They apparently ran too deeply. Three days later, she concluded a somewhat discouraging, but still successful, first war patrol at Midway Island. Second war patrol Following a month there for refit and training, Threadfin embarked upon her second war patrol on 14 March. She initially joined a coordinated attack group composed of herself, and submarines and . During her five-day tour with that wolf pack, Threadfin made two attacks on enemy shipping. On the afternoon of 28 March, she came across two Japanese destroyer escort-type warships and apparently dispatched one with a single hit from a spread of six torpedoes. The stricken warship's screws stopped while her colleague's depth charge attack deprived Threadfin of definite knowledge of the ultimate result. That evening, the submarine tangled with a convoy composed of two small trawlers and four luggers. During the ensuing surface gun engagement, the submarine inflicted serious damage on two of the luggers, moderate damage on the trawlers, and minor damage on the remaining pair of luggers. Though disconcerting, the Japanese return fire proved ineffectual. On 31 March, that group was dissolved, and Threadfin received orders to join and near Bungo Suido, the primary entrance to the Seto Inland Sea which separates Honshū from Kyūshū and Shikoku. The new attack group's primary assignment was to guard against an undetected sortie of the remainder of Japan's fleet during the Allied assault on Okinawa. On the evening of 6 April, Threadfin made radar contact with what later proved to be an enemy task force built around Japan's super battleship, . Passing up a tempting opportunity in order to carry out her prime directive, Threadfin flashed the warning to Fifth Fleet headquarters afloat off Okinawa. Completing that phase of her mission, the submarine tried desperately to regain attack position on the force, but its speed denied her a second chance. On the whole, however, her radio was probably more valuable than her torpedoes would have been. Her timely warning enabled the planes of Task Force 58 to ambush and sink Yamato and to destroy most of her consorts as well. A second mission of the submarine consisted of lifeguard duty to rescue downed American airmen. Her first war patrol afforded her no opportunity to pursue such a humane mission; but, near the end of the second she rescued a half-drowned P-51 Mustang pilot. Though he had swallowed large quantities of water, artificial respiration soon