Forward crossarm rigging

Discussion in 'Multihulls' started by mikereed100, Jun 17, 2009.

  1. mikereed100
    Joined: Jan 2007
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    Location: Borneo/California

    mikereed100 Junior Member

    Sometime during the rebuild of my cat the rigging from the forward crossarm (don't know the proper term) wandered off and the time has come to replace it. I never looked closely at it so am not sure what diameter wire to replace it with. The forestay and cap shrouds are 14mm so I imagine I could replace it with that size and be safe, but there is a large price jump between 12mm and 14mm rigging ends and wire. My question is, is there a formula to calculate the tension that these wires could be expected to withstand assuming a breaking strength of 33,000lbs for the forestay? Or, more to the point, can I get away with 12mm wire if the forestay is 14mm?
    I've tried dredging my brain, looking for any remains of what I learned about vectors in college physics but all I come up with is muck.

    Thanks,
    Mike
     
  2. waikikin
    Joined: Jan 2006
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    waikikin Senior Member

    Mike, I doubt if the forestay is 14mm that the 12 would be good enough, if the forestay goes to a bridle with a monkey face to connect the 3 wires & the included angles are 120 degrees all round the load is equal all round, if the bridle is "flatter" than that the load is increased, if "steeper" it reduces. If I was you I'd go to a yacht rigger with a drawing/diagram & get them to do the calcs & make the wires up for you too. Hope that makes sense & all the best from Jeff.
     
  3. mikereed100
    Joined: Jan 2007
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    Location: Borneo/California

    mikereed100 Junior Member

    Thanks Jeff, that makes good sense. The forestay does not go to a bridle but rather directly to the crossbeam which is supported by an A-frame strut with rigging running from the strut to the ends of the beam. Here's a drawing showing what I've got. Having a rigger do some calculations is sound advice.

    Thanks,
    Mike
     

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  4. catsketcher
    Joined: Mar 2006
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    Location: Australia

    catsketcher Senior Member

    You can do some calcs

    Hello Mike

    Jeff is right in that you will need a husky bridle wire. Assuming the angle at the bridle ends is 20 degrees then you have a triangle with the bridle wire forming the hypotenuse, half the beam the base and the seagull striker to opposite side. To find the load in the wire (hypotenuse) make the seagull striker side the vertical load - 33 000lbs. To make the rest of the triangle equivalent you get a load in the bridle wire of 97 000lbs. Even 1 inch diameter wire is 89 000lb breaking strain. What type of cat is it? Sound bloody big to me.

    That is a huge load. So check with the rigger. Maybe the forestay is overspecced so you don't have to go this way.

    Cheers

    Phil
     
  5. catsketcher
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    Location: Australia

    catsketcher Senior Member

    Maybe wire is thicker

    Mike there may be a reason your forestay is so stout. Hanks and roller furlers must pose huge loads on the thing so I would guess that going huskier copes better with the wear and tear on the forestay. That may be a reason.

    Phil
     
  6. Steve W
    Joined: Jul 2004
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    Location: Duluth, Minnesota

    Steve W Senior Member

    Mike, my first course of action would be to contact the designer or builder, if thats not possible for some reason then as Waikikin suggests, have a rigger do the calculations and make up the assembly.
    Steve.
     

  7. mikereed100
    Joined: Jan 2007
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    Location: Borneo/California

    mikereed100 Junior Member

    Thanks all. It's looking like bigger is better!

    Phil, the boat seems big to me but probably not all that big in the grand scheme of things. 46' with a typical fractional rig supported by a forestay and 2 caps which are all 14mm or 9/16". Mainsail is less than 700ft/sq as far as I can figure. I don't know if this rigging is typical for a cat this size as I live in the multihull vacuum of the west coast N. America and don't see much to compare it with.

    Steve, this is a custom design by a group of French mechanical engineers with a fellow named Capelli doing the naval architecture. I don't have contacts for any of them so I will do as you guys suggest and have a rigger look at it. By the way, I have just finished building a pair of "rudders in a drum" similar to what you have done, and will be installing them in the next couple weeks. I will let you know how they work when she launches next spring.

    Thanks again to all for the help. I have had to do a lot of reverse engineering on this rebuild and the help I have recieved on this forum has been invaluable.

    Mike
     
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