At 09.26 hours on 7 Oct 1942 the unescorted Firethorn (Master Paul F. Schultz) was hit by two torpedoes from U-172 while steaming on a non-evasive course at 13 knots about 60 miles northwest of Capetown. The first torpedo struck on the port side just below the bridge, tearing open the hull. The second struck in the engine room a few seconds later, causing the ship to sink within two minutes with a heavy list to port. No lifeboats could be launched and the 40 crew members and 21 armed guards (the ship was armed with one 4in, one 3in and four 20mm guns) abandoned ship in four rafts and a yawl boat which floated free. The master, nine crew members and two armed guards died. The U-boat surfaced after the ship sank and questioned the survivors before leaving the area. The yawl boat was found bottom up, was rightened and used to pick up the men swimming in the water and place them on the rafts which were eventually lashed together. Six men in the yawl boat set forth for the coast. They were spotted by an aircraft the next day and picked up four hours later by HMS Rockrose (K 51) (Lt E.J. Binfield, RNR). On 9 October, the remaining survivors were picked up by the same corvette and 21 survivors by the South African M/S whaler HMSAS Springs and taken to Capetown, arriving later that day.

Six armed guards and two crewmen from Firethorn were lost during repatriation on Zaandam, which was sunk by U-174 (Thilo) on 2 November. One crew member from Firethorn survived 82 days on a raft together with two survivors from the Dutch ship.