Holes in plywood affect on strength

Discussion in 'Fiberglass and Composite Boat Building' started by fallguy, Nov 13, 2022.

  1. fallguy
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    fallguy Senior Member

    Does anyone know how holes in plywood affect the strength of the panel?

    I used 1/2"/12mm plywood cores as knees on a cantilever section of the bridgedeck. And I'd like to remove some of the weight in circle cutouts without sacrificing too much strength.

    I used two pieces of plywood to keep the section from sagging. It is about 3' of overhang about 24" high. It seems like I could take 2" circles out with say 1.5" of meat left and not really affect the performance of the panel. Hindsight; the panel should have been corecell, but everything is tabbed and tight..and no room to change. I would start about 3" from the edges.

    I can take a picture, but these are lockers on the front of a bridgedeck.
     
  2. fallguy
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    fallguy Senior Member

    General idea in a 30 second sketch.

    488284BE-0811-4F1E-8EF6-375617A56928.png
     
  3. Tops
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    Tops Senior Member

    PS Is the plywood in question inside a locker or under a deck? The idea is to make a trestle that fits rather than 'drillium' if it would be any easier on you.

    Could you do something like this? Might be easier to do and re-finish than a bunch of holes.
    fallguy_gusset1.png
     
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  4. fallguy
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    fallguy Senior Member

    Ocean hits bottom in big seas..
     
  5. Tops
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    Tops Senior Member

    Would a pole be better than a plate?
     
  6. Barry
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    Barry Senior Member

    Is one of the purposes to reduce impact loads from any sideways seas that may hit the current support?

    I am curious how much you will save in weight if you drill a bunch of holes and then have to fiberglass up all the exposed edge ply that you are exposing when cutting the holes

    What you will remove
    20 plugs at 2 inches
    If 3/4 inch ply 8 pounds
    1 inch ply 12 pounds
    1 1/2 inch ply 18 pounds

    I had originally thought you would have to add back some glass and resin but I guess that you are also removing some glass weight from the existing structure during plug removal. So perhaps
    this is awash

    Seems like a lot of work for up to 18 pounds of reduction

    You could subtract a 24 pack of beer in cans to get the same weight saving. mmm
    Alternatively if you drill the holes you can carry another 24 pack.
    I see where you are going with this
     
    Last edited: Nov 13, 2022
  7. Rumars
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    Rumars Senior Member

    You mixed up your units somewhere, it's 0.8lbs, so only one can of beer.
     
  8. fallguy
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    fallguy Senior Member

    It is these two inside panels in a cantilever of the bridgedeck. I don't care if it is 2 pounds. Please stick to the question..most appreciated.

    51452DFE-AF36-4456-AFBB-3A58DC06A87C.jpeg
     
  9. Chuck Losness
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    Chuck Losness Senior Member

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  10. fallguy
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    fallguy Senior Member

    Didn't find much.

    It seems like something like perforations effects should be well understood. Airplanes use it all the time.
     
  11. Ad Hoc
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    Ad Hoc Naval Architect

    It all comes down to how many what size, how close together and in what thickness v diameter size.

    But the bottom line is, there is a trade off between, the weight you 'think' you can save v the loss of shear strength.
    Since holes, in webs (no matter the material) is a general loss of shear strength. So, you need to ask yourself, how much shear strength do you currently have - for the load cases considered - and how much excess do you have, if any?
    Then, finally, how you make those holes, will influence the performance...as it can be the site of crack initiation, if poorly cut..etc etc.
     
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  12. fallguy
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    fallguy Senior Member

    The 12mm ply was a guess. I was worried a massive head sea at 20 kts could crush a light core (knee). But I'm pretty sure the ply is excessive. Of course, this is all just going by gut. I tried to find a datasheet and cannot.

    It seems like panel perforation should be well understood, but googling led to little.
     
  13. Ad Hoc
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    Ad Hoc Naval Architect

    Well, that depends whether you have designed the structure with the plywood core - to take the shear loads - or whether the plywood core, is there merely to created the shape you require?

    It is, as noted above.
    But...you just need to calculate the effects owing to your preferred arrangement. It is not a formulae or a set of tabulated guides in that sense.
     
  14. fallguy
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    fallguy Senior Member

    Well, it was not an overly complex approach.

    Just figured a plywood core would be stronger, less likely to buckle. Same approach with the core change on the sides..not a lot of meat above the beam, so was sort of staying on the page is all. But I would like to perforate the panels. I've done the math; each 2" cutout ain't much, but I aim to make it a wee bit lighter.
     

  15. Ad Hoc
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    Ad Hoc Naval Architect

    Given the size of the knee/bkt, the only weight you'll really be losing is that of your own, from all the hard work attempting to cut, sand, smooth and glass a few small holes.
    A few calories burnt there me thinks .. :oops:

    Ergo...such a small amount, if this is what you need to do, there are bigger fish to fry than a few grams.
     
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