At 04:45 on 4 October 1939, U-23 scored one of the Kriegsmarines early successes of the war when she torpedoed and sank with gunfire, the merchant ship Glen Farg about south-southwest of Sumburgh Head (southern Shetland).
In 16 patrols U-23 sank seven ships for a total of including two warships, as well as damaging a warship and an auxiliary warship.