat. No. 4,782,779, than when not equipped with such skegs. 
     In the past, hydrofoil skegs for barges were manufactured in a highly labor-intensive manner, with skin plates for the skegs being cut oversized from plate material of the required thickness, and then bent to the required curved shape for the location of the skin. Thereafter, the bent skin plate was burned into three parts, a nose part, a middle part, and a tail part, and the edges of each part were then scarfed by hand so that a V-groove weld could be used to rejoin the parts of the skin plate during final assembly. Each skin plate part was welded to several supporting web members to form a structural section of the skeg, and girders were welded to the web members of the middle section. Finally, the nose and tail sections were welded to the middle section, which required numerous passes of hand welding to create the V-groove welds rejoining the sections of the skin parts for each side of the skeg, another multiple pass V-groove weld along the nose of the skeg, and welds along the tail edge of the skeg, to completely interconnect the skin plates of the opposite sides of the skeg to each other. 
     Cutting torches could be guided mechanically to follow lines scribed or drawn on the surface of the skin plate. For example, the cutting torch could be guided by tracks fastened to the metal being cut, but the tracks often shifted as a result of the heat distortion encountered during the burning process, but after being scarfed by hand the skin parts frequently did not meet closely when the structural sections were assembled, and gaps that resulted between the edges of the plate pieces were too deep to permit the plate sections to be simply welded together. 
     What is needed, then are methods by which to cut and bevel a plate to form parts of a skin for a skeg or hydrofoil more accurately than has been previously possible, and to assemble a hydrofoil structure more precisely, more quickly, and with less labor than was previously required for construction of such a hydrofoil structure. 
     SUMMARY OF THE DISCLOSURE 
     As an answer to some of the needs mentioned above the present disclosure provides a method and apparatus for use in manufacture of structures having curved outer surfaces, such as hydrofoil structures for waterborne vessels, as defined by the claims appended hereto. 
     In one embodiment of a method disclosed herein a skin plate for a hydrofoil structure is bent to a required shape, and then, in a single cutting operation, the skin plate is separated into skin parts, and margins of each of the skin parts are formed with a predetermined configuration. 
     According to one embodiment of the method the margins of a skin part may be appropriately shaped to form the sides of a groove along which the skin parts can be welded together efficiently in connection with joining structural sections including the skin parts. 
     In one embodiment of the method structural sections of a hydrofoil structure include respective ones of the skin parts, and after the structural sections are initially fastened together, at least two elongate joints between adjacent ones of the skin parts are welded simultaneously to rejoin the skin parts to each other as incorporated parts of a hydrofoil structure. 
     In one embodiment of apparatus that may be used in accordance with the method disclosed, a gantry is equipped with at least one pair of cutting torches and is movable along a predetermined path to carry the cutting torches, in order to cut a workpiece along a predetermined cutting path and also simultaneously to form margins, each having a predetermined configuration, on the resulting separate parts of the workpiece. 
     In one embodiment of such apparatus the gantry is equipped with at least a pair of welders arranged to be carried along a workpiece by the gantry to form simultaneously at least two welded seams to join at least three separate elements of a structure to one another. 
     In one embodiment of the apparatus disclosed such welders are arranged to precede the gantry as it moves along a predetermined path to perform the simultaneous welding operations. 
     In one embodiment of the apparatus such cutting torches are carried on the gantry so as to precede the gantry as it moves along a predetermined path relative to a workpiece while cutting the workpiece into separate parts. 
     The foregoing and other features and advantages will be more readily understood upon consideration of the following detailed description taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an isometric view from below and behind a starboard quarter of waterborne vessel such as a barge, showing a hydrofoil skeg arrangement mounted thereon. 
         FIG. 2  is a side elevational view of a portion of the barge hull and the hydrofoil skeg arrangement shown in  FIG. 1 . 
         FIG. 3  is a sectional view, at an enlarged scale, of a generally horizontal hydrofoil skeg section, taken along the line  3 - 3  in  FIG. 1 . 
         FIG. 4  is an isometric view of an upper, or inner, side of a bottom skin plate for a hydrofoil structure such as that shown in sectional view in  FIG. 3 . 
         FIG. 5  is a top plan view of a parts support table and an associated gantry carrying an arrangement of cutting torches and welders. 
         FIG. 6  is an end elevational view of the parts support table and gantry shown in  FIG. 5 , taken along line  6 - 6  of  FIG. 5  and showing a skin plate such as the one shown in  FIG. 4  being cut into separate parts. 
         FIG. 7  is a detail view at an enlarged scale showing the arrangement of a pair of the cutting torches shown in  FIG. 6 . 
         FIG. 8  is a view similar to that of  FIG. 4  showing the skin plate after it has been cut into separate parts. 
         FIG. 9  is an isometric view showing web members being attached to a middle skin part and a girder member, as a step in assembling a middle structural section of a hydrofoil structure such as that shown in  FIGS. 1-3 . 
         FIG. 10  is an isometric view showing a further step in the assembly of a middle structural section for a skeg including the web members and skin part shown in  FIG. 9 . 
         FIGS. 11A and 11B  show steps in assembling skin members and web members of a nose structural section of the hydrofoil skeg structure shown in  FIGS. 1-3 . 
         FIG. 12  is an end elevational view of the parts support table and gantry shown in  FIG. 5 , taken along line  12 - 12  of  FIG. 5  and showing a nose seam being welded on the nose structural section shown in  FIGS. 11A and 11B . 
         FIG. 13  is a sectional view of the middle structural section of a hydrofoil structure such as is shown in  FIG. 3 , at an enlarged scale, together with portions of the nose and tail structural sections, prior to their being fitted together with the middle section. 
         FIG. 14  is an end elevational view of the gantry arrangement and parts assembly table as shown in  FIG. 12 , showing a pair of seams being welded simultaneously on a hydrofoil structure such as that shown in  FIG. 3 . 
         FIG. 15  is a view of a detail of  FIG. 14 , at an enlarged scale, showing a seam being welded. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     Referring now to the drawings which form a part of the disclosure herein, in  FIG. 1  a starboard quarter portion of an oceangoing barge hull  20  is equipped with a hydrofoil skeg arrangement  22  which may be generally similar to those disclosed, for example, in the previously mentioned Gruzling, U.S. Pat. Nos. 4,217,844 and 4,569,302. Such a skeg arrangement may include two or more vertical hydrofoil portions  24  extending downward from the hull  20  of the barge and interconnected at their lower ends by a horizontal hydrofoil portion  26 . The vertical portions  24  and horizontal portion  26  of the skeg arrangement  22  are attached at selected angles relative to the hull  20  so that they affect the flow of water near the hull as the barge is towed, in order to reduce the energy required to tow the barge. 
     The vertical and horizontal portions  24  and  26  of the skeg arrangement  22  are constructed generally similarly, with each vertical portion  24  and the horizontal portion  26  having generally similar hydrofoil shape defined by a respective skin  28  supported internally by a system of webs and girders. A horizontal portion  26  shown in section in  FIG. 3  is representative of the general structures of any of the vertical portions  24  as well, with the exception of variations in the hydrofoil shape which do not affect the manner of construction and assembly of the skeg portion. 
     The horizontal skeg portion  26  includes a nose structural section  30 , a middle structural section  32 , and a tail structural section  34 . The nose section  30  includes several apart-spaced nose webs  36 , a top nose skin part  38 , and a bottom nose skin part  40 , all of material such as suitably thick steel plate. A backing bar  42  extends along the nose and is located in notches  44  defined in the nose webs  36 . The webs  36  are welded to the top nose skin part  38  and bottom nose skin part  40 , and the backing bar  42  is held in place in alignment with a nose seam  46  along which respective margins of the top nose skin part  38  and bottom nose skin part  40  are aligned with each other and welded together. 
     The middle structural section  32  of the hydrofoil structure of the horizontal portion  26  of the skeg arrangement  22 , similar to the nose section  30 , includes a plurality of parallel webs  50  that may be spaced apart from each other and aligned with corresponding ones of the nose webs  36 . Transverse girder members  52  and  54  may also be of steel plate and may be welded or otherwise suitably attached to the webs  50  adjacent the nose section  30  and the tail section  34 . A top skin part  56  and bottom skin part  58  of the middle structural section  32  are also welded to the webs  50 . 
     The tail section  34  similarly may include a plurality of webs  62  of steel or other metal which may be spaced apart from and parallel with one another and aligned with respective ones of the webs  50  of the middle section  32 , and to which a top tail skin part  64  and a bottom tail skin part  66  may be welded. 
     The webs  36 ,  50 , and  62  may define respective central openings  68 ,  70 , and  72  to reduce their weight, and may define cutouts  74 ,  76 , and  78  along the girders  52  and  54  and near the intersections of the skin parts  38 ,  40 ,  56 ,  58 ,  64 , and  66  with the girders  52  and  54 . 
     The three skin parts  38 ,  56 , and  64  of the top portion of the skin  28  of the horizontal hydrofoil structure portion  26  are interconnected with each other and with the girders  52  and  54  along welded seams  80  and  82 . Similarly, the three skin parts  40 ,  58 , and  66  of the portion of the skin  28  on the opposite, or bottom side of the horizontal hydrofoil structure portion  26  are also interconnected with each other and with the girders  52  and  54  by welded seams  84  and  86 . The nose section top skin part  38  is welded to the nose section bottom skin part  40  along the nose seam  46  at the nose of the hydrofoil structure  26 , as previously mentioned. The top tail skin part  64  and bottom tail skin part  66  of the tail structural section  34  are welded together along the tail margin  89  of the horizontal hydrofoil structure  26 . 
     The hydrofoil structures of the skeg vertical portions  24  or horizontal portions  26  of skeg arrangements  22  may be constructed by assembling sets of nose webs  36 , middle section webs  50 , and tail section webs  62 , all cut from suitable material such as ½ inch thick steel plate, with skins  28  and girders  52  and  54  that may be of steel plate or other suitable metal of a greater thickness, such as ¾ inch. The skins  28  may be manufactured by first cutting a skin blank plate  90  of suitable plate material to an appropriate size, leaving a margin strip beyond the desired final size of the actual skin portion, for use such as in handling the plate  90 . The skin plate  90  may then be bent to the appropriate shape in which it will be used as the skin of a hydrofoil structure, which may be a shape which is uniformly curved along the entire length  92  of the plate  90 . Separation lines  94 ,  96 , and  98  may be etched, chalked, or otherwise defined along a surface of the plate  90  to indicate the proper locations for cuts to divide the plate  90  into separate portions such as the nose skin part  40 , middle skin part  58 , and tail skin part  66  of the bottom skin of the hydrofoil structure  26 , and to separate a margin portion  100  from the nose skin part  40  in preparation for assembly of the nose, middle, and tail structural sections  30 ,  32 , and  34  of the hydrofoil structure  26 . 
     As shown in  FIGS. 5 and 6 , the plate  90  may be cut along the lines  94 ,  96 , and  98  in a single cutting operation to form the bottom skin parts  40 ,  58 , and  66  and to provide simultaneously a desired configuration of each of the edge surfaces or margins of the bottom skin parts  40 ,  58 , and  66  along which to form the welded seams  46 ,  84 , and  86 , as will be explained in greater detail presently. This operation of cutting along the lines  94 ,  96 , and  98  may be accomplished according to the method disclosed herein so as to provide uniformly parallel and straight cuts along the separation lines  94 ,  96 , and  98 , by supporting the bent skin plate  90  on a table  102  equipped with a set of cradle forms  104  built to conform to the shape of the skin plate  90  when it has been bent to the required shape as shown in  FIG. 4 . Several cradle forms  104 , for example five cradle forms  104  for a plate  90  having a length  92  of about 22 feet, are used to provide ample and solid support for the bent plate  90  atop the table  102 . 
     The table  102  may be located beneath a predetermined path of a gantry  110 , defined as by pair of parallel rails  112 . The gantry  110  may include a motor (not shown) so as to be movable at an accurately controllable speed along the rails  112  in a forward direction indicated by the arrow  114  in  FIG. 5 , or in an opposite rearward direction. It will be understood that the gantry may be supported otherwise than on the rails  112 , so long as it is moveable along an accurately defined path with respect to the location of the table  102  and a workpiece, such as the skin plate  90 , supported on the table  102 . The table  102  and the cradle forms  104  are set to support a bent skin plate  90  so that its entire length  92  is oriented generally horizontally and parallel with the rails  112  or otherwise defined path along which the gantry  110  is arranged to move. 
     Sufficient clearance is available beneath the gantry  110  to allow the gantry  110  to be moved along and over the skin plate  90  while it is supported on the table  102 . Thus the gantry  110  has an amply large width  116  between side posts  118  to provide clearance in a transverse direction with respect to the forward direction indicated by the arrow  114  as the gantry moves along the rails  112 . 
     Supported on a transverse member  120  extending along the front of the gantry are an array of adjustably supported cutting torches  122 ,  124 ,  126 ,  128 ,  130 , and  132 . The cradle forms  104  may be supported on individual stands  134  fastened adjustably to the top of the table  102 , as by clamps  136  allowing for shims (not shown) to be held between the stands  134  and the top of table  102  as necessary to align the cradle forms  104  with each other and with the path of movement of the gantry  110 , so that as the gantry  110  moves along the tracks  112  or other predetermined path of movement the individual cutting torches, once adjusted to a required position and orientation with respect to one of the separation lines  94 ,  96 , and  98 , will be carried along the respective separation line by the gantry  110  at a constant distance  138  from a plate  90  being cut, as shown in  FIG. 7 , along the entire length  92  of the plate  90  as the gantry  110  moves along the rails  112 . 
     As shown best in  FIG. 6 , the stands  134  may be constructed so as to support a similar cradle form  140  on an opposite side of a pivot axis  142  defined by the stands  134 , so that the cradle forms  104  or  140  can be selected by simply rotating them about the axis  142 , and thereafter the selected cradle form  104  or  140  can be held in the required position by the use of devices such as latches  144 , so that, when desired, the cradle forms  140  each can easily be placed in the required position to be used to support a bent skin plate (not shown) appropriately shaped to become a top skin portion for the hydrofoil structure  26 . 
     In order to divide a bent plate such as the plate  90 , the plate is placed to be supported by the appropriate cradle forms  104  or  140  mounted on the table  102 , and the gantry  110  is positioned in the location corresponding to the top of  FIG. 5 . The cutting torches  122 ,  124 , etc. are positioned to cut the plate along the separation lines  94 ,  96 , and  98 , with each cutting torch adjusted to burn a narrow kerf through the plate  90  so as to leave a margin surface on the resulting separate skin parts  40 ,  58 , and  66  that is inclined at an angle  150  or  152 , as for example at 20-40 degrees, from being normal to the plate  90  or the respective skin section  40 ,  58 , or  66 . This may be accomplished by arranging the flames of both the cutting torches  124  and  126  to be directed at the separation line  96  and arranging the flames both of the cutting torches  128  and  130  to be directed toward the separation line  98 , on the upper, or interior, side of the plate  90  as it rests on the cradle forms  104 . The cutting torch  126  may be located closer than the cutting torch  124  to the transverse member  120  of the gantry  110 , and, similarly, the cutting torch  130  may be located closer to the transverse member  120  than is the cutting torch  128 , so that the cutting torches  124  and  128  precede the torches  126  and  130  in cutting as the gantry  110  moves forward as indicated by the arrow  114 . The cutting torches may, then, be oriented so that their cutting flames  146  and  148  are directed as shown in  FIG. 7 , at the angles  150  and  152  with respect to a plane perpendicular to the plate  90  and extending along the separation line  98 . The cutting torch  122  is similarly oriented non-perpendicularly with respect to the plate  90  along the separation line  94 , as shown in  FIG. 6 . 
     With the cutting torches  122 ,  124 ,  126 ,  128 , and  130  operating, the gantry  110  is then moved at a controlled speed, such as, for example, 8 inches per minute, along the entire length  92  of the plate  90 , and the torches cut the plate into the separate elongate parts shown in  FIG. 8 , including the narrow margin piece  100 , the nose section bottom skin part  40 , the middle section bottom skin part  58 , and the tail section bottom skin part  66 . Because of the arrangement of the cutting torches, each of the margin surfaces  154  and  156  of the nose section bottom skin part  40 , the margin surfaces  158  and  160  of the middle section skin part  58 , and the adjacent margin surface  162  of the tail section bottom skin part  66  are all oriented at non-perpendicular slopes with respect to the major surfaces of the plate  90 . 
     It will be appreciated that during this cutting operation, as the gantry  110  moves in the forward direction, indicated by the arrow  114 , the cutting torches precede the gantry  110  and are thus clearly in view, so that they can be adjusted, as in the event that the plate  90  appears to have been misaligned, and so that it is generally easier to visually monitor the progress of the cutting operation. 
     When the skin plates  90  for both sides of a hydrofoil structure  26  have been bent and cut into the several skin parts as described above, assembly of the hydrofoil structure is performed by first assembling three separate structural sections, the nose structural section  30 , the middle structural section  32 , and the tail structural section  34 . The structural sections  30 ,  32 , and  34  are then fitted together to form the completed hydrofoil structure  26 . 
     To assemble the middle structural section  32  a skin part such as the bottom middle skin part  58  is placed on an assembly table  166  and clamped to the table top in a predetermined position, as by the use of a suitable hydraulic or mechanical clamp  168 , as shown in  FIG. 9 . The table  166  may be provided with profiled pieces to support the curved shape of the skin section  58  accurately. Additional clamps (not shown) may be desirable to hold the skin part  58  in the desired position and keep it flat upon the top of the table  166 , in order to overcome internal stresses within the middle section bottom skin part  58  which may have been released as a result of cutting the skin plate  90  into the several parts. 
     The table  166  may also be provided with several accurately located alignment stanchions  170  near which the skin part  58  may be placed and which are aligned perpendicular to the top of the table  166  to act as guides for alignment of the several webs  50  and the girder  52 . The girder  52  is located as required along a margin of the skin part  58  and clamped to the stanchions  170 , where it is held while it is securely tack welded to the middle bottom skin part  58 . The webs  50  are then securely tack welded to the inner surface of the bottom middle skin part  58  and to the girder  52  in their predetermined locations, which may have been etched or scribed on the surface of the skin plate  90  before it was cut into the separate nose, middle, and tail skin parts. The girder  52  extends beyond the edge of the margin surface  158  to be used in interconnecting the middle structural section  32  with the nose structural section  30 . Next, the girder plate member  54  may be securely tack welded to the opposite margin of the skin part  58  and to the webs  50 , while the bottom middle skin part  58  remains clamped to the table  166 . This forms a three sided box structure with the parallel webs  50  spaced apart along the structure. 
     Next, as shown in  FIG. 10 , the bottom skin part  58  is released from the table top and the partially assembled middle structural section is then rolled 90 degrees and the webs  50  may be more completely welded to the girders  52  and d 54  and to the bottom skin part  58  to structurally unite the webs  50 , girders  52  and  54 , and the skin sheet  58 . 
     This completion welding may best be accomplished in a careful sequence to minimize the expansion and shrinkage effects of heating and cooling, and to provide a structure that is straight and free from unnecessary included stress upon completion of the welding. For example, welding can be begun along the web  50  nearest the middle of the length of the hydrofoil structure  26 , first welding every second web  50  to the girder plate  52 , in the angle on one side of each such web, then rolling the middle structural section  32  and welding along both sides of each of the same webs  50  to attach them to the girder plate  54 , and thereafter also welding one side of the previously unwelded webs  50  to the girder  54 . Next the middle structural section  32  can be rolled back 180 degrees and the remaining unwelded angles can be welded to connect the webs  50  completely to the first girder  52 , after which the section can be again turned over and the remaining unwelded corners can be welded to completely fasten the remaining webs  50  to the girder plate  54 . 
     Thereafter, the top middle skin part  56  can be placed into position and tacked securely into place, using a portable hydraulic press as necessary to hold the top skin part  56  in the required position with respect to the remainder of the middle structural section  32  until the skin part  56  is securely attached. The remainder of the welds necessary to securely interconnect the webs  50 , girders  52  and  54 , and skin parts  56  and  58  may then be completed by gaining access through the several holes  172  provided in each of the girders  52  and  54 . 
     Assembly of the nose structural section  30  of the hydrofoil structure  26  is generally similar to the assembly of the middle section  32 , although slightly simpler. As shown in  FIGS. 11A and 11B , the nose section bottom skin part  40  may be fastened to the table  166 , supported by cradle forms  176  shaped to correspond with the curvature of the outer surface of the nose structural section  30 . Several of the forms  176  may be located on the table  166 , spaced apart along the table at locations aligned with the alignment stanchions  170 , for convenience. At predetermined locations along the nose bottom skin part  40 , the nose webs  36  are placed on the bottom skin part  40  and aligned with the alignment bars  70  and then tack welded securely into position. Locator bodies  178  may be positioned on the top of the table  166  spaced apart and opposite the alignment stanchions  170 , to help keep the bottom skin section  40  correctly aligned on the table  166 . Portable overhead hydraulic rams may also be used to hold the bottom skin  40  properly aligned with the table  166  and the shaped forms  176  while the webs  36  are aligned with the skin section  40  and tack welded into place. 
     Once the nose webs  36  have been tacked to the bottom skin section  40 , the nose backing bar  42  may be inserted through the notches  44 , to fit snugly against the interior surface of the bottom skin section  40 . When the backing bar  42  is properly located it may be fastened in place by being tack welded to at least the ones of the webs  36  at the ends of the nose section  30 . 
     Thereafter, the top skin part  38  may be placed atop the nose webs  36 , as shown in  FIG. 11B . The top skin part  38  may be held in the required position, aligned with and fitting tightly against the webs  36 , by devices such as an overhead hydraulic ram  180  equipped with a suitably contoured pressing head  182 , while the top nose skin part  38  is tack welded to the nose webs  36 . The pressing head  182  may include sharp hardened teeth to engage the top skin  38  securely while pressing it into the required position against the nose webs  36 . It may been seen that the nose margin of the top skin part  38  is thereby positioned in contact with the backing bar  42 , and that the inclined margin surfaces of the nose skin parts  38  and  40  define a V-shaped groove  184  seen in end view in  FIG. 11B . Sacrificial spacers of material such as ¼ inch×¼ inch steel stock may be placed in the bottom of the groove and tack welded to the round bar  42  along the margin of the bottom skin sheet  40  to assure that the V-shaped groove  184  has a root width sufficient to assure good weld penetration for completion of the nose seam  46 . 
     Once the top skin part  38  has been securely tack welded to the webs  36 , the webs  36  may be welded completely to the skin parts  38  and  40 , as by welding them in a sequence similar to that described above with respect to welding the middle section webs  50  into place between the girders  52  and  54 , so as to complete assembly of the nose section  30  with a minimum of distortion resulting from thermal expansion and contraction of the welds. 
     With the inclined nose margin surfaces  154  of the top skin section  38  and bottom skin section  40  extending to the backing bar  42 , the V-shaped groove  184  is fairly tightly closed at its bottom by the backing bar  42 , and any gaps which do remain along the backing bar  42  may be closed simply by welding them shut by hand if necessary. 
     When welding of the nose skin parts  38  and  40  to the webs  36  has been completed the nose structural section  30  may be lifted from the assembly table  166  by a suitable crane and may be placed nose-up on the table  102  beneath the gantry  110  with the V-shaped groove  184  facing openly upward. The nose structural section  30  may be supported upon the stands  134  mounted on top of the table  102 , with the stands  134  adjusted to provide a flat support for the nose section  30 , as by placing a flat plate  186  atop the stands  134 , extending between them along the length of the table  102 , as shown in  FIG. 12 . 
     A pair of flux dams  188 , which may be narrow strips of metal plate, may be tack welded to the nose skin parts  38  and  40  on either side of the V-groove  184  to form a trough along the groove  184  as shown in  FIG. 12  to hold welding flux. A pair of submerged arc welders  190  and  192  are supported on a suitable transverse structural member  194  extending horizontally across the rear end  195  of the gantry  110 , generally opposite the location of the transverse member  120  supporting the cutting torches  122 , etc., as may be seen in  FIG. 5 . The nose section  30  is supported with the V-shaped groove  184  aligned parallel with the rails  112  supporting the gantry  110 . Once the nose section is properly aligned the gantry may be moved in a rearward direction, opposite the direction of the arrow  114  shown in  FIG. 5 , to move the submerged arc welder  190  along the nose section  30  as it operates to weld together the nose margins of the top nose skin part  38  and the bottom nose skin part  40  of the nose structural section  30 . With each of the skin parts  38  and  40  being of steel plate ¾ inch thick, for example, securely welding the seam  46  thus formed along the V-shaped groove  184  may take one to three welding passes along the nose structural section  30  to form a completely filled welded seam  46  that can be ground flush to provide a smooth surface along the nose structural section  30  when completed. Once the seam  46  is completely welded along the V-shaped groove  184  the flux dams  188  are removed. 
     Each of the submerged arc welders  190  and  192  may be equipped, for example to operate using two electrodes at conventional power settings for the material being welded, such as, a lead electrode operating with direct current at 33 volts, 600 amperes, and a trailing electrode operating with alternating current at 42 volts, and 740 amperes, with automatic wire feed electrodes and with the gantry operating at 18 inches per minute. 
     Each submerged arc welder  190  or  192  includes a flux feed tube  196 , for providing a flow of granulated flux into the trough defined by the flux dams  188 , ahead of the electrodes. A flux removal vacuum tube  198  is provided for removing remaining granulated flux from behind the point at which the welder is actually operating along the seam being welded. A gauge wire  200  may extend forward from each welder  190  and  192 , at a known position with respect to the welding electrodes, in order to verify that the workpiece remains in alignment with the path of the gantry  110  and the welders  190  and  192  carried thereon along the entire length of a seam being welded, and so that adjustments may be made to the welders during the progress of the gantry  110  during the welding operation, if necessary. 
     Assembly of the top tail skin part  64  and bottom tail skin part  66  to the webs  62  of the tail structural section  34  is similar to assembly of the nose structural section  30 , except that a seam may need to be welded by hand along the trailing edge margin  89 . 
     Once assembly of the tail structural section  34  has been completed, the structural sections  30 ,  32 , and  34  of the hydrofoil structure may be fastened to each other. According to one embodiment of the present method, the tail structural section  34  may be placed on an assembly table  204  in a horizontal position with its trailing edge seam  89  facing outboard and the open side of the tail structural section  34  facing toward the middle of the table. 
     The middle structural section  32  is then placed onto the assembly table  204 , aligned with the tail section  34  and is moved into a mating position in which the girder member  54  extends between the margins  162  the top skin section  64  and bottom skin section  66 . Short sections  205  of metal such as ¼ inch by ¼ inch square dimensional stock may be tack welded into place along the top and bottom of the girder member  54  to assure that a sufficient root width is provided in the V-shaped grooves between the margins of the top skin parts  56  and  64  and between the margins  160  and  162  of the bottom skin parts  58  and  66 . As shown in  FIG. 13 , the webs  36  of the nose structural section  30  and the webs  62  of the tail structural section  34  may be cut to be slightly larger than the opposite girder member  52  and  54  to assure that after welding of the top skin part  38  and bottom skin part  40  to the nose webs  36 , and after welding of the top skin part  56  and bottom skin part  58  to the webs  62  of the tail structural section  34 , the margins of the skin parts  38 ,  40 ,  56 , and  58  will have sufficient separation gaps  210  and  214 , despite shrinkage of the web members  36  and  62  because of the welding, so that the margins of the top nose skin part  38  and bottom nose skin part  40  can pass over and fit onto the girder member  52  with a nose skin gap  210  and a girder height  212 , and the margins of the top tail skin part  56  and bottom tail skin part  58  can fit similarly over the girder member  54  of the middle structural section  32  as shown in  FIG. 13  and a tail skin gap  214  and girder height  216 . 
     Appropriate portable rams may be used to push the two structural sections  32  and  34  toward each other as they are tacked securely together along the joint between the top skin parts  56  and  62  and are also tacked together at the exposed ends of the bottom skin parts  58  and  66 . Thereafter the combined middle structural section  32  and tail section  34  are rolled over and the process is repeated to tack weld the bottom skin part  58  and bottom skin part  66  securely together along the entire structure. 
     Next the nose structural section  30  is placed alongside the combined middle section  32  and tail section  34  and the nose structural section  30  is similarly tacked to the middle structural section  32 . The nose structural section  30  may have bowed sufficiently so that initially only the middle part of the length of the nose section  30  will meet the middle structural section  32 . This part of the nose structural section  30  may be secured to the middle section  32  with tack welds and thereafter the nose section  30  will have to be forced toward the middle section and tacked at spaced apart locations progressing toward each end of the hydrofoil structure  26 . 
     When the three sections  30 ,  32 , and  34  are securely tacked together the thus preliminarily fastened hydrofoil structure is placed on the table  102  associated with the gantry  110 , supported on the stands  134  and the cradle forms  104 , that have been aligned with each other atop the table  102 . The submerged arc welders  190  and  192  are adjusted to weld the seams  80  and  82  simultaneously to reunite the nose top skin part  38 , the middle top skin part  56 , and the tail top skin part  64  of the skin  28  of the top of the preliminarily fastened hydrofoil structure as it is shown in  FIG. 3 , using flux dams  218  temporarily tacked to the skin  28 . The submerged arc welders  190  and  192  form weld joints along the V-shaped grooves between the sloped opposed margins  156 ,  158 , or  160 ,  162  of adjacent skin parts, forming the seams  80  and  82 , reuniting the skin  28  and attaching it to the girder members  52  and  54 . Using the previously described dual electrode arrangement these welds can be completed in two or three passes of the gantry carrying the welders along the hydrofoil structure  26  at a speed of, for example, 18 inches per minute, thus completing the welds of the seams  80  and  82  in a much shorter time than was previously necessary when they were made manually and required six or seven passes to complete each seam  80  or  82 . 
     Once the seams  80  and  82  are welded completely, the hydrofoil structure  26  is raised and rolled and reversed, and the cradles  140  are raised into position to hold the hydrofoil structure  26  while the seams  84  and  86  on the bottom side are similarly welded simultaneously with two or three passes of the submerged arc welders  190  and  192 . Once the seam welds are completed the flux dams  218  are removed and the surfaces of the welded seams  80 ,  82 ,  84 , and  86 , and the surrounding surface where the weld dams  218  had been attached are ground smooth and to a required shape. Finally, excess material from the tail edge  89  which may have been used during the process of assembling the hydrofoil structure as a convenient location for attachment of clamps, etc., used to move and reposition the hydrofoil structure, may be trimmed at  222  by using one of the cutting torches  122 , etc. carried by the gantry  110 . 
     The terms and expressions which have been employed in the foregoing specification are used therein as terms of description and not of limitation, and there is no intention in the use of such terms and expressions of excluding equivalents of the features shown and described or portions thereof, it being recognized that the scope of the invention is defined and limited only by the claims which follow.