The acronym GUPPY stands for Greater Underwater Propulsion Power. At the conclusion of W.W.II, the United States was left with a very modern submarine fleet. Submarines captured from Germany demonstrated the feasibility of the snorkel system, which allowed for greater underwater speed and maneuverability.
Essentially, the GUPPY I program meant streamlining exterior hull surfaces, increasing battery capacity, removal of deck guns, and compartment rearrangements.
Almost immediately after these conversions, the GUPPY II program commenced,
which added the installation of a snorkel system that allowed the diesel engines
to be operated while the boat was at periscope depth. Twenty four boats were
converted to GUPPY II, about half being Balao Class, and the remaining Tench
Class. In fiscal year 1951, 10 boats were to receive a GUPPY 1A conversion.
This conversion was similar to the GUPPY II, but the equipment updates and
re-arrangements were not as drastic.