Builders' Forum Archives
Re: To Scarf or To Butt?
Posted by Mark Camp on Oct 28, 2005
Warning: long note follows/was reviewed before sending/does not seem to say much of anything, near as I can tell.
Scarf joints ARE probably prettier if you do them well. I have one or two that aren't real pretty, but they were my first tries. My boat don't got any butt joints yet. I think the deck has some, but it ain't on yet, and I don't read ahead cause it ruins the plot.
What I know of butt joint strength is from reading a lot here and other places on the web. Then I sat and over-analyzed it a few dozen times, like you probably have, and came to the same conclusion.
Course, at first it seems intuitively obvious that butt joints should be WEAK, not strong. We are accustomed to lumber plank (strong) and wood glue joint (weak).
But, epoxy and plywood is a whole 'nother matter. "The glue is way stronger than the plywood" as they say. And a plywood plank is weaker than a lumber plank (because some plys are running parallel to the joint). So the joint starts out...way stronger? about as strong? as the surrounding plywood. *CLC*, help us out here with the details, if you're on lunch break. Otherwise, you can go ahead finishing up the XXXXXXXX boat design in time for Christmas. Hope that made it past the censors.
But the biggy is the 'glass-epoxy skin on both sides. So strong (in tension especially, ie, on side opposite the applied force) that it kind of makes everything else irrelevant.
Now, it's easy to make a bad butt joint. (a) Don't apply enough wet epoxy to penetrate the wood fibers. (b) Let the temperature rise during early cure stage, so outgassing from the fibers is pushing the goo out). But those mistakes are easy to avoid. If you are nuts enough you could even warm the wood up to 120 degrees F for a few hours and then glue, letting the temp fall, which draws epoxy into the wood. I'm pretty close to being that nuts, but fortunately, I am lazier than I am crazier.
That is a tour of my limited, overanalyzing mind. You can see why at some point I just go out in the garage and start boiling some epoxy, or gouging the mortal remains of some unlucky okoume tree, and trying to get a boat out of it all, someday.
In Response to: Re: To Scarf or To Butt? by John Short on Oct 27, 2005