Closing the Atlantic air gap

At nine in the morning on 28 April 1943 the submarine U-650, stationed in a patrol line east of Greenland, sighted convoy ONS 5, sailing in ballast from Liverpool to Halifax. Her captain sent a sighting report and kept in touch with the ships, but during the day he had to dive three times to escape approaching aircraft. Only a few weeks earlier U-boats in this part of the ocean had been able to shadow a convoy on the surface round the clock without fear of aircraft. This small episode showed the change that was under way: the Allies were closing the air gap over the mid-Atlantic – the belt of ocean beyond the reach of land-based aircraft in which, for three years, the wolf packs had inflicted their heaviest losses on the convoys.

It was not a single order or a single operation, but a combination of decisions taken in the winter and spring of 1943: the allocation of very long range (VLR) aircraft to maritime patrol forces, new bases in Iceland and Newfoundland, escort carriers, support groups, new radars and anti-submarine weapons, and renewed insight into German communications. In April all these elements began to work at the same time, and a few days later, in the battle for convoy ONS 5, they proved decisive.

The black pit in the middle of the ocean

Aircraft were the U-boats’ most dangerous enemy even when they sank nothing. The submarine of that era was in essence a surface vessel capable of diving: on the surface it was faster than a convoy, under water slower, and it quickly lost contact. A wolf pack therefore had to overtake the convoy on the surface by day, gather ahead of it and attack at night. An aircraft circling over the ships forced the boats to dive and broke up this tactic before any attack could take place.

At the beginning of the war RAF Coastal Command had mainly short-range machines, while the best heavy aircraft went to Bomber Command. From airfields in Britain, Northern Ireland, Iceland and Newfoundland, patrol aircraft covered an ever wider belt of coastal waters, but south of Greenland there remained an area several hundred nautical miles wide which they could not reach. Sailors called it the “Black Pit”; in official documents it was the Mid-Atlantic gap or the Greenland gap. It was there that Karl Dönitz concentrated his packs, and there that the greatest convoy battles of 1942 and March 1943 were fought, above all the battle for convoys HX 229 and SC 122.

The dispute over the Liberators

Only aircraft with a range of more than a thousand miles from base could close the gap. The best suited was the American Consolidated B-24 Liberator, which, with some of its armour removed and extra fuel tanks added, could circle over a convoy for several hours hundreds of miles from land. The British had very few such machines. For most of 1942 the only Coastal Command squadron flying VLR Liberators was No. 120 Squadron RAF, operating from Northern Ireland and Reykjavík; it usually had anything from a handful to a dozen or so serviceable aircraft. In February 1943 a mere dozen or so VLR aircraft in all were operating over the North Atlantic.

The cause was not a shortage of Liberators as such, but the way they were allocated. The US Navy wanted them for reconnaissance in the Pacific, the US Army Air Forces for bombing, and in Britain the head of Bomber Command, Arthur Harris, opposed giving up heavy aircraft for tasks he regarded as defensive. Meanwhile Professor Patrick Blackett, head of the Admiralty’s operational research section, calculated that one Liberator over a convoy saved more ships than many aircraft dropping bombs on German cities or on the U-boat bunkers in France. The latter raids, carried out from January to May 1943, cost Bomber Command more than two hundred aircraft and did not damage a single pen.

The March crisis tipped the balance. Under pressure from Churchill and Roosevelt, Liberators began to be handed over to the maritime patrol forces, the number of VLR aircraft in No. 120 Squadron doubled, further British squadrons were re-equipped, and aircraft from Iceland and Northern Ireland reached the middle of the ocean ever more often. The Canadians, who since the autumn of 1942 had been demanding Liberators for the Royal Canadian Air Force, finally received them in the spring; No. 10 Squadron RCAF began flying them from Gander in Newfoundland in May. The Americans sent Army Air Forces anti-submarine squadrons to Newfoundland and over the Bay of Biscay. In April the Liberators began to reap their harvest: on 5 April an aircraft of No. 120 Squadron sank U-635 near convoy HX 231, and on 23 April another Liberator of the squadron sank U-189.

The convoy conference and a new division of the ocean

From 1 to 12 March 1943 the Allied Atlantic Convoy Conference met in Washington, convened by the head of the US Navy, Admiral Ernest King. It reorganised command in the Atlantic. The Americans withdrew their ships from escorting convoys on the northern route and concentrated on the central route linking the United States with Gibraltar and North Africa, while the British and Canadians took over responsibility for convoys between North America and Britain. On 30 April 1943 the Canadian Rear Admiral Leonard Murray took over the newly created Northwest Atlantic Command in Halifax. He was the only Canadian to command an entire theatre of operations during the war. The conference also recognised that the maritime patrol forces needed many more very long range aircraft, and settled how they should be divided between bases on both sides of the ocean.

Escort carriers and support groups

The second way of closing the gap was to take air power along with the convoy. Escort carriers, built in the United States on merchant hulls, carried from a dozen to a few dozen aircraft: fighters and torpedo bombers armed with depth charges. The first of them joined the North Atlantic convoy routes in March 1943. The American USS Bogue escorted convoys across the ocean three times in March and April, and on 22 April sailed on her fourth voyage. The British HMS Biter covered convoy ONS 4 in April; on 25 April her Swordfish, together with the destroyer HMS Pathfinder, sank U-203 south of Greenland. A carrier sailing with a convoy meant that the air gap ceased to exist wherever the ships happened to be.

Support groups were equally important. A convoy’s regular escort had to stay with the ships and could not pursue a detected submarine for long without weakening the protective ring. The Commander-in-Chief Western Approaches, Admiral Max Horton, had long wanted reserve groups that could be sent to a threatened convoy and would hunt U-boats to the finish. In the spring of 1943, thanks to ships released after the operations in North Africa and destroyers lent by the Home Fleet, five British support groups were formed, two of them built around the escort carriers Biter and Archer, and the Americans sent a group with Bogue to the Atlantic. The 2nd Support Group, made up of sloops, was led by Captain Frederic John Walker, the most successful British submarine hunter of the war, who from June 1943 onwards was to sink more than a dozen U-boats with his group.

Radar, direction-finders and the “hedgehog”

Ships and aircraft also received new technology. The most important was centimetric radar, whose heart was the British cavity magnetron. Shipborne radars of this type, fitted to escorts from 1942, could detect the conning tower of a surfaced U-boat at night at a range of several kilometres. From March 1943 Coastal Command aircraft began to receive the ASV Mk III centimetric airborne radar, which worked together with a powerful searchlight that lit up the target in the final phase of a night attack. The German Metox warning receivers, which detected the older metric radars, remained deaf to the new wavelengths. For a long time U-boat crews did not understand why aircraft were suddenly surprising them in the dark, and even suspected that the Allies were homing in on the emissions of the Metox sets themselves.

More and more escorts carried HF/DF direction-finders, which made it possible to fix the bearing from which a U-boat was transmitting its report and to send a ship or aircraft there. A new weapon was the Hedgehog, a launcher that threw a salvo of 24 projectiles ahead of the bow, with charges that exploded only on striking a hull. Unlike traditional depth charges, dropped over the stern, it allowed an attack without losing sonar contact with the target. At the beginning of 1943 crews did not yet trust the new weapon and its effectiveness was low; only better training meant that by the end of the war roughly one Hedgehog attack in five ended with a submarine sunk.

The cipher war

After a few days of “blindness” in March, the cryptanalysts at Bletchley Park quickly regained access to “Shark”, the Enigma key used by the U-boats in the Atlantic. For most of April, therefore, the Admiralty knew the positions of the patrol lines and could route convoys around them. This advantage was not constant, however. On 26 April the Germans changed the cipher settings, and until 5 May the British were once again unable to read Dönitz’s signals currently. It was in precisely these days that convoy ONS 5 entered the area of the German patrol lines. The German cryptanalytic service, the B-Dienst, on the other hand, was still reading the British convoy cipher, which was changed only in June.

28 April: convoy ONS 5

Convoy ONS 5 left Liverpool on 21 April: more than forty ships, most of them empty and heading to America for cargo, escorted by Commander Peter Gretton’s escort group B7 of two destroyers, a frigate and four corvettes. From the start it battled with gales; the Polish freighter Modlin had to turn back with engine failure, and two other ships collided in the storm. On 24 April a Flying Fortress of No. 206 Squadron RAF from the Hebrides sank U-710 ahead of the convoy.

Several groups of U-boats, almost sixty boats in all, were waiting for the convoy, deployed west and east of the gap. Group Star, formed on 27 April east of Greenland, crossed the convoy’s route on the morning of 28 April. U-650 reported on the ships all day, and in the evening other boats joined her. Gretton, alerted by his direction-finders, sent destroyers and corvettes out along every bearing. During the night of 28–29 April the escort detected the U-boats by radar one after another and drove them off with depth charges. No ship was lost, and three damaged boats had to return to base. Only at dawn on 29 April did U-258, lying submerged ahead of the convoy, torpedo the American freighter McKeesport, being damaged herself in the process. The following days brought even heavier gales, the arrival of further support groups and a new, much larger group of U-boats.

Its place in the war

The battle for convoy ONS 5 lasted until 6 May. When a gale scattered the convoy, the Germans threw more than forty boats against the stragglers and sank 13 ships, but on the night of 5–6 May, in fog, the escort and the support groups, guided by centimetric radar, sank or drove off more U-boats. The Germans lost six boats in the battle, and several others were badly damaged. Both the British and the German commands realised that a wolf pack, however large, could no longer defeat a well-equipped escort supported from the air. Historians regard the battle as the turning point of the Battle of the Atlantic.

May 1943 went down in history as “Black May” for the German U-boat arm. In that month the U-boats lost more than forty boats, about a quarter of the force at sea, and sank few ships. Dönitz’s son was also killed. On 24 May Dönitz withdrew his boats from the North Atlantic, and in his war diary he admitted that he had lost the battle. In May Canadian Liberators began flying from Newfoundland, and the air gap practically ceased to exist. In the autumn of 1943 Portugal made bases in the Azores available to the Allies, closing the last gap on the central route.

The U-boats did return to the Atlantic with new weapons and new boats, but they never regained the initiative. The closing of the gap over the middle of the ocean, combined with the Allies’ technical and intelligence superiority and the enormous output of American shipyards, secured the route by which food, fuel, weapons and troops reached Britain. Without it, neither the bombing offensive against the Reich on the scale planned nor the invasion of France in 1944 would have been possible.