I agree with you on that. I can't see anything but problems bolting a PW deck to steel.
But the difference is that L. Francis COULD do the math when he needed to. He just believed that the math had to be balanced with experience. I would certainly agree with that. Can't imagine I know any succesful designer who doesn't think that.
I have another question for you. I was never comfortable spraying fowm inside the shell. IIt scares me that I can never se that part of the shell again. I'd worry what was happening under the foam. Hoiw do you deal with that?
Many coats of epoxy tar or wasser tar. I once salvaged some galv pipe out of an old building. the side facing SE against the prevailing rainy weather was severely rusted; the side facing the NW and dry winds looked almost new. Inside a steel hull it doesn't take a lot to tip the balance, but given paint is cheap and you don't get a second chance, why not give it an overkill on thickness. Most of the boats built by Foulkes and Fehr have zero paint on the inside, and predictable results, with severe inside rusting. Any time you see rust on a flat surface on the outside of a steel boat, the paint was never thick enough. Moitessier makes this point in pointing out how many coats the French navy insists on before launching. Many! No such thing as too thick, the thicker the better. If you can still see the weld pattern ,it is not thick enough. Too thin a paint job is the main reason for steel boat owners having maintenance problems ( along with wood over steel on the outside).
On older boats, you can tell how well the foam has stuck to the steel, and any other problems under the foam, by dragging your fingernails over it. Where it has separated from the steel, you can hear a distinct hollow sound. That is where you should dig it out and have a look. If you see smooth ,clean epoxy, the rest is probably the same.
I use Hereshoff's methods of calculation, with models as double checks. The results, decades of successful boats, with some owners on their third, as testimony.