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 U‑boat losses climbed to 15 vs. 39 Allied kills (235,000 tons). In May 1943 U-boat num­bers reached a peak, with 240 U‑boats opera­tional, of which 118 were at sea. But on this date in 1943, after losing 43 U‑boats in con­voy battles with escort ships (18 U‑boats), ship- and shore-based air patrols (21 boats), 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.” U‑boats returned to the Atlan­tic during the last four months of 1943, but they never regained their former domi­nance, sty­mied in part by the loss of ex­peri­enced crews but even more so by the Allies’ in­creas­ingly sophis­ti­cated anti­sub­marine wea­ponry. 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 the 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. By D-Day, June 6, 1944, the date the Allies returned to France, U‑boats had become irrele­vant to the outcome of the war.

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Important Allied Technological Advances in Antisubmarine Warfare

Convoy PQ-17 assembling in Iceland, mid-1942Mk 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 Ger­man U‑boats and planes in 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.

Hedgehog antisubmarine mortarWing-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