Tube is roughly 18" in diameter (9" radius) and, while it is 11' long, there is an 6' section that makes a nice cylinder. So the volume of a 72" x 9" (radius) cylinder, is 18,312 cubic inches. There are 231 cubic inches in a gallon, so this is 79 gallons. At room temperature, water is about 8.3# per gallon. So the weight of water this tube could displace is 657#...per pontoon. Now to reach the maximum, the tube would have to be submerged entirely (hard to row that way).

But, if 50% submerged, I'm still displacing 675# (50% of the displacement of both pontoons). Or is this relationship non-linear? Some ship design sites I found, seem to indicate planning displacement is a linear activity. I assume at some point the strength of the pontoon shell must come into play (frame pressing on the pontoon, with a relatively small contact area could result in very high forces on the pontoon).

But I cannot figure out how to get from displacement capacity to anything like the rating on the pontoon boat. With a total displacement of about 1300#, assuming less than 50% submerged, I still come up with almost double the advertised weight rating.

Any smarty smart pants out there capable of explaining to me how to reconcile this? More of an engineering question than one of physics? In the long run it does not really matter, but it tickles my curiosity.

I need to go fishing...(tomorrow!)