Soon afterward, "Sea Dog" cleared the Niigata area and headed north to patrol off Sakata and Akita. On 10 June, she closed Oga Hanto, and that night she patrolled northwest of Kisakata and northeast of Tobishima, Minato Island. On 11 June, she returned to the vicinity of Oga Hanto. Shortly after 1300, she made contact with another coastal freighter, surfaced, and commenced running west and north to intercept. At 1519, she dived. At 1555, she fired one torpedo. Forty-three seconds later, the torpedo hit; and the target, cargo ship "Kofuku Maru", broke in two, up-ending both the bow and the stern.