Black May

On 24 May 1943 the German U-boat command signalled to its boats in the North Atlantic an order it had not issued at any point in the war so far: break off attacks on convoys and move south. Grand Admiral Karl Dönitz, since January Commander-in-Chief of the Kriegsmarine, who had nevertheless kept direct command of the U-boats, recorded in the war diary that losses had reached an unacceptable level and that the fight against convoys in the North Atlantic was not possible for the time being. Most of the boats were to move to an area south-west of the Azores, where weaker defences were expected, and some were to return to base.

The order closed a month that the Germans called “Black May” (Schwarzer Mai). In the space of a few weeks the U-boat arm lost more than forty boats, roughly a quarter of those operating at sea, while sinking far fewer ships than in the preceding months. After the war Dönitz wrote briefly in his memoirs: “We had lost the Battle of the Atlantic.” Historians regard May 1943 as the turning point of the Battle of the Atlantic, the longest campaign of the Second World War.

Peak strength and the beginning of defeat

In the spring of 1943 the U-boat arm was stronger than ever. In May Dönitz had some 240 operational boats, of which about 120 were at sea at any one time. In the North Atlantic he deployed them in long patrol lines named after animals and rivers – in May these included the groups Fink, Elbe, Rhein, Donau and Mosel – which cut across the routes of convoys sailing between North America and Britain. The German radio monitoring and cryptanalysis service, the B-Dienst, was still reading the British convoy cipher and often knew where convoys were to assemble and what changes of course they had been given.

In March this machine had achieved its greatest successes. In the battle for convoys HX 229 and SC 122 a wolf pack sank 22 ships, and total Allied losses that month exceeded 120 ships. At the British Admiralty there was serious debate as to whether the convoy system still worked. In April, however, while the German boats were returning to base to replenish, the Allies brought further means into the fight: very-long-range aircraft, escort carriers and support groups, which closed the air gap over the mid-Atlantic. The first major trial of strength after this change was the battle for convoy ONS 5, which began at the end of April.

ONS 5: a night in the fog

Convoy ONS 5, made up of more than forty ships sailing in ballast from Liverpool to Halifax, was escorted by the British escort group B7 under Commander Peter Gretton. From the end of April the convoy battled a gale that scattered the ships and exhausted the destroyers’ fuel; Gretton himself had to leave in his destroyer Duncan for Newfoundland, and command passed to Lieutenant Commander Robert Sherwood in the frigate Tay. The Admiralty sent two support groups, the 3rd and the 1st, but some of their ships also had to turn back for lack of fuel.

On 4 and 5 May Dönitz sent more than forty boats of the Star and Fink groups against the convoy. Within twenty-four hours the U-boats sank more than ten scattered ships, and it seemed that a repeat of the March success awaited him. On the evening of 5 May, however, the convoy entered a belt of fog over the Grand Banks of Newfoundland, where the warm Gulf Stream meets the cold Labrador Current. Visibility fell to a hundred metres. The Germans, sailing on the surface, could no longer see anything, whereas the escorts had the new centimetric radar, which could detect a submarine’s conning tower several kilometres away. On the night of 5–6 May corvettes, sloops and destroyers went out time and again after radar contacts, ramming and attacking with depth charges and Hedgehog throwers boats that had no idea where the attack was coming from. On the morning of 6 May Dönitz called off the operation.

The outcome was unprecedented. The convoy lost 13 ships, but the Germans lost six boats in the battle for it, and several others returned to base badly damaged. Until then a wolf pack had paid for success with one boat, two at most. After ONS 5 both sides understood that a well-trained escort with new equipment did not merely defend a convoy but killed its attackers.

HX 237 and SC 129: a carrier with the convoy

The following battles produced a similar result. Convoy HX 237, sailing from New York to Britain, was escorted from 7 May by the Canadian escort group C2 and a support group with the British escort carrier Biter. Her Fairey Swordfish aircraft circled over the convoy and forced the U-boats of the Rhein and Elbe groups to dive before they could reach attacking positions. The Germans sank three ships, all of them stragglers that had fallen behind the convoy, and lost three boats. One of them, U-456, was hit by a Liberator on 12 May with a new weapon – an air-dropped torpedo that homed in on the sound of propellers, known as “Fido” – in one of its first combat uses.

At the same time, 200 miles to the south-west, twelve U-boats attacked convoy SC 129. The escort commander, Commander Donald Macintyre in the destroyer Hesperus, lost two ships on 11 May, but during the night sank U-186 with depth charges and badly damaged other boats. When Biter arrived with her aircraft on 13 May, U-boat command called off the attack.

SC 130 and the death of Peter Dönitz

The change was shown most clearly by the battle for convoy SC 130, which left Halifax on 11 May with 37 ships. From 15 May it was again escorted by Gretton’s B7 group, this time at full strength, and from 19 May also by the 1st Support Group. Dönitz sent some 25 boats against the convoy, mainly from the Donau group. This time, however, aircraft appeared over the ships from the very first day: Liberators of No. 120 Squadron of RAF Coastal Command from Iceland and Hudsons from other bases. Boats that tried to overtake the convoy on the surface had to dive again and again, lost contact and ran into escorts guided by radar and radio direction-finders.

On 19 May the sloop Sennen and the frigate Jed of the support group attacked U-954 with Hedgehog projectiles. The entire crew was killed, including 21-year-old Leutnant zur See Peter Dönitz, the younger son of the U-boat commander, who was serving aboard as a watch officer. (Some recent studies attribute the sinking of U-954 to a Liberator or to other ships of the escort; this does not alter the fact that the boat was lost at convoy SC 130.) Dönitz learned of his son’s death in the course of a battle he was himself directing from his headquarters. A year later, in May 1944, his elder son Klaus, serving in a torpedo boat, was also killed.

On 20 May a Liberator of No. 120 Squadron sank U-258, and U-boat command called off the attack. Convoy SC 130 did not lose a single ship, while the Germans lost at least three boats against it and several others were damaged. Two and three days later, on 22 and 23 May, the attacks on convoys ON 184 and HX 239, escorted by escort carriers – the American Bogue and the British Archer – ended in the same way. Grumman Avenger aircraft from Bogue destroyed U-569 on 22 May, and on 23 May a Swordfish from Archer sank U-752 with rockets – the first U-boat destroyed by a rocket-armed aircraft. Not a single ship was lost from either convoy.

Why the wolf packs lost

Black May was not the work of a single invention or a single commander. It was the result of several changes that had been maturing for months and began to take effect at the same time.

The most important was air power. Very-long-range (VLR) aircraft, above all modified Liberators, flew from Northern Ireland, Iceland and, from May, also from Newfoundland, where Canadian Liberators began operating. The air gap over the middle of the ocean, in which the wolf packs had until then operated with impunity, shrank to a narrow strip, and where land-based aircraft could not reach, escort carriers appeared. An aircraft rarely sank a submarine, but by forcing it to dive it robbed it of its speed and its contact with the convoy – and without these the wolf-pack tactic ceased to work.

The second change was in the ships. Until the spring of 1943 a convoy’s escort had usually been too small to hunt a detected submarine to the finish: every escort sent off in pursuit weakened the protection of the merchant ships. Now the Admiralty, and especially the Commander-in-Chief Western Approaches, Admiral Max Horton, had support groups at its disposal – groups of destroyers, frigates and sloops, often with an escort carrier – which were responsible for no particular convoy but went wherever the U-boats were gathering and could hunt them for hours. The escort groups themselves were by now well drilled and experienced, and their commanders, such as Gretton, Macintyre and Captain Frederic John Walker, had worked out effective procedures for co-operation.

The third change was technical. Centimetric radar, made possible by the cavity magnetron, detected a surfaced boat at night and in fog, and the German Metox warning receivers could not pick it up. Shipborne HF/DF radio direction-finders allowed escorts to determine the bearing from which a U-boat was transmitting a report to headquarters – and wolf-pack tactics required it to transmit often. The Hedgehog thrower fired a salvo of charges ahead of the ship’s bow, so that the escort did not lose sonar contact at the moment of attack, as it did with conventional depth charges. Aircraft received more powerful bombs, Leigh lights for night attacks, rockets and acoustic torpedoes.

The fourth element was intelligence. From December 1942 the codebreakers at Bletchley Park were again reading the Triton cipher used by the U-boats in the Atlantic, albeit with delays and interruptions. Ultra intelligence made it possible to route convoys away from the patrol lines and to send support groups where they were needed. Throughout May the Germans continued to read the British convoy cipher, but this advantage was not enough when even a convoy that had been located could defend itself. In any case the Admiralty changed the cipher in June 1943, and from then on the B-Dienst practically lost its insight into convoy movements.

To this was added the air offensive over the Bay of Biscay, through which the U-boats sailed from and returned to their bases in France. In May Coastal Command aircraft sank several boats there and forced many others to turn back. Behind it all, finally, lay industrial superiority: American shipyards were building ships faster than the U-boats could sink them, and the tonnage war on which Dönitz had based his strategy was in practice already lost.

24 May: the retreat

After the defeats at SC 130, ON 184 and HX 239 Dönitz had no illusions. On 24 May he ordered the boats in the North Atlantic to move to the area south-west of the Azores, where they were to attack convoys sailing from the United States to the Mediterranean; boats short of fuel returned to base. Only a few boats remained on the convoy routes between America and Britain, with orders to keep up lively radio traffic so that the Allies would believe the wolf packs were still waiting there. The command recorded in its war diary that this was a pause, and that the boats would return to the North Atlantic once they received new weapons: better anti-aircraft armament, receivers to warn of the new radars and acoustic torpedoes for use against the escorts.

Dönitz went to report to Hitler. According to the record of their conference of 31 May, he declared that the U-boat war must go on even without its former successes, because it tied down enormous Allied forces that would otherwise be used against the Reich. Hitler agreed. The retreat was thus meant to be temporary, but in reality it marked the end of the period in which the U-boats could threaten to cut Britain’s supply lines.

The frequently encountered statement that Dönitz withdrew most of the U-boats “from the Atlantic” needs qualifying: what was involved was a withdrawal from the North Atlantic, from the main convoy route between North America and the British Isles. The boats continued to operate in the central and South Atlantic, and further afield in the Indian Ocean.

The month’s balance sheet

Loss figures vary depending on the source and the method of counting. The American historian Clay Blair gives 41 boats lost in May; other tallies, which include accidents and boats lost to unknown causes, give 43 – and this is also the figure most often quoted. Fewer than half of them were lost in convoy battles; a dozen or so fell victim to aircraft while on patrol, several of them over the Bay of Biscay. An estimated almost two thousand sailors were killed or captured. This was twice the losses of any previous month, and more than the shipyards could replace: in 1943 they were commissioning about 20 boats a month.

On the Allied side the picture was the reverse. In the Atlantic the U-boats sank some 34 ships of about 134,000 gross register tons in May – and this is the figure usually set against the German losses. On all seas submarines sank some 50 ships of about 265,000 GRT, and including the victims of aircraft, mines and surface ships the Allies lost some 58 ships of almost 300,000 GRT in May. In March the figure had been more than twice as high. For the first time in the war the Germans were losing roughly one submarine for every ship sunk in the main theatre – whereas in earlier years the rate of exchange had been a dozen or more ships, and in the best periods several dozen, for each boat.

Place in the war

Black May ended the crisis of the Battle of the Atlantic, which had lasted since the autumn of 1942 and reached its peak in March 1943. In June almost no convoy was attacked in the North Atlantic, and ships crossed between America and Britain without loss. The U-boats that moved to the Azores area ran into American escort-carrier groups there in the summer and suffered further heavy losses. In the autumn of 1943 Dönitz sent them back to the North Atlantic with new anti-aircraft armament and Zaunkönig acoustic torpedoes, but this attempt, although it cost the lives of many escort sailors, did not bring a return to the successes of 1940–1942. Later, boats with snorkels, which allowed them to charge their batteries underwater, came into service, and towards the end of the war new electric boats, but they came too late and in too small numbers.

The defeat of the U-boats mattered far beyond the ocean. A safe route across the Atlantic meant that American troops, equipment, fuel and food needed for the bombing offensive against the Reich and for the invasion of Western Europe in 1944 could reach Britain unhindered. Convoys also sailed on to the Mediterranean and to the ports of northern Russia. The Kriegsmarine waged submarine warfare until the last days of the war and lost in it, in all, almost three-quarters of the more than 40,000 men of the U-boat arm, but after May 1943 it could no longer win the tonnage war. For Dönitz, who in January had taken command of the whole navy and staked everything on the U-boat war, May was a personal defeat – a double one, since his son was also killed that month.