Plywood as core material

[23feet]

I just redid the center decks of my Tiki 21. They were 1/4 plywood/epoxy with stringers (no glass) and were very flexible. I redid them with 6 oz glass on BOTH sides, and they are now amazingly stiff. Makes me wonder why thinner plywood sections are not more often used as core with glass on both sides. Is it because of weight - the decks still feel light, although they are heavier than they were of course?

 

[latestarter]

Years ago I did a crude experiment with 4mm plywood.
The outcome was that there was a major improvement at right angles to the grain of the ply, some improvement in the stiffness parallel to the grain but very little in the strength. 6 oz/yd2 is 142 grams/m2.
3-ply plywood https://www.boatdesign.net/threads/3-ply-plywood.50493/

 

[23feet]

This is amazing data, thank you Latestarter.

You interpret it as being negligible improvement, but it strikes me that the difference in stiffness (for glassed both sides) is considerable even in the parallel grain test (>2X). so, the question is your test of strength. I interpret your test method as continuing the stiffness deflection to destruction, but is that what "strength" usually means in such test? The Gougeon Bible appendix shows fatigue tests and penetration tests, but there is no protocol for "strength" unless I missed it. Overall, your "stiffness" test seems to show that glassing both sides in effect makes the plywood composite more "brittle" (stiff but easily fractured).

Another interesting aspect of your data would be to compare it to other composites of similar weight, i.e. do glass and foam composites of similar weight have similar stiffness and are they "stronger" in your use of the term (force to failure in deflection)?

One thing is for sure that there is little data readily available about this question on the interwebs. There is this one here Plywood Composite Strength Testing | Bateau2 https://bateau2.com/howto/ply_test.php which is similar to yours but does not distinguish between grain directions; and this one at Gougeon's Boatworks Plywood Boat Construction https://www.epoxyworks.com/index.php/plywood-boat-construction/?hilite=%273%2F16%27%2C%27plywood%27%2C%27stiffness%27. This last one shows that stiffness in 7 ply Luan increased by 32% for an 11% weight gain when glassed on both sides with 6oz e-glass. But there is no test for differences in PSI force to failure.

 

[latestarter]

I would not put too much weight on my data, certainly would not design anything based on the data, at most it points to the issues involved.
I have been lurking on this and other boat related forums and recall someone who knew what he was talking about, said that if an element was stiff enough usually it was found to be strong enough.

Only yesterday on another site there was the comment "Are you going to sheath the ply with glass fibre to finish the boat? If so then the ply is just the core and the strength comes from the glass and resin."
I do not think it is that simple.

 

[Yellowjacket]

A layer of glass on both sides dramatically stiffens a piece of thin plywood. Typically it takes between two and three coats of epoxy to seal a piece of plywood. For the same weight, instead of using just epoxy alone, if you use the same thickness of epoxy and use glass in it you'll generally double the stiffness and the strength. That's pretty much the tradeoff. If you're going to waterproof with epoxy you're much better off putting glass in it. Really thin (3 layer) plywood is obviously going to be much stronger in one direction, but adding a single layer of glass improves the weak direction more than it does the strong direction. If you went to more cloth (either 2 layers or heavier cloth) then eventually you'll get to the point where the strength is in the glass and the wood is acting more as a core. Here is some test data that was done with a testing machine. As you can see in the data a small amount of glass really improves the stiffness to weight ratio. Obviously the thicker the plywood the more improvement that you get from glassing it. Also note that increasing the thickness of the plywood is more effective than adding glass on a strength to weight ratio basis. This is because the strength and stiffness go up as a cube of the thickness. But you can get similar strength to weight from the thinner plywood that is glassed, and that glassed plywood is waterproofed (if done properly) To waterproof the thicker plywood would add more weight, and wouldn't improve the strength or stiffness. For 1/4 plywood adding 4 ounces of cloth on both sides increases the weight by 20%, but it increases strength and stiffness by 50%.