These are boats are little seen on the gaming table, and that is a shame. The Greater Underwater Propulsion Power Program (GUPPY) and their Fleet Snorkel were the US Navy’s response to the surprise found in the German Type XXI U-boats.
When the war ended in 1945 the US Navy and Royal Navy both had large battle tested fleets. Unfortunately the American vessels had experienced four years of war while the British ships were battered by an additional two years of war. The designs were tested but old.
The US started the GUPPY program in 1947 and the British followed a similar upgrade with their T and A class boats. The reason for the conversions was that the submarine hulls existed and were needed. The down side was the cost. Congress was not willing to spend to bring all of the existing submarines to the higher standards.
While there is an alphabet soup of conversions (GUPPY I, GUPPY II, GUPPY IA, Fleet Snorkel, GUPPY IIA, GUPPY IB, and GUPPY III) they all involved improvements in hull and battery life. The GUPPY II had the snorkel and improved radar. Only nine of the GUPPY IIs were converted to the GUPPY III with the improved BQG-4 PUFFS passive ranging sonar. Once again it was a budget issue.
This link will be helpful to anyone interested in the GUPPYs. Also I suggest looking at U.S. Submarines Since 1945, it is a great reference that I use often.
Do not worry about the stats when designing a scenario, as there are few out there that know of all of the differences between the seven groupings or even the quirks within them. Now gamers, why do you need this ancient submarine? These were the front line boats in the 1950s into the 1990s with our allied navies. One was even active in the Falkland War, but that is another blog post.
Miniature is 1/6000 scale from Figurehead.