Shipbuilders aluminum plate is designed for salt water immersion. One of the best alloys used is #5086, which was used for Holy Grail, which has been built for strength first and lightness second, unlike most aluminum sailboats.
Lofting lines were carefully drawn on the shop floor before the deck plates were rolled to achieve an 8″ deck camber. They were then shimmed 8″ all the way around in advance of setting the framing upside down on them. Frames were true and plumb, then secured with temporary supports. Ring frames, 3″ by 3/8″ are 16″ apart, and each fitted with 3/8″ flange. Every station frame was band sawed to a slightly different curve and shape for true true round bilge construction.
Left is a view of the framing, showing the keel web, frame stations including floors in the engine room.
After this stage was complete, two 8″ by 3″ stringers fore and aft per side were installed, each positioned just below settee seat height to avoid encroaching on useable interior space. The result is ice-breaker hull strength with secure furniture attachment and maximum storage uimpeded by structure.
The final process in the upside down stage was adding the hull plating. Quarter inch flat plates 5 feet wide and 20 feet long were hoisted to the frames and were tack welded wherever they touched. Then temporary “handles” were welded to the inside of the plate as attachment points for chain hoists to apply pressure. Slowly the plate was pulled in and welded at each point where it touched the frames, until no more pulling could be applied without damage. At that point a triangular cut was made in the plate so it would come to the frame and together, much like a seamstress would add a dart to a garment.
After the 1/4″ plating was on, the upper portion of the keel was framed and she was ready to be turned right side up, wheeled out of the shop and readied for the crane. Note that the open part of the keel is the diesel tank with baffles every 16.” The ballast portion of the keel was added after she was right side up. 3/8″ plating of the full keel and the 24″ zone around the keel were then applied. This modified, elongated fin keel is 14 feet long. Once the hull was turned over and the ballast section added, the resulting “wing” became 4 feet wide at the widest point. Consequently, Holy Grail will sit upright on her flat bottom section of keel.
After she was turned upright, the ballast secttion of the keel was added, including 13,000 pounds of lead. The flared section is bullet shaped 49″ wide at the widest point, originally designed by Henry Sheel. The wing shape enables Holy Grail to have great stability with only 6 foot draft, with a modern, speedy underbody.