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Why a Cutting Allowance for the W17 Skin?

One question that's starting to crop up fairly often is this. Why do the plans not give precise offsets for the skin panels … why the 5‑10 mm margin on the Nesting Plans?
A few builders even want to plug the panel offsets into a computer and have numerical controlled (NC) machines cut out their plywood for them! In a perfect world, that might indeed be a fine way to go, but first let's look at the issues here.

If the part can be totally defined in a panel less than 8' long, then we could give exact dimensions with a low risk of major problems, but none of the shell panels for ANY of the three W17 hulls fit that criteria. Even for parts cut from one sheet, give 3 first‑time builders the same offsets and dimensions and I highly doubt they'd end up with identical boats by the time it's all fitted together and faired off. It's my experience from building some 20 small boats that persuaded me to have the long shell parts initially cut with a small margin for the W17, so that the wood was there to make up for small, relatively unimportant errors, even if it means a little more trimming work on the building frame.

Building a 7'‑9"pram dinghy is certainly possible with NC but to use NC for the shell panels of the W17 would first require that 8' plywood sheets be scarfed together into sheets of 15'‑10"x 4' in order to have a sheet large enough for even the ama skins. (The longer main hull shell panels would first need about 2 feet of a 3rd sheet attached.)

Here is where the problems start.
Making a successful scarf across the full 4ft width of a plywood sheet is very difficult and would be a real challenge for the first‑time or even average boat builder. So when working through the design of the W17, I choose to dimension just parts of the skin that would fit on a standard 8ft long plywood sheet. These could then first be cut out BEFORE joining them so that scarfs would not be longer that necessary—not longer than the required width of a skin panel. But scarfing has its own hazards.
If you read my article on Scarfing on this website, you'll note that if the plywood is not held down firmly to the work surface while being machined, the router blade will simply remove what's lifted up in its way and in seconds, your sheet is now anywhere from 10‑20 mm shorter than planned! It's true that a butt joint could remove that risk, but such a joint is ugly, heavier, rarely as strong and causes enough problems of fairing and fitting, that I now rarely use it—especially when good short scarf joints are not difficult with the right technique.
There is also the question of how well the two 8ft long parts of each shell panel are aligned to each other. They have to be 'perfectly aligned' if they are to be precut on an NC machine or they will either not fit the frames at the ends or fit in a way that causes buckling and unfairness to the panel. By providing a datum line down each sheet that needs to be aligned when scarfing and also a small margin on the sides of each sheet, one at least has a better chance of creating long panels that are usable.

So NC cutting and precise definition of skin panels is not for me at this stage. Scarfs that are not straight (or not perpendicular to the datum line) and have to be recut, will quickly shorten a sheet by up to 50 mm and if cut 'exact' they will no longer fit unless the boat is then made shorter than the plans call for. Couple that to the other issue of panel alignment and I hope you will come to see the wisdom of providing ply panels with a cutting margin. As I said earlier, in a perfect world this would not be required, but experience has shown me that we are clearly still far from perfect.
Having clarified that, cutting bulkheads, stem pieces and certain other small parts to exact dimensions, should work just fine—even the beam parts and fairings, as these were checked by building a ¼ scale model.

mike


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