Structural Interior Furniture Options

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

  1. fallguy
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    fallguy Senior Member

    Pretty heavy layup in the foam I'd say, but I am an amateur builder.
     
  2. rxcomposite
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    rxcomposite Senior Member

    Something wrong here. Typical okume ply density is 510 kg/m3 while H80 foam is 80 kg/m3.

    Also H80 has too much shear strength. I can use that for the bottom 16-20mm thick. If I use it for the panel with 10 kN/m2 of pressure, I need only to use 5 mm thick.
    H45 will do at 12 to 16mm thick.
     
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  3. KnockedDown
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    KnockedDown Junior Member

    Not replacing the bulkheads for the reason you mention. Sticking with ply there. Only considering replacing the cabinet fronts and tops (not sides which are the bulkheads).

    Would be worried about creating a hinge but since there's cabinetry and other structural elements running nearly the length of the boat bow to stern, don't believe this is much of an issue. And the small bit aft of bulkhead 5 can be reinforced with panel tying in the transom.
     
  4. wet feet
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    wet feet Senior Member

    If the old ply lasted a long time in spite of neglect,why not replace it with a newer,prettier ply?Do you have experience of preparing effective composite sandwich panels and bonding them into hulls?It will be simpler and quite effective to use ply and realistically a tiny amount of weight that might be saved at a low point in a displacement hull isn't likely to make a huge difference.How much weight would you be hoping to save 12 lbs,25 lbs?
     
  5. fallguy
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    fallguy Senior Member

    I will never forget the video on youtube of the woman who made great efforts to reduce her sailing cat by 10kg. I was laughing at her until the end. She said, my reward is a wine locker with 10 bottles of wine. Then, I was impressed by her; a lot.

    22 pounds is 10 bottles of wine I guess
     
  6. KnockedDown
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    KnockedDown Junior Member

    Yes to the question of experience. Give me a recipe and I'll follow it. And I'm going to have to do the bonding whether I use ply or composite panels. Figuring 80 ft^2 to replace so 65lbs of 9mm ply at 26lbs/ft^3 (okume is new benchmark since the heavier marine ply is being replaced regardless - and 26 is the number from Boulter Plywood but may actually be too light). Believe half-inch H80 with two layers each side of 24oz/yd^2 plus resin results in 6lbs/ft^3 and an all-up weight for the 80 ft^2 or around 15lbs for a 50lbs saving or almost 1.5% of total displacement. Worth it? Not sure but that's starting to sound meaningful.
     
    Last edited: Dec 7, 2020
  7. KnockedDown
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    KnockedDown Junior Member

    I was asked to provide boat details...
    SA: 371 ft^2
    Disp: 3,600lbs
    SA/Disp: 27.6
    Ballast: 1,500lbs
    Bal/Disp: 40
     
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  8. Rumars
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    Rumars Senior Member

    I didn't propose such a heavy panel, but with that layup he can not save weight. As for okoume weight, my supplier quotes 500kg/m3 +-40kg.
     
  9. rxcomposite
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    rxcomposite Senior Member

    Ok 500 kg/m3 compared to 80 kg/m3. That is 6.4 X heavier than foam.
     
    Last edited: Dec 8, 2020
  10. fallguy
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    fallguy Senior Member

    The okume is correct. The panel is not.

    A panel made with H45 and 1700 db which is very close to 1mm is lighter. I don't have time to do the calculation now.
     
  11. Rumars
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    Rumars Senior Member

    1700db is not "close" to 1mm. It will be 0.5-0.7mm depending on how it's made. Close means 0.95mm or 1.05mm but that needs ~1000gr/m2 (29.5oz/sqyd) of fiberglass.

    I agree that foam furniture can save weight over ply, but only if the calculations are done. When using a "rule of thumb" approach of making a foam panel equivalent in strength to ply, the result does not save any weight over thin okoume ply.
     
    Last edited: Dec 9, 2020
  12. KnockedDown
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    KnockedDown Junior Member

    First off I appreciate all the input and I've learned a great deal. For one thing, fallguy correctly pointed out my idea of testing a panel wouldn't have tested what I'm trying to achieve. So it's clear I'm not an engineer. But I enjoy thinking about making racing sailboats, lighter, stronger, easier to sail and sometimes, prettier. But I'm having a hard time with "making a foam panel equivalent in strength to ply, the result does not save any weight over thin okoume ply." Two things... Believe fallguy had proposed multiple layers of db aggregating to 1mm and two.... I'm thinking (hoping) you're working with the assumption I'm constrained to making a composite panel of roughly the same thickness as the 9mm ply. At that thickness, my understanding (West Systems manual) is that the panel gains little from structure and has to be close to a solid piece of glass resulting in no weight advantage. But once I go up to 12mm (or 18mm) as proposed, I don't believe I need to spend all of the weight savings (core vs. ply) on the lamination to get a panel with improved characteristics versus the okume.
     
  13. fallguy
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    fallguy Senior Member

    1700 db is about 0.035"

    one millimeter is 0.039"

    difference is 0.004"

    0.004"/0.0039" is 10% off....

    I realize I suggested 24 oz; that would exceed one millimeter by quite a bit.

    I use 1700 db quite a lot and generally gauges at 0.033" here. I used West System's number for the 0.035" citation.

    Determining Laminate Thickness • WEST SYSTEM • Fiberglass Repairs https://www.westsystem.com/the-105-system/reinforcing-materials/determining-laminate-thickness/
     
  14. fallguy
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    fallguy Senior Member

    so curious

    Too much shear strength?

    Do you mean unneeded shear strength?

    How would a foam panel have too much? Or am I just being a nit?
     

  15. rxcomposite
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    rxcomposite Senior Member

    It is a unit of measure of the engineering properties of the material. H80 has 1.0 N/mm2 ultimate strength while the calculated strength required is 0.8 N/mm2 ultimate. I still could use it provided I use only 5mm foam thickness.

    Now, I could use a cheaper but thicker core if I do not want to use H80 at 5 mm. Increasing thickness will reduce the required shear so at 12 mm, I need only an ultimate shear of 0.27 N/mm2.

    Looking at the tables of core shear strength, H60 has 0.76, H45 has 0.46, H35 has 0.40.

    The core shear strength you must choose must be equal to or near the 0.27 N.mm2 required.
     
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