Which is stronger

I am curious as to what you all think about something.  I am building a sail rig and the plans call for 9mm plywood to be used for the bulkheads and then 9mm ply laminations on a couple of small pieces.  

 I am building from the plans and have a lot of scrap 4mm ply laying around.  My question is which would be stronger, bulkheads made from 9mm or from 8mm made from two laminated 4mm pieces?  I was planning to fiberglass the bulkheads whichever way I go.  The main reason I would go for using the 4mm is because I can save a bit of money not having to buy a 9mm sheet of okume.  



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RE: Which is stronger

Manufactured 9 mm is definitely stronger than home-laminated 8 mm, but for what you're doing the 8 mm should be strong enough.

Plywood manufacturers use resorcinal applied and pressed with machines that maintain optimum conditions. Homebuilders with epoxy need thicker glue lines and in most cases can't control the conditions as well as plywood manufacturers.

The good news is that epoxy is filling and forgiving. As long as reasonable care is used, there won't be any voids. The lamination, though a bit heavier than manufactured plywood, will work fine. Remember, the glue joint doesn't have to be super-strength, just stronger than the wood.

Go for it,




RE: Which is stronger

I built a sail rig and had almost the same question. I just happened to have extra 6mil and 4mil and also didn't want the hassle of getting 9mil. I laminated 6 and 4 together using thickened epoxy, which produced a very stiff laminate. I've also done 3 and 3 when I needed 6 and I can tell you based on a direct comparison with factory 6, the 3 and 3 laminate was much stiffer. I can't say if this is better or worse for this application.

In your situation I would definitely go with three layyers of 4. Not sure why CLC specs 9 for this application but after building the amas I would say 12 would be much better. In fact, the magazine review for the Sailrig on this very site comments that the 9 mil bulkhead cracked and thought they should be doubled.

When laminating the stock for the bulkheads I would do each one individually and use lots of deep throat clamps or curved clamping cauls to eliminate voids in the middle.


RE: Which is stronger

I actually found some 1/4 inch marine okume in my shop so I laminated 3 layers of that for a total of about 19mm.  I think that should be enough!  

RE: Which is stronger


3+3's extra stiffness compared to 6 is to be expected. It's a result of the thicker glue line that epoxy gives you over resorcinal.



RE: Which is stronger

The laminated piece will be stronger in the grain direction of the surface plies and probably weaker in the opposite direction. This is because there are usually more plies and overall thickness in the direction of the outer plies than in the perpendicular direction. Also, since stiffness increases at the cube of the thickness, the outer plies add more stiffness than the inner plies up to their breaking point. This is all assuming equal thickness plies and an equal thickness between the laminated piece and the piece made of heavier material.

RE: Which is stronger

Casey got the answer to the root post of this thread, and is happily building away.  I will add only a postscript.

This thread reminds us that "which is stronger?" can mean quite a number of  different things, and in some cases, two answers will be exactly the opposite, depending on which meaning of "strength" was assumed.

The most important distinction is this: strength can mean

  • Resistance to deformation (stiffness, also called "modulus" or "modulus of elasticity") 


  • Resistance to failing

Making a boat part stronger the first way can make it weaker the second way. A classic case is carbon fiber mast vs. a wood or 'Glass one.  The carbon fiber mast will be stiffer (ceteris paribus), but more likely to break in a gust.

There are many other variations in the meaning of "strength".

  • Donm1753 mentioned one: plywood, and wood in general, has different "strengths" in different directions.
  • Another example: an ash part  is "stronger" than maple in "resistance to breaking when subjected to a shock load".
  • A polyester-fiberglass boat is a little less stiff than epoxy-fibreglass when new, but the gap widens a lot when the boats are old
  • There are three different Young's modulii--one for bending stresses, etc.  Knowing one doesn't tell you the other two (although knowing two of them DOES tell you what the third must be).



RE: Which is stronger

Having built a lot of the SailRigs, I think the 9mm bulkheads are about right.  They won't break once assembled or underway---it's never happened.  The problem is if you torque the bulkhead with the ten-foot long aka during assembly on the beach.  I've done that a few times. 
I was trying to keep the weight down to...nothing.  So those bulkheads are strong enough and not a bit stronger.  I think the way to make them stronger would be to 'glass those bulkheads, which would add more strength per ounce than making them thicker. 

BIG fillets on the bulkheads will add tons of strength.  I'm not sure all the builders do this.  As visible here:



RE: Which is stronger

That's it - I'm retiring from my goal of being the chief pontificator on this forum. "ceteris paribus", indeed. How can I compete with that? It's all yours, Camper. I'm going off to learn to talk like Kurt.


Remember - Lingua Mortua Solum Lingua Bona Est


RE: Which is stronger

My daughter came home from college with a degree in economics plus the phrase "ceteris paribus".

I've got a considerable sum sunk into that phrase, on a dollars per syllable basis, so I'm looking to get some kind of return.

I've been using it a lot around the house,  but the family just roll their eyes with that "Don't you have some epoxy drips to scrape out yonder in the garage?" look.   So I've been itching to use it somewhere else.

I'm glad that it created the right impression.  I've always been a bit envious of the Mauermeister and others who can toss off a completed kayak in less time than it takes me to bring a cup of epoxy mix to a rolling boil.

But Kurt and them, I bet they can't use "ceteris paribus" in their posts.  It's been a real self-esteem-builder, I can tell you.



RE: Which is stronger



Thanks for clarifying the use of 9mm bulkheads.  I probably would have just used that thickness, but it is a little late now!  When holding all 4 extra thick bulkheads in my hand I would say that they weighed about 7 pounds, so I am not too worried.

I have a question about something that I noticed while wiring the hull and in one of your photos.  I noticed that the top and bottom panels are different lengths.  Is this on purpose, or did both I and the builder in the photos somehow mess up?  Remember I am building from plans.  

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