DD951 said:
while submarines under 600 tons
Looks like you missed a thought, here.
DD951 said:
That has to do with the specifics of the treaty- submarines could only be replaced by new construction once they were more than 13 years old, except in the event of accidental loss
For a start, then, that gives you three for the S-boats lost to accidents, plus
S-50. Assuming you replace these, & through
S-10, at the rate of one or two
per yard
per year (which was about the going rate), you're no earlier than 1933 when you've built out your first 14 (max prewar build in any one year was 7, IIRC).
This also presumes the S-boats are retained... They were demonstrably too slow for operation with the fleet (in keeping with the doctrine under development), & too small for the Pacific. (Not to mention damned uncomfortable for work in the Tropics.

) So what happens if PotUS declares them surplus & sells them to Bolivia or the Vatican or somebody?

I can't imagine the Treaty refuses to allow replacement in that instance.
So replace with
Bottlenoses

(8 tubes {4+4? 6+2?}? 10 tubes (6+4? 8+2?}? 2x4"/50cal? 4 Fairbanks-Morses or Wintons) at 4/yr til 1936, then see Japan renounce the treaty, quit it, & increase the build rate to 2-3/yard/year in '36 or '37. Then in '39, surplus off the remaining S-boats to RN & RCN.
DD951 said:
they could get 21 & a half units on that displacement, which even then would be a loss of over a third in total unit numbers, as whatever replacement submarines were built would have to do the same work as the 35+ S-boats eligible for replacement in that timeframe (at a point where USN planners figured that at least ~60 long-range modern submarines to fulfill all the requirements for a war against Japan...)
This is the hardest argument to answer. At the time, I doubt anybody really anticipated a major war against Japan's commerce. Loss of the P.I., Guam, & Wake had long been anticipated, tho, so retaining the S-boats should have been self-evidently a bad idea: fine if you work out of Manila, not if you have to base in Midway or Pearl Harbor (never mind Dago or San Pedro


)
They were right about the number: a sustained force of 65 was about what Nimitz got. Truth is, it was possible to do the job with fewer...but that required more imagination, which is another POD entirely.