Re: Butt or scarf?

Posted by Mark Camp on Oct 3, 2005

Good question, Pete. I'd be curious to know the answer. But...

As a guy who never got past ME 102, I've learned not to assume anything about strength questions in S and G construction. It seems that every actual engineering test on one of these questions has DISPROVED what I assumed.

A good example is the tests LeeG did on failure mode of filleted taped joints. Basically, the joints were SO strong that they made the plywood break at place where the tape ends, not the joint itself!

Another example is, I bought some balsa wood and used it as a "core", to make me a fancy, hi-tech patch on the deck of a Hobie cat. Naturally, I ASSUMED that for strength, you put the grain of the wood PARALLEL to the surface. I read not too long ago that the pros actually make REAL balsa core with the grain PERPENDICULAR to the surface! Turns out, the core is not there to add bending strength, like you would assume if it were planking. It is there to resist the shear forces at the center of the cross section, and balsa has an extremely high shear strength for the weight. Only if you shear it across the grain, of course :-)

I vaguely recall a web page report of a science project to answer exactly your question, 'which is stronger, an S and G butt joint or a scarf joint?' Not done by a real materials lab, but if memory serves, they found that the butt joint was the stronger, just the opposite of what you and I would ASSUME.

Anyway, these S and G joints are all probably way stronger than they need to be. Some of the old European boat-builders used two-to-one scarf joints (!) glued together with paint (!!!), and copper-nailed, to plank a boat. Doesn't apply directly to S and G, true, but I just mean this: I would not have ASSUMED they would do it that way.

In Response to: Re: Butt or scarf? by Pete on Oct 3, 2005

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