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Thread: Curved/offset Kingpost

  1. #1
    Join Date
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    Default Curved/offset Kingpost

    Have you seen any design that incorporated an 'offset king post'? This would be composite structure based on carbon fiber mast layup plans. I'd overbuild it. I understand generally that the downward forces from the mast are enormous.
    The goal is to provide slightly more clearance (to the right) in my very tight trimaran cabin.
    23 foot trimaran. Coach roof is framed/plywood/epoxy.
    I understand that t
    he compressive strength of dry oak, for example, is around 1000 pounds per square inch. Woods such as fir and pine are higher than this, up to 1,500psi, others a bit less, down to 800psi.
    A 4 x 4 will support about 16,000 lbs. The load on the post is likely to be in the 4,500 to 5,000lb range based on 4mm rigging tensioned to 25% of breaking load, plus the weight of the rig.

    IMG_8643.jpg
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  2. #2
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    Default Re: Curved/offset Kingpost

    You may be overthinking this.
    Let me guess that the max. righting moment of a 23 ft. tri might be about 15000 ft-lbs. an that the mast is stayed to the amas, maybe 8 feet away. Unless you go nuts pre-tensioning the rig, this translates to a downward force on the mast of about 2000 lbs. If it blows harder, you’ve capsized.
    I’m not enough of an engineer to specify that column, but i don’t think it’ll be massive. You might also consider a lintel and two posts under the mast step.

    What kind of tri is it? I built a 29’ Newick Spark, but its masts we’re unstayed, which was a whole ‘nother proposition.
    Last edited by JimConlin; 01-12-2020 at 08:57 PM.

  3. #3
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    Default Re: Curved/offset Kingpost

    Thank you so much, Jim. I am renovating a 1990 Farrier Trailertri 720.
    The existing king post directs its force down to the keel. I think I would still need to make that the focus of force.

  4. #4
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    Default Re: Curved/offset Kingpost

    Have you considered a very strong laminated deck beam with a laminated hanging knee with long arms and at each end?

  5. #5
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    Default Re: Curved/offset Kingpost



    This one appears to be offset.

  6. #6
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    Default Re: Curved/offset Kingpost

    Have you posted your question on the Farrier/Corsair group?

  7. #7
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    Default Re: Curved/offset Kingpost

    Talk to a real structural engineer and let him decide.It could get very nasty and painful if a hunch goes wrong with somebody below.

  8. #8
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    Default Re: Curved/offset Kingpost

    Farrier's specs for my F27, which had a very flexy mast, called for tuning the lowers, intermediates and forestay to over 1500 pounds each. That's 5 lines working together to pull the mast down into the boat. On top of that, the capshrouds are adjusted to bend the mast as needed, adding another ton or more. That's just the static loads. Sail into a chop, sharp dynamic loads are introduced and the boat starts to flex this way and that. I don't think your estimates are too low at all. But the Farrier/Corsair group that Jim pointed you too is the place to go. Unfortunately, the designer is no longer with us. If he were, he would answer that question personally.

    My own take on it is that most any curved beam will flex more than you want it to. It might be possible to build in a solid bulkhead with an oval passageway in the center.
    -Dave

  9. #9
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    Default Re: Curved/offset Kingpost

    Do you know the speck of the lower part of the mast? The load on the kingpost and the load on the lower paert of the mast should be identical. Maybe you can use that to speck the post
    Ragnar B.

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