Black May ( 1943 ) ‘ 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 . Background . After February battles around convoys SC 118 , ON 166 , and UC 1 , Black May was the culmination of the March – May 1943 crisis in the Battle of the Atlantic . March . March had seen the U - boat offensive reach its peak , with a series of major convoy battles , first around convoys HX 228 , SC 121 , and UGS 6 ; then followed the battle for HX229 / SC122 , the largest convoy battle of the war . Allied losses for March totalled 120 ships of , of which 82 ( ) were lost in the Atlantic . The German U - boat arm ( U - Bootwaffe ) ( UBW ) lost 12 U - boats during this time . A Royal Navy report later concluded “ The Germans never came so near to disrupting communications between the New World and the Old as in the first twenty days of March 1943 ” . April . April saw some respite for the allies , as UBW was unable to maintain such a large presence in the Atlantic in April . Many of the boats heavily involved in March had withdrawn for replenishing ; nevertheless the boats still operational in the month remained active . A particular shock at the end of April was the attack by " U - 515 " on convoy TS . 37 , which saw the loss of four tankers in three minutes , and another three over the next six hours . Allied losses in April were 64 ships totalling ; 39 ships ( ) were lost in the Atlantic . UBW lost 15 boats from all causes . However , the following month saw the strategic and tactical advantage swing to the allies , where it remained for the rest of the campaign . May . May opened with the battle for ONS 5 , a hard - fought clash which saw heavy losses on both sides ; 12 ships were lost for the loss of six U - boats . But the tactical improvements of the escorts began to take effect ; The next three convoys that attacked saw seven ships sink , for the loss of seven U - boats . Finally , convoy SC 130 saw five U - boats sink ( Admiral Dönitz ' s son Peter was among those lost aboard " U - 954 " ) without loss to the convoy . Shaken by this , Admiral Dönitz ordered a retreat from the Atlantic , in order to recoup ; the U - boats were unable to return to the fray in significant numbers until autumn , and never regained the advantage . Total allied losses in May were 58 ships of , of which 34 ships ( ) were lost in the Atlantic . " Black May " . May 1943 saw the U - boat strength reach its peak , with 240 operational U - boats of which 118 were at sea , yet the sinking of Allied ships continued to decline . May 1943 also saw the greatest losses suffered by U - boats up to that time , with 41 being destroyed in May 1943 — 25 % of the operational U - boats . On 24 May 1943 , Karl Dönitz — shocked at the defeat suffered by the U - boats — ordered a temporary halt to the U - boat campaign ; most were withdrawn from operational service . May had seen a drop in allied losses coupled with a disastrous rise in U - boat losses ; 18 boats were lost in convoy battles in the Atlantic in the month , 14 were lost to air patrols ; six of these in the Bay of Biscay . With losses in other theatres , accident , or other causes , the total loss to the U - boat arm in May was 43 boats . This was the worst month for losses suffered by UBW in the war so far , nearly 3 times the loss in the previous worst month , and more boats than had been lost in the whole of 1941 . Equally significant was the loss of experienced crews , particularly the junior officers who represented the next generation of commanders . Black May signalled a decline from which UBW never recovered ; Despite various efforts over the next two years the U - boats were never able to re - establish the threat to allied shipping they had posed . . Allied success . This change was the result of a combination of the sheer numbers of Allied ships at sea , Allied air power at sea , and technological developments in anti - submarine warfare . These had been introduced over the period ; these came to fruition in May , with devastating results . Tactical and technical developments . The first and most important factor in the Allied success was that the escorts were getting better ; escort groups were becoming more skilled , and scientific analysis was producing more efficient tactics . New weapons such as the Hedgehog , and FIDO , were coming into use ; and new tactics , such as the creeping attack pioneered by Capt . " Johnnie " Walker , proved devastatingly effective . Support groups were organized , to be stationed at sea in order to reinforce convoys under attack , and to have the freedom to pursue U - boats to destruction , rather than just drive them away . The advantage conferred by Ultra , conversely , became less significant at this stage of the campaign . Its value previously had been to enable convoys to be re - routed away from trouble ; now that the escorts could successfully repel or destroy attackers there was little reason to do so . While the Admiralty baulked at using convoys as bait , out of regard for Merchant Navy morale , nonetheless there was no advantage in avoiding U - boat attacks . Air power . Over convoys , the introduction of " Very Long Range " aircraft such as the Liberator and the use of additional escort carrier s to close the air gap had a major effect in both repelling assaults 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 . Operational analysis was used here too , to improve the efficiency both of attack methods and the weapons in use . Numbers . Numbers were a factor in Allied success , though the effect was more than sheer numbers alone ; both UBW and the Allies had many more vessels operational in 1943 than they had at the beginning of the war . The Atlantic campaign was a Tonnage war ; UBW needed to sink ships faster than they could be replaced to win , and needed to build more U - boats than were lost in order not to lose . Before May 1943 , UBW wasn ’ t winning ; even in the worst months , the majority of convoys arrived without being attacked , whilst even in those that were attacked , the majority of ships got through . In HX . 229 / SC . 122 , for example , nearly 80 % of the ships arrived safely . At the start of the campaign , UBW needed to sink per month to win ; this was seldom achieved . Once the huge shipbuilding capacity of the U . S . came into play , this target leapt to per month . However , U - boat losses were also manageable ; German shipyards were producing 20 U - boats per month , while losses for most months prior to Black May were less than half that . What changed in May was that UBW started to lose ; the loss of 43 U - boats ( 25 % of the UBW operational strength ) was a major blow , and losses outstripping production became commonplace , continuing till the end of the war . German response . The Germans tried to turn the campaign in the Atlantic back in their favour by introducing tactical and technological changes . 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 defended . Although the U - boats found fewer escort ships , there were also fewer merchant ships to sink . The far - away U - boats were called the " Monsun Gruppe " . Another tactical change was to try to counter Allied air power by fighting on the surface rather than diving . When " U - 333 " came under attack from an aircraft in March 1943 , rather than diving , she stayed on the surface and shot down the attacking aircraft . It was hoped that this success could be repeated if U - boats were given better anti - aircraft defences . To facilitate this several U - boats were converted to flak U - boats ( such as " U - 441 " ) , but they proved unsuccessful . At first , the flak U - boats gave the Allies a shock but they soon welcomed attempts by U - boats to prolong their stay on the surface . Additional defences against aircraft was 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 strafing aircraft , and Allied pilots often called in surface reinforcements to deal with flak U - boats . Furthermore , the extra anti - aircraft guns caused drag when the U - boat was submerged . The " U - 333 " incident had proved to be the exception rather than the rule and the flak experiment was abandoned after six months ; the best defence for U - boats against aircraft was to dive if attacked . New technologies were also tried as a way to regain the upper hand . In mid - 1943 , two new technologies were introduced to the U - boats : the Wanze radar warning device and T5 Zaunkönig torpedoes . The Wanze warning device was designed to give U - boats advance warning of aircraft by detecting incoming radar waves so that the U - boats could dive before the aircraft started its attack run . The T5 Zaunkönig torpedoes were designed to zigzag in the hope that they would have a better chance of finding a target within a convoy . The Allies , in turn , introduced the Foxer noisemaking decoy in an attempt to defeat the acoustic homing device of the T5 torpedo . As a response , the Germans developed the T11 torpedo that was designed to ignore noisemaking decoys , but the war ended before it could be deployed . The first U - boats fitted with snorkels ( German : " Schnorchel " ) went into service in August 1943 . The snorkel was basically an extendable pipe that allowed U - boats to take in air without surfacing , allowing the U - boat ' s diesel engines to run submerged for longer periods . However , the snorkel suffered from technical problems and did not see wide use until mid - 1944 . Allied radar also became precise enough to pick up even the small target of the snorkel . The UBW also developed a radically new submarine design , the " Elektroboot " ( the Type XXI and Type XXIII boats ) . " Elektroboote " didn ' t need to surface at all during operations , however the first " Elektroboote " were commissioned too late to see combat in the war . None of the new tactics or technologies could turn the tide of war for the U - boat arm and heavy losses of U - boats continued . After May 1943 , the rate of loss of U - boats was greater than the rate at which new U - boats were commissioned , and the number of operational U - boats slowly declined .