Can Anyone Explain the Forces on a Longeron?

Discussion in 'Multihulls' started by Chotu, Jun 4, 2020.

  1. Chotu
    Joined: Mar 2018
    Posts: 52
    Likes: 2, Points: 8
    Location: Florida

    Chotu Junior Member

    Hi,

    I’m looking at fitting a longeron to a 50’ catamaran. It’s currently without a forward cross beam at all. Nothing there.

    Can anyone take me through the forces from the forestay on a modern, 2 furler (screecher and working jib) headsail setup?

    50’ very high performance and lightweight cruising catamaran. 60’ mast deck stepped. Need to account for anchoring forces as well, although the anchoring bridal will be attached to the bows.

    Considering doing something like the pic attached.

    Is the longeron taking any cantilever force or is it held completely by the guide wires running to the bows?

    Why the really high seagull striker? To keep loads on the bows in a more vertical orientation?

    Thanks!
     

    Attached Files:

  2. jehardiman
    Joined: Aug 2004
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    Location: Port Orchard, Washington, USA

    jehardiman Senior Member

    Basically, to support the fore stay loads, a classic bowsprit is built as horizontal (or slightly steeved) mast with shrouds only on one side (the dolphin striker and bobstay) with guys either side . Because you can't fit a bobstay or dolphin striker to a cat bowsprit , the "seagull striker" compression post and its stays transfer the vertical forestay load to the amas, the higher the compression post the more shear as opposed to tension load on the amas. The bowsprit under the compression post is in classical compression against the fwd aka, both from the fore stay and the guys lead to each ama.
    FWIW, many modern cats are not rigged this way. They have a single forward tube similarly jumper stayed but to itself, making the load into the amas all shear.
    Fore stay loads can be calculated from GL Guidelines for Design and Construction of Large Modern Yacht Rigs.
     
  3. Chotu
    Joined: Mar 2018
    Posts: 52
    Likes: 2, Points: 8
    Location: Florida

    Chotu Junior Member

    Thank you for helping confirm and visualize the forces in this type of arrangement. I wanted to be sure I wasn’t missing any important concepts.

    Yes. The standard bow tube ( stayed to itself) with all loads in sheer on the amas is more common. I’ve seen these longeron-only type designs with some forestay force translated to tension in Morelli and Melvin designs and a few others, such as the example in the attachment.

    My thinking is it might be easier to put together this type of arrangement than a typical bow tube given the single attachment point. Also a bit more length is available to carry a larger screecher, since it extends past the bows a little.

    Does that seem reasonable?

    What are the advantages to this arrangement over the more traditional ones with a bowsprit added?

    Could I even use an off cut from a properly sized aluminum mast as the longeron? Since it’s all under compression, I’m guessing so.

    Am I missing anything in this decision?
     
  4. Chotu
    Joined: Mar 2018
    Posts: 52
    Likes: 2, Points: 8
    Location: Florida

    Chotu Junior Member

    Also, WOW. What a resource in the document you suggested for calculating the forestay loads! Thanks so much.
     
  5. ziper1221
    Joined: May 2018
    Posts: 25
    Likes: 1, Points: 3
    Location: florida

    ziper1221 Junior Member

    Wow, that render looks a lot like a beachcat with a bridle on the forestay. It doesn't look very practical for a cruising boat, what happens when you drop the rig?
     

  6. Chotu
    Joined: Mar 2018
    Posts: 52
    Likes: 2, Points: 8
    Location: Florida

    Chotu Junior Member


    I imagine you just unhook the forestay and the rest stays put?
     
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