Cat Scantlings

Discussion in 'Multihulls' started by TealTiger, Mar 26, 2012.

  1. TealTiger
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    TealTiger Junior Member

    In 'Boat Strength', Mr. Gerr excludes multihulls. I would expect the book still apples to each hull by simply putting in the dimensions for one hull. Obviously doesn't cover anything related to the bridge deck. Does anyone know if I'm correct, or how to calc. cat scantlings? Thank you.
     
  2. Tad
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    Tad Boat Designer

    The scantlings used in any boat are based on anticipated loads, if you don't know the loads you cannot calculate the required scantlings.

    Mr. Gerr's book concerns very conservative (heavy) monohull scantlings. Multihulls are typically high performance vehicles, (higher boatspeed and righting moments) which impose very different loads on a structure than those of a typical mono lead mine......Thus the book will indicate an overweight structure with no indication of how to tie it together.......

    To calculate multihull scantlings go back to first principals, and add appropriate safety and material factors. See some long threads here on cross beam design.......
     
  3. TealTiger
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    TealTiger Junior Member

    I appreciate your response Tad. It's quite clear and logical. I may repost after I've digested them. Thank you.
     
  4. baeckmo
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    baeckmo Hydrodynamics

    Are you looking at sailing or motor-cats? Back in the end of the 70-ies we did some testing with an instrumented outboard driven "workboat cat prototype", just to get a few facts on dimensioning of deck and connecting structure; there were no cat rules available back then.

    Generally speaking, we found the maximum vertical acceleration in very rough conditions to be about plus one g, loading the structure diagonally. That means the hull landed in a through after flying, with one stem and the other stern taking the load.

    We thus dimensioned the cross structure for ~double boat weight, carried by one hull forward and the other hull aft. When checking the loads at other conditions, f.i. when landing on one hull, heeled in the worst position, they always were lower than the diagonal load condition.
     
  5. ImaginaryNumber
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    ImaginaryNumber Imaginary Member

    baeckmo,
    When considering the diagonal load between opposite ends of opposite hulls, are there torsional loads on cross beams (assuming no bridge deck)? Or are all the loads on the beams bending loads?
     
  6. CatBuilder

    CatBuilder Previous Member

    Very good thread. I suppose that diagonal loading is why many people suggest blocking your newly built catamaran on the 4 corners, then removing one of the blocks to test the boat before launch. That is the 1g load.
     
  7. baeckmo
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    baeckmo Hydrodynamics

    There must be a combination of bending and torsional loads.
     
  8. baeckmo
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    baeckmo Hydrodynamics

    Yes, that is one g, but for the speeds we aimed at, the acceleration corresponded to two g:s when coming down in the wave through (I wrote "plus one g", maybe that was a bit unclear).

    In my view, the one g test is insufficient, because even when the hull is only "platforming" in a seaway, the acceleration will vary from zero (on a breaking wave crest) to roughly 2 g in the through. The maximum values depend on speed and hullshape, but the structure should be tested in diagonal load with double displacement to be safe in "real life" seas. This seemed to hold true when we later made capsizing tests in wavetank; superstructure dimensioning according to established rules did not cope with the forces from a breaking plunger.
     
  9. ImaginaryNumber
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    ImaginaryNumber Imaginary Member

    baeckmo,
    I wondered whether there are torsional loads in the cross beams because of a discussion on this thread,

    http://www.boatdesign.net/forums/mu...ather-cruiser-project-41658-4.html#post540841

    http://www.boatdesign.net/forums/mu...ather-cruiser-project-41658-2.html#post528229

    http://www.boatdesign.net/forums/mu...ather-cruiser-project-41658-2.html#post528224

    http://www.boatdesign.net/forums/mu...weather-cruiser-project-41658.html#post526396

    where it was noted that the planar beams used by Jan Gougeon in his catamaran Strings suggested that there are not torsional loads in the cross beams. Not being an engineer, I was looking for conformation or refutation of that line of thought.
     
  10. CatBuilder

    CatBuilder Previous Member

    Ah, thank you for the explanation of the 2g force on the diagonal. I did misunderstand it to be 1g total rather than 1g+1g. Makes sense.

    Now, I suppose we also have box beams (rather than flat beams) across all of our catamarans to handle the torsional loads at the beams.

    The design I am currently building has a very large main box beam and a standard size aft box beam. The other beams are non-critical, but do help take up in-plane loads.

    Great information here.
     
  11. baeckmo
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    baeckmo Hydrodynamics

    Ahah, don't make the wrong interpretation here: if a rectangular bridge structure is bent over a diagonal, a transverse cross beam has to twist in relation to the demihulls. That does not mean that the beam has to be designed to resist torsion per se; in fact it may well be freely rotating in one end (or having an easily twisting shape), not taking up any torsional LOAD at all. Instead, the combined bending modulus of the deck and beams may be designed to cater for the complete load.

    Or, vice versa: the deck structure has no bending strength (like a membrane). Then all the load may be taken by bending and torsion in the beams. Ergo: there is not one single solution to the problem, it may be solved in different ways, provided you understand how deformation is resisted.
     
  12. ImaginaryNumber
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    ImaginaryNumber Imaginary Member

    I understand better now. There are torsional forces between the two hulls, but those forces can be managed in different ways, by different components of the structure, at the discretion of the designer. Thanks for the explanation.
     
  13. Ad Hoc
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    Ad Hoc Naval Architect

    These images may also help in understanding too. This one shows a typ arrangement:

    Box beam raft structure.jpg

    And this shows a full deck typ, under such a load using FEA:

    Box beam raft structure - FEM.jpg

    And this is the classic "S" shape one finds at the extreme ends during such a load:

    Box beam raft structure - S shape.jpg
     
  14. upchurchmr
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    upchurchmr Senior Member

    Ad Hoc,

    Perhaps I just can't see close enough but the S in the cross beam structure will cause the hull to twist around the longitudinal axis also, there is nothing to actually hold the hull in a vertical orientation in the presence of enough force to S bend the beams (I assume the cross beams are not rubber).
     

  15. Ad Hoc
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    Ad Hoc Naval Architect

    Yes, techcinally that is correct.

    When i perform these calculations i restrain the hulls from such a twist as you describe. Since you have to consider how stiff the hull is, compared to the raft, or bridging structure. And the purpose of the analysis to to establish the strength of the raft structure, not the hulls.
     
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