# plywood and foam sandwich comparison

Discussion in 'Boatbuilding' started by tsanakou, Nov 10, 2011.

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### CharlySenior Member

but the foam version does not have stringers

2. ### CatBuilderPrevious Member

That's right, it doesn't. Some extra stiffness there.

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### tsanakouJunior Member

I found it: http://multihullblog.com/2010/11/plywoodglass-strength-comparison/
Kurt Hughes on Catamarans, Trimarans, and Boat Design

Plywood/Glass strength Comparison

How strong is glass compared to plywood? Here is a table comparing strengths with out of plane loads (like water pressure).

Plywood/Glass Thickness Conversion comparing bending strength. (Assumed 60,000 psi bending strength for glass and 10,000 for plywood)
Assumed that triaxial amount is on both sides of a core

Ply thickness Triaxial thickness Core

3mm ply equals (12 oz) 400 gsm w/12 mm core

4mm ply equals (17 oz) 600 gsm w/12 mm core

6mm ply equals (22 oz ) 750 gsm w/12 mm core

9mm ply equals (34 oz) 1150 gsm w/19 mm core

12mm ply equals (2) (22 oz ) 1500 gsm w/19 mm core

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4. ### CatBuilderPrevious Member

Cough. Cough! ahem! Look at the second post in this thread. Agrees 100% with what tsanakou and Kurt Hughes posted.

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### tsanakouJunior Member

I wonder:
9mm ply equals (34 oz) 1150 gsm w/19 mm core = weight of foam = foam 1.52kg/sqm + 2X ( 1150 gsm + matrix 1150gr) = 6.120kg/sqm ???
The plywood is 5.2kg/sqm + 220gsm + 220gr matrix = 5.64kg/sqm ???
Does that make the plywood construct lighter?

6. ### CatBuilderPrevious Member

It does, but to compare more accurately, you must investigate the method of construction off the cross beams and include stringers, as 9mm ply needs stringers for stiffness.

There is a reason Kurt's cylinder mold boats are among the best performing of their kind in the world. It is a very light way to make a boat. Look at his personal trimaran (now sold and in Europe). You can't do much better than that.

Lastly, a big surprise for me was that the Kurt Hughes I'm building has better weight numbers than the carbon fiber Gunboat 48, even though the lwl is the same and the KH is beamier. (gunboat 48 is a 45 where they count the bowsprit for loa)

The man knows how to make a performance boat without blowing your budget.

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### david@boatsmithSenior Member

well I am not an engineer but I have built boats of 3/8 (9mm)plyand am now building a boat of 1 1/2 oz CSM, 1708,1808, with 5/8 H60 and 1708,1808, 1 1/2oz CSM. I do not consider a 3/8" ply to be anywhere near as stiff or as strong as the sandwich.

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### tsanakouJunior Member

Well, I am happy that I found the comparison again; there are doubtlessly better production methods than the plywood method. But when it comes to prototypes it is still unbeatable for my humble opinion and it is still fun to work with wood.
I am glad that groper wasn't that right with his post: 'You do realize you are trying to compare apples with oranges dont you? ...compared with the foam sandwich of same thickness you get - roughly 1/3 the strength in plywood of the same thickness and 3 times the weight...' I am happy to compare apples and oranges when there is a good reason for it ;-)) Thanks for all your help!

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### Corleyepoxy coated

Its worth keeping in mind that you can bag core to the inside of a ply structure as well instead of stringers to help with panel stiffness you get some of the positives of both methods then.

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### tsanakouJunior Member

Composites have a big variety of methods available; we have constructed a very light proa with a much smaller size of plywood thickness demanded because we were using Aramid/Kevlar from the inside and glass from the outside. Everything is possible ;-))

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### idkfaSenior Member

Catbuilder, somehow 9mm equals 21mm (19+1+1)?

I might be hesitant betting against 9mm solid carbon.