Bulkhead thickness modification

Discussion in 'Boat Design' started by billie, Jan 4, 2007.

  1. billie
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    billie Junior Member

    Hello,

    Imagine a existing bulkhead sandwich : carbon biax/12MM airex foam/carbon biax
    and that I want to
    have the same stifness and mechanical caracteristic of a sandwich carbon biax/15mm airex/carbon biax.

    How should I do it ?
     
  2. Greybarn
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    Greybarn Junior Member

    Pretty hard to have the smae mechanical characteristics when you increase the core thickness. Basically, with the same skins, when the core T goes from 12 mm to 15 mm, the stiffness is going go up by a factor of about 1.95. Even if the skin thickness goes down (using the same material), the stiffness (and strength) will be higher). If the skin thickness on the 12 mm panel is low, then the only way to get back to similar stiffness and strength will be to use E-glass skins instead of the carbon.
     
  3. billie
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    billie Junior Member

    Txs Greybarn,

    In fact (sorry for my english) I want to convert a 12mm to a 15mm, or to convert the existing biax carb/12mm/biax car to a bulkhead with the same mechanical caracteristics as with 15mm core. How can I do it, can it be by increasing skin thickness ?
     
  4. Raggi_Thor
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    Raggi_Thor Nav.arch/Designer/Builder

    You can't increase the thickness from 12 to 15mm without having a much stiffer panel. Is that bad? Isn't stiffness normally good?

    Is it correct that you want to use your existing bulkhead, but make it 3mm thicker? If you don't want it to be much stronger or stiffer you could use something light and cheap that looks good, for example 2.5mm of wood?
     
  5. Raggi_Thor
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    Raggi_Thor Nav.arch/Designer/Builder

    Sorry, I think I didn't quite understand.
    As Greybarn told you (15/12) squeared is almost 2.
    So a solid lamiinate (or plywood) would be twice as stiff at 15mm as at 12mm.
    With a sandwich is not that simple, I think.
    the core contributes almost othing to the stiffness, it's juste there to make a distance between the skins.

    I think you can have the same stiffness with 12mm if you increase the skin thickness (the fibre content) wit approx 25%.
     
  6. billie
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    billie Junior Member

    Thanks,

    Yes that's what I'm looking for ; to increase skin thickness in order to have same stifness as with 15mm laminate.

    Is 25% enough ? (in my case just by adding 100gr/m² layer on both sides?)

    Anyone more precise data ?

    Is there any tool around to calculate this ?
     
  7. Raggi_Thor
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    Raggi_Thor Nav.arch/Designer/Builder

    I think this is accurate enough:
    The "arm" to make a moment is reduced from 15mm to 12mm, approximately.
    Say your 15mm lminate had 400g reinforcement.
    Then:

    15 x 400 = 12 x NewThickness
    NewThickness = 400 x 15/12 = 400 x 5/4 = 400 x 1.25

    I think that's it :) add another 25%, yes.
    Maybe you can do a bending test also?
     
  8. billie
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    billie Junior Member

    Great. Many thanks for your help.

    Will do a test. Good idea. Keep you informed.
     
  9. tom28571
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    tom28571 Senior Member

    I am still confused about what Billie wants to do. Does he want to decrease the core from 15mm to 12mm, or vice versa? What axis is the controlling one for stiffness? Even biax carbon is not very stiff at 45 degrees. About 25% the stiffness of zero or 90 degrees, if I remember correctly.

    If you are adding more layers of biax carbon, they should be oriented at 45 degrees to the other to get optimal overall stiffness. If stiffness is mostly wanted in one axis, uni-directional carbon or even glass might be better.
     

  10. billie
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    billie Junior Member

    Hi,

    Trying to make it clear: goal is to 'convert' a 12mm to a 15mm bulkhead.
    Can it be done by increasing skin thickness ?
     
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