March – May 1943

What Winston Churchill called the ‘Battle of the Atlantic’ was a major struggle waged by the Royal Navy, abetted by the Canadian and US navies, against the submarine arm of the German Reichsmarine for control of the trans-oceanic shipping lanes. The German plan was to sink enough merchant shipping to blockade Britain and undermine any attempt to invade mainland Europe; the Allied strategy was to break the stranglehold of the U-boats and so maintain the flow of supplies, resources and troops to Britain. The course of the battle was determined not only by the naval vessels assigned to the convoy operations, but by the introduction of very long-range aircraft, advanced radar equipment and a scientific system for cracking German naval codes. The breakthrough for the Allies came at the last moment in spring 1943, in time to avoid a growing crisis.

A Vought SB2U Vindicator naval dive-bomber from the carrier USS Ranger flies alongside Convoy WS-12 on its way through the Atlantic to Cape Town in South Africa. The use of aircraft in reconnaissance and anti-submarine roles was essential to the Allied success against the U-boats.

In the course of 1942 German submarines sank 7.8 million tons of shipping – 1,662 ships in all. They hunted the convoys and single vessels in ‘wolfpacks’ of anything up to thirty or forty vessels. A lucky convoy might avoid contact altogether, and many did so, but the real danger to the Allies was an attrition rate so high that shipping lost could not be replaced in time. This meant finding an active way to inflict losses on the submarines. By 1942, many innovations had been introduced: large, armed escorts protected the merchantmen and hunted for submarines; improved air-to-surface radar helped Allied aircraft to locate submarines more accurately; a large spotlight (the Leigh Light) was introduced so that submarines that surfaced at night could be illuminated for attack. Better anti-submarine projectiles were developed, including the Hedgehog, which fired twenty-five small bombs fitted with contact fuses, so that an explosion would indicate clearly a submarine hit. The German submarine arm, commanded by Admiral Karl Dönitz, reacted to these changes, but the main effect was to push the wolfpacks to the so-called Atlantic Gap in the central part of the ocean, between Greenland and the Azores, where long-range aircraft could not yet reach. By late 1941, the German B-Dienst serving naval intelligence had cracked Anglo-American codes and could track convoys, while in February 1942, after a long period in which the German encrypted Enigma messages could be decoded and read (known in Britain as Ultra intelligence), the German navy changed its Triton cypher and kept the Allies in the dark until December 1942, when German naval traffic could be read again, with a brief pause in March 1943, and the submarine packs tracked.

The battle was poised by the end of 1942. Submarine losses were at last starting to mount. Only three had been sunk in the Atlantic between January and June, but between July and December the total was thirty-two. Allied shipping losses were high in 1942 and the morale of merchant seamen, thousands of whom lost their lives in the cold and remote seas of the mid-ocean, was low. The cost of building new ships challenged even the wealthy economy of the United States. But over the first months of 1943, the balance began to tilt towards the Allies, thanks not only to the painstaking work of directing convoys onto safer routes, and the growing expertise of the aircraft and escort vessels hunting the enemy, but also to a number of vital technical changes in the Allied force that came together all at once, and at the moment of greatest crisis.

The first innovation was a new radar set, ASV-III, using the newly discovered cavity magnetron that permitted much more accurate sets based on narrow centimetric wavelengths. The Germans had been able to block the ASV-I and ASV-II sets that used long radar waves, but they had nothing to stop the new sets. The new radar could even detect a periscope in calm seas, transforming the capacity of aircraft to detect submarines.

The second change was the advent of very long-range aircraft, principally the B-24 Liberator bomber, designed to close the Atlantic Gap and give air cover for the whole convoy voyage. This was something the Royal Navy had wanted for a long time but they had been frustrated not by the enemy but by the resistance of the US navy chief-of-staff, Admiral Ernest King, and the commander-in-chief of RAF Bomber Command, Air Chief Marshal Arthur Harris, neither of whom had been willing to divert long-range aircraft from other operations. But this change came only after a fierce battle between submarines, ships and aircraft in the middle of March 1943 that signalled the peak of the Battle of the Atlantic.

The battle was fought against thirty-eight submarines, organized in three wolfpacks, Raubgraf, Stürmer and Dränger, in the heart of the Atlantic Gap. Intercepted signals warned the Germans that two large convoys, SC122 and HX229, were on their way, a total of ninety-one vessels. The weather was appalling, with squalls of snow and hail, fog, and heavy grey seas. Contact was made with the convoys when U-653 sighted the ships of HX-229 and alerted the rest of the packs. Radar was less useful with the high swell of the waves and the submarines pressed home their attacks, sinking a total of twenty-one ships of 141,000 tons. Only one submarine was lost, sunk by a B-17 ‘Flying Fortress’ on duty from the Hebrides Islands. But during the battle, two B-24s were used for the first time over the Gap; neither scored a kill, but their presence caused the submarines to be more cautious.

The British commander of the Western Approaches in the eastern Atlantic, Admiral Sir Max Horton, sensed that, despite the high losses, the balance of technology was swinging the Allies’ way. Rather than urging greater caution, Horton insisted that the next convoys sail into the waiting jaws of the wolfpacks, where long-range aircraft and escorts armed with centimetric radar would seek out and fight the submarines.This was risky and convoy sinkings continued, but at only half the losses experienced in March. More significant, the losses of submarines escalated. Between March and May 1943, fifty-six submarines were sunk, with thirty-three in May alone, one submarine for every merchantman sunk. In mid-May, convoy SC130 with thirty-seven merchant vessels set out across the gap.

Supported by a large naval escort and very long-range Liberators, the waiting submarines were decimated, losing six of their number but sinking not a single ship. Dönitz, whose son died on one of the submarines in this battle, bowed to reality. On 24 May, he withdrew his submarines from the Atlantic so that they could be refitted and he could assess the new Allied tactics and technology, but their battle was all but over. It took time for the Allies to realize what had happened, but by June, when only four ships were sunk for the loss of twelve submarines in all waters, it was clear that the battle had been won. The submarines never posed a serious threat again.