25858Re: another Birdsmouth spar thread.....
- Jan 3, 2003I think this is a bad idea if the idea is mostly practical. I have
been recently dueling with a friend who wanted to do birdsmouth
joints on the spar for a 38' wharram. The only ready supply of Sitka
we had was 8.50 a Bft, and only in 2x6. To cut out the birdmouths
required an extremely wasteful scenario that doubled the cost of the
spar, versus just strip planking the spar. But he had the idea he
wanted the birdmnouth experience. In the end he decided on a
different boat. This might not happen in your case, but there are
three, possibly four sources of waste in the B mouth: One is the
waste in the joints themselves, two is the waste that comes when
cutting to a particular section rather than creating a wood fiber
strip which can work in a multitude of sizes that a BM strip
wouldn't. Third is the fact that a polygon is an inefficient shape
versus the tube. Four may or may not be a problem depending on how
you break out the strips is a taper situation.
Further issue would be the fact that tubes of whatever configuration,
with uniform wall thickness (best case if not the BM case) are good
for keeping a spar in collumn with a stayed spar where loads are in
compression along the tube. This is not the situation with the
micro, it is loaded in a cantilever mode, and mostly in one
direction. Bolger understood this when he built the box section mast
I also feel that if you must have an octagon, you should consider
biscuits, and vacuum bagged assembly (shop vac).
For the tappered plug, jam a ball or celophane covered shuttlecock up
the tube, pour in some epoxy bog. When that kicks pour in some more
epoxy, but use a piece of cheap wood like any 2x stock, to take up
most of the space. The butt end of this shouldn't require much
tapering out, since there will be little bending there, but if you
want to, you could jam a lump of styro in there to feather it out.
Anywho... It is getting increasingly hard to make an economic case
for a noble wood spar.
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