Structural Interior Furniture Options

Discussion in 'Materials' started by KnockedDown, Dec 5, 2020.

  1. rxcomposite
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    rxcomposite Senior Member

    Post your boat particulars ie LWL, displacement, beam, depth, draught, max speed ect.

    Cored composite properly engineered is lighter.
     
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  2. fallguy
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    fallguy Senior Member

    I understand the basic principles of shear. In composites, and here, the concern would be panel shear failure from stresses. What I am still not understanding is lowerin the shear rating.

    For semantics, let's say he uses 12mm plywood. Stresses on the hull have less effect than before, right? Or suppose he uses quarter inch steel plate and (pick any sufficient) glue. Surely the steel plate does not need the shear rating in the analysis.

    I greatly respect your work and effort and I apologize for any tone. I simply don't understand and wish to.... From my perspective, I see no reason exceeding 0.27 Nmm2 is a problem; just that is is not needed.

    Unless that shear rating is simply driving to the better composite. That part I can understand.

    I guess the trouble is mostly comparing foam and ply in my brain. Shear is not really used in the ply calc or is it?
     
  3. rxcomposite
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    rxcomposite Senior Member

    Shear is the driving force in all cored composite and in a way also for solid laminate. In plane shear is greatest at the neutral axis where the plies tend to go against each other. In solid laminate, the laminate always has greater shear available. It is the bondline that is the culprit. Resin is always the weakest link.

    That is why cored composites is always better and lighter. That is what composites is all about. You fill in the core with something light and bulky like CSM, felt, foam core, or balsa (thus raising the moment of inertia) and leave the tension/compression forces in the skin to do its job.

    The best way to understand is to go back to 1st principle calculation of a flanged beam (the equivalent of a cored composite). The calculations follows the same method.

    By the way, the material data sheet shows Okume has the same shear rating as H130 core at 2.0 N/mm2
     
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  4. rxcomposite
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    rxcomposite Senior Member

    I search my design database. That 9mm Okume wit a 300 x 600mm long panel will withstand a 16 kN/mm2 of pressure. 16-18 kN/mm2 is normally used for superstructure side. The core shear can take it but the skins will fail first as the tensile strength is only 27.0 N/mm2
     
  5. fallguy
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    fallguy Senior Member

    Do you mean the hull skins will fail because the material is too stiff? We don't know the hull skins.

    I apologize. Hopefully this is edifying for the OP.
     
  6. rxcomposite
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    rxcomposite Senior Member

    Stiffness has nothing to do with it. I already said tensile strength.

    Skins because plywood is composed of plies like a laminate. I am referring to the outermost skin where tension/compression is greatest.
     
  7. fallguy
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    fallguy Senior Member

    Ah, so you mean the plywood skins. Got it.

    if op gives you some more details; you can spec? So good of you.

    Will that include bonding? I will wait and see.
     
  8. rxcomposite
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    rxcomposite Senior Member

    If it is in my database or I can come up with something close. Too busy with other work.
     
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  9. Rumars
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    Rumars Senior Member

    Fallguy, I'm curious, what is the finished weight per square foot or yard of one of your panels with 1700 each side of whatever thickness of foam you use?
     
  10. fallguy
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    fallguy Senior Member

    21.8 pounds 12#Gurit Corecell M80
    170z glass at 100% resin both sides

    calc
    32 square feet
    3.55 sqyd
    7.11 sqyd both sided
    241.78 ounces resin and glass at 100% resin
    15.11 pounds resin
    6.68# core
    21.788# total
    0.6808 pounds per sqft
    0.309 kg per sqft
    10.764 sqft per sqm
    3.33 kg per sqm

    using vac reduces the resin by say 30% or gives

    2.98 kgsm

    using lower density core saves some more; not the original idea

    2.54 kgsm

    using 45# density core

    Let me know if I erred. My favorite mistake is switching 2.2 kilos for a pound. I slept poorly last nite. My father is sort of gravely ill we think.

    In my original estimates, I used a sheathed plywood iirc.

    But I think he saves about 50% of the plywood weight if he does it right.

    So, pretty close to that 10 bottles of wine. Just for fun.

    I should link the video. Classy lady.
     
  11. fallguy
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    fallguy Senior Member

    My panels I build with vac are running closer to 65% resin. I am always terrified they are dry, but rx and others have assured me those are decent numbers. Intuitively, a 20 pound panel seems about right.
     
  12. Rumars
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    Rumars Senior Member

    Your calculations are indeed correct, but I was more interested in actual measured weight from a sample. If we know the actual dimensions and weight it's easy to scale it up to whatever we want.
    The bigger question is what thickness of plywood equals your sample panel. The rule of thumb I know is that 17oz each side of 12mm H80 corresponds to 6mm ply, but maybe I know wrong.
     

  13. fallguy
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    fallguy Senior Member

    My bulkhead panels were all made with the layup I cited under vac and the three tested averaged 65% resin or the 2.98 kgm number. They are light. Bonded into the hull rather well.

    Here is an interesting prior thread that goes on and on and ultimately compares an 80 kg foam layup to 9mm ply and the foam layup calls for double the glass. The problem with the comparison is that all of the loading on these panels is way different than a hull.

    plywood and foam sandwich comparison https://www.boatdesign.net/threads/plywood-and-foam-sandwich-comparison.40533/

    And I think it is still important to recognize, 9mm ply was never an ultralight solution just as I am not an engineer. The more likely reason the 9mm was chosen is because it is the smallest thick ess of ply one would be comfortable sitting on...

    I would like to compare the wood cleats to glass tapes just for semantics, but I have to go for now.
     
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