Beach Catamaran Beams from Aluminum to ply/composite?

Discussion in 'Multihulls' started by Burnside Style, Jul 16, 2021.

  1. Burnside Style
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    Burnside Style Junior Member

    I am building a small beach catamaran (duo 425 by Bernd Kohler)
    I was wondering if it would be an option to make the crossbeams from ply and carbon or fibreglass.
    A box beam wrapped in biaxal cloth with unidirectional running the length top and bottom.

    My question is how do you determine how much composite reinforcement the ply beam needs?
    I can plug in the aluminum beam specs: (70mm od, 64mm id, 2.2m long) and get the numbers that the ply composite beam would need to match. I have done experiments with ply and solid timber beams designed to match an aluminum tube, but I am not sure where to start when it comes to how many layers of what would need to be applied to a plywood box type beam to match an aluminum tubes specs.

    Do you think that the box beams would end up being heavier than the aluminum? How do you go about comparing the two? Again, the aluminum is easy to look up, but "X" amount of layers of carbon + epoxy + plywood =?
     
  2. TANSL
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    TANSL Senior Member

    Why do you want to put wood? Use it, wood or another element, if you like, as a mold to shape the composite and then remove it.
    Do not make solid crossbeams as, in that case, more than 70% of the material does not work practically and increases the weight and bear in mind that these elements are subjected to torsional forces, so there is no "up" or "down" but it is advisable that have the same properties throughout their perimeter.
     
  3. Burnside Style
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    Burnside Style Junior Member

    Making a mold and using composite only would be fine with me.
    Are you saying that the composite beam should have the same wall thickness and layup on each face?
    (Imagining a rectangular box made from two composite "C" channels when I refer to the faces)
     
  4. Ad Hoc
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    Ad Hoc Naval Architect

    Like most things - it depends.
    It wont be any lighter... but the key driver is not the stress, but the deflection.

    Composites with a much lower Young's modulus will flex more, thus you end up having to add more material to reduce the deflection, and/or change the design of the structural member.
    That's where the weight goes ...
     
  5. TANSL
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    TANSL Senior Member

    I mean that a closed profile would be preferable, with a uniform laminate scheme around its perimeter.
     
  6. Ad Hoc
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    Ad Hoc Naval Architect

    Not usually done this way.
    Because the sides, if an RHS, will be carrying the shear load, thus you need +/- 45 degree fibres, whereas the flanges will be in-plane 0 degree fibres.
    You tailor the fibre orientation to the applied load.
     
  7. Burnside Style
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    Burnside Style Junior Member

    Ok, just quickly using an online calculator.. That gives an idea on size comparisons between the two materials right?

    Screen Shot 2021-07-16 at 7.30.33 PM.png Screen Shot 2021-07-16 at 7.30.45 PM.png Screen Shot 2021-07-16 at 7.36.18 PM.png

    I understand what you are saying about the fiber orientation differences on the two planes. But do the vertical sides have uni as well or just biaxl on the sides and just uni on the top?
     
  8. Ad Hoc
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    Ad Hoc Naval Architect

    In general not really.
    Just biaxl on the sides with some CSM as "filler" and uni on top/bottom. Since you want the dominant fibres and their orientation to work as they are designed to do.
    Thus you don't ant 'competing' fibres... in that sense.
     
  9. guzzis3
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    guzzis3 Senior Member

    2 interlocking C sections.

    45/45 coes right round the C
    uniaxial top and bottom. Interleave the 45/45 to aid resin flow if you are infusing.

    Top and bottom should have a total of 3mm of uni, so say 5 layers of 600 gsm uni or whatever you buy. Edit: This is the TOTAL of both "C"'s so say 2 layers on one C and 3 on the other, not 5+5! So 5 layers total top plus 5 layers total bottom!

    maybe 3 layers of 45/45 in each "C". 300 gsm might be enough.

    epoxy not poly imo. I'd infuse. Above is all glass not carbon. You could probably halve the weight of uni for carbon but I'd stick to that for the 45/45.

    The beam should end up 2/3 the weight of aluminium. Make it 70mm high and wide as per Al.

    Should be near enough. Make a test beam and test it. 3mm of glass uni is definitely enough but the twisting forces might need more 45/45. In carbon that should be enough 45/45. Nothing like trying it and measuring it to know for sure.
     
  10. fallguy
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    fallguy Senior Member

    So, just lurking...

    If YM is 11 GPa for wood, 17 for FRP, 40 for CF, 69 for Al, isn't it a little off to suggest FRP is lighter than aluminum? Just as an example, if I built my beams for my cat which are aluminum from FRP; they'd be lighter same size, but same size is about 25% of the needed I...or less

    Something seems amiss with saying 2/3rds lighter to me is all..
     
  11. Burnside Style
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    Burnside Style Junior Member

    Could the beam be made this way?
    Laminate the two "C" shapes, and top and btm pieces individually - then bond them together with epoxy.

    PXL_20210717_134445756~2.jpg PXL_20210717_135448451~2.jpg

    I have on hand 130z unidirectional fibreglass, 1708 bi axial cloth, and 9 0z bi axial cloth with no mat.
    Sounds like I could uses the 9 oz and uni cloth to make a test piece.

    How did you get to that layup schedule is what I want to learn. Is that from gained knowledge of looking at other structures or another way?
     
  12. gonzo
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    gonzo Senior Member

    I think the first question you need to ask yourself is what is the goal of the change. Are you looking for less weight, an experiment in design, learn to work with exotic fibers, etc. ? The goal will define what design will fulfill it. It is hard to beat aluminum for low price and simplicity.
     
  13. TANSL
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    TANSL Senior Member

    What you are concerned about cannot be deduced by looking at your neighbor. The problem of the cross beam of catamarans is complex but, as always, it is a matter of calculating the existing loads on the beams, knowing the bending moment and with it, and the properties of the materials to be used, determining their scantlings.
    I enclose some notes on all this problem.
     

    Attached Files:

  14. rxcomposite
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    rxcomposite Senior Member

    This is how it is laminated.

    Define the Load Model. This is a simply supported beam. Read DNV rules to familiarize yourself with what is needed. It is the most simple.

    No I don't think so. Designing a composite crossbeam is a very complicated procedure.
     

    Attached Files:


  15. Ad Hoc
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    Ad Hoc Naval Architect

    This is also addressed in Professional Boatbuilder Magazine, issue 182, Dec/Jan 2020 - how to design the box beams for a catamaran.
     
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