I think mine is 3/16", but I have a 20 gallon tank under the console in my boat. It had a 45 gallon, which I also think was 3/16" thick at ~24" x 60" x 8" tall. It sat between the two big stringers in a potter hull and stopped under the console. That leaves room for a "head" in a 20 foot Master Angler.
Pics of my 45 gallon removal from my 20 footer here:
http://s1119.photobucket.com/profile...tretcher/index
With a small 4 stroke, I put the 20 gallon in it, I can't motor far enough to need more with a 100 HP 4 stroke. The boat had a 175 HP 2 stroke in it when I got it, and I think it probably had the same range with a 45 gallon tank.