Submarine warfare had some impact in World War One but became vastly more significant in World War Two as the German U-boat packs aimed to blockade Europe. Merchant ships took to sailing in large convoys, protected by screens of destroyers and corvettes armed with depth charges and sonar. Daring U-Boat commanders carried out torpedo attacks within the defensive screen, and when several submarines attacked at once, the defenders had little chance of striking back. In the end, the Battle of the Atlantic was eventually won by technology. Radar to detect U-Boats from the surface, radio interception, and code-breaking all played a part. By the end of the war more than 3 000 merchant ships had been sunk, as well as almost 800 U-Boats.
The Battle of the Atlantic comprised the assault by the sea and air forces of Germany against the merchant shipping of Britain and her Allies. Control of the Atlantic Ocean was vital not only to the survival of Britain but also to the successful execution of the defeat of Nazi Germany. In this monumental struggle the U-boats introduced a powerful element of terror and diverted Allied effort into anti-submarine campaigns.
Admiral Karl Dönitz, commander of the U-boats, devised new tactics and strategies, the best known being that of the U-boat wolf-packs, where multiple submarines would stay close together, making it easier for them to sink specific targets. He achieved the highest standards of seamanship among his men and developed good man-management skills, remembering most U-boat captains by name. He ensured that crews had superior ration allocations and that they received a special operational pay allowance. When each U-boat returned from a patrol, he attempted to personally welcome home the crews and ensured that any decorational awards due to the crew were presented by him as soon as the boat docked. He arranged for a special luxury express train to ferry the crews back to Germany when they went on leave. The affection for his men was reciprocated in full by his crews, who referred to him as Uncle Karl.
The Battle of the Atlantic, which lasted from September 1939 until the defeat of Nazi Germany in May 1945, was the war’s longest continuous military campaign. During six years of naval warfare, German U-boats and warships were pitted against Allied convoys transporting badly needed military equipment and supplies across the Atlantic to aid the survival of Great Britain. This battle to control the Atlantic shipping lanes involved thousands of ships and stretched across thousands of perilous miles of ocean.
Although unable to challenge the Royal Navy in regard to surface ships, the German Navy (Kriegsmarine) overseen by Grand Admiral Erich Raeder employed a mix of surface raiders and Uboats. Though he favoured the surface fleet, Raeder was urged by Dönitz, to use U-boats as the main naval weapon. Initially ordered to seek out British warships, Dönitz’s U-boats had early success sinking the old battleship HMS Royal Oak, docked at Scapa Flow and the aircraft carrier HMS Courageous, off the Irish coast. Despite these victories, he vigorously advocated using hunting groups of U-boat wolf-packs to attack the Atlantic convoys.
Other instruments used in the conflict were specially selected merchant ships which were powerfully armed and skilfully disguised to act as commerce raiders. These were very soon joined by aircraft and minelayers (both airborne and seaborne). Sometimes one instrument acting in conjunction with another and sometimes each acting independently.
The German surface raiders scored some early successes, and drew the attention of the Royal Navy who sought to destroy them or keep them in port. Well before the outbreak of war on 3rd September 1939, the Germans sent out into the Atlantic two of their powerful 'pocket battleships', the Deutschland (renamed Lützow in November 1939) and the Graf Spee. The Deutschland's cruise accomplished little, and she was recalled after sinking only two ships in three months. The Graf Spee ranged the oceans far more widely and captured or sank 9 ships (50 000 tons) before she was trapped by three British cruisers off the River Plate on 13th December 1939, and scuttled herself. The pocket battleship Admiral Scheer made a successful sortie in the Atlantic between October 1940 and April 1941 and got home safely after accounting for 16 ships (99 059 tons). The heavy cruiser Hipper, a short endurance ship, made two comparatively brief cruises in 1940 and 1941, but found only nine victims (40 078 tons). By far the most successful of the warship raiders were the battle-cruisers Scharnhorst and Gneisenau (32 000 tons displacement) which cruised in the north and mid-Atlantic between January and March 1941, sinking 22 ships (115 622 tons), thereby, seriously disrupting the entire convoy system before returning safely to Brest, in occupied France, thus posing a constant threat to Atlantic shipping.
The peak of warship raiding came, however, with the sortie of the giant battleship Bismarck (42 345 tons displacement) and the heavy cruiser Prinz Eugen in May 1941, which resulted in the sinking of the British battle cruiser Hood in the Battle of the Denmark Strait on the 24th May. This was followed by a dramatic pursuit by the Home Fleet and other forces, which ended three days later when the Bismarck was sunk almost within reach of air cover from western France. This sortie, though in British eyes menacing in the extreme, achieved nothing against merchant shipping; and its outcome made the Germans unwilling to risk their heavy warships in further Atlantic operations. In February 1942, the Scharnhorst, Gneisenau and Prinz Eugen escaped home up-Channel from Brest in a skilfully planned and executed dash. Thereafter, the large German warships were only employed occasionally against the Allied convoys to Russia, whose protection was a British responsibility.
With the fall of France in June 1940, Dönitz gained new bases on the Bay of Biscay from which his U-boats could operate. Spreading into the Atlantic, the U-boats were supported by Focke-Wulf 200 Condor aircraft which aided in finding Allied ships as well as attacking them. Through the remainder of 1940 and into 1941, the U-boats enjoyed tremendous success and inflicted heavy losses on Allied shipping. As a result, it became known as the ‘Happy Time’ among the U-boat crews. As technology evolved, the Kriegsmarine produced many different types of U-boats. Most notable are Type VII, known as the ‘workhorse’ of the fleet, which was by far the highest number produced. Type IX boats were larger and specifically designed for long-range patrols, some journeying as far as Japan. .
The Germans organized and trained their air force (Luftwaffe) primarily for close support of the army, and had, relatively speaking, ignored both strategic bombing and cooperation with naval forces. Moreover, Reichsmarschall Göring's selfish insistence that 'everything that flies belongs to me' frustrated any possibility of the Kriegsmarine creating its own air arm. Relations between the higher ranks in the Luftwaffe and the navy were notoriously bad, and remained so virtually throughout the war. In the early months, German aircraft were used occasionally to reconnoitre British naval bases, and locate units of the Home Fleet at sea. A few were employed on mine laying, and it was not until the opening of the Norwegian campaign in April 1940 that the Luftwaffe began to exercise substantial influence on the war at sea. That 'ramshackle campaign', as Churchill described it, did however, prove that within the comparatively short range of the Ju 87 dive-bombers and the longer-range Ju 88, Do 17 and He III bombers, British naval forces could not operate successfully in coastal waters without fighter cover; and off Norway that cover was almost completely lacking. The heavy losses inflicted on supply convoys and on their escorting warships, taught the British a harsh lesson, a lesson which was to be driven home even more harshly after the German conquest of the Low Countries and France. Convoys passing through the English Channel, up and down the east coast, all came within range of German shore-based bombers.
The grievous shortage of escort vessels was much aggravated by the heavy losses suffered in the evacuations from Norway and France. This, combined with the preoccupation of RAF Fighter Command on the defence of the homeland and the lack of fighter aircraft in Coastal Command, rendered such targets as the slow-moving coastal convoys invitingly easy. It is therefore, not surprising that during the second half of 1940 the German dive-bombers inflicted heavy losses on British coastal shipping.
Gradually however, Fighter Command accepted the need to defend offshore shipping. Emergency measures such as the diversion of as much shipping as possible to the north of Ireland and to west coast ports, eased the situation. The creation of specially trained anti-aircraft crews to join merchant ships during the most dangerous parts of their passages, and the instruction of merchant seamen in the rudiments of self-defence against air attack bore fruit. The losses inflicted by enemy aircraft in 1940 totalled 192 ships (580 074 tons), but a large proportion of them were suffered during the evacuations already mentioned.
Throughout the war an arms race evolved between the Allied navies and the German Kriegsmarine, especially in detection and counter-detection. Sonar (ASDIC) allowed allied warships to detect submerged U-boats but was not effective against surfaced vessels; thus, early in the war, a U-boat at night or in bad weather was actually safer on the surface. Advancements in radar became particularly deadly for the U-boat, especially once aircraft-mounted units were developed. As a countermeasure, U-boats were fitted with radar warning detectors, to give them ample time to dive before being attacked. However, the Allies switched to centimetric radar which rendered the radar detectors ineffective.
In 1941 the Focke-Wulf four-engined Condor began to make sorties from western France much farther out into the Atlantic in order to report convoy movements to the U-boat command headquarters at Lorient, and to attack merchant ships themselves far outside the range of British shore-based aircraft. Thence arose the need for convoys to carry their own fighter defences along with them. As the navy had no escort aircraft carriers, improvisation was once again necessary. It took the form of converting a number of merchant ships (19 in all) to carry a few aircraft. These became known as merchant aircraft carriers
(MAC’s). Others were fitted with a catapult from which a Hurricane fighter could be launched and called catapult aircraft merchantmen, or CAM’s. A third type, known as fighter catapult ships (FC’s), was also produced. They were of course 'one-shot weapons', and the pilots of their Hurricanes, once they had been catapulted, had to parachute into the sea or ditch in the hope of being picked up by one of the escorts. Everyone realized, however, that these hazardous and demanding improvisations were no more than stop-gap measures, and that the real need was for small aircraft carriers which could provide continuous air escort throughout the convoys' voyages; but time was needed to build such vessels and train their aircrews, and in 1940-41 the pressure on the building yards was so heavy that the British had to rely chiefly on the United States to produce such vessels. Nonetheless the improvisations described above did check the depredations of the Focke-Wulfs.
To counter improvements in British operations, Dönitz pushed his wolf-packs further west forcing the Allies to provide escorts for the entire Atlantic crossing. United States President Franklin Roosevelt countered by extending the Pan-American Security Zone nearly to Iceland. Though neutral at the time, the US provided escorts within this region. Despite these improvements, U-boats continued to operate at will in the central Atlantic outside the range of Allied aircraft. This ‘air gap’ posed serious issues until more advanced maritime patrol aircraft arrived.
Other elements that aided in stemming Allied losses were the capture of a German Enigma code machine and the installation of new high-frequency direction-finding equipment for tracking U-boats. With the US entry into the war after the Japanese attack on Pearl Harbor, Dönitz dispatched U-boats to the American coast and Caribbean under the code name ‘Operation Drumbeat’. The U-boats began enjoying a second ‘happy time’ as they took advantage of unescorted American merchant ships as well as the US failure to implement a coastal black-out.
As losses mounted, the US eventually implemented a convoy system in May 1942. With convoys operating on the American coast, Dönitz withdrew his U-boats back to the mid-Atlantic that summer. Losses continued to mount on both sides as the escorts and U-boats clashed. In November 1942, Admiral Sir Max Horton became commander-in-chief of the Western Approaches Command. As additional escort vessels became available, he formed separate forces which were tasked with supporting the convoy escorts. As they were not tied to defending a convoy, these groups were able to specifically hunt U-boats.
The event which undoubtedly had the greatest influence on the air side of the Atlantic battle, and which brought about a sharp fall in the losses inflicted by German bombers, was Hitler's invasion of Russia on 22nd June 1941. The opening of the gigantic land campaigns on the Eastern Front necessitated large-scale diversion of Luftwaffe squadrons from Western Europe. The autumn of 1941 saw the first proper escort carrier (a German merchant ship which had been captured and converted) enter service. Though she was soon sunk by a U-boat her performance wholly confirmed the view that such ships provided the required solution. However, the need to employ the first generation of escort carriers to cover the North African landings in November 1942 prevented them joining in the Atlantic battle until early 1943. The success they then achieved fully justified the confidence placed in them. The heavy losses suffered by some Arctic convoys - notably by PQ 17 in July 1942 when the Admiralty unwisely ordered it to scatter and the escorts to withdraw in anticipation of attack by the powerful German naval and air forces based in Norway, accounted for a large proportion of the losses. By the end of 1942 such losses had dropped sharply. This improvement was attributable chiefly to the increasing number of escort vessels and of improved weapons entering service as American production got into its stride. Thus the strength available to the Western Approaches and Coastal Commands steadily grew, and the training of the escort groups and aircrews improved.
The German mine laying campaign started with the outbreak of war, when their magnetic mine found the Royal Navy unprepared - despite the fact that the British had themselves produced and laid mines of that type towards the end of World War One. In 1939 British minesweepers were only equipped to deal with moored mines, and the magnetic mine thus inflicted substantial losses during the early months and produced a serious situation by forcing the temporary closure of many harbours and offshore channels. However, by October 1939 the necessary antidote in the form of magnetic minesweepers had been produced and all warships and merchantmen were demagnetized to give them a measure of self-contained immunity. In 1939 Allied losses to mines totalled 78 ships (262 542 tons) and in the following year 201 ships (509 889 tons). Not long after the magnetic mine had been mastered the Germans began to lay a different variety, which was detonated by the sound waves produced by a passing ship; but the Admiralty had anticipated such a development and had designed the necessary counter-measure, with the result that the acoustic mine never achieved results commensurate with the magnetic mines of the early months. The production of many variations of the magnetic and acoustic mine, or of a combination of the two, initiated the struggle between enemy minelayers (which might be surface ships, U-boats or aircraft) and Allied minesweepers which was to last throughout the war.
The ingenuity and persistence with which the Germans executed their mine laying campaign forced on the Allies a very great minesweeping effort, and a large number of the little ships which toiled to keep the channels clear were destroyed by the mines they were endeavouring to sweep. After 1941 the actual merchant ship losses suffered were comparatively small: 51 ships (104 588 tons) in 1942, thirty seven ships (108 658 tons) in 1943, twenty-eight ships (95 855 tons) in 1944 and the same number (93 663 tons) in the last year of the war. The most serious threat after the magnetic mine of 1939 actually arose in 1944, when the Germans produced a pressure-operated mine and laid it in the channels and harbours used by the ships supporting the invasion of Normandy. Though the Allies never managed to produce a sweep capable of exploding such mines harmlessly, precautionary measures, such as steaming at slow speed, and complete air supremacy enjoyed by the Allied air forces, prevented this new type of mine from becoming a serious threat.
But, it was the U-boat campaign that constituted by far the greatest threat to Allied control of the vital shipping routes and inflicted the greatest losses. Prime Minister Winston Churchill wrote “The only thing that really frightened me during the war was the U-boat peril". Though the instruments already described were certainly not negligible in their influence, it is no exaggeration to say that the Battle of the Atlantic was chiefly fought between the U-boats and Allied (and in particular British) surface convoy escorts and aircraft. One clear lesson of World War One was that the convoy system was much the most effective strategy against submarine attacks; and the fact that it saved Britain from disaster in 1917 was not forgotten between the wars. The Admiralty always insisted that if another war broke out merchant ships not capable of steaming at a fairly high speed must be sailed in convoy - despite all the well-worn arguments against doing so. However, as the probable influence of air power became clearer in the1930’s, the Air Ministry, in reviewing the part that the RAF should play in war, argued that to mass ships into convoys would present enemy aircraft with easier targets and so opposed the introduction of the convoy system.
Not until nearly the end of 1937 did the Air Staff accept that such a system should be introduced, and allocate a large proportion of the strength of Coastal Command to convoy escort duty. On the Admiralty's side the invention in the early 1920’s of the ASDIC submarine detection device (called SONAR by the U.S. navy) proved over optimistic. Certainly ASDIC was a very valuable development; but it did not, as the Naval Staff argued as late as 1937, by itself mean that the submarine threat had been mastered. Furthermore, the Admiralty long held that as the various naval treaties signed between the wars had forbidden unrestricted submarine attacks on merchant ships. Nazi Germany had become a party to those arrangements in 1935. This was presented to avoid a campaign such as had brought Britain to the verge of defeat in 1917 from being repeated. Fortunately this illusion in naval circles was dissipated at about the same time as the adoption of the convoy system was agreed by the Air Ministry. In 1938 the Admiralty accordingly set about the difficult task of organising the worldwide control of merchant shipping which would fall to its lot in war, and started to create the administrative machinery necessary to put shipping into convoy - initially only on the most important routes, namely those crossing the Atlantic. But the actual introduction of convoys, even on a limited number of routes, was bound to take time. Therefore, for some months after the outbreak of war many ships were still sailing independently, even in the most dangerous waters. Furthermore the Allies did not possess sufficient escort vessels and aircraft to give convoys the necessary protection - except at the very end of a homeward journey or the beginning of an outward one; and such strength in escort vessels as they did possess was too often wasted by sending ships and aircraft out to hunt for U-boats instead of using them to escort convoys.
The British had a major advantage in their ability to read some German naval Enigma codes. Codebooks and equipment were captured by raids on German weather ships and from captured U-boats. A team including Alan Turing used special purpose ‘Bombes’ and early computers to break new German codes as they were introduced. The speedy decoding of messages was vital in directing convoys away from wolf-packs and allowing interception and destruction of U-boats. This was demonstrated when the German naval Enigma machines were altered in February 1942 and wolfpack effectiveness greatly increased until the new code was broken.
The conception that the convoy system was a 'defensive' strategy and therefore inferior to 'offensive' measures such as hunting was very long-lasting, and found a powerful supporter in Churchill as First Lord of the Admiralty. In the Air Ministry the doctrine that strategic bombing was the primary function of an air force and the only answer to enemy bombing was equally enduring; and the Air Staff remained markedly reluctant to reduce its bombing effort in order to provide convoy air escorts. Taken together these false arguments and stubbornly held convictions resulted in the U-boats finding large numbers of comparatively easy targets in 1939-40 among ships sailing independently or in weakly escorted convoys. By the Anglo-German Naval Agreement of 1935 Germany was allowed to build a tonnage of submarines equal to Britain's if she herself considered it necessary, and on the outbreak of war she actually possessed 56 U-boats. Thirty of them, however, were of a small coastal type suitable only for use in the North Sea. By the end of August 1939, thirty-nine U-boats had put to sea to take up their war stations; but only 17 of them were of the ocean-going type. Their orders were to obey 'for the present' the rules laid down in international law governing attacks on merchant ships; but the sinking of the British liner Athenia on the day war was declared showed that unrestricted submarine warfare had begun and strengthened the Admiralty's conviction that the strategy of shipping defence had to be primarily based on the convoy system. In fact homeward-bound convoys from Halifax, Freetown (Sierra Leone), Bergen (Norway) and Gibraltar, and outward convoys from the Thames ports, Merseyside, and the Clyde, as well as coastal convoys between the Thames and Firth of Forth, were all started between September and October 1939.
Though anti-submarine escorts based in British ports (including those in Northern Ireland) and in Halifax could only remain with their charges for about the first 400 miles of their trans-Atlantic voyages the results of the first phase of the struggle (September 1939 to May 1940) were not unfavourable to the Allies. After inflicting comparatively heavy losses in September and October 1939, when many ships were still sailing independently, the U-boats did less well until the cataclysmic month of June 1940; and they themselves suffered quite heavily, losing no less than 24 of their number from various causes.
The German occupation of Norway in April and May 1940, followed quickly by the conquest of the Low Countries and France, caused the pendulum of the Battle of the Atlantic to swing very strongly in favour of the Germans; not only had the Royal Navy suffered heavy losses of destroyers and smaller ships in the evacuations, but the U-boats were now able to use bases in Norway and France, so enabling them to reach much further out into the Atlantic and increase the duration of their patrols. Furthermore, new production, though not yet on the massive scale achieved later, had sufficed to replace losses. During their first 'happy time', the so-called 'Aces' Prien, Kretschmer, Endras, Frauenheim, Schepke, Lemp and others - made their names. They operated independently and generally attacked at night while on the surface. The graph representing Allied shipping losses rose steadily and steeply until October 1940.
The second winter of the war saw a slight improvement, which can confidently be attributed to the gradual increase in the number of surface escorts as ships damaged in the evacuations of 1940 returned to service. Churchill's 'destroyers for bases' deal with the Americans, whereby 50 of the oldest American destroyers (of World War One vintage) were transferred to the Royal Navy in exchange for the use of sea and air bases in the western hemisphere, helped to mitigate the shortage of escort vessels. Moreover, despite the heavy sinking’s of merchant ships, there were several encouraging instances in which well-trained escort vessels, sometimes working in conjunction with shore-based aircraft, struck back hard and successfully at the U-boats. Between October 1940 and March 1941, Dönitz gradually introduced new tactics. The number of boats available to him enabled him to replace the single-handed work of the `Aces' by coordinated attacks by a group of U-boats controlled from his own headquarters at Lorient. The system employed was that the first U-boat to sight a convoy would report its position, course and speed by wireless, whereupon the closest supporters would be ordered to join the sighting boat, whose duty it was to keep in touch with the convoy.
When the `wolf pack' had assembled, Dönitz would order it to begin attacks, operating on the surface, undetected by ASDIC. The British reply was therefore to try and turn night into day by the use of illumination, so forcing the U-boats to submerge, break off their attacks, and perhaps offer a target to the escort vessels, which could now detect them. The fitting of searchlights (called Leigh lights after their inventor Squadron-Leader H. de V. Leigh) in Coastal Command aircraft was another development which helped to counter the wolf pack threat, and later became a vitally important instrument. The most promising new counter-measure was the introduction of short-wave radar sets which could detect a surfaced U-boat and could be fitted in escort vessels and aircraft. The fitting of these sets began early in 1941, and the British commands and authorities involved in the Atlantic battle at once gave their production high priority - because the new radars, used in conjunction with the Leigh light or sent up from surface ships, filled the yawning gulf produced by the uselessness of the Asdic against U-boats operating like torpedo boats on the surface.
At the time when the campaign by the wolf packs in the north-western approaches to the British Isles was at its height, the Admiralty made an important change in the shore organization which controlled and directed the Atlantic battle, namely the Western Approaches Command. Its headquarters were shifted from Plymouth, where they had been situated since the beginning of the war, to Liverpool. There the naval authorities were in much closer touch with No.15 Group of Coastal Command. In the same month of February 1941 Admiral Sir Percy Noble took over the Western Approaches Command, and it was he who extended the practice of forming groups of escort vessels, which would as far as possible always work together. He also invigorated the training of key officers and men serving in those ships, and created improved coordination between the naval and RAF headquarters and units involved in the struggle. In April 1941, operational control of Coastal Command aircraft was transferred to the Admiralty, thus establishing unified responsibility at the summit of the command organization, and in November of the following year Admiral Sir Max Horton, a former submarine specialist who had achieved great distinction in World War One and thoroughly understood the capabilities and limitations of such vessels, took over the Western Approaches Command. Though he reaped much that his predecessor had sown he was the ideal opponent to pit against Dönitz, and he held that command until the end of the war.
Throughout the first half of 1941 Allied losses remained high, averaging 44 ships (241 930 tons) per month; but the shorter nights of summer and the new developments already mentioned brought a substantial reduction during the second half, which produced average losses of only 28 ships (120 027 tons) per month. Furthermore, the losses inflicted on the U-boats rose from12 in the first six months to 23 in the second six months; and March 1941 produced the end of the dominance of the U-boat 'Aces' when the boats commanded by Prien, Schepke and Kretschmer were all destroyed. In May these successes were reinforced by the capture of Lemp's command boat U-110, and although she sank while being towed into port her captors' search of her after she had been abandoned by her crew produced material of priceless value to the Allied intelligence and cryptographic organizations. U-559 was also captured by the British in October 1942; three sailors boarded her as she was sinking, and desperately threw all the code books out of the submarine. Two of them, Able Seaman Colin Grazier and Lieutenant Anthony Fasson continued to throw code books out of the ship as it went under water, and they went down with it. The summer of 1941 also saw a great extension of the convoy-south routes between Freetown and Gibraltar and British home ports.
In May 1940, when the loss of the whole of Norway was plainly imminent, British forces were sent to occupy key points in Iceland, whose geographic position made it an ideal place to establish naval and air bases from which the north Atlantic could be covered. Between that date and April 1941 the Allies developed the air and sea escort system which was, with only minor modifications, to last until the end of the war. Early in July 1941 American forces took over responsibility for the defence of Iceland - despite the fact that their country