In the late evening on 16 November 1939 U-28 stopped the unescorted and neutral Sliedrecht about 120 miles southeast of Rockall and examined the documents of the ship. They showed that the tanker was ordered by the British authorities in Gibraltar to proceed to Kirkwall for a closer inspection of the cargo. The commander of U 28, Kuhnke, therefore ordered the crew to abandon ship and sank her with a torpedo at 00.20 hours on 17 November. Five crew members in one lifeboat were picked up by the British trawler Meresia and landed at Castlebay, Isle of Barra on 23 November.
The other lifeboat with 26 occupants was never seen again.