Bruce's Pontoons

We were planning to use Bruce's pontoon boat that he scored somewhere around the Lake Shasta area as the base for our shantyboat.  In fact, it was Bruce's pontoon boat score that gave us the greenlight for this project.  Not so sure about this now.  Every boat has a buoyancy, a certain weight that it can support safely.  Bruce writes:
I don't know how easy it is to calculate this. During high school geometry I had a crush on the Castellucio twin sisters (Karyn and Kathleen) and most of my mental energies were taken up by trying to flirt with the two of them rather than on the substance of the coursework. I haven't really revisited the subject of geometry (nor, alas, the Castellucios) since that time.

The pontoons are sort of irregular shaped, they are flat on the top and sort of u-shaped on the bottom. The top to bottom dimension is 17 1/2". They are 18 1/2" wide.  As I've mentioned before, the platform is 16' long. The pontoons extend another 18" to the rear of the platform. They also extend about 24" to the front of the platform. However, in the front they are tapering down to be much more narrow, expecially in that last foot.

Okay, I'm gonna make some assumptions about Bruce's pontoon boat.  Doing some crazy math that requires me to get out my geometry book again, I guesstimate the total cross-sectional area of the pontoon is about 2 sq ft.  To get the volume, we multiply that times the length. We'll say that is 16 feet + another 1.5 ft at the back, and another maybe 1 foot at the front. For 18.5 feet. So the volume of one pontoon is perhaps 37 cu ft.

We just look at one pontoon, because we want to make sure that the capacity we work with never exceeds the floatation of one pontoon, lest a shift in weight drive it under the water and flip the craft. This situation even has an ominous-sounding name:  Pontoon Effect.  Plus, if you think about two pontoons, you want them both no more than half in the water.

To get the floatation, we multiply the volume times the weight of the same amount of water. Thanks, Archimedes.  One cu ft of fresh water weighs 62.4 lbs. So the flotation of one pontoon is 37 cu ft  * 62.4 lbs/cu ft = 2308 lbs. So the weight of the boat, plus the passengers, plus their gear and stuff, plus some margin of safety 10 to 25% should not exceed 2308 lbs.

Probably too tiny for our heavy shantyboat. However, we can build Bruce's pontoon out to be a little hillbilly sun porch to accompany our shantyboat.  Something like this...