At 22.43 hours on 30 March 1942 the Muncaster Castle (Master Harold William Harper) was hit by a two torpedo spread from U-68 south-southwest of Monrovia. The U-boat earlier observed how the ship was dispersed from convoy ST-18 at 13.35 hours and followed the heavy zigzagging ship, running at 13 knots, until she stopped zigzagging in the darkness. The vessel settled immediately after the hits but did not sink and Merten had to use his last torpedo as a coup de grâce at 23.16 hours. After the ship sank by the bow, the Germans observed more than ten lifeboats and questioned the survivors in one of them before leaving the area on home course.
Four crew members, one gunner and 19 passengers were lost. The master, 69 crew members, eleven gunners, two naval signalmen and 246 passengers were picked up by HMS Aubretia (K 96) (LtCdr V.F. Smith, DSO, RD, RNR) and the Greek steam merchant Ann Stathatos and landed at Freetown.

The master, Harold William Harper, was awarded the Lloyds War Medal for bravery at sea.