During World War II, diesel-electric submarines made up a majority of the propulsion systems used by the U.S. Navy. This was a departure from earlier submarine power systems, which were direct drive types, little changed from the earliest boats.


submarine direct driveIn this direct-drive power system, the diesel engine is directly connected to the propeller shaft. Between the engine and the propeller shaft, there is a large, combination electric motor/generator. A clutch connects the engine to the motor/generator. A second clutch connects the motor/generator to the propeller shaft.

When the engine is connected to the motor/generator, the m/g functions as a direct current generator. Its output is directed through the switchboard, with the switches on charge, and into the batteries, keeping them charged. Since there are likely to be times when it will be necessary to charge the batteries when the boat is tied up, or not moving, a second clutch connects the m/g to the propeller shaft.

Throwing the switches to the battery position takes power from the batteries and directs it into the m/g, which now functions as a motor, driving the propeller shaft.

Direct drive systems are somewhat limited since the narrow hull of a submarine generally precludes more than two shafts. As a consequence, direct-drive subs were limited to one or two engines. This was fine with the relatively short-range designs of World War I and the inter-war period. German submarines retained this system throughout the war, at least in part because smaller submarines could be built in greater numbers, and the relatively narrow Atlantic didn’t require extremely long range.

One virtually universal design feature of direct-drive boats was that they had no reversing gear. When it was necessary to go astern, the electric motors were used.


diesel electric submarine drive systemWhen the United States started to build the long-range fleet submarines, it was decided that more power was needed. Since the boats were considerably larger—a Gato Class attack submarine was about the same length as a World War I destroyer—four engines were used instead of the previous two. As there were still only two shafts, this presented an obvious problem with transmitting power from the second pair of engines.

The solution was to use the full-time electric drive for the propeller shafts. Adapting a system originally developed for trains, the diesel engines were directly coupled to a large direct current generator. This power could then be used for charging the batteries or powering the motors. Since there were four engines and generators, it was possible to use the full power output for the motors, charging the batteries, or a combination of the two.

When submerged, of course, the motors drew their power from the batteries. The illustration shows a boat with a high-speed motor, coupled to the shaft with a reduction gear. Some boats used this system, and others drove the shaft directly off the motor. In the latter case, obviously, the motors were designed to run slower. The Gato Class boats had four electric motors, two per shaft.

Diesel-electric submarines were powered by one of three engine types at the beginning of the war. One of these engines, the Hooven-Owens-Rentschler (H.O.R.), which was a license-built version of a German M.A.N. diesel, proved to be completely unreliable. There were a few comments that M.A.N. had provided deliberately faulty design specifications and drawings. In fact, the Germans had only slightly better luck with the same engine design. Ultimately they were removed from all American boats and replaced with more reliable engines.

The H.O.R. engines were a double-acting design, in which a solid piston moves up and down inside a double-ended cylinder. Double acting designs were common for reciprocating steam engines, but when it came to diesel engines it was one of those ideas that sounded a lot better than it worked. Wartime is rarely a good time to experiment, though—particularly when failure may mean the deaths of an entire submarine crew—so the engines were scrapped. Something of how the crews felt about them is, perhaps, illustrated by their tendency to read H.O.R. as a word and not as initials.

The two reliable designs were a 40° V-16 diesel built by Winton Motors Company (later bought out by General Motors, and commonly referred to as the “GM-Winton,” “GM-Cleveland”—because they were built at General Motors’ Cleveland Engine Plant—or just “GM”), and a nine or ten cylinder (18 or 20 piston) opposed-piston design built by Fairbanks-Morse. An example of the Fairbanks-Morse engines may be seen in the motion picture Down Periscope, where the Balao Class boat U.S.S. Pampanitostood in for the fictional U.S.S. Stingray. (There actually was a Stingray, by the way, a Salmon Class boat launched in 1938.)

fairbanks morse pistons

Most people are familiar with the one piston per cylinder design of the majority of engines. The Fairbanks-Morse engines, however, used an opposed piston design, which had two pistons in each cylinder, and two crankshafts. The upper and lower pistons come together at the midpoint of the vertical cylinder, their dished tops forming the combustion chamber. This design effectively doubles the number of cylinders while keeping the engine relatively compact.

Both of these engines continued in production even after they were no longer needed for submarines, as they were also well suited for locomotive use. The railroads also provided a source of employment for many former submarine enginemen, who came already skilled in maintaining these big diesel engines.

Gato Class fleet submarines were powered by four direct-current electric motors. Two motors were attached to each shaft. Motors and generators were provided by General Electric, Westinghouse, Allis-Chalmers, and Elliot. GE and Westinghouse also built the main control units (switchboards), along with Cutler-Hammer. The presence of a GE switchboard usually meant the boat would also have GE motors and generators. Westinghouse controls were used with their own motors and generators, or with the Elliott motors and generators. The Cutler-Hammer controls were generally linked with Allis-Chalmers motors and generators.

Some indication of the amount of power consumed by these motors was the necessity to include water cooling systems to keep the operating temperatures within safe limits. The high-speed type motors (used with a reduction gear) were rated at 1370 horsepower and ran at 1300 rpm, pulling 2600 amps at 415 volts. (By way of comparison, an automobile starter motor runs on 12 volts at a maximum draw of around 45 amps.)

Located at the aft end of the maneuvering room, the main control unit consists of a number of levers, indicator dials, and switches. Since the engines run at a constant speed when the boat is surfaced—and not at all, of course, when submerged—and the electrically-driven screws are controlled from this station, the “engine-room” telegraphs are located here.

By moving the big switches on the main panel, power is directed where needed.

Gato Class submarines had two main batteries (in a submarine, a battery is the entire collection of cells wired together in the battery compartment), one forward, under the officers’ staterooms, and another aft, under the largest crew berthing area. Each of these contained 126 cells, with each cell being about 54 inches high, 15 inches deep, and 24 inches wide. These cells weigh about 1650 pounds or about 208 tons between the two batteries.

When batteries are charged, they produce hydrogen gas. This is hardly noticeable in a car, where the battery is contained in a well-ventilated space, but can present a problem in the sealed hull of a submarine. The cells are all connected to a vent system designed to safely draw off the hydrogen and discharge it through the main exhaust.

A means is also provided to seal off the battery compartments, isolating them from the air supply in the rest of the boat. This is a safety feature and is particularly important if salt water gets into the battery compartment since it can combine with the electrolyte to produce chlorine gas.