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 Agwimonte (Master James William Beattie) was in station #53 (last ship of the starboard column) and the lookouts spotted the wake of a torpedo but it was too late. It struck on the starboard side between the #2 and #3 hatches, causing the ship to heel to starboard and the sea washed over the foredeck and the bridge. The watch below secured the engines and the ship settled on even keel. Ten officers, 36 men and 23 armed guards (the ship was armed with one 5in, one 3in, eight 20mm and two 30cal guns) abandoned ship in two lifeboats and three rafts.
The first boat swamped in the swells when the after fall failed to release. The men tumbled into the sea and climbed back on board the Agwimonte. Several of these men left the ship with the overcrowded second lifeboat. The master and five men bailed out the swamped boat and picked up two other men from a raft. About one hour later they witnessed the second torpedo attack, which caused the boiler to explode and the ship to sink bow first off Cape Agulhas at 01.25 hours with the general alarm still blaring. Two of the survivors on one of the rafts later transferred to an abandoned lifeboat of the other torpedoed ship.

The South African armed trawler HMSAS Vereeniging (T 72) picked up 61 survivors from a lifeboat and two rafts and landed them at Port Elizabeth, South Africa on 30 May. The other two boats were sighted by a South African Army aircraft in the afternoon of 29 May. An Army crash boat rescued the occupants of those boats 18 hours after the attack and landed them two hours later at Gordon Bay.