Building the Rushton Catboat

Beveling the Laps and Cutting the Gains

The entire length of the plank lap has to be beveled so that the next plank can lie flat against it. This is imperative in glued lapstrake construction because you don't want any gaps in the glue joint. I use several different planes to do this but one I especially like is my antique rabbet plane. I fitted the steel fence arm with a block of hardwood which is even with the sole of the plane. I can rest this block on the adjoining ribband and as I plane the bevel I am guaranteed that I am cutting the proper angle. I first strike my 3/4 inch mark for the lap up from the plank edge and then I plane until I reach that mark. I sometimes start a rough cut using my spokeshave, which can remove a lot of material very quickly. With two hands it is easy to approximate the angle with the spokeshave, but the rabbet plane and arm finish it off. I do any final trimming with a block plane. By looking carefully at the pattern created by the veneers I can see if my bevel is even and transitions smoothly.

The edges of lapstrake planking fore and aft come together such that right at the stem and transom the hull is actually smooth. The bevel on each plank basically become a rabbet at either end that the adjacent plank lies in. These rabbets are called gains. The gains want to taper gradually from their starting point so that the lap disappears in a fair line. On the Rushton catboat I made the forward gains 18 inches long and the after gains 12 inches. I clamped a batten along my lap line and used a small Japanese saw called a azebiki to cut the gain. This saw has a curved edge and a very short blade (they are used in Japan to start a cut in the middle of a piece of wood) and I find it a very handy tool. In the case of the catboat I cut a half-depth gain on the outside face of the plank previously hung on the boat and cut the same half depth gain on the inside of the plank to meet it. I just glance at how much the teeth show to gauge the depth of my cut.

On the planking bench I will clamp a piece of straight scrap to the edge of the gain's kerf and then my small rabbet plane does quick work planing the rabbet. For this boat I cut half-depth gains on the outside of one plank (after it was hung on the boat) and on the inside of the plank I was about to hang (at the planking bench). The two fit together to form the gain.

Note: an alternative would be to cut the gain on just one plank. This gain would have to taper to a feather edge in order for the next plank to lie fully flush at the stem and transom. With this thin plywood was concerned about planing the gain's rabbet to such an extreme, and that it might break through at the stem or transom.

Beveling the lap with a rabbet plane. The hardwood block is mounted on the plane's fence arm and the bottom of the block is even with the sole of the plane. The block rides on the ribband and ensures that the bevel cut by the plane is accurate. The pencil line marks the width of the lap. I plane until the cut reaches the mark.

The Japanese saw I like to use to cut the gains. It is called an azebiki and is used in Japan for starting cuts in wood, and for cutting sliding dovetails.

I could freehand the cut, but the work goes so much faster if I clamp a fence to the edge of my line.

I gauge the depth of my cut by watching the teeth: when they disappear I know I have cut to the correct depth.

I do the same thing cutting the gain on the next plank on the bench.

The layers of the plywood nicely show how evenly I have planed the taper.

The finished gain.

Looking at the face of the inner stem and the plank ends. You can just make out the gains, which are like half laps when the planking is done.

Back :: Next: Spiling the Planks

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.

© Copyright 2007 by Douglas Brooks

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