On the morning of 21 June, she sighted three armed trawlers, one under tow, headed from Shantung toward Korea. At 09:43, she surfaced and, three minutes later, commenced firing with her five-inch (127 mm) gun, 40 millimeter and 20 millimeter guns. The 40 millimeter soon jammed, but fire from the five-inch (127 mm) gun and 20 millimeter guns sank the trawler being towed and left a second burning and settling. The third, the towing vessel, cast loose and headed west under full steam. "Sea Devil" picked up three survivors and moved south to patrol in another area.