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25858Re: another Birdsmouth spar thread.....

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  • proaconstrictor <proaconstrictor@yahoo.c
    Jan 3, 2003
      I 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
      for Anhinga.

      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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