Work out how tall to cut each companionway dropboard so the set fills the opening once the joints overlap, and how thick the boards will be stacked in their stowage bag or rack.
Use it when you are making a new set of washboards, switching from one big board to two or three smaller ones, or replacing a cracked board and want the new set to close the opening with the same lap at each joint. It also tells you how much room the set takes up in a cockpit locker or a bag on the companionway ladder.
The calculator works on the clear opening height measured inside the slides, at the centre of the opening. It does not cut angles: most companionways taper, and the slides are rarely parallel. Use the face height here for the board count and joint position, then trace the taper and the slide bevels with a cardboard template.
Each joint between two boards needs a lap: the lower board’s top edge and the upper board’s bottom edge overlap so rain driven against the boards runs out rather than in. Every lap eats into the height a board contributes, so each board face has to be taller than the opening divided by the number of boards.
With n boards there are n − 1 joints. The faces together must cover the opening plus every lap:
face height = (opening + (n − 1) × lap) ÷ n
stowed stack = n × board thickness
The check line under the result multiplies back: n faces minus the overlapped height at each joint returns the opening exactly. The stowed stack is simply the boards lying flat on each other; rabbets and laps nest when the boards are stacked the same way up.
Two boards in a 28 in opening, ½ in lap. face = (28 + 1 × 0.5) ÷ 2 = 14.25 in. Cut both boards to 14.25 in tall at the centre line. Stowed, two ¾ in boards are 1.50 in thick.
Three boards in a 30 in opening, ¾ in rabbet lap. face = (30 + 2 × 0.75) ÷ 3 = 10.50 in. Three boards make the top one small enough to lift out alone for ventilation while the lower two keep green water out of the cabin.
Four ½ in boards for a 32 in opening, ⅝ in lap. face = (32 + 3 × 0.625) ÷ 4 = 8.4688, rounded 8.47 in. The stowed set is only 2.00 in thick, which fits in a narrow slot beside the ladder.
| Opening | 2 boards, ½ in lap | 3 boards, ½ in lap | 3 boards, ¾ in lap |
|---|---|---|---|
| 20 in | 10.25 in | 7.00 in | 7.17 in |
| 24 in | 12.25 in | 8.33 in | 8.50 in |
| 28 in | 14.25 in | 9.67 in | 9.83 in |
| 32 in | 16.25 in | 11.00 in | 11.17 in |
| 36 in | 18.25 in | 12.33 in | 12.50 in |
All values from the same formula as the calculator. Add your saw kerf and planing allowance when you cut blanks.
Dividing the opening height by the number of boards and forgetting the laps leaves a gap the size of the laps added together at the top. Using the height of the old boards as the opening carries their wear and swelling into the new set. Measuring at the slide face instead of the clear opening between the slides gives a board that cannot enter the track.
Somewhere between ⅜ and ¾ in is common. Thicker boards can carry a deeper rabbet. More lap sheds water better but leaves less face on small boards.
Yes, for square-edged boards butting together. The face height becomes opening ÷ n. Expect leaks at the joints in rain.
If the lap equals or exceeds the face height, the boards would overlap entirely. Reduce the lap or the number of boards.
Yes. Enter every length in millimetres instead of inches and the result is in millimetres. The labels still say inches.