There are lots of sources of materials density out there.

http://www.rfcafe.com/references/gen..._materials.htm
shows:

Aluminum at 171 lb/cu ft. Divide by (12*8) to get approx. 1.78 lb/SF for
0.125 aluminum.
Plywood (5/8 thickness construction grade) is 1.77 lb/SF (or 34 lb/cu ft)

So I stand corrected. 1/8 aluminum weighs about the same as 5/8 construction
grade ply. Maybe, when I calculated this a few years ago, I used a
different grade or species of plywood, or alloy of aluminum - I honestly
don't recall. But either way, it gets you into the ballpark. There are
relatively few elegant small boat designs using material this heavy.

Sal's Dad

--

as 1/8 aluminum weighs about the same as 3/4
ply.

If you divide 2.7, the density of aluminum, by 6, you get .45. I
thought plywood was heavier than that.