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H-019-4: "Black May"—The Tide Turns in the Battle of the Atlantic

German U-boat U-118 attacked and sunk 12 June 1943 by aircraft from USS Bogue (ACV-9) (80-G-68694).

H-Gram 019, Attachment 4
Samuel J. Cox, Director NHHC
June 2018 

There were many factors that caused the sudden turn of fortune in the Allies’ favor during May 1943, and to say that any one of them was the decisive factor would not be accurate. However, to briefly re-cap the Battle of the Atlantic prior to May 1943, the battle had see-sawed throughout the course of the war to that point. In the months after the outbreak of World War II in September 1939, German U-boats had great success (the “First Happy Time”), but an insufficient number of submarines at the start of the war was a major factor in Great Britain’s ability to survive the first year of the conflict despite heavy losses of both merchant ships and warships. However, effective use of convoys, improved ASW sensors, weapons, and tactics enabled the British to decrease their losses and make the U-boats pay a higher price. Nevertheless, the period 1940 to 1941 was very desperate for the British as the island nation faced acute shortages of just about everything, including fuel oil and food, which at times reached critical levels.

When the United States entered the war after Japan attacked Pearl Harbor, Germany immediately (unlike in World War I) sent U-boats to attack shipping right off the U.S. east coast. With the United States unprepared for the onslaught of “Operation Drumbeat,” the result was unprecedented carnage at sea. For the U-boats, the first months of 1942 were the “Second Happy Time.” For the Allies, they were a near-disaster that cost more lives, and many more ships, than Pearl Harbor. As the United States got its act together along the eastern seaboard in the late spring of 1942, the Germans shifted their operations to the Gulf of Mexico and the Caribbean, with initial great success. In the summer of 1942, Arctic convoy PQ-17 was also a disaster in which most of the ships carrying critical supplies to Russia were lost. (Although it is true that the amount of war material produced by the Soviet Union by itself dwarfed that provided to the Soviet Union by the United States and United Kingdom, the material that did get through—at great cost—came at a critical time when the Soviet Union was hanging on by a thread. Had it not been for the Allied supplies, the Soviet Union might not have survived long enough to build the hordes of tanks that eventually took the war to Berlin.)

Throughout late 1942 and early 1943, convoys fought their way across the Atlantic, and although U-boat losses increased, they were still manageable and the submarines were still inflicting serious losses on the convoys. However, increasingly capable and longer-range Allied air cover made operating in certain areas extremely dangerous for the U-boats, and the mid-Atlantic gap, where convoys were most vulnerable because they had no air cover, was getting smaller and smaller. The peak of pitched Atlantic convoy battles occurred in March 1943, when U-boats sank 567,000 tons of Allied shipping. (Note that statistics in the Battle of the Atlantic should be taken with a grain of salt, because many different sources use different frames of reference for how they count things.) Nevertheless, a lot of Allied shipping went to the bottom in March 1943, and the rate of loss would make a build-up for an early Allied invasion of Europe very problematic. In the space of one ten-day period in March, 40 Allied merchant ships would be sunk by U-boats in the Atlantic.

One key factor in the Battle of the Atlantic was the continuing war for intelligence by both sides, and, as in the battles at sea, it was very much a back-and-forth affair, sometimes without either side knowing it. One reason for the Germans’ success was that their naval radio intelligence and code-breaking organization, “B-Dienst,” was very good, and, from 1941 into most of 1943, the Germans were breaking and reading the British convoy codes fast enough to take operational action (although, like the Allies, the Germans were very careful in how they used code-breaking intelligence so as to not give away the fact that they were doing it). In fact, the British had no idea the Germans were reading their convoy-routing traffic. The British finally changed their convoy-routing code in June 1943 at the urging of the U.S. Navy cryptologic organization, OP20G.

The British had good initial success against the German Enigma encoding machine (which was an extraordinary electro-mechanical device that was close to impervious to being broken, even if a machine was captured). Nevertheless, the British had caught some early breaks with captured code books and development of “cribs” resulting from rare but periodic German communications security lapses that enabled the British to read a fair amount of Enigma code traffic at a useful pace, which they shared with the U.S. However, in February 1942, the German navy added a fourth rotor to their Enigma machines, which resulted in an astronomical (literally) increase in possible numeric combinations, making it impossible to break except by developing electro-mechanical devices even more sophisticated (and costly) than the Enigma machines. The new naval Enigma system was known as “Triton” by the Germans, and “M4 Shark” by the British. Partly as a result of the loss of ability to read Enigma traffic, convoy losses in the Atlantic in the latter half of 1942 were three times that of the same period in 1941.

The counter to the Enigma were massive machines, known as “bombes,” which were a marvel of technology at the time, because they required extremely high quality control for components, which were in constant high-speed motion in order to brute-force their way through many millions of possible combinations. Even then, the machines, even when operating in large numbers, could not do it themselves. Code-breaking art by skilled cryptanalysts was still required to give the machines a chance of success. Nevertheless, with the advent of the four-rotor Enigma in the German navy, Britain’s best source of Intelligence mostly dried up. (The German army and air force kept using the three-rotor device, so the Allies had much greater success breaking their communications.) The British could still read a German weather signals code and gain some intelligence about U-boat operations, but on 10 March 1943, the Germans switched to a new set of weather codes, and the British lost even that tenuous hold, which was one key factor in the high convoy loss rates in the latter half of March 1943.

Meanwhile, while the British were having serious production difficulties developing and building bombes (and enough of them) to work the four-rotor Enigma, the U.S. Navy had embarked on a hugely expensive crash program to do the same thing. In fact the U.S. bombe effort was given the same resource priority as the Manhattan Project (atom bomb development) and the technological challenges were arguably as great. The bombes were developed at the U.S. Naval Computing Machine Laboratory on the National Cash Register (NCR) compound in Dayton, Ohio. Cost overruns in the program were in the millions (in 1942 dollars). The first two “Desch” bombes, known as Adam and Eve, followed by Cain and Abel, were all prone to serious technical difficulties. For many months it appeared as if the Navy was pouring huge sums of money and resources (advanced materials) down a rat hole, all to the frustration of U.S. Navy cryptologist, who for the first year of the war were highly dependent on British intelligence.

Of note, the type of code-breaking employed against the Japanese was completely different than that employed against the Germans. The Japanese used old-fashioned paper-and-pencil code systems and didn’t have Enigma machines or an equivalent (except for the diplomatic “Purple” code) in any kind of scale operation. However, success against the Japanese contributed nothing to success against the Germans. Nevertheless, by the spring of 1943, the United States had overcome most of the technical challenges of the bombes, and was starting to build them in large numbers (hundreds would be required to perform the necessary calculations). The Navy also enlisted a small army of several hundred women (WAVES) to mind the hundreds of bombes that were put in operation in a commandeered women’s college on Nebraska Avenue in Washington, DC (in what for many years after the war was the headquarters of Naval Security Group). This duty was not without danger, as the bombes were prone to throwing metal components at high velocity. In theory, the WAVES did not know the true purpose of the machines they were minding, other than that it was a extremely important top secret program to which they were sworn to a lifetime of secrecy (and the WAVES kept their oath until the end). Although the U.S. Navy bombes didn’t play a significant role in “Black May,” by the end of June 1943 they were having significant effects and results. For the duration of the war, the U.S. Navy code-breaking effort emerged into the lead against the Germans. (More on this in a future H-gram.)

Like the British, the Germans didn’t think their codes were vulnerable either. As a result, the Germans were prolific communicators, which created vulnerability through techniques of traffic analysis and radio direction finding even during periods when the Allies could not read the contents of the messages. Grossadmiral Karl Dönitz, who had moved from commander of the German submarine force to supreme commander of the German navy (after Grossadmiral Erich Raeder had been relieved when Hitler became unenthused about the performance of the German surface navy), was in frequent communications with German U-boats, providing locations and routing of Allied convoys, and forming wolf packs of multiple submarines to attack the convoys. The Allies were trying just as hard to route the convoys away from where the wolf packs were forming, and the failure to do so in late March 1943 was a factor in the high convoy losses.

Led by the British, the Allies embarked on a crash program for high frequency radio-direction finding (HFDF) on both ships and aircraft, with such considerable and rapid success that the Germans did not really grasp their vulnerability to the technology. The Germans ascribed the sudden appearance of Allied anti-submarine ships and aircraft in unexpected places to a wide variety of other factors, particularly airborne radar and a belief that the British had developed an infrared (IR) sensor (abetted by false reports deliberately planted by the British as deception), which led the Germans to invest considerable resources in special paint for their submarines to imitate the optical properties of sea water, as well as a crash German program to develop IR sensors of their own, with some limited progress before the war ended.

For all the importance of intelligence in the overall Battle of the Atlantic, during the turning point in May, Allied ntelligence capability against the U-boats was at the weakest point of the entire war. Fortunately several other factors and technologies came to fruition at the same time, resulting in a radical shift of fortune.

The first factor was Allied air power, with significantly greater numbers of longer-range aircraft operating from increasing numbers of bases around the Atlantic, as well as from a growing number of small escort carriers, such as the USS Bogue (CVE-9), which played a significant role in operations against German submarines in May 1943. The modified, very long-range B-24 Liberator bombers closed the mid-Atlantic gap, and the escort carriers could provide near continues air cover to the convoy.

The U-boats were acutely vulnerable to aircraft attack, and the Germans even developed “flak” U-boats, designed with a heavy defensive anti-aircraft armament intended to duke it out on the surface with aircraft. The Allies received a rude shock (due to surprise) during the first encounter with a flak U-boat, before they quickly adapted and it became readily apparent that this was not one of the Germans’ better ideas. The British, in particular, also resumed air attacks (after early heavy losses) against U-boats returning to their bases in occupied France on the Bay of Biscay. The last few miles of a U-boat patrol quickly become the most dangerous, and many were lost almost within sight of the safety of their massively reinforced U-boat pens. Increasingly equipped with airborne radar, ASW aircraft became significantly more dangerous and could attack at night when the U-boats preferred to surface to recharge their batteries. The preferred tactic by wolf packs was also a night surface attack; taking away the U-boats’ sanctuary in the darkness had a major adverse effect on U-boat success and survival.

Another factor was sheer numbers. The anti-submarine production effort was given top priority (and U.S. Navy commanders felt the resulting shortages first-hand during the brutal battles around Guadalcanal in late 1942) but, by 1943, Allied (primarily U.S.) shipyards were cranking out more of everything: ships, aircraft, weapons, sensors. Even during the worst of the U-boat attacks, Allied ship building was keeping pace with losses (sometimes barely) while German shipyards were also keeping pace with their submarine losses. By May 1943, those curves had changed radically. U.S. shipyards were producing many more ships than the Germans could sink, and the Germans were losing more submarines that their shipyards could replace—hence, Dönitz’s recall order on 24 May 1943. The Germans didn’t make a concerted effort to resume submarine operations in the Atlantic until the fall of 1943, and from then on they were almost constantly on the defensive.

Rapidly improving Allied technology, and the tactics to use it effectively, coupled with advances in scientific operational analysis, also reached a key inflection point in May 1943. The new U.S. Navy short- range communications system, “Talk-Between Ships” (TBS), developed just before the start of the war, represented a significant advance over the CW 936 radio-telephones that had proved very effective when first introduced in World War I. New weapons technology included better sonar, the advent of sonobouys, and FIDO, which was termed the Mark 24 mine, but was actually a U.S. air-dropped passive acoustic homing ASW torpedo, introduced in March 1943.

Another weapons technology was the proliferation of the British-designed forward-throwing ASW Hedgehog depth bomb aboard both British and U.S. ships. (In the Pacific, in May 1944, USS England (DE-635) sank six Japanese submarines in as many days using Hedgehog). With the Hedgehog, the attacking surface ship did not have to first steam over the contact and drop depth charges in her wake, thus allowing the submarine much less time to take evasive maneuvers. Quickly recognizing the deadly threat posed by the Hedgehog (one in five Hedgehog attacks resulted in a kill, compared to one out of 80 for conventional depth charges), the Germans introduced the Falke (“Falcon”) acoustic homing torpedo in mid-1943, which was in turn countered by the Allied Foxer noisemaking decoy. The Germans also developed the Wanze (“Tick”) radar-warning device to try to counter increasingly deadly attacks by Allied aircraft equipped with microwave anti–surface vessel (ASV) radar. The Allies countered with radars at a frequency Wanze could not detect. In early May 1943, this 10-centimeter radar aboard aircraft detected all 26 attempts by U-boats to attack convoy ONS 5, which were driven off.

Greatly improved Allied tactics were primarily a function of hard-won lessons learned and experience. Commanders and crews had simply gotten better than they were earlier in the war. Scientific analysis was also used to refine tactics. With more assets, independent support groups could be placed at strategic points along the convoy routes, where they could more rapidly reinforce a convoy’s organic escorts in the event the convoy came under wolf-pack attack. The German wolf-pack tactics (later used with great effect by U.S. submarines against the Japanese) and Allied support group tactics are arguably early examples of “swarm” tactics.

Allied operational capability had so improved by mid-1943 that it would have actually made more sense to draw the U-boats to the convoy rather than using Ultra (broken Enigma traffic) to avoid them, under the theory that the more U-boats that attacked, the more that could be sunk. Nevertheless, U.S. and British commanders balked at the idea of using convoys as “bait,” instead choosing to employ hunter-killer task groups, often driven by intelligence on U-boat locations. Although use of the hunter-killer groups was raised to a fine art, they were resource-intensive and less efficient than letting the U-boats come to the target, but much better for the morale of those on the troopships.

Another major development in May 1943 was the creation of the U.S. Tenth Fleet on 20 May. CNO/COMINCH Admiral King directed that the responsibilities of technological development, scientific operational analysis, ASW doctrine, and training be combined in one command, along with significant intelligence capability. Although the Tenth Fleet did not have its own forces, it would serve as the central command responsible for protecting convoys and hunting down U-boats. Despite being stood up too late to effect the convoy battles in early 1943 or the turning of the tide, Tenth Fleet would have significant effect on the remainder of the Battle of the Atlantic, and would serve as a forerunner of the operational intelligence (OPINTEL) concept that served the U.S. Navy throughout the Cold War, integrating all types of intelligence into a very close operational cycle.

(Sources include The Secret in Building 26: The Untold Story of America’s Ultra War Against the U-boat Enigma Codes, by Jim DeBrosse and Colin Burke, 2004, “Turning Point in the Atlantic” by Commander In H. Ha, USN, in the April 2018 issue of Naval History magazine, and Information at Sea: Shipboard Command and Control in the U.S. Navy from Mobile Bay to Okinawa by Timothy S. Wolters, 2013, as well as a host of U-boat books in my personal library.)

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Published: Wed May 08 10:39:09 EDT 2019