Builders' Forum Archives
Re: Gaps in Stitching
Posted by Laszlo on Mar 20, 2007
I'm guessing that your stitches are too tight in some spots. Loosen them a bit where there's no gaps and tighten them a bit where there are gaps. As the CLC post said, tiny gaps will disappear when you fillet the seams.
Fairness is much more important than no gaps, as long as the gaps are 1/8 inch or less. If the boat has developed panels - panels that only curve in one direction - it's actually better to use as few stitches as possible and to keep them as loose as possible. The only time you need close tight stitches is where the plywood is "tortured", that is, being forced to bend in 2 directions at once.
The actual strength of a seam comes from the glass cloth. All the putty does is fill the gaps and provide the minimum bend radius necessary for the cloth. The kind of fiberglass tape we use here will pull away if the bend radius is less than 5/8" (about the curvature of a US nickel). Even if we can force the cloth to stay in a tighter corner through various means, the glass fibers are now folding instead of laying smooth, so the corner becomes a stress concentrator and the joint is weak. Hence the fillets which smooth the transition from one panel to the other.
There is even a school of thought that a constant 1/8" or smaller gap is actually better than tight contact. The idea is that wood bumping wood makes for a "hard" spot which concentrates stress. A putty-filled gap is supposed to distribute stress evenly across the entire joint. For boats the size of the ones we're building, I don't think that makes that much difference, but I've built boats both ways and haven't had problems with either.
And that's my point - adjust your stitches to get the fairest shape and if that results in a few gaps, don't worry about it. You'll have a strong good-looking boat, anyway.
In Response to: Gaps in Stitching by Norm Brunelle on Mar 19, 2007