Category: Construction Molds

The construction molds will contact various parts of Justine, and support the entire hull as it is being planked. For example, Justine will have a centerboard and consequently the molds must accommodate the centerboard trunk. The same is true forward, where the stem will lie on the molds, and aft, where the transom will be supported. Justine’s substantial keel plank must also be supported at each station along its length. Finally, the edges of the molds need to be beveled to conform to the curvature of Justine’s hull.

Centerboard  and centerboard trunk

The centerboard trunk is a water-tight case that will be bolted to Justine’s keel plank. It consists of two “bedlogs” at the bottom, marine plywood sides, and posts that form the front and back of the narrow space into which the centerboard will fit.

The centerboard itself made by gluing two pieces of marine plywood together to achieve a blank 1 1/4″ thick, then cutting the blank to shape. To keep the centerboard down, it needs to be weighted (otherwise the plywood would tend to float). So I cut a hole of the appropriate size (specified in the plans), beveled the internal edge into a “V” shape (point facing the center of the hole), put on a backing piece of plywood over the rear side of the hole, and poured lead into the hole, filling it level with the face of the centerboard. After the lead froze and cooled, I removed the backing piece and used a block plane to smooth off the surfaces of the lead casting.

The centerboard sides fit into a rabbet in the bedlogs. The posts join the port and starboard sides of the trunk assembly. All joints used 3M 5200 adhesive bedding compound and bronze screws as fastenings and to keep water out.

The centerboard trunk will project into the cockpit, so the construction molds need to be modified to both accommodate and support the trunk. I cut out the tops of molds 13-17 to match the trunk’s shape at each location, and added additional pieces as needed to support the trunk.

With molds 13-17 modified, I set the molds back in position.

Transom and transome knee

The aftermost molds 22 and 23 need small modifications to accommodate the transom and transom knee. (The transom knee reinforces the joint between the transom and keel plank.) Slots of the appropriate width, depth, and bevel were cut.

Justine’s transom needs its own support. It lies at about 45 degrees to horizontal, so I added pieces between the mold at station 23 and the barn floor.

Construction mold bevels

Beveling the edges of the construction molds to match the hull’s curvature sounds straightforward and I expected it to go relatively quickly. Long battens extending fore and aft along the molds are used to gauge the bevel angle at each point on the molds. If a batten does not lie fair, the high spot on the edge of the mold is shaved away. This is an iterative process because the aim is to get every point on every mold to conform to a batten’s curvature. Shaving away material on one mold generally requires adjustments of the bevels on adjacent molds. All the while you strive to refrain from cutting material away from the mold’s station edge in order that the hull’s designed shape is not altered.

I began by first taking a black marker and going over the very edge of the station side of each mold so that I would not bevel away any of that wood. (At least, not until that became absolutely necessary; and ideally, it wouldn’t.) I also used my knowledge of geometry to calculate the ideal bevel angle at a lot of points from my computer lofting. I wrote those numbers directly on the molds. Once that was done, I made some very conservative bevel cuts along the mold edges using my band saw, to cut down on the amount of beveling I’d need to do with hand tools. From there on, I worked with a block plane and spoke shave.

The early stages of mold beveling seem to go quickly as there is lots of material, in obvious places, that can be removed. It gets more challenging  as the bevels begin to take shape. One is constantly shifting the locations of battens, looking for gaps between molds and battens as well as for battens that while contacting the molds, do not take a fair curve.

As the beveling proceeds, you are looking for places where there are small gaps between the battens and the molds, then deciding where material should be pared away from the molds.

Beveling the molds is tedious. It can’t be rushed. It never seems to be perfect. When you believe you’ve done your very best, or close to it, you’re done. I began beveling the molds in January 2008 and finished in June 2009. I did accomplish some other parts of the construction in parallel with beveling, but not a great deal.

In constructing Justine’s molds, I followed established procedure and cut the bottom of each mold in the plane of the construction baseline shown on the plans. With the baseline positioned near the barn floor, this placed the molds at a convenient working height for construction. As a first step in setting up the molds I needed to lay out the construction baseline in our barn.

I decided to make the baseline using long lengths of 1 x 2″ lumber that could be screwed to the barn floor and shimmed into position using opposing pieces of cedar shingles. My laser level was the ideal tool for positioning the baseline. The two photos below show what I did. I used about 21′ of 1 x 2 for my construction baseline as it would be needed over Justine’s entire length.

Each of Justine’s molds needs to lie perpendicular to the construction baseline. I drew a very accurate perpendicular line about midway along the baseline, using the method I was taught in my high-school plane geometry class. Then I’d used a ruler and a compass. Now, I’d need a long compass and I made one up using  a small lumber off-cut fitted with trammel points. One of the trammels has a pointed end, and the other can hold a pencil. So I had an adjustable compass about 5′ long with which to draw the arcs to erect a perpendicular to the baseline.

With a straight and level baseline, and an accurate perpendicular reference line, I was ready to start fastening the molds in place. I began near the center of the boat with the mold at station 14. This mold was supported with diagonal bracing to keep it exactly vertical. All the other molds were secured in the vertical plane with battens that tied in to the mold at station 14.

The photo above was taken on September 15, 2008. I began construction of the molds in the fall of 2006. The project’s pace was slow. I was working full-time in Boston, and building Justine in Georgetown, ME, a three-hour drive away. So much of the construction work was done on long weekends, holidays, and vacations. I didn’t retire and move to Maine until 2013.

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.

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.

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.

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.