1939 1940 1941 1942 1943 1944 1945 Black May refers to a period (May 1943) in the Battle of the Atlantic campaign during World War II, when the German U-boat arm (U-Bootwaffe) suffered high casualties with fewer Allied ships sunk; it is considered a turning point in the Battle of the Atlantic.

The tactical improvements of the escorts began to take effect; the next three convoys that were attacked resulted in just seven ships sunk and an equal number of U-boats.

This month had the most losses suffered by the U-boat Arm in the war so far, nearly three times the number of the previous highest, and more boats than had been lost in the whole of 1941.

New weapons such as the Hedgehog, and Mark 24 FIDO Torpedo, were coming into use, and new tactics, such as the creeping attack pioneered by Captain "Johnnie" Walker, proved devastatingly effective.

Support groups were organised, to be stationed at sea, ready to reinforce convoys under attack, and to have the freedom to pursue U-boats to destruction, rather than just drive them away.

The Admiralty balked at using convoys as bait, out of regard for Merchant Navy morale but there was considerably more safety in sailing through U-boat patrolled waters by mid-1943.

Over convoys, the introduction of "very long range" aircraft, such as the Liberator, and the use of additional escort carriers to close the air gap had a major effect in defeating attacks and destroying U-boats.

The re-introduction of air patrols over the Bay of Biscay by long range Beaufighters and Mosquitoes, to attack boats as they came and went from base, also began to take effect at this stage of the conflict.

At the start of the campaign, the UBW needed to sink 700,000 long tons (710,000 t) per month to win; this was seldom achieved.

The first tactical change saw U-boats starting operations in new waters, such as the Indian Ocean, in the hope that their targets would be less well-defended.

Additional defences against aircraft were offset by the U-boat having to remain on the surface longer, increasing the chance of the submarine's pressure hull being punctured.

The gunners' effectiveness was limited by the lack of protection from return-fire and Allied pilots often called on ships to deal with flak U-boats.

The U-333 incident had proved to be an exception, and the experiment was abandoned after six months; the best defence for U-boats against aircraft was to dive if attacked.

The Allies introduced the Foxer noisemaking decoy in an attempt to defeat the acoustic homing device of the T5 torpedo.