The "Balao"s were similar to the "Gato"s, except they were modified to increase test depth from to . In late 1941, two of the Navy's leading submarine designers, Captain Andrew McKee and Commander Armand Morgan, met to explore increasing diving depth in a redesigned "Gato". A switch to a new High-Tensile Steel (HTS) alloy, combined with an increase in hull thickness from to , would result in a test depth of and a collapse depth of . However, the limited capacity of the trim pump at deep depths, and lack of time to design a new pump, caused Rear Admiral E. L. Cochrane, Chief of the Bureau of Ships, to limit test depth to . Fortunately, in 1944 a redesigned Gould centrifugal pump replaced the noisy early-war pump, and effective diving depth was increased.