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Thread: Bird's Mouth Heel-Step interface and Scantlings Affirmation

  1. #1
    Join Date
    Aug 2016
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    Redondo beach CA
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    Default Bird's Mouth Heel-Step interface and Scantlings Affirmation

    I'm about to start ripping staves for my alternative rig. 76 square feet of boom-less Standing Lug. Mast and spar, 11 feet each, fitting neatly within my 16 ft. Sid Skiff leaving me plenty of room for rowing, gear, passengers, etc.

    I am a little anxious:
    I used what I presume to be oft-used formulas (from Ian Oughtred?) for mast and spar diameters: 1/64 length of spar, and 1/52 length of mast PLUS 10% for Bird's Mouth construction. Then I used 0.4 and 0.2 as multipliers of that diameter for the stave dimensions (a la Duckworks).

    So my mast will be 2 1/2 inches at its widest and the spar 2 1/4 inches. Does that seem right? I question it because my current Sprit Sail has a mast 3 1/4 and the sprit is only 1 3/4. (The sprit rig is 100 sq. feet total with jib).

    My other concern is making the bird's mouth mast heel mate with the mast step. Can I make the plug at the heel 2 inches longer and that will be strong enough? My skiff has a socket at the step about 1 1/4 inch diameter and about that deep which accepts the solid Sprit sail mast that tapers to 1 1/4 inches.

    thanks.
    Jeff

  2. #2
    Join Date
    Oct 2008
    Location
    Walney, near Cumbria UK
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    Default Re: Bird's Mouth Heel-Step interface and Scantlings Affirmation

    The sprit is purely a compression column, whereas the yard is both in compression and bending so it needs to be thicker.
    As to the question about the heel and step. Two inches longer than what?
    It really is quite difficult to build an ugly wooden boat.

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  3. #3
    Join Date
    Aug 2016
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    Redondo beach CA
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    Default Re: Bird's Mouth Heel-Step interface and Scantlings Affirmation

    Quote Originally Posted by Peerie Maa View Post
    As to the question about the heel and step. Two inches longer than what?
    Good question. How long should the plug be? Stem to thwart-partners is 13 inches if that matters.

  4. #4
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    Default Re: Bird's Mouth Heel-Step interface and Scantlings Affirmation

    Quote Originally Posted by Classtime View Post
    Good question. How long should the plug be? Stem to thwart-partners is 13 inches if that matters.
    Not really relevant unless you mean the height from the top of the stem knee. The mast should be solid at the partners, so is it worth the faff of fitting two plugs or will one be excessively long? If you are talking height your plug will ne 14 to 15 inches, not too big an issue.
    It really is quite difficult to build an ugly wooden boat.

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  5. #5
    Join Date
    Oct 2005
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    The Netherlands
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    Default Re: Bird's Mouth Heel-Step interface and Scantlings Affirmation

    Quote Originally Posted by Peerie Maa View Post
    Not really relevant unless you mean the height from the top of the stem knee. The mast should be solid at the partners, so is it worth the faff of fitting two plugs or will one be excessively long? If you are talking height your plug will ne 14 to 15 inches, not too big an issue.
    I think the plugs end should be tapered: A friend got his mast, Caledonia Yawl, broken where the plug abruptly ended. Frank van Zoest

  6. #6
    Join Date
    Feb 2010
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    Everett, WA
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    Default Re: Bird's Mouth Heel-Step interface and Scantlings Affirmation

    I used Duckworks math for a similar length mast and came up with 3.275" maximum diameter. I used Iain's tapering ratios. Sitka spruce, epoxied octagon made externally round. For the plugs I feathered them to avoid a hard transition point. I put crinkled aluminum foil inside for better radar detection.
    Gerard>
    Everett, WA

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  7. #7
    Join Date
    Aug 2016
    Location
    Redondo beach CA
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    Default Re: Bird's Mouth Heel-Step interface and Scantlings Affirmation

    Gerarddm,
    What maths did you use for the diameter? I thought I was being conservative using 1/50 of mast plus 10% for the birds mouth and rounding to the next 16th for 2.5" (11' total length)

    And yes feather the plug above the thwart. I'm planning to make the plug with 8 staves tapered on the insides.

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