Black May
Part of The Battle of the Atlantic
Depth charges exploding from the destroyer HMS Vanoc during an Atlantic convoy in May 1943
Date 29 April – 24 May 1943 Location Atlantic Ocean Result British victory
Belligerents
United Kingdom  Canada	Germany
Commanders and leaders
Max Horton John Slessor	Karl Dönitz
Strength
Royal Navy Merchant navy	240 submarines
Casualties and losses
58 ships sunk	43 submarines destroyed 37 submarines damaged

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.

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.

The U-boat offensive reached its peak in March, with a series of major convoy battles, first around convoys Convoy HX 228, Convoy SC 121, and Convoy UGS 6; then followed the battle for Convoy HX 229/Convoy SC 122, the largest convoy battle of the war. Allied losses for March totalled 120 ships of 693,000 long tons (704,000 t), of which 82 (476,000 long tons (484,000 t)) were lost in the Atlantic. The German U-boat arm (U-Bootwaffe) (UBW) lost 12 U-boats during this time. Post-war, the official Royal Navy history - by Capt. Stephen Roskill - 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."^[1]

Some respite for the Royal Navy came in April, as the UBW was unable to maintain as many U-boats in the Atlantic. Many of the boats involved in March had withdrawn for replenishing; nevertheless the boats still operational in the month remained active.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 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 345,000 long tons (351,000 t); 39 ships (235,000 long tons (239,000 t)) were lost in the Atlantic. The UBW lost 15 boats from all causes. In the following month, the strategic and tactical advantages swung to the Allies, where it remained for the rest of the campaign.

In May 1943, U-boat strength reached its peak, with 240 operational U-boats of which 118 were at sea, yet the sinking of Allied ships continued to decline.^[2] May 1943 also had the greatest losses suffered by U-boats up to that time, with 41 being destroyed, 25 per cent of the operational U-boats.^[3] The month opened with the battle for Convoy ONS 5, which was costly for both sides: 13 merchant ships and six U-boats. 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. Finally, five U-boats were sunk attacking convoy SC 130, with Admiral Dönitz's son Peter among those lost aboard U-954, while no convoy ships were lost.^[4]

Total Allied losses in May were 58 ships of 299,000 long tons (304,000 t), of which 34 (134,000 long tons (136,000 t)) were lost in the Atlantic. 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. The U-boats were unable to return to the fray in significant numbers until autumn, and never regained the advantage. During May there had been a drop in Allied losses, coupled with a tremendous 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, accidents, or other causes, the total loss to the U-boat arm in May was 43 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. 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 the U-boat arm 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 previously posed.

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 and came to fruition in May, with devastating results.

The 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 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 advantage conferred by Ultra, conversely, became less significant at this stage of the campaign. Its value had been to enable convoys to be re-routed away from trouble, but now the escorts could repel or destroy attackers. 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. Operational analysis was used here, too, to improve the efficiency of attack methods and the weapons in use.

Numbers were a factor in Allied success, though the effect was more than that; the 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; the 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, the UBW was not winning; even in their worst months, the majority of convoys arrived without being attacked, while even in those that were attacked, the majority of ships got through. In Convoy HX 229/Convoy SC 122, for example, nearly 80 per cent of the ships arrived. 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. Once the huge shipbuilding capacity of the U.S. came into play, this target leapt to 1,300,000 long tons (1,300,000 t) per month. 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 the UBW started to lose; the loss of 43 U-boats (25 per cent of the UBW operational strength) was a big defeat and losses outstripping production became commonplace until the end of the war.

The Germans replied 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 well-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, it 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. Several U-boats were converted to flak U-boats but they proved unsuccessful. At first, the flak U-boats gave the Allies a shock but they soon welcomed attempts by U-boats to 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 return-fire and Allied pilots often called on ships to deal with flak U-boats. The extra anti-aircraft guns caused drag when the U-boat was submerged. The U-333 incident had proved to be the exceptional and the experiment was abandoned after six months; the best defence for U-boats against aircraft was to dive if attacked.

In mid-1943, the Wanze (Bug) radar warning device and T5 Zaunkönig (Wren) torpedoes were introduced. Wanze was designed to give U-boats advance warning of aircraft by detecting radar waves so that the U-boats could dive before the aircraft could attack. The Zaunkönig torpedoes were designed to zig-zag in the hope that they would have a better chance of finding a target within a convoy. The Allies introduced the Foxer noisemaking decoy in an attempt to defeat the acoustic homing device of the T5 torpedo. The Germans developed the T11 torpedo that was designed to ignore noisemaking decoys, but the war ended before it came into use. The first U-boats fitted with snorkels (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. The snorkel suffered from technical problems and did not see wide use until mid-1944. Allied radar also became precise enough to pick up the snorkel.

The UBW also developed a radically new submarine design, the Elektroboot (the Type XXI and Type XXIII boats). Elektroboote did not 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 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.

• Miller, David. U-Boats: the Illustrated History of the Raiders of the Deep. Washington: Brassey's Inc, 2000.
• Neistle, Axel: German U-boat Losses during World War II (1998). ISBN 1-85367-352-8
• Roskill, S. W. (1956). The Period of Balance. History of the Second World War United Kingdom Military Series: The War at Sea 1939–1945. Vol. II. London: HMSO. OCLC 174453986 – via Hyperwar Foundation.
• Stern, Robert C. U-Boats in action. Squadron/Signal pub., 1977.
• Vat, Dan van der (1988). The Atlantic Campaign. Harper & Row. ISBN 0-06-015967-7 – via Archive Foundation.