The US navy got two German Type XXIs after WWII, U-2513 and U-3008. It was
obvious that what was needed were larger batteries, a streamlined shape, a
snorkel and improved fire control. The result was the Tang Class. However
these were not cheap and the Navy had several hundred older fleet boats, some
still on the ways.
Enter the Guppy program.
The first two converted were Odax (SS-484) and Pomodon (SS-486). They were
Guppy I. They did not get snorkels and were the only two Guppy I's.
Next were the Guppy IAs. These were interim modifications (the Guppy II
conversion was pretty expensive) and the lessons learned from the Guppy I's
The Guppy IIs had a snorkel induction mast, a snorkel exhaust mast and an
ESM mast. There were two types of sails which were called the EB (Electric Boat) and Portsmouth (Portsmouth Naval Shipyard)
step sails. Viewed from the side, the EB sail is a straight slanted back, and
the back of the Portsmouth sail slants down aft, then bulges out just before
reaching the deck. Both sails had round or rectangle windows. The early Portsmouth sails also were thinner at the top,
that's the cause of the bulge for the radar at the top of some boats' sails.
The main difference between the Guppy IIs and the IIAs was the
elimination of one main engine. That's way the IIs have 4 exhaust ports and
the IIAs have 3.
Some of the Guppy II boats received the socalled "North Atlantic Sail" later but were not modified to a Guppy III.
All Guppies have rounded bows, as opposed to the pointed bows of the
There was one other group, the Fleet Snorkel boats. These were similar to
the Gupp IIs but without most of the internal changes. Batteries, engines,
fire control, etc. There were about 17 of this type. Jim Christley's book
Naval Submarine Force Information Book"