Birdsmouth Masts/?new idea? on hard-spots

Discussion in 'Wooden Boat Building and Restoration' started by Robert Miller, Jul 19, 2004.

  1. Robert Miller
    Joined: Dec 2003
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    Location: Rhode Island

    Robert Miller Junior Member

    I'm considering a birdsmouth mast for the lug-rigged boat we are now building. Have been wondering (worrying?) about best way to place solid pieces inside for external mast fittings. ... And still fabricate a plug that avoids stress risers.

    It seems to me that the irregular hollow interior of the birdsmouth method, would make this more difficult than it would be with a retangular or true round hollow mast. (Where, of course, stress risers are still an issue and must be engineered out of the design.)

    I've come up with what MAY be a solution. This will be my first attempt at a birdsmouth mast (first boat I ever built as well) and would very much appreciate the thoughts of those more experienced.

    I've thought of mixing up a batch of epoxy containing at least 50%, or more, sawdust. This should be pretty thick.. and would be troweled into place at the proper points before closing up the birdsmouth strakes. This would seem to offer several advantages. First, the "plug" will conform on its own to the irregular shape of the interior of the mast (eliminating the labor of that task). Second, it will be strong enough to provide the intended function for external mast fittings. Three, it could be run out along the strakes, in both directions... in contact with the strakes, but using less of it as you go... the intent being to avoid the hard-spots that create stress risers... at least as much as possible.

    Any thoughts on this idea?

    Robert
     
  2. PAR
    Joined: Nov 2003
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    Location: Eustis, FL

    PAR Yacht Designer/Builder

    I've built several masts with this construction method and have adopted this technique to address the "hard point" issue.

    I build the mast as normal, but place clear box tape on two of the joints opposing each other in the section. When dry, I have two halves, that can be opened and tapered hard points fitted, just like a box or built up sectioned stick would be.

    Yes it does take a little more time to do all the pieces for the tapered mounts, but not as much as you'd think. I do them all up as a single tapered piece, then rip them to size on the table saw and insert them into the stick with thickened epoxy.

    Then the stick halves get buttered with goo and put together. Everything else is as done on any stick of this construction method. I don't plug the ends of the mast by reaming out the hole and fitting a plug, but rather like to fit up a cap to fit the hole. My thinking is, if I have a slight opening of a joint there, it will be under the cap, not directly exposed to weather. The cap is nothing more then a hard wood mushroom shaped plug, designed to fit the opening on the end of the stick. I bed the underside of the cap to mast top surface with 3M 5200.

    I don't like the idea of just smearing in thickened epoxy, seems I'd be defeating the idea of tapering the hard point. I use as little epoxy as I can. In fact most of the masts I've built used very little and were much lighter for it.
     
  3. Aharon
    Joined: May 2011
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    Location: Israel

    Aharon Junior Member

    tapering of hard points

    Par, sir, I don't follow.
    You said "I do them all up as a single tapered piece, then rip them to size on the table saw and insert them into the stick with thickened epoxy."
    Aren't they supposed to be tapered in both directions? Or is it ok to insert them tight (a snug fit) in the right points? How long is each "hard point" insert?
    Sorry for the bombardment, but I intend to build either a round, tapered mast or a rectangular one, witch cross-section resembling an inverted waterdrop.
     

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  4. PAR
    Joined: Nov 2003
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    Location: Eustis, FL

    PAR Yacht Designer/Builder

    This is quite an old thread, but I make the "swallow tails" (hard point reinforcements) as long, tapers in a fairly thick piece of stock, then I mill it to stave width dimension as required on the table saw. Yes, both sides of the swallow tail are tapered, meaning I'll butt the milled pieces together inside the mast during assembly. Sometimes I'll make a double ended tapered piece, but it's more trouble for an out of sight piece.

    Your proposed mast section shows eight odd shaped pieces, which looks like it could easily be 6 or possably 4 pieces. There also doesn't appear to me much gluing surface available for that section as well.
     
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