At 06.24 hours on 11 June 1944, U-539 fired one torpedo at the unescorted and unarmed Casandra (Master Theodore Meimen) west of Curaçao and heard a detonation which was not noticed by the tanker. The U-boat surfaced to overtake the ship, but was spotted by a lookout who alerted the vessel and made it possible to evade a spread of two FAT torpedoes fired at 07.31 hours. Seven minutes later, U-539 fired a Gnat torpedo and observed a hit after 11 minutes, but apparently the torpedo detonated in the wake on the port side aft and caused no damage. After a half an hour the U-boat opened fire with the 37mm AA gun, fired one T-3 torpedo at 09.36 hours and ceased the attack when the gun malfunctioned. Casandra alerted the nearby naval station by radio, evaded the torpedo by zigzagging at full speed and escaped slightly damaged after being hit by three rounds, arriving safely at Willemstad later that day. Two crew members had been wounded by shell splinters and the second mate jumped overboard when the U-boat began to shell the vessel, but he was picked up after about five hours by USS SC-1309.