Catamaran main mast beam

Discussion in 'Multihulls' started by windfiderthai, Nov 30, 2016.

  1. windfiderthai
    Joined: Nov 2016
    Posts: 3
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    Location: thailand

    windfiderthai New Member

    I have built several 50 foot aluminium catamarans using a multichine flat panel design.
    These boat where quite successful but the build process was difficult and expensive.
    I am considering changing the materials from alloy to cold molded mulichine ply to make it easier to build here in Thailand.

    My question to the forum is regarding the beams.
    The calcs for the bulkheads/stringers and hull skins are easy to work out but I cannot find any real reference books or info regarding the possible structure of the aft and main mast beams.
    I have an old copy of Roger Simpsons book which is full of great info ,,but not specific enough re the calculation's for the beams.
    The concept would be similar to Simpsons "truss beam" and "box beam" arrangements.

    In the alloy plans I used 6mm with vertical stiffening section every 200mm welded at 100mm centers ,,but how to make this in ply!!!!

    The calcs i have worked out so far are 12mm hull skins over 18mm ply bulkheads spaced at approx 1 mtr spacings with 40mm x 19mm dressed timber stringers at approximately 300mm centers all exoxied together with all items fully epoy sealed.
    This seems to be the basic scantlings as referenced from the Gougeon Brothers excellent book.
    Anyway I am rambling on now ,,
    would be great-full for any comments or assistance .
  2. catsketcher
    Joined: Mar 2006
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    Location: Australia

    catsketcher Senior Member

    Reverse engineering

    Can you not use the current design as a base rather than the Gougeons book? If you check the beam material for stiffness, Aluminium has an E of 69 (but check your alloy type) and the E for Douglas fir is about 12. So aluminium is 5.75 times stiffer. So if you increase the depth of the flanges by 6 times you should be fine for stiffness. 4mm alloy means 24mm Douglas fir.

    Check next for stress. Douglas fir has a modulus of rupture of 86 MPa and Aluminium 276MPa. So if you size for stiffness you will be safe from overloading the flanges. Use epoxy and uni/DB glass where possible.

    Get an engineer to check but you should be able to do it easily.


  3. Emerson White
    Joined: Aug 2012
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    Location: Nordland, WA, USA

    Emerson White Junior Member

    Won't help with specifics but for a "sanity check" you might look for plans for a 50 foot ply cat and make sure you are in the same general area in terms of scantlings.
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