Thus, only in 1944 did the U.S. Navy begin to use its 150 submarines to maximum effect: installing effective shipboard radar, replacing commanders deemed lacking in aggression, and fixing the faults in the torpedoes. Japanese commerce protection was "shiftless beyond description,"[nb 14] and convoys were poorly organized and defended compared to Allied ones, a product of flawed IJN doctrine and training – errors concealed by American faults as much as Japanese overconfidence. The number of U.S. submarines patrols (and sinkings) rose steeply: 350 patrols (180 ships sunk) in 1942, 350 (335) in 1943, and 520 (603) in 1944. By 1945, sinkings of Japanese vessels had decreased because so few targets dared to venture out on the high seas. In all, Allied submarines destroyed 1,200 merchant ships – about five million tons of shipping. Most were small cargo-carriers, but 124 were tankers bringing desperately needed oil from the East Indies. Another 320 were passenger ships and troop transports. At critical stages of the Guadalcanal, Saipan, and Leyte campaigns, thousands of Japanese troops were killed or diverted from where they were needed. Over 200 warships were sunk, ranging from many auxiliaries and destroyers to one battleship and no fewer than eight carriers.

When did the US Navy begin to use it's submarines to maximum effect?