When a kayak hull is flexed from passing over a wave, the individual strips do not run the whole range of stresses such as shear, tension and compression. The whole hull acts as a beam. When lifted by bow and stern the deck of the kayak is in compression (as is each strip), the bottom is in tension and the sides of the kayak are in shear thus each strip is exposed to very simple stresses. Even if the hull was to be twisted, the torsional stresses of the hull are manifest in the strips only as shear forces.

As a butt joint performs adequately in compression and shear, only tension forces need concern us when dealing with primary hull stresses. If the butt joint was to seperate when under applied tension load then the surrounding strips and the skin must absorb those forces the strip can no longer take. This is a stress raiser and if the hull was to be exposed to near ultimate loads this is the area where it would fail. We all appreciate that butt joints work adequately but it just goes to show that the hull is overdesigned for 99.99 percent of its life.

Another load applied to the hull which is probably more of a concern is impact. This happens when a wave drops us on a rock or we forget to tie the yak down and we drive away :-) never happens right? In this load case the hulls outer skin is in compression at the point of impact, the wood is in shear and the inner skin is in tension. Some distance away fom the point of impact the tresses are reversed. If the point of impact were to be applied to a section of the hull where there is a seperated butt joint then the two skins must take all three stresses as the butt joint is not able to transfer the shear stresses. Agreed the odds of impacting on a joint are small but we are discussing principles here. Here I think the butt joint severely compromises the hull's strength.

One of the main reasons for strip building was to create a light, easy to build, beautiful "wood" boat. To some a joint in the wood is unsightly, to others it doesn't matter. If one takes the time to properly grade the strips and match for grain and color, a scarf joint becomes nearly invisible. I know this is important to some as they look for 20 foot wood and even refuse to accept staple holes in the strips.

So in practice a butt joint is adequate, in theory it isn't, so the only governing concern would be esthetics.

Bram