Aluminum to plywood?

If I want to duplicate an aluminum design but build it in plywood/epoxy, any rules of thumb for scantling conversion? If the hull's plated with 3/16" aluminum, what would an equivalent thickness of plywood be to achieve similar stiffness and strength, assuming similarly spaced ribs and stringers?

I understand there are a lot of considerations and "it depends" answers, but I'm assuming that this conversion is done often enough that people have gained experience with it.

 

We are going to build a 16-17 feet outboard boat in 9mm plywood. It was designed for 4 to 5mm aluminium. One problem you may consider is the weight of tha boat and stability when it's empty.

 

So you're about doubling the thickness. How did you arrive at that ratio?

Good points about weight & stability. How much more do you estimate your ply boat will weigh over the aluminum?

 

Aluminium is about eight times as stiff as wood I think, thats the E-modulus.
So theoretically you should increase the thickness with the square root of 8 which is a little less than 3. BUT aluminium is subject to fatigue, much more than wood, so I think you are safe if you double the thickness.

The plywood version will weigh a lot less than the original aluminum boat. The density of plywood (Okume) is one fith (20%) of aluminium, less than 500kg/m3 while alu is 2.7(?).

 

I had to take look for the numbers, as it's late at night here
http://www.fpl.fs.fed.us/rwu4706/mechproperties.html
Modulus of elasticity: 6.89-13.1 GPa (1-1.9 x 106 lb/in2)
I suppose this is fir plywood?
Okume is not so strong, but we will add 300g (10oz?) of glass on both sides.

 

You're right. Aluminum is 2,700 kg/m^3.

The plywood in the web site you referenced is a general construction sheathing grade - typically known as CDX around here. Marine grade fir is considerably better - fewer voids, better adhesives.

So your new hull will weigh about half as much as the aluminum hull. I wouldn't have guessed it would be that significant. Interesting!

Thanks for your insights.

 

I think E modulus of 8 to 12 GPa is typical for many kinds of plywood. The outside plies gives extra stiffness in one direction. The difference etween the two directions is quite small for 7-ply but significan for 3-ply.

Yes, we think we can use a 50HP outboard on the plywood verson while 90HP is typical for the original. This is 18 degrees v bottom skiff (I think you call it), mostly open with a selfbailing sole and benches all the way round + two seats behind a small wind screen.

DNV has rules for aluminium and grp, but not plywood.
There will be an ISO standard for scantlings in small plywood boats this year or next.