Marine ply for a small launch
A four and a half metre launch built by one person in a rented unit. Twenty eight pieces, eight of them 2400 mm long, and marine ply at a price that makes you count twice before you cut once.
The job
A hard chine plywood launch, 9 mm marine ply throughout, on 2440 by 1220 sheets. The builder works alone, buys ply a few sheets at a time, and has nowhere to store a mistake.
The hull is longer than a sheet, so each bottom and side panel is scarfed from two pieces and glued up on the bench before it goes anywhere near the boat. That is ordinary practice for anything much over two and a half metres, and it is why the list has eight long pieces rather than four panels.
The cutting list, once every curved edge is reduced to the rectangle its piece is cut from:
- 4 bottom panel halves at 2400 by 520
- 4 side panel halves at 2400 by 420
- 2 sole panels at 1600 by 400
- 1 transom at 1180 by 600
- 3 bulkheads at 1100 by 480
- 3 frames at 900 by 260
- 3 seat tops at 900 by 300
- 8 knees at 320 by 240
28 pieces, 14.7 square metres. The hull pieces, sole panels, transom and bulkheads all have their face grain locked along the length, because on a structural panel the veneer direction is a strength decision rather than a looks one. The frames, seat tops and knees are free to turn.
How it was going before
Sheet by sheet, drawn out in chalk on the ply itself. The hull pieces came first because they are the frightening ones, and they went on one piece per sheet so there was room to fair the curve without running into another part.
That is a defensible way to build a boat, and it is why ten sheets were on the order. Eight of them carried a single long hull piece each and were then most of a sheet of marine ply leaning against a wall.
Ten sheets against 14.7 square metres of pieces is 50.5 per cent waste. On marine ply that was the largest single line in the build.
What changed
The rectangles went in, with grain locked on everything structural. The kerf was 3.2 mm and the trim 12 mm, because marine ply arrives banded and the outer 10 mm of an edge is often bruised.
The important setting was not a setting at all. It was accepting that two long pieces can share a sheet if you cut it in the right order. Two 520 mm bottom halves side by side come to 1043 mm, which leaves 153 mm of the sheet, and two 420 mm side halves leave 352 mm, which is where the seat tops went.
Six sheets.
Job sheet
- Material9 mm marine ply
- Sheet size2440 x 1220
- Sheet area2.977 m2
- Pieces28
- Piece area14.7 m2
- Kerf3.2 mm
- Edge trim12 mm
- GrainLocked on 14 pieces
- Sheets before10
- Sheets after6
The layout
Six sheets. The ones that matter are where a long hull piece shares its sheet with something else, because that is where eight sheets became five.
Cutting order matters more here than anywhere else on this site. A 2400 mm rip in 9 mm ply on your own is a two trestle job, and it has to be the first cut on the sheet. See cutting order on a panel saw.
The result
6 sheets instead of 10. Waste down from 50.5 per cent to 17.6 per cent. At around 95 dollars a sheet for 9 mm marine ply, four sheets is about 380 dollars, on a build where the ply was the largest single cost.
Why that waste figure flatters us
17.6 per cent is the second best number on this site, and in one sense it is earned: eight long hull pieces are big clean rectangles, and big clean rectangles tile a sheet beautifully. The builder really did buy four fewer sheets.
It is also not the whole truth. Every hull piece goes in as the rectangle it is cut from, and the real panel is a curve inside that rectangle. The ply between the curve and the edge of the box is waste, and Cut Solver never sees it, because it never sees the curve. It sees a 2400 by 520 box.
So do not put this 17.6 per cent next to the 13.9 per cent on the kitchen job and conclude that boatbuilding wastes less board than kitchen fitting. On the kitchen the parts really are rectangles and the number means what it says. Read this one as: six sheets is the right number of sheets to buy, and what happens inside each rectangle is between you and the jigsaw.
The builder's own view was that the number they cared about was ten becoming six, and the percentage was our problem rather than theirs. That is probably the right attitude.
How long it took
Reducing the panels to bounding rectangles off the plans took about forty minutes, which is the real work and would have had to happen anyway. Typing twenty eight pieces took ten minutes. The layout ran in under a second, and it was re-run four times while the builder tried different trim values to see what the bruised edges were costing. It was 12 mm in the end.
Bounding rectangles, not shapes
For any curved panel, enter the smallest rectangle the piece fits inside, then add your own margin for fairing the curve. Cut Solver will not do this for you and does not pretend to.
Scarf before you cut, not after
The eight hull halves are cut oversize on their scarfed end so there is something to plane the bevel into. That allowance is in the numbers above, and it is the sort of thing to add to the part size rather than to the trim.
Face grain on structural ply
Locking grain on a hull piece is a strength decision. Do not leave those parts free to turn because the layout looks tidier. See grain direction.
The other five examples
Every job here is written up the same way, with the sheet count before, the sheet count after and the time it took.
- Eighteen kitchen carcasses from melamineKitchen fitting. 108 parts, 13 sheets down to 11.
- Acrylic letters nested for a sign shopSignmaking. 41 letters and roundels onto a single sheet.
- A sideboard cut with the grain running one wayFurniture making. 14 grain locked parts, 4 sheets down to 3.
- A term of plywood for a school workshopEducation. 216 parts for 36 pupils, 6 sheets down to 4.
- Engineered oak floor cut from lengthsFlooring. 81 pieces from 2200 mm boards, 64 down to 58.
Read on
The settings behind this job, and how to price the next one.