Destruction of the Japanese Merchant Fleet

Japan's merchant shipping fleet, was not only a key link in the logistical support of her armed forces in the field, but also a vital link in her economic structure. It was the sole element of this basic structure which was vulnerable to direct attack throughout a major portion of the war.

Japan entered the war with some 6,000,000 tons of merchant shipping of over 500 tons gross weight. During the war an additional 4,100,000 tons were constructed, captured or requisitioned. Sufficient information was secured by the Survey in Japan concerning this 10,100,000 tons to tabulate ship by ship, (a) the name end tonnage, (b>) the date, location; and agent of sinking or damage, and (c) the present condition and location of such ships as survived. The sources from which evidence was obtained were in some respects conflicting. Where possible these conflicts have been resolved. The Joint Army and Navy Assessment Committee has tentatively arrived at similar results and is continuing its efforts further to refine the evidence. The Survey believes that the figures included in the following breakdown will not differ significantly from the final evaluation of the Joint Army and Navy Assessment Committee.

Eight million nine hundred thousand tons of this shipping were sunk or so seriously damaged as to be out of action at the end of the war. Fifty-four and seven-tenths percent of this total was attributable to submarines, 16.3 percent to carrier based planes, 10.2 percent to Army land-based planes and 4.3 percent to Navy and Marine land-based planes, 9.3 percent to mines (largely dropped by B-29s), less than 1 percent to surface gunfire, and the balance of 4 percent to marine accidents.

Due to their ability to penetrate deeply into enemy-controlled waters, submarines accounted for approximately 60 percent of sinkings up until the final months of the war. During 1941, carrier task forces made deep sweeps which accounted for large numbers of ships. After April, 1945, when Japanese shipping was restricted to the Korean and Manchurian runs and to shallow inland waters, mines dropped by B-29s in Japanese harbors and inland waterways accounted for 50 percent of all ships sunk or damaged. In isolating areas of combat from ship-borne reinforcements land-based aircraft also sank large numbers of barges and vessels smaller than 500 tons gross weight, not included in the tabulation prepared by the Survey.

In the Survey's opinion those air units which had anti-shipping attacks as their prime mission and employed the required specialized techniques, equipment and training achieved against ships the best results for the effort expended.

The Japanese originally allocated two-thirds of their shipping fleet to the logistic support of their military forces in the field. They expected that after their original advance had been completed they would be able to return increasing numbers of ships to the movement of raw materials for their basic economy. After the beginning of the Guadalcanal campaign, however, they were kept under such constant and unexpected military pressure that the contemplated returns after that date were never possible.

Up to the end of 1942, ship sinkings exceeded new acquisitions by a small margin. Thereafter, the aggregate tonnage sunk increased far more rapidly than could be matched by the expansion of the Japanese shipbuilding program. The size of the usable fleet thus declined continuously and at the end of the war amounted to little more than 10 percent of its original tonnage. The Japanese belatedly attempted to build up a convoy system, to re-route freight movements to rail lines, and to abandon more distant sources of supply, but these measures acted only as palliatives and not as cures. Furthermore, convoying and re-routing decreased the freight moved per ship by a factor amounting to 43 percent in the closing months of the war. In 1944 tanker losses became particularly heavy and were thereafter the first concern of the Japanese shipping authorities.

The basic economic consequences of ship sinking will be discussed in a later section. From the standpoint of the Japanese armed forces in the field it will be noted that 17 percent of army supplies shipped from Japan were sunk in 1943, 30 percent in 1944, and 50 percent in 1945. A shortage of fleet tankers was a continuing limitation on the mobility of the Japanese fleet and contributed to its defeat in the two crucial battles of the Philippine Sea. Inadequate logistic support, due in large part to lack of shipping, was one of the principal handicaps of the Japanese air forces.

Attacks by submarines, long-range search and attack planes, mines, and carrier and land-based planes were mutually supporting and complicated the Japanese defenses. Long-range air search found targets for the submarines; convoying which offered some protection against submarines increased the vulnerability to air attack; ships driven into congested harbors in fear of submarines were easy prey for carrier strikes; and mines helped to drive ships out of shallow water into waters where submarines could operate. Had we constructed more submarines, earlier concentrated on tankers and more fully coordinated long-range air search and attack missions with submarine operations, the ship sinking program might have been even more effective.