On 16/17 October 1941, HMS Gladiolus (K 34) (LtCdr H.M.C. Sanders, DSO, DSC, RD, RNR) was reported missing after a night action fought between five U-boats and the escorts of convoy SC-48. She was on her station astern of the port wing of convoy when the U-boat attacks began and disappeared with all hands. The commander, four officers, 59 ratings and one survivor from Empire Heron picked up one night earlier were lost.

Between 00.00 and 00.05 hours on 17 October, U-553 fired three carefully aimed single torpedoes at three different tankers in the port wing of convoy SC-48, but missed. Thurmann assumed that the torpedoes ran too deep, so the fourth torpedo was set to 2 meters depth and fired at a fourth tanker which was seen to explode in a ball of fire after being hit amidships. More likely is that all torpedoes missed their targets because he had underestimated the speed of convoy, but the last apparently hit HMS Gladiolus by chance astern of convoy, igniting her magazine and causing her to sink immediately with all hands.