Radio Intelligence Support of Submarine Operations

   Early in the war, specific bits of radio intelligence information of value to operating submarines would be picked up by Hypo.  Relaying this information through the Fleet Intelligence Officer by official messages often lost valuable time.  As a result, blanket authority was obtained for the Combat Intelligence Center to deal directly with ComSubPac.  At first, this was done by personal visits to the SubPac’s Chief of Staff.     As the volume of decrypted traffic increased, the amount of such information of interest to submarine operations likewise increased.  Early on, our submarines were few and scattered.  Often, decrypts could guide submarines to intercept major men of war in transit although such interception was difficult in the wide expanse of the Pacific Ocean.

   After the Battle of Midway, the “Maru Code” or JN-11, a four numeral code with a superimposed cipher was broken, which provided details on the routing of Japanese naval convoys.  This information included predicted noon position reports, which enabled submarines to be directed to mid-ocean contacts eliminating long and frequently fruitless searches. In December 1942, Lieutenant Commander Richard G. Vogel, the Pacific Submarine Force Operations Officer, was fully indoctrinated and was permitted access to radio intelligence decrypts.  Each morning, he reviewed the previous days’ Japanese traffic for information of value.  The Submarine Force provided Combat Intelligence with a daily overlay plot of the location of U.S. submarines.  It could be easily determined whether any submarine on patrol could use a recovered ship or convoy position.  Arrangements were also made to pass spot radio intelligence information to ComSubPac by secure telephone. ComSubPac and Combat Intelligence worked together in a smooth working relationship.  Since U.S. submarines were often reluctant to transmit their positions to prevent enemy direction finder fixes on them, the reverse situation also worked.  When a submarine attacked a maru or convoy, it sent out standard emergency signals like “submarine sighted”, “torpedo attack” or “I am sinking” together with a position report.   This information was also sent to ComSubPac, who then could perfect the actual location of his submarines.  To illustrate the advantage of radio intelligence, there were nights when nearly every American submarine on patrol in the central pacific was working on information based on cryptanalysis.  Similar arrangements were made with submarines operating out of Brisbane and Perth, Australia.  These reports became so reliable, submarine C.O.’s were known to complain when convoys were ten minutes late or convoys were rerouted by subsequent messages.  When the Japanese Army Water Transport code for Army transports was solved in April 1943 by SSA and the Central Bureau, this information was also passed to submarines, particularly those operating out of Perth and Brisbane.

   Between May and July 1943, American submarines made contact with Japanese fleet units eighteen times and actually make eight attacks through radio intelligence directives. The carriers Hiyo, Unyo, and Taiyo were damaged and the submarine I-24 was sunk. The reading of the Truk portmaster’s code greatly increased the enemy naval targets available to U.S. submarines.  Submarines were assigned to certain lagoon channels and were alerted as to ships entering or exiting Truk by that channel.
   No definitive records of Ultra messages alerting U.S. submarines to Japanese ships were kept.  However, on 14 April 1945, Vice Admiral Charles A. Lockwood, ComSubPac, advised Nimitz that “a high percentage of submarine sinkings is based on Ultra information” and he wrote in 1947 “… a common saying in Singapore (was) that you could walk from that port to Japan on American periscopes.  This feeling … was created, not by the great number of submarines on patrol, but rather by the fact, thanks to Communication Intelligence, the submarines were always at the same place as Japanese ships.”