Plywood specification questions

Discussion in 'Wooden Boat Building and Restoration' started by erik818, Apr 8, 2010.

  1. freeboatrsrce
    Joined: Apr 2010
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    Location: Delaware

    freeboatrsrce Junior Member

    I have been told marine grade is the best hands down. Exterior grade plywood with internal voids contains air that is bad, especially if you are using epoxy with it. If you do decide to use exterior grade plywood you should cut test samples from it and test the samples to make sure there is no de-lamination. I also have no information on the strength properties of the marine grade plywood, but the delamination possibility of exterior grade plywood could make the project a nightmare, far outweighing the strenght property differences between marine grade plywood and exterior grade plywood. I would think that following Scantling Rules would resolve any problems related to hull thickness using marine grade plywood. When using Exterior Grade Plywood, you should always test the plywood to ensure you do not have delamination problems.
  2. ancient kayaker
    Joined: Aug 2006
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    Location: Alliston, Ontario, Canada

    ancient kayaker aka Terry Haines

    The strength of plywood is determined by the strength of the wood it is made from. There is some glue between the veneers so in practice it's slightly less, but not significantly so, except perhaps for very thin plywood such as those used for aircraft and modelling.

    Unlike solid wood the lengthwise vs widthwise strengths are not widely different as far as tensile strength is concerned, being determined by the thickness of veneers running in each of those directions.

    As a simple example, 3 mm okoume ply with equal veneer thicknesses would have slightly more than 2/3 of the grainwise strength of solid okoume in one direction and only 1/3 in the other direction. A plywood with different thickness of face and core ply or more than 3 plies would be different of course.

    Stiffness and bending strength also differ across vs parallel to face grain direction, and are more difficult to calculate. Not that big a deal theoretically but I don't really want to go into the math. Unfortunately the simple theory that works for materials like metals with properties that are the same in all directions does not work precisely for wood. Plywood breakage is not the same as (dry) solid wood breakage, more like green wood breakage in that it does not necessarily break cleanly across the entire width at the same time.

    Few of us have the machinery to perform pull tests on wood to destruction, so we are more likely to compare different woods by bending samples until they break. Unfortunately the breaking mechanism is more complicated for bending than for pulling as the wood fails at different levels of stress in tension and compression and results are affected by crushing in the thickness direction. The failure levels also change with variations in moisture and the history of stresses previously applied and removed even without damage. Historical stresses can actually increase wood strength, I understand. This is in addition to the strength variation within a single wood species from tree to tree and within the tree as well as how the wood has been cut and treated.

    Maybe the above simplified (!!) account explains why plywood manufacturers do not seem to publish strength figures for their products, as far as I know, although they do publish rated loads for various constructions of floor for example.

    Here are the modulus of rupture and compression parallel to grain of douglas fir and okoume, both at 12% moisture content, for comparison purposes.

    d-fir MoR 12,800 CPtG 7,260
    okoume MoR 7,400 CPtG 3,970

    See for more data on wood properties.

    caveate: I am not a wood or plywood expert, just repeating what I have gathered over a few years on the assumption I understood what I read. I think it at least illustrates the complexity of the subject.

  3. magwas
    Joined: Oct 2009
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    Location: Hungary

    magwas Senior Member

    I did seen plywood manufacturer publishing strength figures for its okume plywood.
    (Boasting that the figures are well above the standard, but I can't remember which.)
    So it is not without precedent, however admittedly not common.
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