22. American code breakers supported the submarine war by identifying enemy ship movements.
In 1942, submarines were dispatched to sea with assigned patrol areas. After reaching their assigned area they conducted operations at the will of the commanding officer, remaining within their designated area. The areas were designated and assigned based on analysis of shipping needs within. That changed in 1943. America’s code breakers, as they had with several other Japanese communication codes, cracked what submariners called the “maru code”. Using the information, intelligence analysts identified Japanese shipping convoys, what they carried, the number of escorts, shipping times, and destinations. The information allowed commanders to route submarines to the areas to intercept the Japanese.
Using the coded information intercepted from the Japanese allowed submarines on patrol to receive updated information while at sea. Japanese convoys at sea provided daily updates of their position, course, and speed, overheard by Americans and relayed to the submarines. Ships sailing individually did likewise, allowing certain targets to be designated as high value to American analysts at Pearl Harbor. The intercepted messages also confirmed Admiral Lockwood’s concerns with the defective Mark 14 torpedo in mid-1943. The Japanese captains informed their commanders of being struck with torpedoes which failed to explode, and American codebreakers intercepted the messages and forwarded them to higher authority.