Michael Daly wrote:

And other people have done the same with different results. However, we are
talking about fiberglass boats here, not plywood. When you load a butt joint in
tension, the asymmetric, single-sided joint will fail with a significant flexure
in the joint. If it is symmetric, double sided, it will be in pure tension.
The difference in strength is considerable. The difference in fatigue
performance is considerable.

Single-sided seams along a sheer line will _buckle_ if the hull is subjected to
serious load - I've seen this happen. It buckles because the joint is not
symmetric. Under those conditions, the single sided joint can split - I've seen
this happen too.

Why do you assume that _your_ experiments count for more than the experience
that many others have had with single-taped joints? Good kayaks use a lapped
and double-taped seam. That is much stronger than a butted joint and much, much
stronger than a single sided seam.

A sheet of paper is also thin enough to fold. Once a tear is repaired
with tape that part is stronger than the rest of the sheet. That's what
you see in a butt join. It's thicker and more rigid than the rest of
the hull. It won't flex. It won't fold. It can take more stress than
the original hull before failing. If the hull's going to fold and fail
it won't be at the butt join.

Longitudinals and butts aren't comparable. Before there was
firbreglass or plywood plank-on-frame hulls would work open along the
seams but not at the butts where the planks were joined end-to-end.
Longitudinals, being long and thin, are weaker.

Butt joins are strong. The reason people scarf plywood and feather
fibreglass is to get sufficient surface for adhesive strength. With
butt joins the whole butt is the adhesive surface. A 2" butt is
stronger than a 1/2" feather. As I wrote, I would probalyhy use a 4"
over 2" butt.