Building the Rushton Catboat

Planking the Hull

Once the plank is scarfed I clamp it onto the moulds. Working from inside the frame I trace the lower edge (nearest the keel) with a series of short hashmarks. The other edge I trace continually along the ribband. When I lay the plank down on the bench the hash marks tell me I need to add 3/4" for the lap (the hash marks mean I have less pencil to erase, because this boat is varnished on the inside). I carefully strike a batten through my marks and then saw my two planks simultaneously. I have always liked using a hand-held circular saw to cut out planking. The circular saw will cut a long, smooth curve and will resist a sudden turn, unlike a bandsaw or jigsaw. With practice the plank edges will require very little planing to achieve a nice fair curve. My cordless saw is able to cut the two layers of 6mm okume, and I like it for slower rpm's and less noise.

With its wineglass transom, the Rushton catboat presents some planking challenges. I have seen glued lapstrake plywood boats in which the builders faceted the transom for the planking, something that frankly, looks terrible to me. There is no need to do this but it requires some forethought to plank such a hull with plywood. First, when lining off for the planking I made sure that I divided the most extreme curves along the transom edge with more than one plank. In other words, I worked to divide the transom so that my planks would have to make the least bend possible. If the boat were planked with lumber the boat builder might steam the planks ends to get them to curve or possibly leave the material a little thicker in this region so that the planks could be backed out with a hollow plane. With plywood, even a narrow plank, let's say four inches wide, is going to be pretty resistant to bending. Since the planks is already bending over its length, making it curve around the edge of the transom creates a compound curve. What I do is score the outside surface of the plywood with my circular saw, cutting a series of kerfs to a depth of about 3/4's of the thickness of the planking. These kerfs are about 1/4 inch apart and between six and twelve inches long. Kerfed like this the planks ends will easily take a concave or convex curve along the transom. Later I fill these kerfs with thickened epoxy, sand them and go over them again with epoxy mixed with fairing compound. Needless to say this can't be done if you plan on varnishing the outside of the hull.

I always have a reference point marked on my planking stock from the time I spile it, so I can be sure I hang it on the boat in the same spot. For this boat I made sure that all the interior scarfs lined up on mould six. These scarfs will be very visible when I varnish the interior, but because they are lined up precisely, I can lay a frame over them to hide the scarfs.

Once again it is important not to edge set the plank when you hang it on the boat. If you have been careful about this during spiling then you should not have any trouble, but an edge set plank will tend to pull away from the hull rather than lie tight against the moulds and ribbands. I used spring clamps along the bottom and drove small finish nails through the lap and into the ribbands to clamp the seam. Epoxy does not require much clamping pressure, but I made sure that there were no gaps showing in the seam and I didn't spare the nails. I feel that the small holes that they leave won't be noticed with this dark okume is eventually varnished on the inside.

When I am convinced that I have the plank nailed and clamped firmly in place I then take a scrap of plywood and go inside the mould and carefully wipe off the excess glue that has squeezed out of the seam. You learn pretty quickly just how much epoxy you need to put on the laps by the amount you need to scrape off. I try very carefully to cleanly wipe away the excess. It is hard work sanding the inside of the hull, so time spent here more than offsets harder work later. And, of course, I don't want to glue the hull to the mould setup. With this boat I waxed the ribbands for my first two planks, but then realized that I was cleaning the seams well enough that this wasn't necessary. As I kept planking I began to use the material I had scraped off to begin filling the kerfs I had cut in the planking aft, and filling nail holes and other dings. I'm thrifty, epoxy is expensive and I don't like waste.

The scarfed plank clamped in place, ready to be marked by tracing off the shape from the ribbands.

A batten connects the marks traced off from the hull to draw a fair curve.

A cordless circular saw does a nice job cutting out both planks with less noise and dust than a conventional saw.

The lap on the adjacent plank. By looking at the exposed veneers I can tell if the bevel is planed smoothly, with no bumps or hollows.

The stern showing the kerfs I cut in the planking aft so it could bend against the wineglass transom. These kerfs are filled with epoxy and sanded. Later I will do a final smoothing with fairing compound. Note that only three planks needed this treatment, two bent concave and one convex next to the sheer.

The completed hull. I am in the process here of sanding epoxy residue from the inside. I just want to smooth it, as I will give the entire hull inside a coat of epoxy, which will mask the residue from the planking. Then I will varnish the interior before I begin to finish it out.

A look at the interior of the hull at the stern. Note how the planks I kerfed on the outside look inside. My method seems worth it for the final results. Of course kerfing the planks means that I could not finish the outside of the hull with varnish....

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Douglas Brooks (www.douglasbrooksboatbuilding.com) is a boatbuilder, writer and researcher specializing in the construction of traditional wooden boats for museums and private clients. He lives with his wife Catherine in Vergennes, Vermont.