Is this a sea craft?

[KenB]

For a 17' outboard they say 20 cubic feet (or about 5 gallons of uncured foam). For a 20' inboard, they say 22 cubic feet. P. 26 suggests pouring foam down the gunnels.

How much floatation should I add? I assume what is in the stringers is enough to pass the restrictions, but how much is enough to float the boat?

Can we convert that to plain english for 20's and 23's?