From Ultra in the Pacific: How Breaking Japanese Codes and Ciphers Affected Naval Operations Against Japan, 1941-45 (The Secret War), by John Winton (Sapere Books, 2022), Kindle pp. 190-193:
‘PRESS home all attacks,’ wrote Rear Admiral James Fife USN, Commander Submarines South-West Pacific, in his standing orders. ‘Pursue relentlessly, remembering that the mission is to destroy every possible enemy ship. Do not let cripples escape or leave them to sink — make sure that they do sink.’
The American submariners in the Far East, very ably assisted by the British and the Dutch, put Admiral Fife’s orders faithfully into effect and achieved devastating results. By VJ Day, 1945, Allied submarines in the Far East were actually running out of targets. By that time, although submarines still constituted only 2 per cent of the American war effort on the Pacific, American submarines had sunk two-thirds of the total Japanese merchant ship tonnage sunk during the war, and had also sunk one out of every three of the Japanese warships sunk.
The United States and the Imperial Japanese Navies were roughly equal in submarine strength in the Pacific at the outset of the war. Neither navy had had any operational experience of submarines in the First World War. Both had prepared for submarine warfare on a long-range scale, and primarily for use against enemy warships. The crucial difference in the Second World War lay in the US Navy’s technological advances, its readiness to profit by tactical experience, and its proper strategic deployment of its submarines. In all three areas the Americans were superior.
The one advantage the Japanese submarines had was the quality of their formidable 40-knot, oxygen-powered, longer ranged torpedoes, with twice the explosive charge of the American torpedoes. American torpedoes were frequently defective and, incredibly, it was nearly two years before the US Navy established the causes of the defects and remedied them. Under operational patrol conditions American torpedoes nearly always ran eight to ten feet below their proper depth, so that their magnetic detonators, designed to be activated by the target ship’s metal hull, failed to work properly. Similarly, the contact detonators only worked best after an oblique impact, thus, ironically, penalizing the very submarine captains who aimed best and hit their targets broadside on.
Design faults were compounded by bureaucratic obstruction: shorebound officers and bureaucrats continued to insist that the whole fault lay with incompetent submarine captains who could not aim properly, and refused to believe submarine captains who said they had heard their torpedoes hitting the target and failing to explode.
For the first months of the war Japanese submarines had considerable success in sinking Allied warships, especially in ‘Torpedo Junction’ in the summer of 1942. But the fatal Japanese tendency to indulge in non-profitable peripheral activities soon began to drain away their submarine patrol strength.
The Japanese diverted their submarines to carry midget submarines, to no tactical purpose, or to act as communication links, or to wait at rendezvous to refuel flying-boats, or to carry out unimportant surface bombardments, which had no more than pinprick nuisance value, of Midway, or Canton Island or Johnston Island, or (in 1942) the coasts of Vancouver and Oregon.
The largest Japanese submarines carried aircraft — requiring an hour after surfacing to assemble and launch — which they transported thousands of miles for valueless reconnaissance flights. One submarine, I-25, launched her aircraft loaded with incendiaries with the serious intention of setting light to the forests of North America. As the war progressed more and more Japanese submarines were taken off patrols and used to carry men, ammunition and food to beleaguered Japanese island garrisons bypassed and left to ‘wither on the vine’ by the Allied advance.
Unquestionably the best strategic use the Japanese could have made of their submarines would have been to make a determined effort to cut the supply lines from Pearl Harbor to Micronesia and Australia. They made no such effort. There was never any submarine war in the Pacific remotely comparable with the struggle against the Atlantic U-boat. The US Navy began by escorting their ships in convoys in the Pacific, but by the end of 1943 there was so little enemy submarine activity that single ships were steaming across the Pacific unescorted.
To misuse of submarines in exotic sideshows the Japanese Navy added an almost complete failure to safeguard their own surface ships against submarines. The Japanese were obsessed by the idea of an ‘offensive’ war. Like the British in the First World War, they regarded convoys as ‘defensive’ and therefore somehow demeaning and unworthy of a warrior nation. Convoy did not appeal to the Samurai spirit.