I did this three or four days ago. And then checked my work today and had to sand clean and remark it all.

If you want to make a decked boat, with trapezoid-like bulkheads below kissing the bottom of the deck, the starting point is the deck line. 18' foot strips of clear vertical grain fir cut to 1/4" inch thick are easy to bend and they make a good marking line. But at 1/4" they not only bend in and out they bend all too easily up and down too.

The second time around I made more layout lines: 3' feet and 6' feet from the bow, dead middle and 3' feet and 5' feet from the transom and then used deep-reach clamps to pin the marking strips in three places only. When the top of the marking strip up to the bottom of the gunwale was the same on both sides, at each layout, I ran the pencil again. Pressing hard.

I had to buy this deep-reach vise-grip-like clamp. 36 bucks. But it was worth it. Then I made one bulkhead (no photo yet) and called it a day.

I'll make the deck and seating depressions out of OSB sub-sidiing which I can afford to make mistakes with. Once that looks right I'll cut the more expsive Meranti Hydrotech. I'm still thinking about some Cape Falcon like skin on frame stuff at the ends. Those parts can bulge way up too, so the boat would be easier to right again, if and when it gets flipped. I'm not planning on flipping it. But you never know. I did flip the Dayak once. Two years ago. And had to swim. I was 69 then. I guess I'd better not flip this big boat. I'm 72 now. The Dayak I can lift. This one I can't.

Clear fir 18 20 and 22 feet long is almost extinct. When my lumber yard gets it i buy it.

You can make a long line marking strip from a 20' masonite-like siding board. Cut off a 3/4" strip. Bend that.