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Thread: asymmetrical birdsmouth taper

  1. #1
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    Default asymmetrical birdsmouth taper

    I've finished up the birdsmouth main mast for the Campion Apple and I'm moving onto the boom and mizzen next, but the plans call for an asymmetrical taper on the boom at either end. To accomplish this do I simply need to cut a taper on 4 of the 8 staves and arrange it so the tapered staves are on one side of the spar or is it more complicated than that?

    The asymmetrical taper goes from 65mm to 40mm at either end starting 750mm from end.

  2. #2
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    Default Re: asymmetrical birdsmouth taper

    I see you didn't get any answer to this

    Can you perhaps show us a drawing of what you mean? Im not sure what you mean by an asymmetric taper...
    R
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  3. #3
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    Default Re: asymmetrical birdsmouth taper

    It's really that simple.

    For example, if you're keeping the foot straight (top face of the boom) flat but making some taper at the ends, you put a bit of taper in each edge of each end of the top and bottom staves, some taper on the bottom edge each end of the two side staves, and smaller symetrical tapers in the edges of the four corner staves. Given boom stresses, you may have engineered the side staves a bit wider than any others making the spar more ovoid. After those tapers are in, you router or table say out the one notch per stave.

    You'll want to draw a bunch of sections to determine the exact dimensions you'll need but the connecting lines should be straight so probably you need only drawings for fattest part of the boom, the two ends, and perhaps just one mid section per end to be sure it looks right.

    Since it's the bird's mouth mating the corner of the right angle edge that makes the turn, no matter what you do, it will fit together.

    G'luck

  4. #4
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    Default Re: asymmetrical birdsmouth taper

    It seems to me that one would taper all the staves equally throughout the length. Doing so would maintain symmetry in every cross-section. The spar's asymmetry is introduced only during glue up. The staves would be blocked into a fixture that holds the upper edge/upper stave straight. Then all the other staves will conform. I suppose that to be mathematically correct, one would make the lower staves a bit longer to account for their greater path from one end to the other. That would be sort of gilding the lilly though.

    Jeff

  5. #5
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    Default Re: asymmetrical birdsmouth taper

    No need to reinvent the wheel... or write a new essay, when the topic is already well-documented --

    http://www.duckworksmagazine.com/04/...es/birdsmouth/

    https://cse.google.com/cse?cx=partne...sts&gsc.page=1

    http://rosslillistonewoodenboat.blog...llow-mast.html

    http://www.storerboatplans.com/wp/de...e-recommended/

    Once you understand the process... it's quite easy to extrapolate moving from a round spar to an elliptical spar, and/or to an asymmetrical spar. For the latter, jpatrick touches on the most basic solution.

    HOWEVER -- are you sure you want and need a hollow boom? Since a boom is down low, your extra effort won't gain you savings in 'weight aloft'... one of the prime motives for a hollow mast (or even a yard). And... often... for sail shape, a heavier boom is desirable. Though, stiffer is often an asset, as well, and one can make a stiffer boom - without making it so heavy as to be unwieldy - by going hollow.

    This is the balanced lug yawl version?

    ETA - and if you are set on a hollow boom, that taper would be far easier to accomplish with a hollow rectangle.
    Last edited by David G; 03-20-2017 at 12:15 PM.
    David G
    Harbor Woodworks
    http://www.harborwoodworking.com/boat.html

    "It was a Sunday morning and Goddard gave thanks that there were still places where one could worship in temples not made by human hands." -- L. F. Herreshoff (The Compleat Cruiser)

  6. #6
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    Default

    Thanks for the replies, I had forgotten about this post. As it turns out, the asymetric taper on the plans was actually a small mistake in the plan's line drawing, but it wasn't in the narrative. I emailed Tom D. And he confirmed the issue.

    I ended up building it as a birds mouth, but with a symmetrical taper.

  7. #7
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    Default Re: asymmetrical birdsmouth taper

    <sigh>
    David G
    Harbor Woodworks
    http://www.harborwoodworking.com/boat.html

    "It was a Sunday morning and Goddard gave thanks that there were still places where one could worship in temples not made by human hands." -- L. F. Herreshoff (The Compleat Cruiser)

  8. #8
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    Default Re: asymmetrical birdsmouth taper

    Quote Originally Posted by galleywench View Post
    Thanks for the replies, I had forgotten about this post. As it turns out, the asymetric taper on the plans was actually a small mistake in the plan's line drawing, but it wasn't in the narrative. I emailed Tom D. And he confirmed the issue.

    I ended up building it as a birds mouth, but with a symmetrical taper.
    Good, I was having trouble with the ''asymmetrical ''thing.
    '' You ain't gonna learn what you don't want to know. ''
    Grateful Dead

  9. #9
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    Default Re: asymmetrical birdsmouth taper

    I have a plan for a hollow birds mouth oar that call for an elliptical section to the oar shaft to give a wing shape to the blade end to beef up the stiffness of the shaft during the stroke of the spoon blade oar. It was given to me free and I will provide it to anyone who emails a request for it. It is really simple 2 staves flare and 6 taper normally, the two flared are the 12 and 6 position 2 of the tapered are on the left and 4 are on the right side giving the wing shape. It is strong and attractive looking behind the spoon blade.

    Tom...
    "Lord, please keep your arm around my shoulders and your hand over my mouth."

  10. #10
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    Default Re: asymmetrical birdsmouth taper

    Quote Originally Posted by Tom Hoffman View Post
    I have a plan for a hollow birds mouth oar that call for an elliptical section to the oar shaft to give a wing shape to the blade end to beef up the stiffness of the shaft during the stroke of the spoon blade oar. It was given to me free and I will provide it to anyone who emails a request for it. It is really simple 2 staves flare and 6 taper normally, the two flared are the 12 and 6 position 2 of the tapered are on the left and 4 are on the right side giving the wing shape. It is strong and attractive looking behind the spoon blade.

    Tom...
    OMG this ^^ is EXACTLY how and why I found the original thread started by Galleywench. I'm looking for plans/ideas to build some hollow birdsmouth oars. I need everything to be as light as possible in my build...
    R
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  11. #11
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    Default Re: asymmetrical birdsmouth taper

    Quote Originally Posted by PeterSibley View Post
    Good, I was having trouble with the ''asymmetrical ''thing.
    Here is a screenshot of the line drawing mistake:

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