Submarines

When viewed in its entirely, the Allied naval war on the Japanese merchant fleet was a complex relationship between submarine, air and mining operations. As a whole, the campaign can be defined as war of attrition waged by submarines, intermittently supported by crippling air raids and capped by a debilitating mining effort designed to figuratively—and literally—starve the Japanese state and people. The Allied submarine fleet was the most consistent sinking agent from the war’s inception until late 1944. The fleet struggled early with a small flotilla equipped with faulty torpedoes and skippered by largely inexperienced captains. As these factors improved, so did their results. Beginning in the summer of 1943, the U.S. submarine fleet started to rapidly increase in effectiveness. Airpower, primarily of the United States Navy, struck decisive blows at key points in the war. These strikes not only sunk Japanese merchant vessels; they also shaped the battlespace which allowed the sub fleet to operate more effectively. Lastly, mines eclipsed air and submarines at the end of the war when the Japanese empire was isolated and approaching defeat.

This graph plots totals of total tonnage sunk by sinking agent, per month. Notice the different patterns between submarines, aircraft, and mines. The submarine campaigned waxed and waned but grew steadily until Oct 1944. Aircraft sinkings spiked periodically with major operations and the mining effort started late and look precedence until the end of the war. Major aircraft operations, while few in number, shaped submarine operations, particularly after major bombing campaigns in Feb 1944 and Jan 1945.

The historiographic focus on the Allied submarine effort is not without its merits. Allied subs sunk 1149 merchant vessels which accounted for 52% of all lost Japanese merchant vessels. These vessels comprised over 4.8 million tons of merchant shipping, 59% of all Japanese merchant tonnage sunk by the Allies during the war. These numbers are even starker when juxtaposed with the Allied submarine fleet’s relatively small size. The United States Navy fielded 263 submarines (179 of which sunk at least one merchant vessel). These boats made 1,147 combat patrols in the Pacific. Throughout the war the Allied submarine fleets had the longest reach of any naval asset. The U.S. sub fleet suffered significant casualties, fifty-two U.S. submarines were lost due to enemy action, accident, friendly fire, or, for many, still unknown causes. 8 From 8 December 1941 until late 1944 Allied submarine fleet sunk the highest totals of Japanese shipping. During that time U.S. subs prowled the Japanese Sea Lines of Communication (SLOC) which connected the core of the Japanese empire to its positions on Truk Lagoon, the Palau Islands, Guam, and Rabaul, New Britain. Over half of all sinkings completed by U.S. subs occurred on these SLOCs in support of the Allied advance though the Pacific. Starting in July 1943 U.S. submarines began their steady ascent into operational success due to an increase in the quantity and quality of available boats, the fielding of the more dependable Mark 18 torpedo, and the breaking of the Japanese merchant fleet’s coded communications.

The submarine war accelerated during the spring of 1944 and reached its crescendo during the fall of that year. This acceleration occured, as least partially due to the collapsing front initiated by U.S. Navy’s surface fleet and the success of the New Guinea Campaign. The Allied advance freed up more subs to operate within the Luzon Straight, a natural chokepoint which divided the Philippine island of Luzon and the island of Taiwan. The straight hosted a significant SLOC, a critical maritime route which teemed with large vessels, particularly tankers that ferried Near East Asia oil to the Japanese home islands. The average merchant vessel sunk in these waters weighed in at approximately 4,600 tons, 26% heavier than elsewhere in Japan’s maritime empire. With the advance of the Allied fleet to the east, U.S. submarines focused their efforts on severing this artery and thereby strategically weakening the empire and supporting the Allied invasion of the Philippines (October - December 1944). Of the October sinkings caused by U.S. submarines, 82% of these events occurred in the South China Sea and in the waters surrounding Taiwan, waters directly connected to the Luzon Straight.As the battle space collapsed so did the rate of sinkings caused by U.S. subs. October 1944 was the most active month for U.S. submarines; 36 different boats sunk 68 Japanese merchant vessels which totaled 349,672 tons of shipping. By this stage of the war, U.S. sub commanders utilized “wolf pack” tactics to aggressively attack Japanese convoys while in self-supporting units of 3-4 subs. During a single two-day span, 23-24 October 1944, the USS Drum, USS Icefish, USS Seadragon, and USS Snook sunk nine vessels, weighing in at a combined 48,000 tons, and did so in an area about the size of Delaware. By the end of December this push by the submarine fleet, coupled with the Allied invasion of Luzon, interdicted this critical shipping route.