Guppyization by Mike Keating

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.

Type XXI U 2540 Wilhelm Bauer
Deutsches Schifffahrtsmuseum Bremerhaven

SS 563 USS Tang
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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 were applied.

SS 484 USS Odax as Guppy I
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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.

SS 522 USS Amberjack with NA-Sail
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The Guppy IIIs were lengthened about 15 feet, have North Atlantic sails and, the most distinctive feature, the three shark fin like Puff domes, topside.

SS 346 USS Corporal as Guppy III
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All Guppies have rounded bows, as opposed to the pointed bows of the Fleet boats. 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

"United States Naval Submarine Force Information Book"

covers this in more depth.