At 08.00 hours on 15 Oct 1941, U-553 entered convoy SC-48 on the surface from ahead between column 7 and 8 and at 08.15 hours sank the Silvercedar in station #72 with the first torpedo, while the second torpedo fired two minutes later missed the intended target but was observed to hit a ship behind it, a claim that is not confirmed by Allied reports. Due to a misunderstanding, the torpedo data computer was no longer set correctly and the remaining three torpedoes were fired with wrong settings. However, the third torpedo missed its target and hit the Ila in station #74 at 08.23 hours. The U-boat then passed ahead of the British motor merchant Silverelm in station #83, which unsuccessfully tried to ram her and missed this ship with the stern torpedo. Eventually Thurmann fired the last bow torpedo at W.C. Teagle in station #103, missing her due to the wrong settings and passed behind her stern to outrun convoy on its starboard bow. The U-boat had been observed by at least three ships in convoy but they could not open fire due to the danger of hitting other ships and the two corvettes on the starboard side did not spot the enemy.

The Silvercedar (Master Thomas Keane) had originally been in station#73 but changed to #72 after the ship ahead of her straggled from convoy. She was hit by one torpedo on the starboard side, just forward of the engine room and sank after about 7 minutes on an even keel. Both starboard lifeboats had been destroyed by the explosion that had blown off the #3 hatch and much debris into the air, but the crew managed to launch one of the port boats and a raft. The seven men on the raft were soon rescued by the lifeboat, along with three survivors from Ila swimming in the water. Later the lifeboat capsized but all 29 occupants were picked up by FFL Mimosa (K 11) (Cdr R.R. Birot, FNFL) at 09.30 hours. 19 crew members and seven gunners (the ship was armed with one 4in, one Bofors AA gun and four machine guns) were landed at Reykjavik on 20 October. The master, 17 crew members, two gunners and one passenger (DBS) were lost.