On 16 July 1943, U-306 shadowed convoy SL-133 from grid EK79 to EK49 and reported two days later four ships of 27.000 grt sunk and one other ship of 5000 grt probably sunk in two attacks. The first attack was carried out at 03.52 hours, firing five single torpedoes and reporting one ship sunk, one probably sunk and three misses due to the great distance. In fact, only the Kaipara was damaged in this attack.
At 08.01 hours, the U-boat attacked a second time and fired two spreads of two torpedoes and two minutes later the stern torpedo. Trotha observed three hits and could not see the targets anymore, but none of the reported hits are confirmed by Allied reports.