Math 1, Tom 0

I got this from Fred:

For the rub rail scarfs, maximize the gluing surface.

All along, I had been thinking that I wanted the scarf joints to go across the rails, rather than up and down. It seemed logical to me that by scarfing them across the vertical axis, the stresses put on by the horizontal tension of trying to bend them around a curving boat would not work to pull the joints apart. Despite all of the scarf joints I had seen on the pictures, I thought I knew better.

Maximize the gluing surface.

I did some maths (with the help of Charlie and a calculator), and found that a six-inch scarf on the 3/4" axis will create 4.5 square inches of surface area (6 x 0.75 = 4.5). Whereas a six-inch scarf on the 1.5" axis will create double that. Sure, laugh at me now, but that was a real shocker when I did 6x1.5 and came up with nine.

So I cave to the superior math.
I have seen the light.

Fred went on to say

Ideally when you attach them to the rest of the structure, the joints will be placed in a gentler part of the gunwale curve, closer to the bow or stern, rather than at the apogee of the arc amidships. With the length of material you are using, you might even consider giving each rail two scarfs, adding a small extension at each end of an unmolested 10' or 12' piece, setting the joints in a relatively unstressed part of the plan curve.

If your plans include a provision for a full-scale layout of the boat's shape, you could determine the best spot for the scarfs. Draw out the curve of the gunwale in chalk on the ground, then find the straightest sections.

I'm afraid that's not going to happen. I am not going to double my scarf joint labor. I will have a 9'6" piece scarfed to a 6' piece. That will put the scarf joint not exactly at the center of the curve. It won't be too far off, but it won't be at the center. And, just to be safe, when I make the butt joint in the plywood for the bottom and sides, I will add a layer of fiberglass around the rub rail scarf joints, as well.

The bad news: it's going to rain all weekend. I won't get to work on the boat outside. Rats.

The plus side, the Samurai is coming along well: