At the south entrance to Makassar Strait, SS-271 intercepted a convoy of three ships and sank the cargo ship "Kōshū Maru". Nine days later, she scored damaging hits on a transport and sank cargo ship "Zuisho Maru". On 18 August, off northern Balabac Strait, Philippines, "Ray" closed a large convoy protected by surface escorts and planes, fired six "tin fish" at a tanker, and dived as a destroyer raced in to counterattack. Heavy explosions were heard, and a 2½-hour depth charge pounding followed. During the action, "Ray" heard another violent explosion and the sounds of a ship breaking up, as the tanker "Nansei Maru" went down. The submarine surfaced that evening and pursued the convoy into Palawan Bay.