Hmm, OK, this is how I understand it:

Take something which weighs 1# out of the water. When you submerge it in water it weighs less, and just how much less is a function of its relative density.

This difference is its "reserve buoyancy".

I am going to do some number-rounding here for ease - since math makes my head hurt.

According to the Skene's chart on Baldwins site, 700#'s of lead (one cubic foot) weighs only 636#'s in the water - a ratio of .908 (or: 636 divided by 700). So the 2300# ballast in an Ariel would weigh 2,088#'s submerged, right?

An empty Ariel displaces 5120#'s of water, so that is it's weight. Subtract that 2300#'s of lead ballast, and that leaves 2,820#'s of miscellaneous other materials, the largest majority of which is fiberglass.

According to Skene's, a cubic foot of fiberglass weighs 96#'s, yet underwater the same cubic foot only weighs 32#'s - exactly 1/3, or .333. Fiberglass is denser than most everything on the boat, so for simplicity's sake lets just say that the whole rest of the boat weight is fiberglass. Underwater this mass of fiberglass would weigh 940#'s (2,820 x .333)

Add the 940 to the 2,088, and you have 3,028#'s, which would be the amount of flotation you would need to provide in order to suspend the boat in the water, just at/below the surface.

Right?

3028 divided by 58 (the flotation of a cubic foot of polyurethane foam, from Skene's) equals 52.206, the amount of cubic feet of foam needed to barely float an empty Ariel.

(~8 more cubic feet of foam (to make it an even 60 cu/ft) would provide for an additional 464#'s of flotation - which is more than 464#'s of stuff, taking into account that that stuff would weigh less in the water...)

Note how close this is to the "1 cu/ft per 100#'s of displacement" guesstimate - neato.

So - Am I figuring this right, even though loosely?

If I am, I will be amazed.