The submarine lurked on the surface off Hokkaidō on 31 July and again tangled with some of the numerous Japanese patrol craft. She sighted a patrol boat at and closing. "Thornback"’s five-inch (127 mm) deck gun was trained out, but her crew could not keep their sights on the attacker through the telescopes and switched to open sights. Meanwhile, the 40 millimeter gun opened a devastating barrage at the enemy craft as it came steadily on a collision course. These shells continually hit the escort ship along the waterline and in the pilot house, probably killing the occupants on the bridge.