Although the U. S. Navy had 68 submarines in the Pacific at the start of the Pacific War, they sank only 93,300 tons of Japanese shipping. This was < 10% of what 100 operational German U-boats sank in the same period.
Initially, U.S. submarine operations were hindered by the loss of bases in the Philippines and a doctrine that concentrated on heavy enemy warships rather than merchant ships.
In 1943, smaller U.S. submarine were gradually replaced by the larger Gato, Balao, and Tench classes. Additionally, SJ surface search radar installation further enhanced U.S. submarine effectiveness. SJ radar provided information regarding direction, distance, surface contacts and low-flying aircraft.
Because the Japanese navy failed to organize transports and cargo vessels into convoys, Japan’s merchant shipping losses totaled 1,668,000 tons in 1943 (1.34 million by U.S. submarines).