At 23.53 hours on 28 May 1943, U-177 attacked convoy CD-20, fired a spread of two bow torpedoes at a freighter and two minutes later a spread of two stern torpedoes at a tanker. The U-boat had already missed the same ships with four torpedoes at 21.25 hours. After about five minutes, one hit each were observed on Agwimonte and Storaas. At 00.46 hours on 29 May, the U-boat fired a coup de grâce that hit and sunk the now abandoned Agwimonte and at 01.10 hours a coup de grâce at Storaas, which had to be sunk by a third torpedo at 01.53 hours.

The Storaas (Master Arnold Morits Jensen) was struck on the starboard side in the engine room despite of an immediate evasive action after the other ship had been torpedoed. The explosion stopped the main engines and all lights went out. First the stern settled very quickly, but then stopped. The crew abandoned ship without orders in the aft lifeboats of which one then also took the master, the first and third mates and the radio operator on board. Two men on watch below and the steward were lost. The boats stayed near the tanker with the intention to reboard the vessel at daylight, but after the second torpedo hit the South African armed trawler HMSAS Vereeniging (T 72) approached the boats and took the survivors on board to rejoin convoy. They were landed in Port Elizabeth on 30 May, then travelled by train to Capetown.