Scantlings for Catamarans: Waiting for the ISO 12215-7.

Discussion in 'Class Societies' started by TANSL, May 1, 2016.

  1. TANSL
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    TANSL Senior Member

    While this standard appears and enters into force this paper provides a method of direct calculation for the scantlings of the "wet deck" and cross beams conecting hulls in catamarans.
    Comments and suggestions will be greatly appreciated and will help to correct and improve the procedure.
     

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  2. Ad Hoc
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    Ad Hoc Naval Architect

    In general, all you've done is just copy and past BV rules and others - each of which often use very different methods at arriving at formulae, don't mix and match. For those that do not understand or know how to calculate structures of multihull going to any Class rule is far simpler!

    Also, you can't split a moment. No such thing as "half" a moment. Like no such thing as half a hole! A moment is a moment. The number of beams supporting said moment can reduce its effect by increasing the modulus, not the other way around.
     
  3. TANSL
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    TANSL Senior Member

    Thank Ad Hoc. Your comments make always meditate ....... on the human condition.
    Thanks again.
     
  4. TANSL
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    TANSL Senior Member

    I think the Lloyd's make the same mistake I did: to calculate the stress, they divide the bending moment by the number of "transverse primary stiffeners".
    I repeat again that I did not invent anything, just adapt what already exists to my needs. If the interpretation I have done is not right, I greatly appreciate someone, aim, pull me out of my error. Thank you.
     

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  5. rxcomposite
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    rxcomposite Senior Member

    TANSL- you should have included the link posted by Alik about the forces acting on the crossbeam/mastbeam. It is in Russian but I managed to translate it with google except for some technical words.

    A crossbeam is a beam and LR or ISO tabulation can be modified to design a beam ply schedule. Or you can use the formula F=Mc/I for a non constant cross section.

    For the bending moment, you can use partially distributed load, equally distributed load, and mast point load in simply supported model and the cantilever load when one hull is out of the water.
     
  6. TANSL
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    TANSL Senior Member

    rxcomposite.-I do not know the letter of Alik and do not know how to get it. Is there any link to access it?. I'm sure it's very interesting.
    As for the beams, there have been some simplifying assumptions :
    1. Hulls are rigid.
    2. The beams are embedded in hulls
    3. The beams have a constant section
    This may not be true in all boats, but they are the premises from which I work.
    There are other methods involving a model for the cross beams which, being symmetrical, are embedded in the hull and the center line. It is not the case for the procedure described here.
    Nor I study some loads which result in a bending moment, on the contrary: there are a certain bending moments, giving rise to stresses in the beams.
    The ISO 12215 standard establishes maximum design pressures to calculate the properties of each beam. In this case design pressures are not defined but bending moments in the beam induced by the torsional moments and, from them, the stresses in cross beams are calculated.
    Once determined bending stresses and shear in each beam, one need to define the scheme laminates meet them. That is, it should be determined according to the analysis by layers required by ISO 12215, the minimum thickness, the minimum content of dry fiber, and perform stress analysis on each layer to check that in none of them maximum values are exceeded.
    In short, this is a different, easy programming process, which can be as good or as bad as any other. If there is in it a misconception, I would have to correct it, of course.
     
  7. Ad Hoc
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    Ad Hoc Naval Architect

    Nope, they didn't. You did. You state thus:

    moment-beam DC-Cat.jpg

    You show a moment being divided by the number of beams.

    LR shows it correctly:

    Pages from LR.jpg

    As noted above you can't half a moment, it is what it is...any basic text book will tell you that. Thus, the number of beams supporting said moment can reduce its effect by increasing the modulus, not the other way around, which is exactly as LR shows.

    If you're going merely cut and paste other rules...at least understand basic structural analysis and how and why they arrive at their own formulae.

    You seem to enjoy pulling at Gonzo for his grammar etc (which is hard since like you English is not his mother tongue). Thus if I could be bothered I could easily pick endless holes in your 'technical' documents. For someone who claims to be professional, as you constantly throw at Gonzo, these are basic errors only a student would make, not a professional!
     

  8. TANSL
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    TANSL Senior Member

    I think, modestly, you should read more carefully what I have written, and then if you want, express your opinion, but first analyzes things. Your vast knowledge will allow you to do this easily. My ignorance leads me to insist that I'm saying the same thing as Lloyd's. Thanks, again, for your lessons and your sincere interest in clarifying things. Thank you.
     
    Last edited: May 4, 2016
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