Hunter-Killers Chasing down an Axis Submarine—USS Pillsbury (DE 133) and USS Guadalcanal (CVE 60),
U.S. Navy photograph in the collections of The Mariners' Museum
F-21 Chart from May 7, 1944,
National Archives, Record Group 457

In the central and southern Atlantic, F-21 and Tenth Fleet served as the brains while the ships of the Atlantic Fleet provided the brawn for the U.S. Navy's antisubmarine warfare offensive against Axis submarines. Smaller sized escort carriers were already sailing near Allied convoys, providing air coverage and thwarting U-boat attacks. After 1943, U.S. Navy escort carriers shifted to the offensive. While the British deployed escort carriers with convoys in the North Atlantic, the Americans formed autonomous "hunter-killer" antisubmarine task groups. A typical U.S. Navy hunter-killer task group consisted of a number of escort vessels like Destroyers (DD) and Destroyer Escorts (DE), which were centered on an escort carrier (CVE). Usually, the hunter-killers would sortie from Hampton Roads to a designated operations area. Afterwards, hunter-killer formations would either return to home port or continue on to alternate ports such as those in North Africa for refits, refueling, and rearmament. Maintaining a continuous circuit along the Allied convoy routes and in U-boat operations areas, U.S. Navy hunter-killers were a constant threat to U-boats after 1943.

Grand Admiral Karl Dönitz Receiving a Salute,
U.S. Navy photograph in the collections of The Mariners' Museum
U-Tanker U-117 and U-66 under Attack, August 7, 1943,
U.S. Navy photograph in the collections of The Mariners' Museum
"Vultures Row" aboard USS Bogue,
U.S. Navy photograph in the collections of The Mariners' Museum

Maneuvering on intelligence information supplied by the Knowles F-21 tracking room through Tenth Fleet, U.S. Navy hunter-killer task groups attacked concentrations of U-boats in the central and southern Atlantic. Knowles or a member of his F-21 staff customarily met with hunter-killer task group commanders before they embarked on war patrols, providing them with detailed estimates of enemy trends and activity. Over time, Knowles and his F-21 staff developed great rapport with the escort commanders. For security reasons, it was unnecessary for the hunter-killer commanders to have direct access to special intelligence sources. However, Knowles remembered that they regarded F-21 information as "more right than wrong and, therefore, [the hunter-killer commanders] listened very carefully to everything we sent out." Confirming this assessment, one hunter-killer task group commander stated in a postwar assessment that "I treated the [F-21] estimate as Bible truth every day." Thus, with F-21 providing the means for locating and fixing the enemy, U.S. Navy hunter-killers aggressively chased U-boats from Allied convoys and destroyed their logistical support network.

In the summer of 1943, the German naval staff under Admiral Karl Dönitz struggled to support those U-boats that had successfully traversed the Bay of Biscay to reach the open sea. To undermine U-boat logistics, F-21 trackers concentrated hunter-killer attacks against submersible refueling vessels known as "U-tankers." Using all the various forms of special intelligence to establish a constant presence in U-boat resupply areas, F-21 trackers aggressively exploited cryptologic information to destroy the U-tankers. In June 1943, the Germans had a total of nine operational U-tankers. By August 1943, U.S. Navy hunter-killers had sunk eight U-tankers and a number of combat U-boats during refueling operations—always maneuvering on information from F-21.

Photographic Essay on U.S. Navy Hunter-Killers

Sonobuoy recording of an actual attack against the Japanese submarine I-52, which was sunk in the Atlantic during this action on a voyage to Europe in July 1944.

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