Building construction molds

The lofting I’ve described so far produced 22 half-sections that represent the shape of the planked hull at stations 2-23. Two factors arise in determining the actual shape of the construction molds on which Justine would be built. I’ve mentioned the first in my previous post: The dimensions of the molds need to be reduced from the lofted sections to allow for the thickness of Justine’s planks (5/8″) and the thickness of her frames (7/8″). One could simply reduce the dimension of the half-sections by the sum, 1 1/2″, at every point. This would only work well amidships, at points where the plane of the construction mold is perpendicular to the hull’s surface. Because the hull shape narrows toward the bow and the stern, the hull lies at an acute angle to the plane of the molds. By taking angles off the lofting it’s possible to  calculate a more accurate dimension by which to correct for the difference between the lofted dimension (outside surface of the hull) and the mold dimension. (I did this using vector algebra. Feel free to contact me if you’d like details.)

The illustration below shows an example of how I altered the shapes of the lofted sections to produce drawings of the construction molds near Justine’s bow. Because of the narrowing of the hull forward, the molds in this part of the hull need to be smaller than the sections by about 1.65″, rather than the 1.5″ combined thickness of the frames and planking.

Lofted sections at stations 2-5 (light blue lines). Red circles have diameter equal to calculated reduction for frame and planking thickness. Darker lines are drawn tangent to the corresponding circles to give the shape of the construction molds. The baseline corresponds to the common plane on which all 22 construction molds will rest.

The second factor concerns the hull’s curvature: the stations at which the sections were lofted are planes at precise distances along the hull, but the construction molds have a specific thickness (mine would be built from pine boards that were planed on one side and about 7/8″ in thickness). Because the hull is curved, and the frames necessarily have to lie fair against the hull’s planks, the construction molds have to be beveled along their edges. Furthermore, it will be a “winding bevel” whose angle changes along the molds’ edges. The construction molds generally have a wider side and a narrower side because of the bevels. The wider side is placed exactly on the plan’s stations (where the mold shape was drawn). Consequently, the molds are positioned so that the beveled edges face forward on the bow end of the boat, and face aft on the stern end. (This is shown clearly on the Flatfish plans.)

Once I had the mold shapes drawn on my computer, I printed them out full-size on an inexpensive HP ink-jet printer. I’d added registration marks on my computer drawing, and the my software was able to print the full-size drawing on separate 8 1/2 x 11″ sheets that I then taped together. I was very careful to position the sheets so that the registration marks were aligned and the grid on which I’d positioned the marks remained perpendicular.

I fastened some sheets of plywood to the barn floor on which to position and connect the numerous sheets on which I’d printed out my mold drawings. Here I’m in the process of fastening the 8 1/2 x 11″ sheets together. Each sheet was carefully positioned and tacked to the plywood before joining adjacent sheets with tape. I started with the smallest mold at station 2 and worked aft. Completed molds for stations 2-5 are visible in the background.
To economize on printing and aligning 8 1/2 x 11″ sheets, I overlaid four mold drawings in a single computer file before printing them. This is the drawing for molds 17-20 after aligning the sheets and tacking them to the plywood building surface.

With the mold drawings assembled and in position on the plywood, it was time to cut into some lumber! I’d bought pine boards that were planed on one side and joined on one edge from a local supplier. There’s a clever technique for transferring the line defining the mold’s shape on the drawing to the wood, using some common nails. A nail is placed on the drawing, with its head exactly on the line and its shank perpendicular to the line. Nails are placed at suitable intervals, about 6″ in my case. A hammer is used to tap the edge of the head into the drawing and plywood, so that the entire nail shank contacts the drawing. Then a piece of mold stock is placed in position over the drawing and on top of the nails (planed side of the stock facing down). Next, you walk along the mold stock so that the exposed portions of the nail heads are pressed into the mold stock. When you pick up the board you have a series of impressions that reproduce a series of points on the line on the drawing. Use a batten to connect these impressions with a smooth curve, and you’re ready to make your cut!

The newly-cut curved piece is laid in place on the drawing and marked across the construction baseline and centerline, and those cuts are made. The pieces are fastened temporarily in place to the drawing and plywood, then joined together with a separate piece across the top and at the sheer line.

Molds 2-6 are made of a single thickness of pine board, and the remaining molds, which are larger, more highly curved, and support more loads, are made of  a double thickness.

The construction mold at station 6 is being assembled directly on the full-size drawing. Because the port and starboard sides of the mold have the same shape, the boards for each side can be fastened together temporarily, marked, and sawn to shape simultaneously. The pieces are carefully aligned with the construction baseline at the bottom, and about the centerline, and temporarily fastened to the plywood surface I’d attached to the barn floor. A cleat fastens the two halves together, and the top edge of the cleat is positioned at the position of the sheerline for that station. A separate piece will be added at the top of the mold to secure the pieces there.
Larger molds with more curvature are made with several pieces. Here the mold for station 19 being assembled. A second layer of planks will be added above the long cleat. Molds for stations 2-18 are already assembled and stacked nearby.
Here is a larger mold close to completion. The upper part of the mold has two layers to add strength, and it’s made from 8 pieces of pine. The second layer of planks serves as cleats that back up joints in the first layer. Brackets made from pieces of angle iron fastened to the bottom of the mold will serve as attachments to the shop floor.

At this juncture the molds are sawn to shape but not beveled. Beveling is left until the molds are in place, attached to the shop floor. Also left for later is making some modifications to the molds amidships (stations 13-17) to accommodate the centerboard trunk. I’ll describe both procedures in future posts.