For a reference with some degree of science "The Gougeon Brothers On Boat Construction" page 180 has some data on a small strength/weight experiment.

For those who have not spent the money on the book (it has a a VERY short discussion on strip canoes and kayak, but is still pretty interseting)here is a synopsis.

Western red cedar with 6oz cloth both sides:
Thickness-----ave. # to Failure------Ave wt/sqft----Ave deflection at failure
3/16----------------214-----------------8.0oz---------------.73"
1/4-----------------221-----------------9.8oz---------------.49"
5/16----------------300----------------11.6oz---------------.48"
3/8-----------------298----------------13.1oz---------------.32"

for a comparison they tested a piece of 1/4" okume bare plywood it failed at 225# weighed 10.7oz and had a deflection of .63".

I found the information, such as it is, informative enough to convince me that I could use 3/16 on my Cape Ann Double.

For what it is worth according to the USDA Wood Handbook, the 12% moisture specific gravity of Red Oak is 0.61. You can compare the weight to an equal volume of water which has the specific gravity of 1. (I down loaded and printed the whole damn book.(???)

Russ