Pivoting Spreaders

Discussion in 'Sailboats' started by Scott Carter, May 25, 2007.

  1. Scott Carter
    Joined: Oct 2006
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    Location: Annapolis

    Scott Carter Senior Member

    If my spreaders pivot (please don't ask me why I want them to pivot) in the horizontal plane, will this affect their ability to keep the shrouds spread? I think I've determined the answer for myself already, but wondered if any of you have any experience with pivoting spreaders.
    It's been cautioned to me that their ability to pivot will prevent them from keeping the shroud directly alongside the mast, which will give the mast leeway to bend, counter to the intended purpose of the shroud to begin with.
    The mast has a 5 degree rake, 2 sets of spreaders keeping a single topmast shroud spread. There are also 3 lower shrouds per side terminating just below the lower spreader hounds.
    The line which defines the plane of the shroud between the mast head and the chainplate leaves the spreaders raked aft about 4 degrees.
    Any thoughts?
     
  2. Omeron
    Joined: Feb 2007
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    Location: Istanbul

    Omeron Senior Member

    Ok i am not asking you why you need your spreaders to pivot,but i can
    tell you a story about why you should not.
    Many years ago, i was crewing on a boat, returning to the marina, just before sunset. All of a sudden, a not so loud bang, and the mast is
    gone over the side.
    The mast had pivoting spreaders,with a pin securing it in the
    desired position. We never found the pin, but apparently it sheared
    and made the spreader collapsed.
    If something is allowed to move in an undesired way, it is almost certain
    that one day it will. And it will do so at the most inconvinient place and time.
     
  3. Robert Gainer
    Joined: Jul 2004
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    Robert Gainer Designer/Builder

    Depending on the geometry it works fine. If the wire gets tighter as the spreader pivots then it will work. If not the spreader will want to move and everything just comes down. Even with a fixed base you want to design the system so that the base has no twisting moment in it.
    All the best,
    Robert Gainer
     
  4. yipster
    Joined: Oct 2002
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    Location: netherlands

    yipster designer

    some books discussing spreaders:
    "riggs and rigging" by Richard Henderson
    "the complete riggers apprentice" by Brion Toss
    on page 124 in "skene's" the forces
     
  5. Frosty

    Frosty Previous Member

    I want to know why you want your spreaders to pivot?

    Personally I think it will be unstable in its tension direction, especially if they are wide--as wide as the boat. Just another thing to worry about in a force 9 at 4 am in the morning.
     
  6. fastwave
    Joined: Jan 2007
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    Location: europe

    fastwave Junior Member

    Pivoting spreaders have been done and are nothing strange. Check out the Open60 Ecover and Maximus just to name a few.
     
  7. yipster
    Joined: Oct 2002
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    Location: netherlands

    yipster designer

    axial with the mast rotating speaders in a pitching boat swing
     
  8. garydierking
    Joined: Sep 2004
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    Location: New Zealand

    garydierking Senior Member

    All of the cutter rigs on Jim Brown Searunner trimarans have "single pin ended limited swing spreaders". I put my set through 20,000 miles of the Pacific with no problem. The pins must be big and the shape of the spreader end is such that it can only swing maybe ten degrees either way. I see this as a great way to reduce high transient loads at the spreader base.

    Gary
     

  9. yipster
    Joined: Oct 2002
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    Location: netherlands

    yipster designer

    pins are great, scanned a few pages from Richard Hendersons good book "understanding rigs and rigging"
    if the scans are readable notice the logic and realise that even the weakest link can bring a rig down

    ooch, if anybody want a better scan yell ;-/
     

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