Berlin, Germany May 24, 1943

In June 1942 German submarines sank 637,000 tons of British shipping—a greater total than in any pre­vious or sub­se­quent month. So many prowling U‑boats made it hard for mer­chant con­voys sailing in the major trans-Atlantic traffic lanes to evade detec­tion. The next year, in March, Atlantic U‑boats sank 82 ships (476,000 tons) out of 120 ships sunk world­wide; U-boat losses were 12. In April 1943 losses to Atlantic U‑boats climbed to 15 (235,000 tons) vs. 39 Allied kills. In May 1943 U‑boat num­bers reached a peak, with 240 U‑boats operational, of which 118 were at sea.

On this date, May 24, 1943, after losing 43 U-boats in con­voy battles with escort vessels and small escort air­craft carriers based on merchant-ship-sized hulls (18 U‑boat kills), ship- and shore-based air patrols (21 boat kills), and acci­dents—representing a fifth of his fleet during “Black May”—Grand Admiral Karl Doenitz, com­mander-in-chief of the Kriegs­marine, with­drew almost all U‑boats from the North Atlantic. He is reputed to have said, “We have lost the Battle of the Atlantic.” (Doenitz also lost his son, Peter, during “Black May” when two ships of the Royal Navy sank U‑954.) To Doenitz it became clear that his goal of sinking more trans-Atlantic mer­chant ships than the Allies could build would never be met. (The U.S. alone was turning out stan­dard­ized, easy-to-manu­facture mer­chant ships called “Liberty Ships,” some­times three a day.) British Prime Minister Winston Churchill wrote of the summer of 1943: “The convoys came through intact, the supply line was safe.” U‑boats returned to the Atlan­tic during the last four months of 1943, but they never regained their former super­i­ority, stymied in part by the loss of experi­enced officers and crews but even more so by the Allies’ increasingly sophis­ti­cated anti­sub­marine weaponry, convoy escort leadership, convoy crew training, and the sheer number of convoy merchant ships.

Technological advances progressively intro­duced in the 45 months since the start of the war came to frui­tion in May 1943 with devas­tating results. At this stage in their anti-U‑boat cam­paign, the tech­no­logical advances reduced the signi­fi­cance of the remark­able achieve­ments the Allies had made in un­locking the secrets of the German naval Enigma coding/­decoding ma­chine and using the resulting intel­li­gence to huge advan­tage. (Ultra, as the intel­li­gence was called, had the very prac­ti­cal bene­fit of allowing Allied con­voys to evade U‑boat patrols, plus it guided hunter-killer forces to the location of U‑boats at sea.)

The Germans lost the technological race to new wea­pons such as the multi­head, con­tact-fused Hedge­hog. A sonar-equipped ship could tail a U‑boat and lob Hedge­hog mor­tars over its bow; the Squid could be fired forward or to the side. Together these wea­pons raised the per­cent­age of kills by sur­face ships from 1.6 per­cent for con­ven­tional depth charges to nearly 20 per­cent, the suc­cess rates rising over time. Morale among the remaining U‑boat crews sank to rock bottom, as a further 54 U‑boats were lost in the first quarter of 1944. By D-Day, June 6, 1944, the date the Western Allies returned to Nazi-occupied France, U‑boats had become irrelevant to the outcome of the war.

Important Allied Technological Advances in Antisubmarine Warfare During the Battle of the Atlantic (19391945)

Battle of the Atlantic: Convoy PQ-17 assembling in Iceland, mid-1942 Battle of the Atlantic: Mk VII depth charge

Left: Escorts and merchant ships of Convoy PQ‑17 at Hval­fjord (Hvalfjörður), Iceland, before sailing north to the ports of Mur­mansk and Arch­angel in July 1942. The route was the shortest for sending supplies to the Soviet Union, but it was also the most dan­gerous due to large con­cen­tra­tions of U‑boats and planes in German-occu­pied Norway. PQ‑17 was dec­i­mated during a series of heavy enemy day­light attacks that lasted a week, losing 24 of its 35 mer­chant ships. Just eleven months later, Doenitz with­drew his U‑boats tem­po­rarily from the North Atlan­tic, attrib­uting the major­ity of his May 1943 losses to the “supe­ri­ority of enemy loca­tion instru­ments and the sur­prise from the air which is possible because of that.”

Right: An antisubmarine Mk VII depth charge being loaded onto a Mk IV depth charge thrower on board the Flower-class cor­vette HMS Dian­thus. Most Mk VII depth charges used con­ven­tional explo­sives and a fuse set to go off at a preselected depth in the ocean.

Battle of the Atlantic: Hedgehog antisubmarine mortar Battle of the Atlantic: Wing-mounted Leigh light

Left: The Hedgehog was a 24-barreled anti­sub­ma­rine mor­tar mounted near the bow of a ship. The Hedge­hog fired con­tact-fused bombs ahead of the ship when the tar­get was with­in sonar (ASDIC) range. Unlike depth charges, which were launched behind and to the sides of the attacking ship, dis­turbing the water and making it hard to track the tar­get because sonar con­tact was inter­rupted, Hedge­hog charges exploded only on im­pact. Hedge­hogs allowed the attacking ship to change course and maintain contact as the target maneuvered to avoid its pursuer.

Right: A powerful searchlight, the British Leigh light was auto­ma­tically aligned with an air­craft’s radar to illu­mi­nate tar­gets on the ocean’s sur­face in the final stages of an attack run. A mile or so from the tar­get, the search­light would auto­ma­tically come on, giving the tar­get a five-second warning before it was hit by depth charges. Owing to the intro­duc­tion of this device in June 1942, Allied monthly shipping losses dropped from 600,000 to 200,000 tons.

Black May, 1943: Beginning of the End of the U‑Boat Scourge