The Elements of Boat Strength for a Catamaran

Discussion in 'Fiberglass and Composite Boat Building' started by BlockHead, Jul 3, 2019.

  1. Dejay
    Joined: Mar 2018
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    Dejay Senior Newbie

    So... is there any system that can be used by an amateur to estimate scantlings for foam core on a multihull? Without adding excessive weight, as a means to roughly estimate displacement and material costs so you can iterate over the design. Obviously there will always be caveats.

    I'm guess the problem is very different (and hopefully much easier) for a power trimaran than for a sailing catamaran.

    Catamarans would seem to be almost impossible with the large variations in the connecting structure of the hulls.
     
  2. TANSL
    Joined: Sep 2011
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    TANSL Senior Member

    The structure of a catamaran is not an easy subject, even for professionals. So, in my opinion, an amateur will have great problems to define it correctly. If it's only a matter of making a preliminary weight estimate, in my opinion, you can use Gerr's data, or comparative data with other ships, if you have them. This will certainly lead to a greater than optimal weight but that, in the early stages of the project, is not bad. But you should be aware that your structure may not be correct.
     
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  3. Deering
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    Deering Senior Member

    I suggest seeking out the scantlings of a few successful commercial cats (if available) and compare to Gerr’s numbers. Perhaps you could derive a rough ‘correction factor’ for a long, narrow hull. Ignore the multihull issue at first. Treat each hull as a single hull as a starting point. After that you can attend to the structural complexity of attaching them together.
     
  4. TANSL
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    TANSL Senior Member

    In addition for transversal beams connecting both hulls, special studies must be carried out for the wet deck and for the inner zones of the sides (between both hulls) that are subject to great loads. In general, extrapolating the procedures for calculating a monohull for a catamaran is not good advice.
     
  5. Deering
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    Deering Senior Member

    Exactly. Which is why I stated “After that you can attend to the structural complexity of attaching them together.”

    As an aside, did the OP state that the intent was to build in FRP? Perhaps the goal is to build with aluminum.
     
  6. TANSL
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    TANSL Senior Member

    You are right, the OP can build his boat with different materials but neither the design pressures, nor the calculation procedures, will change for that.
    In addition to "structural complexity of attaching them together" in a catamaran there are areas that, although they do not have a special complexity to join them, at least the same complexity as in a monocoque, are subject to certain loads, pressures, that in a monocoque, in the same area and with the same complexity, do not exist.
     
  7. gonzo
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    gonzo Senior Member

    Unless your design is very unusual and bizarre, looking at similar boats that are successful is a tried and safe way of getting a baseline. If your calculations are far off, it is time to question why. Otherwise, you could use the scantlings of the similar boat and modify the shape, within limits.
     

  8. TANSL
    Joined: Sep 2011
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    TANSL Senior Member

    In PRF hulls it is practically impossible to know the composition of the laminates that have been used in their construction. On the other hand, as you very well know (or should know), putting a thickness equal to the neighbor's boat does not guarantee that you get the proper resistance. Therefore, as you very well know (or should know), observing similar catamarans is not useful to determine the scantlings of a new PRF hull.
     
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