At 14.49 hours on 15 Sep 1939, U-53 tried to stop the unescorted Cheyenne (Master Hugh Kerr) with a shot across her bow about 150 miles west-southwest of Fastnet, Ireland. But the tanker used its radio and ignored a second warning shot 10 minutes later, so the U-boat opened fire with the deck gun until the ship stopped at 15.20 hours. After the master and 36 crew members abandoned ship in two lifeboats, a coup de grâce was fired at 15.44 hours which hit underneath the bridge, but the tanker remained afloat even with a broken back after being hit at the same place by a second torpedo at 15.50 hours. U-53 went to the lifeboats to question the survivors and ordered them to stay near the wreck while they tried to stop the sighted British motor merchant Rothesay Castle to assist in the rescue, first by stern signals and then with two warning shots but the vessel fled. The U-boat then returned to the survivors and told them that they will be towed closer to the Irish coast. Just as U-53 was firing three rounds at the tanker wreck to finish it off, the destroyer HMS Mackay (D 70) (Capt P.L. Vian, RN) arrived at the scene, forced the U-boat to dive by gunfire and drove it off with a few depth charges. The distress signals were also heard by the Norwegian motor merchant Ida Bakke (Master Anton Zakariassen) which arrived to pick up the survivors and subsequently landed them at Baltimore, Co. Cork near Fastnet Light. The destroyer then scuttled the wreck of Cheyenne with gunfire. Six crew members were lost.

The previous day, the Ida Bakke had rescued all survivors from the British Influence, which had been sunk by U-29 (Schuhart).