May of 1943 was the turning point in the Battle of the Atlantic, during which the Third Reich lost 40 U-boats which constituted 25% of its operational strength at the time.
Germany never recovered from this disaster and Dönitz withdrew all but token number of U-boats from battle. This month has been called "Stalingrad of the U-boat arm" by some.
40 U-boats lost during May 1943 (1784 men died and 183 men survived those losses).
+ means that the boat was lost with all hands.