How to Scribe to a Wavy Wall or Hull with a Washer and Pencil

This is catching the MOAM's attention today….. how many good carpenters and boatbuilders own a fancy contour gauge and still reach for a flat washer and a pencil. Scribing a part to a wavy wall or a curved hull looks like magic the first time you see it. It's really just a bit of geometry, and once you understand that geometry you'll know where the washer trick shines and the one spot where it'll fool you.

Why it matters: Nothing in an old Marblehead house is plumb, and nothing in a boat is straight. Gougeon Brothers' Epoxyworks calls the washer scribe "an old boat builder's method" for fitting wood to a gently curved surface, and it's every bit as handy on the job site. On the Chamberlain Marblehead gunning dory, it's the quick way to fit a floorboard, a riser or a thwart knee where it meets the planking. On the GC side, it's how a cabinet end panel, a filler strip or a countertop ends up tight to a plaster wall that's been wavy since Coolidge was governor, with no fat bead of caulk to hide the gap. The washer costs a few cents and lives in your apron. A Fine Homebuilding reader who keeps a whole collection of sizes for scribing moldings points out another bonus: a washer rarely mars a finished surface.

Materials

• A handful of flat washers in different sizes. Common USS flat washers, as examples (nominal sizes, so measure yours): a ⅜″ washer is 1″ outside and 0.438″ inside, a ring about 0.28″ wide; a ½″ washer is 1⅜″ outside and 0.562″ inside, a ring about 0.41″ wide; a ⅝″ washer is 1¾″ outside and 0.688″ inside, a ring about 0.53″ wide. Fender washers give you a wider ring for bigger gaps.
• Wide painter's tape, so the line shows up on dark or finished stock
• Cardboard or ⅛″ door skin (lauan) for a template, or thin pine or plywood strips
• Shims and a scrap with a straight edge for a test scribe

Tools

• A sharp pencil (an ordinary one; the point has to sit in the washer's hole)
• Level, square and a bevel gauge
• Clamps or a couple of temporary screws to hold the part in position
• Jigsaw with a tilting base, or a handsaw
• Block plane, rasp, and a belt sander or sanding block for the final fit
• A pencil compass or dividers for the steep spots (more on that in step 5)

Steps

1. Set the part the way it will finish. Plumb, level, square to its layout, whatever "right" means for that part. Family Handyman shims the cabinet until the top is level and the side is plumb before drawing a line. Then slide it in until it touches the high spot. Put index marks on the part and on the wall or hull so every test fit goes back in exactly the same spot.
2. Find the widest gap and pick a washer. The offset, call it s, is the distance from the washer's outer edge to the pencil point. With the point pressed against the side of the hole nearest the wall, that's the ring width: outside diameter minus inside diameter, divided by 2. Epoxyworks says to pick a washer that is just large enough, measuring from the outside diameter to the inside diameter, to cover the widest part of the gap. If s is smaller than the gap, the line runs off the edge of the part. Remember the part is going to lose s at the high spot and move s toward the wall, so leave extra stock. If it has to land on a layout line, hold it back exactly s from that line with spacer blocks before you scribe.
3. Test the washer before you trust it. Hold the pencil point against the wall side of the hole and you get the ring width. Let the pencil sit in the middle of the hole and the offset grows to about half the outside diameter. Pick one way and stick with it. Run the washer along a straightedge on a scrap and measure the line. That's your real s, not what the package says.
4. Scribe. Tape the edge, lay the washer flat on the face of the part with its outer edge against the wall or hull, and draw the line. Popular Woodworking's tip is to hold the far side of the washer down with a finger while you trace. Keep the washer touching the surface the whole way and take it slow. Epoxyworks is straight with you about the limits: the washer works well on gently curved surfaces but won't accurately follow sharp outside corners, sharp dips or rough, lumpy surfaces. On a lapstrake hull like the dory, the plank laps are little steps, and a round washer rounds them over. Mark each lap separately with a block or dividers.
5. Mind the direction. This is the part most folks never think about. When you cut to the line and move the part in, every point on it travels the same distance in the same direction. A washer doesn't work that way. It always puts the pencil s away at right angles to the surface. Where the surface is about square to the direction the part moves (a gently wavy wall, a cabinet sliding straight back), that's the same thing and you're fine. Where the surface runs steeply, it isn't. Measured along the travel, the washer's line ends up s ÷ cos θ from the surface, where θ is how steeply the surface runs compared with the part's edge. Move the part s and you're left with a gap of s × (1 ÷ cos θ − 1). With a ½″ offset, that's about 0.02″ at 15°, about 0.08″ at 30°, about 0.21″ at 45°, and a full ½″ at 60°. A compass or dividers with the legs held at one fixed bearing, parallel to the direction the part will move, puts every mark exactly s along the travel, steep or not. That's why Fine Homebuilding says to hold the compass level along the whole run of a wall panel, and warns that a fit that's good in spots and way off in others usually means the compass wandered. It's also the heart of WoodenBoat's method for fitting a bulkhead: square level lines off the high points, then lay off the same distance on each one, all parallel to the line of travel. On the dory, a floorboard or knee meeting the hull at a gentle angle is washer work. A bulkhead lowered past the flared sides is steep work, so use a compass with the legs held plumb, or level lines and dividers. To shrink the error with a washer, get the part closer and use a smaller washer.
6. Make a template first on anything costly or complicated. Scribe a piece of cardboard or door skin, cut it and test it, then trace it onto the good stock. WoodenBoat's "skeleton" pattern of thin pine strips, tacked in place and joined together, is the go-to for thwarts, floor timbers and bunk flats. For countertops in old houses, JLC's Roe Osborn builds a template from 3″ to 4″ plywood strips and records the wall with a spiling block. You can wreck all the cardboard you want. Mahogany costs money.
7. Cut to the line and back-bevel. Saw just outside the line with the jigsaw base tilted, so the face edge is the long point and the back is relieved. Fine Homebuilding says to back-cut "a few degrees." Family Handyman says to take more off the side that won't show. A Woodweb cabinet installer cuts loose end panels at a 30° bevel so he can tune them with a block plane and let the sharp edge bite into the drywall. Then plane, rasp or sand to the line. On a boat part, the edge usually needs the real bevel to match the hull, so take it off the hull with a bevel gauge instead of just cutting a relief.
8. Slide it in, check it, tune it. Move the part exactly s, straight in, and look for light. Popular Woodworking's tip is to scribe again with a smaller washer to tighten up the fit. Take the high spots down a few strokes at a time.

MOAM's shop note: The most common mistake I see is scribing a part that isn't sitting the way it'll finish. If a cabinet's out of plumb, or a floorboard's cocked a hair from its layout, the line is drawn for a slanted move, and when you push it home straight the fit's good in spots and lousy in others. Set it right, mark it, then scribe. A close second: grabbing a big fat washer for a ⅛″ gap. Bigger offset means more wood off, more travel, and more error on the curves. Use the smallest washer that covers the gap.

Sources: Brian Knight, "Flat Washer Scribe," Epoxyworks (Gougeon Brothers); "Scribing with Washers," Popular Woodworking; "Roundabout Scribing," Woodcraft Magazine Tips & Tricks; "A Scribing Tool You Can Find Around the House," Fine Homebuilding #297; "Basic Scribing Techniques" and "Scribing Made Simple," Fine Homebuilding; "How to Scribe for a Perfect Fit," Family Handyman; "Fitting Bulkheads," WoodenBoat; Roe Osborn, JLC Q&A (January 2020); "Scribing Loose End Panels," Woodweb Knowledge Base; ASME B18.22.1 USS flat washer dimensions.

MOAM Out.

Tools for this tip

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• General Tools 843/1 pencil compass and scriber: for steep curves where a washer won't follow
• General Tools 450-12 12-inch dividers: stepping off a fixed scribe distance

Use whatever you already have in the shop; these are just the ones that fit this job.