XPS/Luan Sandwich for Deck

[pcfithian]

I am about to install the decking on a Tolman Jumbo I am building and think a sandwich construction of 1" XPS Foam (Pactiv Greenguard) bonded with epoxy to skins of 1/8" exterior luan would make a very strong and light deck. This has often been used by others for cabin side and roof panels.

Once installed, the top surface would be glassed with 10 oz cloth and then finished with a non-skid like Durabak.

See attached for some calculations I've found on this site, but I am unsure of how much deflection I would get across the 30" span between the stringers.

Help and advice are greatly appreciated!

[gonzo]

I have made several hard tops like that. They were used for towers on which the helmsmand would stand on. You need to fill the edges. The easiest way is to chip the foam back about an inch and fill with thickened epoxy.

[pcfithian]

Good advice on filling the edges, I had planned to do that. The only exposed edge will be at the rear. The other edges will but up against the hull sides and be glued to it.

How far between supports could this span and still deflect less than 1/8"?

[gonzo]

You only need to support a 4X8 panel in the middle. I assume you mean with one person standing on it.

[pcfithian]

Thanks for the advice, Gonzo. But maybe the attached sketch will make it clear what I would like to do.

My question is, can I expect the XPS foam core with the 1/8" luan skins to be rigid enough to span the 30" between the stringers? It will need to support a person standing in the middle between the stringers.

[srimes]

Don't you expect more than 1 person in the cockpit?

[gonzo]

We used the panels supported only on the edges with no problem. That should work.

[pcfithian]

I was able to complete the equations and here's how it worked out. This type of panel is incredibly stiff!

[srimes]

Nice. But what's the stress? Won't these types of panels fail before excess deflection?

[pcfithian]

I've decided against using a foam sandwich and will be using 3/4" ply instead. The weight savings don't justify the extra work and drawbacks of a foam sandwich. Here's what the deck cross section will look like:

[pcfithian]

I've modified my design to use XPS foam for floation under the deck. Here's what it will look like: