A key design feature of many Herreshoff boats is a bright-finished (varnished) sheerstrake with a molded shape that is both visually pleasing and functional as a rub rail. Making the sheerstrake is the most challenging part of the planking: The molded shape changes along the sheerstrake’s length, and the nonuniform cross section complicates the spiling process.
The Flatfish construction drawing includes a full-size drawing of the sheerstrake shape amidships and the shape it takes as it approaches the stem and transom. Also included are the sheerstrake widths at the stem, station 14, and the transom. I used the width information for the sheerstrake when I lined off all the planks.
When spiling a plank of uniform cross section, one uses a spiling batten of uniform thickness. If you were to use a uniformly thick spiling batten for the sheerstrake, and make the sheerstrake to the spiled shape, it would not fit. Maynard Bray’s book How to Build the Haven 12 1/2 Footer provides a spiling method that allows for the sheerstrake’s nonuniform cross section: use a spiling batten of nonuniform cross section approximating the sheerstrake’s molded shape. Specifically, one makes a batten from two pieces, one thin and wide and the other thicker and narrower and placed where the thicker part of the actual sheerstrake is.
I decided to first place the thin, wide spiling batten in place on the construction molds, then bend the 5/16 x 3/4″ piece into place onto the wide spiling batten. With the two pieces in place on the construction molds, I fastened them together with screws, placed near each of the station molds. Then I proceeded to use my dividers and mark the spiling batten for the sheerstrake width at each station.
Because I bent both pieces of the spiling batten into place before fastening them together, I built in some residual bending stresses. When I removed the batten from the molds, it came off with a significant bend and some twist.
I proceeded to clamp the spiling batten down flat onto my planking stock, then transfer the marks representing the plank widths at each station. I was not 100% confident this method would work, so I first made a “test” sheerstrake from a piece of cedar. This involved cutting out the spiled shape from a 1″ thick piece of cedar, then planing away enough of the plank to give it a good approximation of the molded sheerstrake’s shape.
When the test sheerstrake was clamped in place on the molds it fit extremely well, so I was confident that using my spiling on mahogany planks would give me sheerstrakes that would fit.
