|Diesel electric drive advantages|
Sixteen submarines of the Salmon class of 1935 and Sargo class of 1936 used the new industrial Diesels; two were direct drive and two drove generators. Starting with the Tambor class of 1938 all four Diesels drove generators.
Diesel-electric gave the skipper great flexibility. When he needed speed all four engines could be put on propulsion, while if the batteries were almost depleted two or three Diesels could charge batteries while the submarine moved more slowly from the power of remaining engines. One engine on charging often meant several hours before batteries were fully charged.
In the Pacific war after a long underwater evasion from enemy escorts, once the submarine surfaced, it often sped away from a dangerous area on all four engines, though the batteries might be empty, then switch one engine to recharge the hungry batteries, adding a second then third until the batteries were fully charged. If the radar and men on watch reported the seas clear, some Diesels could be put on charging for a while, though that meant the submarine moved more slowly.
When one Diesel broke down and had to be off line while it was being repaired, the submarine still had three quarters of its power available for any mix of propulsion or re-charging of batteries.
Later in the war industry developed a new electric motor that had full torque at slow speeds. When introduced into our submarines these motors eliminated the need for gears to reduce the speed and increase the power output of the electric motors for propulsion. Elimination of those gears made our submarines quieter for evasion from enemy hunters.
Having two motors to drive each screw provided redundancy, so when a motor was damaged, the screw still had the full power of the other motor, as happened on the tenth patrol of the Sailfish when a very close depth charge knocked one motor loose from its attachment to the hull.
Railroad Diesel locomotives use Diesel-electric drive with the traction motors driving the wheels on the track while the Diesel and generator are above in the body of the locomotive.