From the New 'S' class design the Americans developed the 'T' class submarine, a dozen of which were launched in barely 13 months, mostly in 1940. They differed primarily in receiving two extra tubes forward (10 in all) and later substituting a speciallymodified 127-mm (5-in) deck gun for the earlier 101.6-mm (4-in) gun, or 76.2- mm (3-in) gun in some cases. This gradual evolutionary process was successful and produced at the right time a submarine with acceptable characteristics for the Pacific war. What was needed was a long endurance and self-sufficiency. Because of the distances involved, patrols were much longer than those in the European theatre and more boats were needed to maintain numbers on station.
Thus the 'Gato' class was an improved 'T' and went into volume production, the first of class, the USS Drum (SS 228), being completed shortly before hostilities commenced. Officially capable of operating down to 91 m (300 ft) they often went deeper. The earlier boats had a large, solid looking sail, similar to those of pre-war designs. These were soon reduced as boats came in for repair but, although the structure could be lowered the very high standards ('shears') demanded by the long periscopes remained a lofty feature. Operating on the surface more than would have been possible in European waters, they began also to accumulate varied outfits of automatic weapons, regular and nonregular, the structures gaining various platforms to support them. Even extra main-calibre deck guns appeared, all in the cause of making the 24 torpedoes aboard last longer. After 73 boats the hull was secretly improved by the adoption of HT steels and advanced sections, increasing their official limit to 122-m (400-ft) depths. No less than 256 of these were ordered and known as the 'Balao' class, but only 122 were actually completed, a further 10 unfinished hulls being scrapped.
Specification 'Gato' class
Displacement: 1,525 tons surfaced and 2,415 tons submerged
Dimensions: length 95.02 m (311.75 ft); beam8.31 m (27.25 ft); draught 4.65 m (15,25ft)
Armament: 10 × 21-inch (533 mm) torpedo tubes (six forward, four aft) 24 torpedoes
1 × 3-inch (76 mm) / 50 caliber deck gun four machine guns