Block Strength

Discussion in 'Sailboats' started by Cleetus, Sep 15, 2007.

  1. Cleetus
    Joined: Aug 2006
    Posts: 27
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    Location: Baltimore, MD, USA

    Cleetus Junior Member

    Howdy,

    I was wondering if there is a formula or rule of thumb to figure out the strength of a block used on a running back. I'm helping a buddy re-rig his boat. It's a Tahiti Ketch we cut down and re-rigged as a gaff-rigged cutter. We're installing running backs and were wondering (based on mast height, sail area, and wind speed) if there's a way to figure out what the pressure is on the running backs. We want double and maybe even triple blocks on them but we're not sure what weight the block should be rated for.

    Thanks a bunch!

    -Cleet
     
  2. Cleetus
    Joined: Aug 2006
    Posts: 27
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Baltimore, MD, USA

    Cleetus Junior Member

    wow, have I stumped everyone? :/
     
  3. PAR
    Joined: Nov 2003
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    Location: Eustis, FL

    PAR Yacht Designer/Builder

    Nope, it's a difficult equation based on the information provided. A fair guide would be to insure the breaking point, of the weakest member of the assembly, can tolerate twice the displacement of the boat. This includes rigging pins (a typical weak link) swivels, tangs and other attachments, as well as the blocks themselves. Generally, you try to size things so everything sort of explodes all at once, after the stress limit has been reached. This never is the case, so you work around the likely weak link and size everything to it.
     
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