The Mediterranean U @-@ boat Campaign lasted approximately from 21 September 1941 to May 1944 . The Kriegsmarine tried to isolate Gibraltar , Malta , and Suez and disrupt Britain 's trade routes . More than sixty U @-@ boats were sent to interdict Allied shipping in the Mediterranean Sea . Many of these U @-@ boats were themselves attacked negotiating the Strait of Gibraltar controlled by Britain . Nine U @-@ boats were sunk while attempting passage and ten more were damaged .