Category: Spiling

The Mahogany Sheerstrakes: Spiling

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.

Sheerstrake profile and width information is provided in the Flatfish construction drawing. I added a few measurements of my own on the drawing.

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.

I made a special spiling board for determining the shape of the sheerstrake. I added a piece of fir about 5/16 x 3/4″ in cross section, positioned about where the thickest part of the sheerstrake would be, to a wide 1/8″ thick piece of cedar.

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.

After removing the sheerstrake spiling board, it had a significant curve due to uneven bending stresses in the two pieces.

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.

I’ve clamped the spiling board down flat onto a piece of cedar, from which I’ll make a test piece of sheerstrake to verify that my spiling method will work.

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.

I wasn’t sure my spiling would be accurate, so I made my first attempt at a molded plank from a piece of cedar rather than to risk ruining a piece of mahogany. The cedar test plank fit very well, so I proceeded to use my spiling to lay out the mahogany planks.

Spiling a Plank

Every plank’s shape is determined through the process known as spiling. A long, thin piece of wood, narrower than the plank to be gotten out, is clamped in place on the construction molds. It is positioned between the adjacent plank (or keel, in the case of the garboard) and the pencil lines that mark where the new plank’s other edge will be. I had several pieces of cedar that I’d planed to about 1/8″ thick that I used repeatedly as spiling battens. I sometimes used strips of 1/4″ plywood to make up segments of a spiling batten.

I’ve clamped the spiling batten in place where Justine’s third plank on her port side will lie. The spiling batten is narrower than the resulting plank will be, but it is shaped so that when clamped down, it will follow any curve the plank needs to take. I made my spiling batten up from several shorter pieces, joining them with thin pieces of wood, well fastened with several very short screws. By using several pieces to make the spiling batten, I could make slight “dog legs” at the joints so as to follow the curve the new plank would need to take.

Spiling is a clever technique that involves placing marks on the spiling batten that can be transferred onto the planking stock to give the plank’s true shape. Because the spiling batten is thin, it can be bent and twisted so that it lies in place against the frames, then when the batten is unclamped and laid flat, it can be used to mark the planking stock while it is flat. This assures that when cut out and bent into place on the frames, the new plank will fit quite well.

With the spiling batten clamped in place, a pair of dividers is set at a suitable distance (here about 1″) and maintained at that distance for the remainder of the spiling process. I’ve put one leg on the edge of the adjacent plank, then I’ll mark the position of the other leg on the spiling batten.
The dividers are repositioned so that one leg rests on the pencil line (made during lining off, not easy to see in this photo but more visible in the previous one) indicating where the new plank’s edge will be. Then a mark is made on the spiling batten where the divider’s other leg falls.
The batten now has two points marked. The upper one, marked “K,” is for the edge of the plank that will be closest to the keel, and the lower one, marked “S,” is the edge closest to the sheerstrake. Note also that I’ve put a line on the batten indicating the location and orientation of the station at which the marks have been made.

I marked the spiling batten with “K” and “S” points at every station. Once that’s done, the spiling batten is unclamped and placed on a suitable piece of planking stock.

The spiling batten has been laid on a suitable piece of planking stock, and is held in place with spring clamps. On this particular spiling, I had made a pattern piece that closely fit the shape of the stem rabbet and attached that to the forward end of the spiling batten while it was still in place on the construction molds.
With the spiling batten in position, all of the station lines and  “K” and “S” points can be transferred to their appropriate positions on the planking stock.
I’ve removed the spiling batten, and clamped a batten along the line of “K” points that define the edge of the plank closest to the keel. Then I used a pencil to mark that line on the stock. I also marked the shape where the plank will lie in the stem rabbet by tracing along the edge of the piece I’d added to the spiling batten. Once the “S” points are similarly connected with a smooth curve, the plank can be cut out.
After fine-tuning the plank’s shape, the plank is fastened to the frames and along the stem rabbet.

All this may sound complicated and like a lot of work. But you have to do enough of it that you eventually get pretty good at it.