Aluminum to plywood?

[Deering]

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.

[Raggi_Thor]

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.

[Deering]

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?

[Raggi_Thor]

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(?).

[Raggi_Thor]

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.

[Deering]

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.

[Raggi_Thor]

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.