AA-1 class and V-boats
The Gato-class boats were considered to be "Fleet Submarines". The original rationale behind their design was that they were intended to operate as adjuncts to the main battle fleet, based on
Standard-type battleships since
World War I. They were to scout out ahead of the fleet and report on the enemy fleet's composition, speed, and course, then they were to attack and whittle down the enemy in preparation for the main fleet action, a titanic gun battle between
cruisers. This was an operational concept born from experience in World War I. In order to operate effectively in this role, a submarine had to have high surface speed, long range and endurance, and a heavy armament.
 Limitations in submarine design and construction in the 1920s and 1930s made this combination of qualities very difficult to achieve.
 The USN experimented constantly with this concept in the post-World War I years, producing a series of submarines with less than stellar qualities and reliability, the
AA-1 class (also known as the T class) and the
V-boats, of which V-1 through V-3 were an unsuccessful attempt to produce a fleet submarine.
Tambor and Gar class
By 1931, the experimental phase of fleet submarine development was over and the Navy began to make solid progress towards what would eventually be the Gato class. By 1940, a much better developed industrial base and experience gained from the
Sargo-class boats resulted in the
Tambor and Gar classes. Finally, the USN had hit the right combination of factors and now had the long-desired fleet submarine.
Timing, however, conspired against the actual use of these boats in their assigned role. The
attack on Pearl Harbor on 7 December 1941 destroyed the Pacific Fleet battle line and along with it the concept of the battleship-led gun battle, as well as 20 years of submarine strategic concept development. It left the fleet submarine without a mission. Fortunately, the very same capabilities that would have enabled these submarines to operate with the fleet made them superbly qualified for their new mission of commerce raiding against the
The Gato-class design was a near-duplicate of the preceding Tambor and Gar-class boats. The only significant differences were an increase in diving depth from 250 feet (76 m) to 300 feet (91 m), and an extra five feet in length to allow the addition of a watertight bulkhead dividing the one large engine room in two, with two
diesel generators in each room. The Gatos, along with nearly all of the USN fleet-type submarines of World War II, were of partial double hull construction. The inner pressure-resisting hull was wrapped by an outer hydrodynamic hull. The voids between the two hulls provided space for fuel and ballast tanks. The outer hull merged with the pressure hull at both ends in the area of the torpedo room bulkheads, hence the "partial" double hull. Operational experience with earlier boats led the naval architects and engineers at the Navy's
Bureau of Construction and Repair to believe that they had been unduly conservative in their estimates of hull strength. Without changing the construction or thickness of the pressure hull steel, they decided that the Gato-class boats would be fully capable of routinely operating at 300 feet, a 50-foot (15 m) increase in test depth over the preceding classes.
The Gatos were slow divers when compared to some German and British designs, but that was mostly because the Gatos were significantly larger boats. Sufficient fuel bunkerage to provide the range necessary for 75-day patrols from
Japan and back could be obtained with only a large boat, which will take longer to submerge than a smaller one. Acknowledging this limitation, the Bureau designers incorporated a negative (sometimes called a "down express") tank into the design, which was flooded to provide a large amount of negative buoyancy at the start of the dive. Based on later wartime experience, the tank was normally kept full or nearly full at the surface, then emptied to a certain mark after the boat was submerged to restore neutral buoyancy. At the start of the war these boats could go from fully surfaced to periscope depth in approximately 45–50 seconds. The superstructure that sat atop the pressure hull provided the main walking deck when the boat was surfaced and was free flooding and full of water when the boat was submerged. When the dive began the boat would "hang" for a few extra seconds while this superstructure filled with water. In an attempt to speed this process, additional limber, or free flooding, holes were drilled and cut into the superstructure to allow it to flood faster. By mid war, these measures combined with improved crew training got dive times down to 30–35 seconds, very fast for such a large boat and acceptable to the boat's crew.
The large size of these boats did negatively affect both surfaced and underwater maneuverability when compared to smaller submarines. There was no practical fix for this due to the limitations of the installed hydraulic systems that were used to move the rudder. Although a point of concern, the turning radius was still good enough to be acceptable. After the war, a few fleet boats were fitted with an additional rudder topside at the very stern.
These boats all had air conditioning, refrigerated storage for food, generous
fresh water distilling units, clothes washers, and bunks for nearly every crew member; these were luxuries virtually unheard of in other navies. The Bureau designers felt that if a crew of 60–80 men were to be expected to conduct 75-day patrols in the warm waters of the Pacific, these types of features were vital to the health and efficiency of the crew. They could be added without impact to the boat's war fighting abilities due to the extra room of the big fleet boat. However, one feature in particular had a very practical side to it. Submerge a submarine for any length of time and the heat generated by the recently shut down engines, electronic gear, and 70 warm bodies will quickly raise internal temperatures above 100 °F (38 °C). High humidity generated by tropical waters will quickly condense and begin dripping into equipment, eventually causing electrical shorts and fires. Air conditioning, acting mostly as a dehumidifier, virtually eliminates this problem and greatly increases mechanical and electrical reliability. It proved to be a key factor in the success of these boats during World War II.
Twelve submarines of this class built by Electric Boat received what would be the final installations of the
diesel engine. The Navy had been tinkering with this engine off and on since 1937 because its unique design promised nearly twice the horsepower in a package the same size as other diesel engine types. Unfortunately, the HOR company ran into severe design and manufacturing problems and these engines proved to be operational and maintenance nightmares.
 Frequent breakdowns and utter unreliability had destroyed these engines' reputation with the Navy and they were all removed at the first opportunity and replaced by
Winton 16-278A V-type diesels. The other Gato-class boats received either the
38D 8-1/8 nine-cylinder
opposed-piston engine or the GM-Winton 16-248 V-type as original installations. These engines were hardy, rugged and well liked by the crews and served the boats quite well.
At the beginning of the war Gato-class boats, as well as the Gar and Tambor classes, had fully shrouded
fairwaters visually similar to modern nuclear submarines. Experience during the war leads to the progressive reduction of this structure to reduce visibility and radar profile at the expense of underwater performance and foul weather operating comfort. Most of the subs in post-war movies show the final result of these modifications. A side benefit of these modifications was the creation of convenient locations for anti-aircraft guns.