Greater Underwater Propulsion Power Program The Greater Underwater Propulsion Power Program ( GUPPY ) was initiated by the United States Navy after World War II to improve the submerged speed , maneuverability , and endurance of its submarine s . ( The " Y " in the acronym was added for pronounceability . ) The navy began the program by testing and reverse engineering two captured German Type XXI U - boat s : U - 2513 and U - 3008 . That analysis led to four goals — increasing the submarines ' battery capacity , streamlining the boats ' structures , adding snorkel s , and improving fire control systems . The navy immediately focused on designing a new class of submarines , but the Bureau of Ships believed that the vast fleet of existing " Gato " , " Balao " , and " Tench " class submarines could be modified to incorporate the desired improvements . In June 1946 , the Chief of Naval Operations approved the GUPPY project . The initial two boat test program , implemented by the Portsmouth Naval Shipyard , eventually grew into several successive conversion programs . Those upgrades proceeded in seven variants , in the following order : GUPPY I , GUPPY II , GUPPY IA , Fleet Snorkel , GUPPY IIA , GUPPY IB , and GUPPY III . Some boats that went through an early phase were then upgraded further in a later phase . A similar programme for the Royal Navy involved modifications to 24 wartime and post - war British T - class and A - class submarines , which were provided with streamlined hulls , sail - type conning towers , and increased underwater performance during 1948 - 60 . GUPPY I program . The prototype GUPPYs , and ( both Portsmouth - built " Tench " boats ) , appeared in 1947 . Externally , they featured improved streamlining of the bridge and shears structures , and periscope and radar mast supports . To reduce hydrodynamic drag , one of the periscopes was deleted . No snorkel was fitted , due to difficulties in adapting the snorkel to the fleet boat . Deck guns and their associated containers were removed . An SV radar aerial was added to the top of the sail , creating a distinctive side bulge . All capstans , cleats , and rail stanchion supports were redesigned so they could be retracted or removed when rigged for dive . Most notably , the sharp V - shaped " fleet boat bow " was replaced with a distinctive rounded " Guppy bow " that improved submerged performance . These modifications changed not only the boats ' appearance , but also their terminology : After a GUPPY conversion , the faired structure around the boat ' s conning tower and mast supports was called the " sail " . Internally , the boats underwent considerable rearrangement to accommodate larger battery wells and batteries of greatly increased electrical power . The batteries were of a new design . Compared with the original Sargo battery , the Guppy battery used a greater number of thinner plates that would generate higher current for a longer time . However , these batteries had a shorter life , 18 months versus the five years of the Sargo battery , and took longer to charge . They also required ventilation to remove hydrogen gas , and required cooling water to the battery terminals and termination bars . Four 126 - cell batteries were installed in enlarged battery wells that replaced former storage , ammunition , and refrigeration spaces . These four batteries could be connected in series or parallel , providing a wide range of voltages and currents , and thus a wide range of speeds . In the maneuvering room , two or four of the earlier high - speed motors and reduction gears were replaced by slow - speed motors . All open - front switchboards were replaced with enclosed splash - proof cabinets . Lighting and other " hotel " electrical loads were converted to use 120 volt 60 hertz alternating current , and ship electronics to use 120 volt 400 hertz AC . A new air conditioning system of greatly increased capacity was also installed . In service , these boats offered greatly improved underwater performance . " Pomodon " reached 17 . 8 knots surfaced and submerged as compared to the previous performance of 20 . 25 knots surfaced and submerged , " Odax " slightly less . GUPPY II program . The GUPPY II conversion ( SCB 47 ) , implemented from 1947 to 1951 , was generally similar to the GUPPY I , except for retention of both periscopes and introduction of the recently perfected snorkel . The addition of three new masts — snorkel induction , snorkel exhaust , and ESM mast — required more room in the upper portion of the sail . BuShips approved two different sail designs : Some boats with a Portsmouth Sail had an SV radar and needed extra room to house the aerial , thus had a bulge at the sail top . Later modifications put the SS or SS2 radars on these and other boats that had a smaller aerial and had an indicator with interlocks , allowing the mast to be housed only with the aerial in certain angular positions . Also , some GUPPY II and GUPPY III boats had their sails extended higher above the waterline , the " Northern Sail " , to raise the bridge , allowing it to be manned in more severe weather . All boats converted during the GUPPY II program that originally had high - speed drive motors with reduction gears had these replaced with low - speed direct - drive motors , producing 2500 horsepower ( 1 . 9 MW ) per shaft . The two GUPPY I boats , " Odax " and " Pomodon " , were modified to GUPPY II standard . GUPPY IA program . BuShips devised the GUPPY IA ( SCB 47A ) program of 1951 as a cheaper alternative to the GUPPY II conversion . While the GUPPY IA conversion included most features of the GUPPY II , it omitted the four - cell battery configuration and extensive internal rearrangement associated with it . Instead , the GUPPY IA retained the original battery wells , fitted with more powerful Sargo II batteries . These batteries featured electrolyte agitation , battery cooling , and open tank ventilation . They also had a longer life than the Guppy batteries , though shorter than the original Sargo battery . The sonar room was relocated from the forward torpedo room to a space under the galley . Compared to the GUPPY II , the GUPPY IA offered lower cost , better habitability , and easier maintenance at the expense of underwater performance . Fleet Snorkel Program . When the navy realized that it would not be able to fund all the GUPPY conversions it desired , it devised the Fleet Snorkel Program ( SCB 47B ) as a means of adding the minimum necessary modifications to fleet boats . This modernization added a snorkel , a streamlined sail , a higher capacity air - conditioning system , and a more powerful electrical system . The deck guns and auxiliary diesel were removed . Unlike the GUPPY conversions , these boats retained their original deck structure , bow , and storage batteries . Submerged performance of the Fleet Snorkel boats was therefore significantly inferior to any GUPPY conversion . Despite their limited features , the Fleet Snorkel boats served almost as long as the more modern GUPPY boats . Three boats , " Piper " , " Sea Owl " , and " Sterlet " , received a large BQR - 4A bow sonar . The GUPPY IA program was succeeded by the nearly identical GUPPY IIA program ( SCB 47C ) , implemented from 1952 to 1954 . The GUPPY IIA , however , further alleviated the cramped internal conditions of earlier conversions by removing one forward engine and replacing it with pumps and air conditioning machinery . Some boats had the high - pressure air compressors relocated to the lower level of the forward engine room . The freezer and refrigerator units were moved to the space under the galley , and the sonar room was relocated to the forward end of the pump room . Sargo II batteries were installed in the existing battery wells . Externally , the GUPPY IIA differed from the GUPPY II and IA by having only three diesel exhaust outlets , whereas the earlier conversions had four . GUPPY IB program . GUPPY IB was an informal designation for a limited upgrade and modernization given to four boats for transfer to foreign navies . These boats had snorkels and were generally similar to the GUPPY IA , except that they were not equipped with the modern sonar , fire control systems , or ESM . The two Italian boats were of the thin - skinned " Gato " class . GUPPY III program . The GUPPY II conversions suffered from very cramped internal conditions due to the four - battery configuration . The GUPPY III program ( SCB 223 ) was devised to address this problem . In 1959 , became the prototype conversion . It was cut in half and lengthened with a 12 . 5 foot ( 3 . 8 m ) section forward of the control room to create space for a new sonar room , berthing , electronics , and storerooms . The removal of the sonar room from the forward torpedo spaces allowed an increase in the number of reloads . Crew spaces were also refurbished . As in the GUPPY IIA conversion , one diesel engine was removed . The tall sail ( or " North Atlantic Sail " , as they were sometimes called ) was a singular characteristic of the GUPPY III boats , distinct from the so - called " step sail " of all of the other GUPPY submarine classes . From 1961 to 1963 , eight more GUPPY II boats were upgraded to GUPPY III standard . These boats differed from " Tiru " by adding a 15 foot ( 4 . 6 m ) section forward of the control room . They also retained all four diesel engines . This increased the boat ' s length to and raised surfaced displacement to approximately 1 , 975 tons . All boats received the BQG - 4 PUFFS passive ranging sonar , identifiable by the three fin - like sonar domes added to the superstructure . The conning tower in the sail gained an additional five foot ( 1 . 5 m ) section to accommodate the Mk 101 fire control system and Mk 37 director . All GUPPY III boats received a plastic sail . The GUPPY III conversion was part of the Fleet Rehabilitation and Modernization ( FRAM ) program . All 24 GUPPY II boats were originally slated to receive the GUPPY III upgrade , but budgetary constraints limited the program to a total of nine boats . Despite their extensive modifications and upgrades , the GUPPY III boats served only slightly longer than the rest of the GUPPY fleet .