Black May (1943)

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Black May
Part of Battle of the Atlantic
The Royal Navy during the Second World War A4570.jpg
Depth charges exploding from the destroyer HMS Vanoc during an Atlantic Convoy in May 1943
Date29 April – 24 May 1943
Result Decisive Allied victory
Flag of the United Kingdom.svg United Kingdom Flag of Germany (1935-1945).svg  Nazi Germany
Commanders and leaders
Naval Ensign of the United Kingdom.svg Sir Max K. Horton
Naval Ensign of the United Kingdom.svg Sir John Slessor
War Ensign of Germany 1938-1945.svg Admiral Karl Dönitz
Royal Navy
Merchant navy
RAF Coastal Command

240 operational U-boats

  • 118 at sea
Casualties and losses
Allies lost 58 ships in the same period, 34 of these (totalling 134,000 tons) in the Atlantic

43 U-boats were destroyed in May, 34 in the Atlantic


  • 25% of operational U-boats
37 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.

Battle of the Atlantic longest continuous military campaign in World War II

The Battle of the Atlantic was the longest continuous military campaign in World War II, running from 1939 to the defeat of Nazi Germany in 1945, and was a major part of the Naval history of World War II. At its core was the Allied naval blockade of Germany, announced the day after the declaration of war, and Germany's subsequent counter-blockade. It was at its height from mid-1940 through to the end of 1943.

World War II 1939–1945 global war

World War II, also known as the Second World War, was a global war that lasted from 1939 to 1945. The vast majority of the world's countries—including all the great powers—eventually formed two opposing military alliances: the Allies and the Axis. A state of total war emerged, directly involving more than 100 million people from over 30 countries. The major participants threw their entire economic, industrial, and scientific capabilities behind the war effort, blurring the distinction between civilian and military resources. World War II was the deadliest conflict in human history, marked by 50 to 85 million fatalities, most of whom were civilians in the Soviet Union and China. It included massacres, the genocide of the Holocaust, strategic bombing, premeditated death from starvation and disease, and the only use of nuclear weapons in war.

<i>Kriegsmarine</i> 1935-1945 naval warfare branch of Germanys armed forces

The Kriegsmarine was the navy of Nazi Germany from 1935 to 1945. It superseded the Imperial German Navy of the German Empire (1871–1918) and the inter-war Reichsmarine (1919–1935) of the Weimar Republic. The Kriegsmarine was one of three official branches, along with the Heer (Army) and the Luftwaffe of the Wehrmacht, the German armed forces from 1933 to 1945.

U-995 Type VIIC at the German navy memorial at Laboe U995 2004 1.jpg
U-995 Type VIIC at the German navy memorial at Laboe


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.

Convoy SC 118

Convoy SC 118 was the 118th of the numbered series of World War II Slow Convoys of merchant ships from Sydney, Cape Breton Island to Liverpool. The ships departed New York City on 24 January 1943 and were met by Mid-Ocean Escort Force Group B-2 consisting of V-class destroyers Vanessa and Vimy, the Treasury-class cutter Bibb, the Town-class destroyer Beverley, Flower-class corvettes Campanula, Mignonette, Abelia and Lobelia, and the convoy rescue ship Toward.

Convoy ON 166

Convoy ON 166 was the 166th of the numbered ON series of merchant ship convoys Outbound from the British Isles to North America. Sixty-three ships departed Liverpool 11 February 1943 and were met the following day by Mid-Ocean Escort Force Group A-3 consisting of the Treasury-class cutters Campbell and Spencer and the Flower-class corvettes Dianthus, Chilliwack, Rosthern, Trillium and Dauphin.


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.

HX 228 was a North Atlantic convoy of the HX series which ran during the Battle of the Atlantic in World War II. It was one of a series of four convoy battles that occurred during the crisis month of March 1943 and is notable for the loss of the Escort Group leader Commander AA "Harry" Tait.

Convoy SC 121

Convoy SC 121 was the 121st of the numbered series of World War II Slow Convoys of merchant ships from Sydney, Cape Breton Island to Liverpool. The ships departed New York City 23 February 1943; and were met by the Mid-Ocean Escort Force Group A-3 consisting of the United States Coast Guard (USCG) Treasury-class cutter USCGC Spencer, the American Wickes-class destroyer USS Greer, the British and Canadian Flower-class corvettes HMS Dianthus, HMCS Rosthern, HMCS Trillium and HMCS Dauphin and the convoy rescue ship Melrose Abbey. Three of the escorts had defective sonar and three had unserviceable radar.

Allied losses for March totalled 120 ships of 693,000 long ton s (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.

Long ton, also known as the imperial ton or displacement ton, is the name for the unit called the "ton" in the avoirdupois system of weights or Imperial system of measurements. It was standardised in the thirteenth century and is used in the United Kingdom and several other British Commonwealth of Nations countries alongside the mass-based metric tonne defined in 1799.

Tonne metric unit of mass

The tonne, commonly referred to as the metric ton in the United States and Canada, is a non-SI metric unit of mass equal to 1,000 kilograms or one megagram. It is equivalent to approximately 2,204.6 pounds, 1.102 short tons (US) or 0.984 long tons (UK). Although not part of the SI, the tonne is accepted for use with SI units and prefixes by the International Committee for Weights and Measures.

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". [1]

Royal Navy Maritime warfare branch of the United Kingdoms military

The Royal Navy (RN) is the United Kingdom's naval warfare force. Although warships were used by the English kings from the early medieval period, the first major maritime engagements were fought in the Hundred Years War against the Kingdom of France. The modern Royal Navy traces its origins to the early 16th century; the oldest of the UK's armed services, it is known as the Senior Service.


April saw some respite, 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.

German submarine <i>U-515</i> German world war II submarine

German submarine U-515 was a Type IXC U-boat of Nazi Germany's Kriegsmarine built for service during World War II. She was commissioned in 1942 and sunk in 1944. U-515 completed six operational patrols and sank 23 ships, badly damaged two ships which later sank, and damaged two additional ships.

TS 37 was a South Atlantic convoy of the TS series which ran during World War II from Takoradi to Freetown. It lost seven ships in "one of the most remarkable convoy attacks of the war."

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. 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 opened with the battle for ONS 5, a hard-fought clash which saw heavy losses on both sides; 13 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. [2]

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 299,000 long tons (304,000 t), of which 34 ships (134,000 long tons (136,000 t)) 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, [3] 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. [4]

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.

Cause of LossNumber lost
Shore-based Aircraft14
Ship-based Aircraft2
Ship + Shore-Based Aircraft4
Ship + Ship-Based Aircraft1
Other Causes1
Bombing Raid3
(Raised and Re-commissioned)
Total Lost43

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 balked at using convoys as bait, out of regard for Merchant Navy morale, 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 carriers 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 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 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. 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.


  1. Roskill, p. 375.
  2. Blair, Clay (1998). Hitler's U-Boat War, The Hunted 1942–1945. Random House. pp. 333–334. ISBN   0-679-45742-9.
  3. Miller, p. 126.
  4. Stern, p. 7.

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