Starting on the day of the Pearl Harbor raid, the US Navy fleet submarines designed in the 1930’s had a standing order: sink whatever you can of the enemy’s military and merchant fleet. Yamamoto had a different standing order for his boats: save your torpedoes and go for the capital ships. Both strategies had very different implications for their navies. The American position was somewhat ironic, given their criticism of Germany’s unrestricted submarine warfare during World War I.
The Japanese torpedo, called the Long Lance, was vastly superior to the American equivalent. The Americans had tested their Mark 14 torpedoes off Rhode Island, where calm waters were nothing like the combat conditions America’s boats would encounter. The heavy seas of the Pacific knocked out the guidance system, and the firing pin was inoperable much of the time, failing to detonate even if the Americans would get a hit. Complaints to the US Naval Command went unheeded. Some American commanders took to modifying their torpedoes themselves, despite orders prohibiting shipboard modifications.
In contrast, when the Japanese submarine I-19 fired five torpedoes, three struck the USS Wasp, which sunk; the USS North Carolina was hit several miles away; and the USS O’Brien foundered after a single hit. Relentless training with the Long Lance was part of every Japanese vessel’s orders before the war, and the Japanese considered them part of any tactical doctrine. Every capital ship except some carriers carried them. Thankfully for the Allies, the order to concentrate on capital ships precluded any sort of unrestricted submarine warfare like the war US Atlantic Fleet fought in earnest.
Worse yet for the sub skippers, the high command tried to use submarines for rescue, supply, and covert missions. Aging, large subs like USS Nautilus were used to transport Carlson’s Raiders to attack Makin in August 1942. While the raid was a huge public relations success, it had little tactical or strategic value, causing the Gilberts to be reinforced by the time of the November 1943 invasion of Tarawa and Makin. Subs were very successful as pilot lifesavers, but this took their torpedoes away from hunting the Japanese ships.
The United States finally listened to their sub commanders and redesigned the Mark 14 torpedo. The new Mark 14 torpedo was a success. By now it was 1943, and the Americans had hampered their forces with bad torpedoes for over a year. Now the fleet submarines took the war to the Japanese merchant fleet operating around the home islands.
Incredible numbers of kills immediately began to be reported. The Americans were sinking large numbers of marus (Japanese transports) and military ships as well. The USS Archerfish sank the IJN Shinano, the largest carrier built during the war. Submarines would go out and come back with three or four kills during a single patrol.
Japan expected to lose some cargo tonnage, but not at the rate they lost in 1944 and 1945. Not only could they not replace their losses, the unrestricted submarine warfare meant that even the fishing fleets were targets.
Combined with minelaying aircraft, the submarines blocked every port, strangling an island nation in a way that the British feared in their worst nightmares. Very little surface traffic arrived or left in 1945.
By war’s end, the American fleet submarines had sunk 51% of the total tonnage of all enemy ships sunk. They were the most effective weapon in the destruction of the Nihon Kaigun and Japan’s merchant fleet.