At 07.35 hours on 23 Feb 1943, U-186 fired a spread of three torpedoes at convoy ON-166 about 310 miles south of Cape Race and observed one hit after 2 minutes 32 seconds on the ship in station #12, the Hastings, which was erroneously reported as Hassop. A second hit, after 2 minutes 35 seconds was possibly on the same ship, which sank within seven minutes. After 2 minutes 41 seconds a third hit was heard but not observed.
At 07.40 hours, U-186 fired a spread of two stern torpedoes and heard two detonations on different targets, but only the Eulima (Master Frederick William Wickera) was hit, the second detonation being a depth charge from USCGC Spencer (WPG 36). At 11.30 hours, the drifting wreck of the tanker was sunk by a coup de grâce and gunfire from U-186.

One lifeboat with 15 survivors from Eulima was encountered by U-409 (Massmann) at 18.45 hours on 24 February. The Germans took the second and third officers as prisoners aboard and provided the remaining British and Chinese men with water, food, a navigation chart and the course to the nearest land. However, they were never seen again: the master, 51 crew members and nine gunners were lost. Both prisoners were landed at Brest on 12 April and taken the POW camp Marlag und Milag Nord.