Aluminum catamaran cross beam height

Discussion in 'Multihulls' started by LePrince, Sep 27, 2022.

  1. LePrince
    Joined: Apr 2011
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    LePrince Junior Member

    Dear all

    I am working on a 30 m aluminum catamaran project and I did all necessary calculation for determining the scantling for the cross beam in terms of strength.

    In my thought, This portion will consist of upper plate (which is the main deck) and lower plate and there are stiffeners welded on both plates. (it look like double bottom on other words)

    In my design I am including some deck transverse with 2.5 m spacing which will be needed to be welded in both upper and lower plates, but I am not sure how it will be welded to the lower plate since this place will be closed and not easy accessible. same thing if I have center girder which go in the vessel center line.

    My 1st question is, should I include these deep deck transverse and center girder in my design ?
    Second question, what is the proper height for the cross beam measured from main deck downward to make it accessible for welding ?

    dose anyone have a sample plan of catamaran cross section to have idea about how this cross beam is reinforced ?

    and please let me know if I misunderstand something.
     
  2. oldmulti
    Joined: May 2019
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    oldmulti Senior Member

    LePrince. Please go to an experienced Multihull designer who has designed and had launched large aluminum catamarans. This is not a strength problem, but a design issue related to twisting between the hulls in a seaway and if you have rigid or flexible structure between the 2 hulls. A 30 meter cat in aluminum is expensive, an experience multihull designer to do the beam design would be very cheap money. EG Kurt Hughes would be a multihull designer option.
     
  3. Alan Cattelliot
    Joined: Jul 2021
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    Alan Cattelliot Senior Member

    Hi Leprince,

    I strongly reccomend to you to follow the advice of old multi. In France, VPLP, Berret-Racoupeau are famous cabinet for yachts of this size. I work with B. Nivelt, who is also experienced in the design of big cats. The boatyard that would built the boat should also be put in the loop, when selecting design principles. Although I do not fully understand your though about crossbeams, I would like to point out, in relation with your questions, that :
    - lboth longitudinal and transveral girders are to be considered, in order to handle pressure on the wet deck
    - For boat of this size, in general, global loads are supported by multiple crossbeams and open bulkheads, that allow weldings to be made. Columns and stanchions are mostly used when local reinforcements are to be made.

    If you have already made the scantling calculations, why don't you ask directly the boatyard for the building details ?
     
  4. LePrince
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    LePrince Junior Member

    Thanks for your reply, I am working on an educational project which will not be achieved in any boatyards, I was curious about what is the value for this height so it can be more logical.
     
  5. TANSL
    Joined: Sep 2011
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    TANSL Senior Member

    As for the minimum height you are looking for, don't worry, use the welding from the outside with the same technique used on rudder blades.
    In addition to the transverse reinforcements on the deck and on the wet deck, which will support the local loads, you should put a few beams, closed profiles welded to both decks and to hulls, to support the global loads induced by the relative movements between the two hulls.
     
  6. Ad Hoc
    Joined: Oct 2008
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    Ad Hoc Naval Architect

    Yes, and no.

    That depends upon how you design the structure.

    Yes, but what do you mean by 'cross beam'.

    You need to describe very clearly what you mean by cross beam, and you need to explain your calculations too, as to your rationale.
    Words can be misleading.... you will need to provide sketches/pictures of what you have and what you are proposing.

    If you are seeking clarification and ideas, then you should read "Essentials of Catamaran Structure" in Professional Boatbuilder Magazine, Dec-Jan, 2020, issue 182.
    That should answer any questions you seek..
     
    bajansailor likes this.
  7. LePrince
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    LePrince Junior Member

    Thanks for your answer, This catamaran is designed for inland water way region when the wave height is 0.75 meters and speed is 12 knots
    In BV rules for inland vessels, the only check needed is check against torsion and since the vessel short in length, no global BM check needed.
     
  8. LePrince
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    LePrince Junior Member


    Thanks for your help
    what I mean with cross beam is that structure connecting the two demi hulls, sorry if I used wrong expression for that
    the calculations I did is according to BV inland rules to check torsion in addition to other local loads calculations.

    Sorry I didn't draw any sketches yet but i think my design will be quite similar to this picture
     

    Attached Files:

  9. Ad Hoc
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    Ad Hoc Naval Architect

    If that's the case, then I don't see you have any issues.
     

  10. ThomD
    Joined: Mar 2009
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    ThomD Senior Member

    The cartel part aside. Lots of interesting build experience for large cat in Aluminum.

     
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