Woodworking Tip of the Day: Spiling Planks

A curved hull doesn't care what your tape measure thinks….. Today's tip: spiling planks — take the shape off the boat with a batten and a compass, then put that shape onto flat stock you can actually cut.

Why it matters: On a Chamberlain Marblehead gunning dory (or any traditional planked boat), the next plank isn't a rectangle. It widens amidships and tucks toward the ends. WoodenBoat and Greg Rössel teach the classic fix: nail a thin spiling batten into the opening, swing fixed-compass arcs from the station points onto that batten, then reverse the process on your cedar (or whatever stock) and connect the dots with a fairing batten. Lost Art Press (Walker & Tolpin) spells out the geometry — same divider setting, arcs back from the recorded marks, intersections recreate the original points. Guessing from a tape is how you burn a good board.

Materials

• Spiling batten — same length as the plank-to-be, thinner and narrower than the opening (thin lauan / plywood strips stapled with a scab or butt strap work well)
• Planking stock laid flat on the bench
• Scrap board or a clear spot on the boat / bench for the "sacred circle"
• Optional: hot glue or small tacks / staples to hold the batten without twisting it

Tools

• Compass or dividers (fixed setting for the whole plank)
• Pencil
• Fairing / marking batten (straight-grained, bends evenly)
• Bandsaw (or handsaw) and plane for final fit

Steps

1. Make the batten. Cut (or staple together) a thin strip the full length of the plank gap but narrower than the opening. WoodenBoat builders often use lauan strips joined with a scab or butt strap.
2. Tack it in place — no twist, no edge-set. Start amidships and work toward each end, as if spreading fabric. Forcing the batten edgewise or twisting it into the gap writes a lie into every mark that follows.
3. Lock one compass setting. Choose a span that reaches every station along the plank. Draw a "sacred circle" on the boat, the bench, or scrap — not on the spiling batten — so you can restore the setting if the dividers bump (WoodenBoat).
4. Swing arcs at the stations. From each reference point (usually frame centerlines on the upper and lower edges of the gap), put one compass leg on the point and draw an arc onto the batten. Keep that same setting for every mark.
5. Transfer to stock. Pull the batten, clamp it flat on your planking stock, check the sacred circle, and reverse the process. Walker & Tolpin: from each recorded arc, swing two small arcs with the same radius — their intersection is the original station point. Connect those points with a fairing batten.
6. Cut and fit. Bandsaw outside the line, then plane to a sweet fit. The batten also helps you place the shape efficiently on the board so you don't waste stock (Rössel).

MOAM's shop note: Biggest miss is changing the compass setting halfway through — or "helping" the batten with a little edge-set so it sits prettier. Both turn a fair plank into a puzzle piece that almost fits. Second miss: drawing the sacred circle on the batten itself, then confusing it with a spiling mark. Keep that circle off the pattern. Joggle sticks and spiling blocks work too, but the compass method is the one the books call more accurate — and it's the one that matches how we think about lofted lines on a Marblehead dory.

MOAM Out.