Construction mold refinements

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.

Centerboard cut to shape. On one side, the hole for the lead insert has been temporarily covered with plywood. The curved after edge of the board has been beveled, revealing the interior layers of the plywood.
The centerboard with its insert after casting the lead. The temporary cleats fastened around the opening prevented most of the lead from overflowing.
The centerboard with its lead insert after smoothing the lead with a block plane. The insert was subsequently smoothed over on both sides with fairing compound (a mixture of epoxy and microballoons).

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.

Centerboard trunk partly assembled. The bedlogs and posts are made of ipe, a tropical hardwood. The inside surfaces of the posts have been painted with red lead and the sides have a thin layer of fiberglass set in epoxy to impart water resistance. Shallow grooves in the posts allow for the 3M 5200 to act as a gasket.
The starboard side of the centerboard trunk is now in place. The bedlogs are curved on the bottom to conform to the shape of Justine’s keel plank.

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.

Mold 14 marked out for cutting the opening for the centerboard trunk. A cross spall has already been added to maintain the correct position of the upper parts of the mold once the original connecting piece is cut away.
Mold at station 14 after cutting the opening to accept the centerboard trunk. The bedlogs will lie against the uppermost flats on the modified mold.
The centerboard trunk temporarily in position on construction mold 14. Here you can see the slot in the bottom of the centerboard trunk. Also note that the bottom of the trunk’s posts project below the bedlogs. These projections serve to index the trunk to the keel plank, once that is in position.

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

Modified molds ready to accept the centerboard trunk.
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.

Slots were cut in molds 22 and 23 to accommodate the transom knee. In addition, the top of mold 23 was beveled that the transom would lie against it.

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.

A small frame was built to support Justine’s transom. The transom will lie against the frame and rest against the two small blocks that are fastened to the sloping pieces of the frame.
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.

Several battens are in place here, fastened with small nails spaced about 3 or 4 stations apart. The mold bevel angles change from station to station, and generally along the edge of any given mold (a “winding” bevel.)

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.

Later stage of beveling. There’s a uniform gap of about 3/32″ between the batten and the third mold in. Shavings on the floor are from paring down the mold bevels. In one or two places, including along the beveled edge of the mold shown here with the two prominent knots, I actually needed to add a thin strip of pine to build up the mold slightly beyond its original profile.
The centerboard trunk’s bedlogs need to lie fair on the molds, and here I’m using a straight piece of lumber as a guide to adjusting those contact points.

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.