Main Crossbeam Bracing

Discussion in 'Multihulls' started by mikereed100, Feb 13, 2008.

  1. mikereed100
    Joined: Jan 2007
    Posts: 87
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    Location: Borneo/California

    mikereed100 Junior Member

    I am rebuilding the main crossbeam that supports the mast on my cat and have some questions concerning the internal bracing.
    First, some info on the boat: Length-46', Beam-25', Mast-59', Mainsail-75m2, Jib-30m2.
    The original beam was 55cm x 55cm, 25mm PVC honeycomb sides and bottom with pressboard(!) top. Not sure of the lamination schedule. On the inside of the beam was a stainless compressin post with struts made of 100mm x 2mm stainless strap.

    The new beam is 55cm x 100cm, 25mm corecell A500 with 2 layers 1802 biax. I would rather provide strength with bulkheads rather than put the old compression post back in. Something like this;
    Crossbeam bracing1.JPG

    The core on the bulkheads beneath the mast would be high density. Since I am not a naval architect and am just going by what "seems' strong I thought I might put it to the forum. Other options I have considered are diagonals but that introduces some complexity to glassing them in without cutting holes in the beam.

    Any thoughts?
    thanks, Mike
  2. catsketcher
    Joined: Mar 2006
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    Location: Australia

    catsketcher Senior Member

    Reverse engineering sounds good

    I think I need more information to be able to help but a few things from my end. If your beam was strong enough then you may reverse engineer it. Work out its second moment of area and use the mechaical properties of the materials to give you a section modulus. Your new beam should at least be the same as your old.

    Two layers of triax seems quite light if you are not going to put any unidirectional top and bottom - a compression post is designed to feed out the compression into shear loads. You will need some way of converting the downward vector into shear. A bulkhead may do.

    I would urge caution and a large amount of research before getting making the new beam.


    Phil Thompson
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