At 21.40 hours on 3 Nov 1940, U-99 torpedoed the unescorted Casanare west of Bloody Foreland. Her distress messages brought the armed merchant cruisers HMS Laurentic (F 51) (Capt E.P. Vivian (Retired), RN) and HMS Patroclus (Capt G.C. Wynter (Retired), RN) to the scene and the U-boat began a dramatic battle at 22.50 hours when the first torpedo struck the engine room of HMS Laurentic (F 51) from a distance of 1500 metres. At 23.28 hours, a second torpedo hit the vessel, but did not explode. A third torpedo was fired at 23.37 hours from a distance of 250 metres into the hole opened by the first torpedo, at this time the lookouts spotted the U-boat on the surface and Kretschmer had a hard time in evading the gunfire.

In the meantime, HMS Patroclus began picking up survivors instead of participating in the fight against the U-boat and her lookouts did not see U-99 only 300 metres away. A first torpedo struck the ship at 00.02 hours, a second at 00.22 hours and a third at 00.44 hours. 14 minutes later, the U-boat opened fire with the deck gun and hit with two of the four fired rounds, before Kretschmer had again to evade the gunfire and hit her with a fourth torpedo at 01.18 hours.

After that, U-99 searched for the Casanare to give the crew time for reloading the torpedo tubes, but only found two lifeboats at her position and questioned the survivors, the vessel had foundered in the meantime. At 02.39 hours, a Sunderland flying boat suddenly appeared over the U-boat, which had to dive, but no bombs were dropped. At 04.04 hours, the U-boat surfaced after reloading the torpedoes, went back to the auxiliary cruisers at high speed and fired at 04.53 hours a coup de grâce from a distance of 250 metres at HMS Laurentic (F 51). The torpedo struck the stern and ignited the depth charges stored there, causing the ship to sink by the stern within minutes. Two officers and 47 ratings were lost.

Around this time a destroyer was spotted and Kretschmer had to sink HMS Patroclus in a short time. A fifth torpedo at 05.16 hours had no significant effect, but the sixth torpedo at 05.25 hours broke the ship in two, the stern capsized and the bow sank slowly. The commander, six officers and 49 ratings were lost. After that, U-99 was attacked by HMS Hesperus (H 57) (LtCdr D.G.F.W. MacIntyre, RN), but the destroyer soon left the U-boat to pick up the commander, 51 officers and 316 ratings from HMS Laurentic (F 51). 33 officers and 330 ratings from HMS Patroclus were picked up by HMS Beagle (H 30) (LtCdr R.H. Wright, RN) and landed at Greenock.