I hope you enjoy this, I wasn't sure where to post but this seems as good as any. I'm a professional engineeer, been lurking a while (years) and have Mike's great book. Got interested after floating the Gulkana with the scouts and taking my family down the Lil Su. Unfortunately I'm interested in the hunting part and it's an expensive hobby.
So I was looking to build a frame myself and use the maxxon tubes. As an engineer I had to calculate the actual load carrying capactiy as best I could since the specs seem to be inconsistent. So I found the tedious equations and beat them out.
Then I ran across this site http://www.copcoltd.com/soft/freeeng02/pareas.htm
you don't have to put in name or date and I just use 1 and 1 for reference and case.
It has a pretty handy user interface to simplify figuring out the actual water displacement for at least a cat. Once you get the gallons of displacement you just multiply by 8.345 (lb/gal). If you choose gallons as your unit.
As an example, for a leopard, I used a 10 ft waterline, I assume that is conservative since as you displace (add weight) your going to bring the tapered ends into play. And assuming that your max displacement is 30% (very close to 8 inches on a 26.5 inch tube), then you would get about 73 gal of displacement per tube resulting in a load of about 1218 lbs (includes boat and load), at 10 inches your almost 100 gals of displacement or about 1670 lbs of load.
A round boat would be more complicated and an inflatable floor more difficult still.
I'm guessing and hoping Mike will have fun with this in small return for the great book!