Offset/Curved Kingpost

Discussion in 'Sailboats' started by dmydlack, Jan 13, 2020.

  1. dmydlack
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    dmydlack New Member

    Have you seen any design that incorporated an 'offset king post'? This would be a 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 a very tight trimaran cabin.

    23 foot trimaran. Coach roof is framed/plywood/epoxy.

     
  2. dmydlack
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    dmydlack New Member

  3. bajansailor
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    bajansailor Marine Surveyor

    I think your best bet in the given circumstances (as per your sketch) might be to have a substantial ring frame in lieu of a king post - no worries then about the kingpost getting in the way, however the disadvantage would be that you would have less head room locally in way of the ring frame.
    But I am guessing that you won't have full headroom anyway, and would still have to duck / be stooped over when going forward past the mast?
     
    Last edited: Jan 13, 2020
  4. gonzo
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    gonzo Senior Member

    Overbuilding is not a good way of designing. It often leads to stress concentrations that will make a structure fail. The offset kingpost can be made to work, together with a ring frame. Depending on the headroom, a ring frame alone could work
     
  5. TANSL
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    TANSL Senior Member

    @gonzo, I'm sorry to say it but none of the four sentences you have written is correct. Not controversial, I only pretend that the OP is not confused with them and makes a bad decision.
     
  6. philSweet
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    philSweet Senior Member

    Gonzo is correct; and both the problem, and the decision to offset the post, is a common one. Try to bond the post to some fore/aft partition as well as transverse framing.
     
  7. TANSL
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    TANSL Senior Member

    @philSweet, sorry to disagree but I think the following:
    1. nobody is talking about oversizing anything.
    2. oversizing an element cannot produce a concentration of stresses, why?.
    3. An offset kingpost does not need a ring frame at all to work, while a ring frame would be desirable..
    4. A ring frame can work perfectly without taking into account the headroom at all.
    Of course if I am wrong, I will be happy to discuss it but I am not sure that it is interesting for the OP.
    That could be a solution without the need for the transverse ring frame. But it is necessary to know the current structure of the ship and the deck, before affirming anything.
     
  8. dmydlack
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    dmydlack New Member

    Here is an example of a bent king post on a Telstar 28 (as you can see no ring frame, no tie-in to bulkhead, and very sexy.)

    Screen Shot 2020-01-14 at 10.24.38 PM.jpg Screen Shot 2020-01-15 at 5.13.12 AM.jpg
     

  9. bajansailor
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    bajansailor Marine Surveyor

    That bent strut does looking rather 'interesting' (but sexy? Ummmm) .
    But be aware that the buckling load to cause it to fail will be considerably less than the buckling load to cause failure on a nice straight strut.
    Maybe you could consider a goal post type of structure, with straight struts port and starboard supporting a top beam / web frame?
     
    BlueBell likes this.
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