Background At the close of WWII, the US Navy had numerous fleet submarines of the Gato, Balao and Tench classes. While these submarines had been enormously effective in carrying the fight to the Japanese in the Pacific, technology was changing quickly. The German Type XII U-boats, of which the US Navy received two for evaluation, suggested a new type of underwater warfare, emphasizing high speeds through streamlining and prolonged submergence through the use of the snorkel. While making plans for a fundamentally new class of submarines (the Tang class), the Navy also embarked upon a program to modify some of the fleet submarines to dramatically improve their underwater performance. This required significant cost and effort, as the modifications had to be fitted into a pre-existing design. Among other issues was the need to redesign the snorkel to accommodate the 2 cycle diesel engines used by the US Navy, which required more volume of air than the German 4 cycle system. The electric drive motors were all changed to the quieter direct drive installed on the Tench class, as opposed to the earlier geared drive on the Balao class boats.

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The Navy program was given the acronym GUPPY (Greater Underwater Propulsive Power). In addition to removal of deck guns and other protuberances, the conning tower was covered with a streamlined sail structure which housed the snorkel and other masts, and the bow reshaped. Not visible was a bow sonar dome that was also fitted on the bottom of the hull. Internal changes were also significant, including the doubling of the number of battery cells from 2 X 126 to 4 X 126, with a new, higher capacity "Guppy" battery. Two submarines (Odax and Pomodon) were converted in a trial run in 1946 (lacking snorkels, which had not been perfected), and subsequently designated as Guppy I. The initial large batch of conversions was designated as Guppy II, and a total of 24 submarines of the Balao and Tench classes were converted between 1947 and 1951. The streamlined "step" sail shape (with a low forward bridge structure and a higher mast housing) varied slightly depending on where the conversion was performed. Those submarines converted at Electric Boat had a step sail with a trailing edge that slanted straight back to the deck, and round navigational deadlights. The government design step sail, converted at either Portsmouth or Mare Island had a curved trailing edge, and the deadlights were rectangular. Both featured a snorkel mast and exhaust at the trailing edge of the sail. The resulting submarines were capable of 18 kts on the surface, and a top speed of 16 kts submerged (vs. a top speed of 9 kts for a fleet submarine). Further battery endurance was greatly increased, as was underwater cruising range between snorkel sessions.

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Unfortunately, this program was very expensive, and the postwar military environment was extremely competitive for funds. The Navy devised a follow on programs to convert ten more fleet submarines (9 Balaos and one Tench) at reduced costs. The Guppy Ia program reduced costs by foregoing the increased batteries, and using an upgraded 2 X 126 battery system with the new "Sargo II" batteries of increased capacity. These also received a new sonar room in the old ammunition magazine space, but it was extremely cramped. The Becuna is one of these conversions. The Navy proceeded to the Guppy IIa design, similar to the Ia but removing one of the diesel engines to provide space for an improved sonar room. There were 16 Balao and Tench class boats in this conversion series. All of these conversions were completed by 1953.

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The final Guppy conversions, the Guppy III series, did not occur until the 1960s. In order to bridge the gap with the lagging nuclear submarine construction programs, nine of the Guppy IIs were upgraded to Guppy III standard. There were three major external changes. The sail was replaced with the "Atlantic High Sail", which had no step in it, and moved the bridge up high (to give the bridge crew additional shelter from high seas in rough northern waters). The sail was made of lightweight fiberglass. The conning tower under the sail was lengthened by 5 feet. The other major change was that the submarines were cut in half forward of the sail, and a 15 foot "hull plug" section inserted between the forward battery compartment and the control room. This provided a suitable sonar room to house the 1960s vintage electronics. The final exterior change was the additional of three shark-fin like PUFFS sonar ranging equipment on the upper decks. These were passive sonar installations, the forerunners of the modern Seawolf and Virginia class SSN conformal arrays.

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The Guppy class of submarines bore the brunt of the Cold War submarine operations during the 1950s and early 1960s. Operating in the cold northern waters in the GIUK gap, as well as in the North Pacific, these submarines kept track of Soviet Naval operations. Snorkeling in the rough waters up north, the frequent dunking of the snorkel closed the head valve, causing the engines to pull a partial vacuum inside the hulls; tough on both the engines and the crews ear drums. In the early 1970s these boats were finally retired as the newer Permit and Sturgeon class SSNs took over these operations. By the mid 1970s the last of the Guppys had been retired from USN service, and with them, a generation of sailors who spanned from WWII into the Cold War era. - Tom Dougherty