The backbone of the submarine fleet during the 1920's were the 51 "S" CLASS boats. These boats had a displacement of 800 & 1,000 tons and were about 210 feet long with a diving depth of 200 feet. They were the first to have stern tubes. Their claim to fame was the frequent headline "S-BOAT sinks.":

S-5 in 1920;
S-48 in 1921;
S-36 and S-39 in 1923;
S-51 in 1925 and
S-4 in 1927.

Even though they were also designed as coastal submarines, another extension/improvement of the HOLLAND Class, they were required to make long surface transits to the Canal Zone, to the Philippines, Hawaii and to China. This long transit requirement caused a major redesign of the bridge and conning tower, away from a more streamlined shape to a larger one with a dryer bridge (causing a 3 knot decrease in submerged speed).

Important Technical Developments During The Inter-War Years

Technically the 1920's saw a number of important advancements in the development of the submarine. There was the remote torpedo gyro angle setter which enabled the course of the torpedo to be changed right up to the time of firing. The periscope was improved and a new device called the electro-acoustic fathometer was developed by the Submarine Signal Company (an active echo ranging sonar was soon to follow). An access hatch that could be flooded to serve as an escape chamber was developed. Many other back-up and safety features were also added. Compartmentation and watertight bulkheads were added so that no single flooded compartment could sink the submarine. Plus, the obvious idea of a truly circular hull finally sunk in. Not to mention, the idea of a smaller submarine was now possible (one with half the displacement [ 1,000 tons ] of the larger V-CLASS; the 'least is best' philosophy). This was helped by the London Naval Treaty which limited the overall tonnage available to new submarine development.

The Introduction of the First Fleet Boat

When these features were incorporated into the V-7 the first steps were taken toward a true 'fleet submarine', at least in the context of the 'fleet boats' of World War II. But, not all progress was forward as the designers of the last two boats of the V-1 CLASS (V-8 and V-9) were still trying to incorporate concepts found in the German U-Boats of 1918.

What was to be the role of the fleet submarine? In the early 1930's, it was to be used for offensive patrol operations in the Pacific, which meant:

1) to scout ahead of the battle fleet;
2) to conduct independent offensive operations against enemy combatant ships;
3) to operate tactically in conjunction with the battle fleet

There was no role in the interdiction of enemy commerce shipping since it was outlawed by international treaty and world opinion. To perform these functions the submarine would need a reasonably high surface speed because of the immense distances to the patrol areas. At 10 knots surface speed, it would take 33 days to cover the 8,000 miles from the West Coast to the Far East; the submarine would use up all its stores during the transit.

Rapid Development Toward the Fleet Boat

New lightweight, high-speed diesel engines (from Navy sponsored railroad technology and further development/experience with the M.A.N. engines) became available in the 1930's, and with it, the idea of an all-electric drive. This produced the diesel-electric drive and yet again another increase in hull length to accommodate the new Maneuvering Room from which the submarine's electrical propulsion plant was operated. Many of the previous boats had a separate generator, and the main motors could double as generators only by reversing the electrical connections. In a diesel-electric submarine, each diesel is connected to its own generator which feeds the batteries. The batteries are connected to the electric motors which are always running the propellers. In this very flexible but heavy arrangement, all electrical power is run through the batteries, even when the submarine is on the surface running on the diesels.

Also in the 1930's the boats began to get the new MARK I Torpedo Data Computer (which automatically kept the torpedo gyros properly set right up to the moment of firing), and most importantly - all welded hulls. Air conditioning became available to allow 90-day patrols. And at this time, the vapor-compression distilled water stills first appeared. Two of these could make up to 1,500 gallons of distilled water for the batteries and for drinking.

Some other small, but important, innovations included the poppet valve to eliminate the release of an air bubble upon torpedo launching by sucking back the air (which was used to eject the torpedo) as the air bubble reached the muzzle of the torpedo tube; stronger battery cell casings to prevent acid leaks during shock loadings (depth charge attacks); and the Negative Tank. This additional tank was kept flooded while the boat was on the surface in order to impart an immediate downward momentum when the main ballast tanks were flooded. As soon as the submarine was underwater, this tank was blown dry to restore the boat to neutral buoyancy. Plus, high power radio transmitters and receivers became available which permitted long range communications.

This was the thrust of the design improvements made during this inter-war period. With the basic design originating in 1931 and with the first article joining the Navy in 1935, the 'fleet boat' had finally arrived. It was a most satisfactory design which served the U.S. Navy, as well as many foreign navies, quite well for the next 20 years. In total, 266 were built. Its general characteristics were:

Length 310 feet
Displacement 1,500 tons
Speed (surface) 20 knots, (submerged) 8 to 10 knots
Depth of dive 300 feet (later on 400 feet)
Power four large diesel engines directly connected to electric generators. The generators powered electric motors and/or recharged batteries
Propulsion to electric motors connected to the propellers
Tubes four forward (later six forward) tubes and four stern tubes with 24 torpedoes carried on board
Guns one or two 5" deck guns
AA Guns twin-barrel 20mm or 40mm cannons
Fire Control allowed torpedo launches out to 4,000 yards from the target

More U.S. Submarines: A Technical History

S-Class (#111)
Pre-WWII Build-up and WWII Developments (#112)
Radar and Countermeasures (#113)
Post WWII Period (#114)
Missiles, Subs, and the USS Albacore (#115)
Early Nuclear Propulsion (#116)
Boomers and Fast Attack Submarines (#117)
Polaris and Other Missiles (#118)
Trident Missiles and Attack Subs (#119)