What exactly is the confusion or conflict ?

heres some info as I understand the issue.


Bouyancy can be calculated by the amount of water displaced by the
hull. (as a rough figure, ignoring salinity etc)

For a cylinder it is pi * radius * radius * length with all figures in
feet. (pi is 3.14 for rough calculation)

A cubic foot of water weighs about 62 pounds.

If your pontoons were square or rectangular cross section the
calculation would be

width * height * length

(* = multiply)

square pontoon 1 foot square and 10 feet long = 1 * 1 * 10 = 10 cubic
feet
10 * 62 = 620 pounds load.
that is 620 pounds including the pontoon will have the water level at
the top of the pontoon.

since you are not going to have the top of the pontoon level with the
water surface the actual load carrying capacity is less than the figure
obtained.

I would divide by 2 for half of pontoon in the water.

The load figure must include the weight of the pontoons.

Square section pontoons carry more load for a given size, but other
structural issues come into play. The square section might need to be
heavier to deal with the stresses.

Also a round section pontoon will sink further under initial loads and
then sink less as the wetted area increases.

Calculation is per pontoon. Double for two pontoons on a boat.

regards.