U-558 was operating in the Windward passage during her fourth patrol.  On the evening of May 21, 1943 she attacked the American tanker William Boyce Thompson near the Pedro Bank, south of Jamaica. The tanker was hit by two torpedoes and stopped.  The U-boat then used her deck gun in an attempt to finish off the vessel.  PCs from Guantanamo Bay arrived and the boat was forced to withdraw.