Radar and the American Submarine War, 1941 -1945: A Reinterpretation Robert Dienesch Cet article examine la guerre sous-marine entreprise par les Américains contre le Japon en 1943, année cruciale car c 'est à ce moment-là que, signe d'une performance sous-marine accrue, le tonnage coulé par mois augmenta considérablement. On attribue traditionnellement cet accroissement à la correction de problèmes de torpille, à l'expansion de la flotte sous-marine, aux améliorations de doctrine et à l'impact du décryptage sur les patrouilles sous-marines. Bien que ces facteurs aient certainement joué un rôle dominant, cet article soutient qu'ils n 'expliquent pas complètement le redressement rapide de la performance. C'est plutôt l'intégration d'un radar de veille de surface efficace qui unifia ces autres facteurs et permit au commandant en poste de patrouiller plus énergiquement et efficacement. At 2105 on 15 April 1943, the silent, shadowy form of the USS Seawolfbroke the surface of the East China Sea after slowly rising from a depth of 250 feet. Since 1954 she had been avoiding a minor depth charge attack brought on by her ambush of a two ship convoy earlier that evening. The first attack on the convoy was launched at 1952 when four torpedoes were fired at a large freighter. The first of those torpedoes detonated prematurely, about fourteen seconds after being fired, which alerted the convoy to the presence of the Seawolf and allowed them to avoid two of the three remaining Mark XIV torpedoes. The fourth torpedo hit the freighter and caused minor damage. The subsequent counter attack by the escort started at about 1954, but it was not as severe as would have been expected. Shells were fired at the periscope and four depth charges were dropped some distance away from the submarine. The Seawolf nonetheless remained submerged for just over an hour before surfacing to pursue the convoy. Seawolf immediately reacquired her targets using her SJ surface search radar at a range of 15,000 yards and proceeded to manoeuvre around the port flank of the convoy. By 2304 the Seawolf had gained a good attack position 10,000 yards (based on radar ranges) ahead of the ships. As the convoy approached the Seawolf'the range decreased rapidly until the submarine was forced to dive at 4,200 yards and launch a submerged attack due to the The Northern Mariner/Le marin du nord, XIV No. 3, (July 2004), 27 - 40. 27 28 The Northern Mariner/Le marin du nord bright surface conditions. During this second attack three more torpedoes were fired at the damaged freighter at a range of roughly 1,650 yards. Set for a depth of sixteen feet with an eight second spread, two of these torpedoes were heard to hit the target which promptly stopped but remained afloat. Because the escort refused to leave the area, the Seawolf was forced to make another submerged attack at 0152 on 16 April. Two more torpedoes, fitted with the secret Mark VI influence detonators, were fired at a now stationary target at a range of 1,400 yards. One torpedo exploded just below the stack of the freighter and the second torpedo either ran erratically or was defective and failed to detonate. Despite this third hit, which broke away the forward partition of the freighter, most of her remained afloat. Later that day, following a periscope observation of the scene the Seawolf finally withdrew because of the continued presence of escorts and aircraft. She left a crippled freighter missing a portion of her bow and with a severe list to port: a meagre result for the nine torpedoes fired, almost 40 per cent of her torpedo load, and all the risks taken in the attack.1 Seawolf s attack on 15 April 1943 encapsulates a critical period for the American submarine effort to destroy Japanese merchant shipping during the Second World War. Seawolf's difficulty is obvious. Silent and almost untraceable, a submarine is nonetheless most vulnerable to detection and counter attack during an approach and an attack on a ship. The need to use the periscope and the tell-tale torpedo wakes reveal the location of the submarine even if the torpedoes miss their target. In the case of the Seawolf the return for a series of such skilful and daring attacks, and the expenditure of expensive ordinance, was paltry at best. In fact, the United States expected much more from its submarine fleet by 1943. Following the shock of Pearl Harbor, the United States Navy was forced to rebuild its surface force which had been savaged in the early hours of 7 December 1941. In an effort to strike back at the Japanese, American commanders turned to the submarine fleet, unleashing it in an unrestricted attack on the Japanese Navy and merchant marine. Unfortunately these attacks failed miserably in 1942. Although ordered to "Execute against Japan unrestricted ...submarine warfare" shortly after the attack on Pearl Harbor, the relatively untouched submarine fleet was severely hampered by a variety of difficulties.2 Prior to the attack on Pearl Harbor, the American submarine fleet was seen as an adjunct to the main battle fleet, an almost ideal scouting force with a devastating capability to attack the Imperial Japanese Navy. With the loss of the battle fleet at Pearl Harbor, the submarine fleet was forced to take a leading roll in the war against Japan. The transition from the pre-war scouting force to a strong and effective anti- 1 Chief of Naval Operations, Communications Division, "The Role of Communication Intelligence in Submarine Warfare in the Pacific January 1943-October 1943," SRH-11, November 1945, National Archives, Washington, DC, (hereafter NA), as found in U.S. Submarine War Patrol Reports and Related Documents, 1941-1945: Reference Documents on Submarine Operations and Submarines, Scholarly Resources Inc., Reel 7, Vol IV, 86- 87. 1 This order was issued by the Navy Department six hours after the attack on Pearl Harbor. Clay Blair Jr., Silent Victory, (New York, 1975), 106. Radar and the American Submarine War 29 shipping fleet was not easy for the submarine fleet. The expectation of many American officers in 1941 was that the submarine fleet would inflict severe losses on Japanese merchant shipping and on the invasion forces expected to be sailing towards the Philippines. As the first months of the war demonstrated, this expectation was never realized. Total Japanese merchant ship losses for 1942 were 977,927 tons, of which American submarines accounted for 612,039 tons.3 This was just slightly less than the 661,800 tons of merchant shipping added to the Japanese fleet by capture in their onslaught that year. By the end of 1943, however, the situation had changed dramatically. Though only conducting roughly the same number of patrols as in 1942 (350), there was a dramatic increase in the amount of Japanese tonnage sunk by US submarines in 1943. Based on post• war records, a total of 1,312,353 tons (from a total of 1,767,624 tons) were sunk by American submarines during 1943 .This represents a total increase of over 100 per cent over 1942. Moreover, during the last quarter of 1943, Japanese tonnage sunk by submarines did not drop below 100,000 tons per month. That was just the beginning. In 1944, the sinkings of Japanese merchant shipping by American submarines skyrocketed to a total of2,3 88,709 tons (see graph following page).4 The cause for this dramatic improvement in the United States Navy's war on Japanese shipping has been the focus of much debate. The purpose of this article is to suggest that it was the successful integration of an effective system of surface search radar into the submarine fleet that produced this improvement in performance. For almost fifty years, historians have debated what caused the dramatic increase in American submarine performance during 1943. The result is a consensus that has been both espoused by the key authors in the field and accepted by most historians. Although authors like Admiral Charles Lockwood (CINCPAC 1943-1945), Theodore Roscoe, Samuel E. Morison and Wilfrid J. Holmes have presented basically the same arguments, the current consensus is best described by Clay Blair Jr. Blair, in probably the most detailed account of the submarine war, presented what has become the accepted explanation for the improvement in submarine performance. According to Blair, the submarine fleet suffered three important problems in 1942. First there was an insufficient number of submarines to 3 It is difficult to assess exactly how much tonnage was actually sunk by the submarine fleet. The figures given by various authors differ substantially. Parillo, in the most recent work on the subject, presents several different figures himself and these differ from other authors like Blair. Parillo states two different figures for tonnage sunk by submarines in 1942. The first is 612,039 tons (Table A.9) and the second in the text of 884,928 tons. No explanation for the differences is given. Parillo also states two different figures for total losses in 1942: 1,095,800 tons and 977,927 tons. The lower figures from table A.9 are used here. See, Mark P. Parillo, The Japanese Merchant Marine In World War II, (Annapolis, 1993), 37-38, 204, 242 Table A.8., 243 Table A.9. 4 Parillo again provides slightly differing figures from Blair. According to Parillo, a total of between 2,065,700 and 1,767,624 tons of shipping were lost by the Japanese in 1943. Of this total, 1,312,353 tons was sunk by the United States submarine fleet.