Battle of the Atlantic (1939-1945)

The Battle of the Atlantic was the longest continuous military campaign of World War II, running from 1939 through the defeat of Nazi Germany in 1945, and was at its height from mid-1940 through to about the end of 1943. The campaign pitted the German Navy’s surface raiders and U-boats against Allied convoys from North America and the South Atlantic to the United Kingdom and Russia, protected mainly by the British and Canadian navies and air forces, later aided by United States ships and aircraft. The German U-boats were joined by Italian submarines after Italy entered the war in June 1940.
The name "Battle of the Atlantic", first coined by Winston Churchill in 1941, is a partial misnomer for a campaign that began on the first day of the European war and lasted for six years, involved thousands of ships and stretched over hundreds of miles of the vast ocean and seas in a succession of more than 100 convoy battles and perhaps 1,000 single-ship encounters. Tactical advantage switched back and forth over the six years as new weapons, tactics and counter-measures were developed by both sides. The British and their allies gradually gained the upper hand, driving the German surface raiders from the ocean by the middle of 1941 and decisively defeating the U-boats in a series of convoy battles between March and May 1943. New German submarines arrived in 1945, but they were too late to affect the course of the war.
==Strategic objectives==
As an island nation with an overseas empire, the United Kingdom was highly dependent on sea-going trade. Britain required more than a million tons of imported food and material per week in order to be able to survive and fight on against Germany. In essence, the Battle of the Atlantic was a tonnage war: the Allied struggle to maintain and the Axis struggle to cut off the shipping that enabled Britain to survive.
From 1942 onwards, the Germans also sought to prevent the build-up of Allied troops and equipment in the British Isles in preparation for the invasion of occupied Europe and to destroy all Allied navies. The defeat of the German threat was a pre-requisite for the invasion.
==Submarine warfare==
Before the war the commander of the German U-boats, Vice Admiral Karl Dönitz, had advocated a system known as the Rudeltaktik or wolf pack, in which groups of U-boats would attack individual merchant ships or whole convoys in mid-ocean and overwhelm any defending warships. In order to be effective, Dönitz calculated that he would need 300 of the latest Atlantic Boats (the Type VII), which would create enough havoc among British shipping that she would be knocked out of the war.
This was in stark contrast to the traditional view of submarine deployment up until then, in which the submarine was seen as a lone ambusher, waiting outside an enemy port to attack ships entering and leaving. This had been a very successful tactic used by British submarines in the Baltic and Bosphorous during World War I, but it could not be successful if port approaches were well patrolled. There had also been naval theorists who held that the submarine should be attached to a main fleet and used in a similar way to a destroyer—this had been tried by the Germans at Jutland with poor results since underwater communications were in their infancy. The Japanese also adhered to the idea of a fleet submarine and never used their submarines either as port blockaders or for convoy interdiction. However, the submarine was still looked upon by much of the naval world as a poor-man’s weapon. This was true in the Kriegsmarine as well, and the Grand Admiral, Erich Raeder, successfully lobbied for the money to be spent on capital ships instead.
The Royal Navy’s main anti-submarine weapon before the war was the inshore patrol craft, armed with hydrophones, a small gun and depth charges. The British Royal Navy, like most navies, had not considered anti-submarine warfare as a tactical subject during the 1920s and 1930s. Unrestricted submarine warfare had been outlawed by the Treaty of Versailles; anti-submarine warfare was seen as ‘defensive’ rather than dashing; and many naval officers believed that anti-submarine work was drudgery similar to mine-sweeping. Though fast destroyers also carried depth charges, it was expected that these ships would be used in fleet actions rather than coastal patrol, so they were not extensively trained in their use.
===ASDIC===
The development of ASDIC, the British secret research on sonar, was as crucial to the Battle of the Atlantic as the development of radar was to the Battle of Britain, and in both cases it was the British who made the crucial breakthroughs. The fact that sound is transmitted effectively by water was well known during the First World War, and microphones placed in water (hydrophones) had been used to listen for submarines at that time. Natural noises and echoes had also been detected using this technique, but the British were the first to develop a working directional 'sound searchlight'.
The acronym ASDIC is often thought to derive from the initials of the British Allied Submarine Detection Investigation Committee; this was given as the official explanation when the system became public knowledge, but it now appears that this was an explanation constructed after the event—no trace of this committee has ever been found. Instead the explanation seems to be that during the secret development of this weapon scientists were encouraged to speak in coded form to avoid spies gleaning the least bit of knowledge. Thus work on sound propagation (ultrasonics) became ASD-ics (anti-submarine detection-ics)
ASDIC comprised a transducer housed in a dome beneath the ship that sent out a narrow beam of sound in a series of pulses that would reflect back from a submerged object within a maximum range of about 3,000 yards. The dome was open to the sea and was to ensure the water around the transducer was relatively still as fast moving water would destroy any signal. The echo produced an accurate range and bearing to the target. But differences in the temperatures at different depths could create false echoes, as could currents, eddies and schools of fish, so ASDIC needed experienced operators to be effective. ASDIC was only effective at low speeds. Above 15 knots or so, the noise of the ship going through the water drowned out the echoes.
The early wartime Royal Navy procedure was to sweep the ASDIC in an arc from one side of the ship's course to the other, stopping the transducer every few degrees to send out a signal. Several ships searching together would be used in a line, a mile or a mile and a half apart. If an echo was detected, and if the operator identified it as a submarine, the ship would be pointed towards the target and would close at a moderate speed, the submarine's range and bearing would be plotted over time to determine course and speed as the ship closed to within 1,000 yards. Once it was decided to attack the ship would close more rapidly, using the target's course and speed data to adjust the course. The intention was for the ship to pass a little way ahead of the submarine, then depth charges would be rolled from chutes in the stern at even intervals and depth-charge throwers would fire further charges some forty meters out on either side. The intention was to lay a depth charge 'pattern' like an elongated diamond, hopefully with the submarine somewhere inside the pattern. But to effectively disable a submarine a depth charge would have to explode within about six meters, in depth as well as in plan. Since early ASDIC equipment was poor on determining depth it was usual to vary the depth settings on part of the pattern.
There were disadvantages to the early versions of this system. Exercises in anti-submarine warfare had been restricted to one or two destroyers hunting a single submarine whose starting position was known in daylight and calm weather, rather than stormy conditions. German U-boats could dive far deeper than British or American submarines, to well below the deepest setting on the British depth charges (A dive depth of over 700 feet against a maximum depth charge setting of 350 feet). More importantly, early ASDIC sets could not look directly down, so the operator lost 'sight' of the U-Boat during the final stages of the attack, a time when the submarine would certainly be manoeuvring rapidly. The explosion of a depth-charge also disturbed the water so that ASDIC contact was very difficult to regain if the first attack had failed.
The belief that ASDIC had solved the submarine problem, the acute budgetary pressures of the Great Depression and the pressing demands for many other types of re-armament meant that little was spent on anti-submarine ships or weapons. Most British naval spending, and many of the best officers, went into the battlefleet. And critically, the British expected that, like the First World War, German submarines would be coastal craft, and only threaten harbour approaches. As a result, the Royal Navy entered the Second World War in 1939 without enough long-distance escorts to protect ocean shipping, and there were no officers with experience of long-range anti-submarine warfare. The situation in the Royal Air Force’s Coastal Command was even more dire, where patrol aircraft could typically only machine-gun the spot where they saw a submarine dive.
==Early skirmishes (September 1939 – May 1940)==
In 1939, the Kriegsmarine lacked the strength to challenge the Allies for command of the sea. Instead, German naval strategy relied on commerce raiding using capital ships, armed merchant cruisers, submarines and aircraft. Many German warships were already at sea when war was declared, including most of the available U-boats and the ‘pocket battleships’ (or Panzerschiff) Deutschland and the Admiral Graf Spee which had sailed out into the Atlantic in August. These ships began an immediate assault on British and French shipping. U-30 sank the liner SS Athenia within hours of the declaration of war – in breach of her orders not to sink passengers ships. The U-boat fleet, which was to dominate so much of the Battle of the Atlantic, was small at the beginning of the war, and many of the 57 available U-boats were the small and short-range Type II U-boats which were useful primarily for mine-laying and operations in British coastal waters. Much of the early German anti-shipping activity involved minelaying by destroyers, aircraft and U-boats off British ports.
With the outbreak of war, the British and French immediately began a blockade of Germany, although this had little immediate effect on German industry. The Royal Navy quickly introduced a convoy system for the protection of trade that gradually extended out from the British Isles, eventually reaching as far as Panama, Bombay and Singapore. Convoys allowed the Royal Navy to concentrate its escorts near the one place the U-boats were guaranteed to be found — the convoys.
But some British naval officers, and particularly the First Lord of the Admiralty, Winston Churchill, sought a more ‘offensive’ strategy. The Royal Navy formed anti-submarine hunting groups based on aircraft carriers to patrol the shipping lanes in the Western Approaches and hunt for German U-boats. But this strategy was deeply flawed because a U-boat, with its tiny silhouette, was always likely to spot the surface warships and submerge long before it was sighted. The carrier aircraft were little help. Although they could spot submarines on the surface, at this stage of the war they had no adequate weapons to attack them. Any submarine found by an aircraft was long gone by the time surface warships arrived. The hunting group strategy proved a disaster within days. On September 14 1939, Britain’s most modern carrier, the HMS Ark Royal, narrowly avoided being sunk when three torpedoes from U 39 exploded prematurely. U 39 was promptly sunk by the escorting destroyers, becoming the first U-boat loss of the war. Failing to learn the lesson, another carrier, the HMS Courageous, was sunk three days later by U 29.
Escort destroyers hunting for U-boats continued to be a prominent, but misguided, feature of British anti-submarine strategy for the first year of the war. The U-boats nearly always proved elusive, and the convoys, denuded of cover, were put at even greater risk.
German success in sinking the Courageous was surpassed a month later when Günther Prien in U-47 penetrated the British base at Scapa Flow and sank the battleship HMS Royal Oak. This was attibuted to insufficient anti-submarine measures and watch officer William Newbigging not being at his post. Prien immediately became a war hero in Germany. 
In the South Atlantic, British forces were stretched by the cruise of the "Pocket Battleship" Admiral Graf Spee, which sank nine merchant ships of 50,000 tons in the South Atlantic and Indian Oceans during the first three months of war. The British and French formed a series of hunting groups including 3 battlecruisers, 3 aircraft carriers and 15 cruisers to seek the raider and her sister Deutschland which was operating in the North Atlantic. These hunting groups scoured the oceans for months with no success until the Graf Spee was caught off the mouth of the River Plate by an inferior British force. She took shelter in neutral Montevideo harbour and the ship was soon scuttled in December 1939.
After an initial burst of activity, the Atlantic campaign quieted down. Karl Dönitz had planned a maximum submarine effort for the first month of the war, with almost all the available U-boats out on patrol in September. That level of operations could not be sustained because the boats needed to return to harbour to refuel, re-stock and refit. The harsh winter of 1939-40, which froze over many of the Baltic ports, seriously hampered the German offensive by trapping several new U-boats in the ice. Finally, Hitler’s plans to invade Norway and Denmark in the spring of 1940 led to the withdrawal of the fleet’s surface warships and most of the ocean-going U-boats to prepare for fleet operations in Operation Weserübung.
The resulting Norwegian campaign revealed serious flaws in the U-boats’ principal weapon, the magnetic torpedo. Although the narrow fjords gave the U-boats little room for manoeuvre, the concentration of British warships, troopships and supply ships provided countless opportunities for the U-boats to attack. Time and again, U-boat captains tracked British targets and fired only to watch the ships sail on unharmed as the torpedoes exploded prematurely or not at all, or ran straight underneath the target. Not a single British warship was sunk by a U-boat in more than 20 attacks. As the news spread through the U-boat fleet, it began to undermine morale. But the director in charge of torpedo development continued to claim it was the crews' fault. In early 1942 the problems were determined to be magnetic problems from the high latitude and a slow leakage of high-pressure air from the submarine into the torpedo's depth regulation gear. Eventually the Kriegsmarine copied some captured British torpedoes which were much more reliable.
=="Happy Time" (June 1940 – February 1941)==
The German occupation of Norway in April 1940, the rapid conquest of the Low Countries and France in May and June and the Italian entry into the war on the Axis side in June transformed the war at sea in general and the Atlantic campaign in particular in three main ways:
# Britain lost her biggest ally. In 1940, the French Navy was the fourth-largest in the world. Only a handful of French ships joined the Free French Forces and fought against Germany, though these were later joined by a few British-built corvettes which played a small but important role in the campaign. With the French fleet removed from the campaign, the Royal Navy was stretched even further. Italy's declaration of war in June meant that Britain also had to reinforce her Mediterranean Fleet and establish a new squadron at Gibraltar, known as Force H, to replace the French fleet in the Western Mediterranean.
# The U-boats gained direct access to the Atlantic. Since the English Channel was relatively shallow and blockaded with minefields by mid 1940, U-boats were ordered not to traverse it and instead travel around the British Isles to reach the most profitable hunting grounds. The French bases at Brest, Lorient, La Pallice and La Rochelle were about 450 miles (720 km) closer to the Atlantic than the German bases on the North Sea. This greatly extended the range of U-boats in the Atlantic, enabling them to attack convoys further west and letting them spend longer on patrol, doubling the effective size of the U-boat force. The Germans later built huge fortified concrete bunkers for the U-boats known as U-boat pens in the French Atlantic bases, which were inpervious to Allied bombing until the development of the Barnes-Wallis tallboy bomb. From early July, U-boats began returning to the new French bases when they completed their Atlantic patrols.
# British destroyers were diverted from the Atlantic. The Norwegian campaign and the German invasion of the Low Countries and France imposed a heavy strain on the Royal Navy’s destroyer flotillas. The Royal Navy withdrew many of its older destroyers from the convoy routes to support the Norwegian operations in April and May and then diverted them to the English Channel to support the withdrawal from Dunkirk. By the summer of 1940 Britain faced a serious threat of invasion. The destroyers were held in the channel where they would be ready to repel a German invasion fleet. The destroyers suffered heavily in these operations when they were exposed to air attack by the Luftwaffe. Seven destroyers were lost in the Norwegian campaign, another six at the Battle of Dunkirk and a further 10 in the Channel and North Sea between May and July, many of them to air attack because they lacked an adequate anti-aircraft armament.Between April and July 1940, the Royal Navy lost 24 destroyers and the Royal Canadian Navy one. Dozens of other destroyers were damaged.
The completion of Hitler’s campaign in Western Europe meant that the U-boats that had been withdrawn for the Norwegian campaign were now released from fleet operations and returned to the war on trade. So at the very time that the number of U-boats on patrol in the Atlantic began to increase, the number of escorts available for the convoys was greatly reduced. The only consolation for the British was that the large merchant fleets of occupied countries like Norway and the Netherlands were under British control. Britain occupied Iceland and the Faeroe Islands to prevent them falling into enemy hands following the German occupation of Denmark and Norway.
It was in these circumstances that Winston Churchill, who had become Prime Minister on May 10, 1940, first wrote to the U.S. President Franklin Roosevelt to request the loan of 50 obsolete U.S. destroyers. This eventually led to the loan (effectively a sale but painted as a loan for political reasons) of the 50 old destroyers under the Destroyers for Bases Agreement in exchange for 99 year leases on certain British bases in Newfoundland, Bermuda and the West Indies, a financially advantagous bargain for the United States, whose population was opposed to entering the war and whose politicians considered that Britain and her allies might actually lose. But the first of these destroyers was only taken over by their British and Canadian crews in September and all needed to be rearmed and fitted with ASDIC. It was to be many months before the relatively obsolete destroyers began to contribute to the campaign.
The Germans too began to get help from their allies. From August 1940, a flotilla of Italian submarines was based on Bordeaux to attack Allied shipping in the Atlantic. The submarines of the Italian Royal Navy (Regia Marina), designed for fleet operations in the Mediterranean, were less well suited to Atlantic convoy operations than the smaller German U-boats. Even so, over the next few years, the 32 Italian submarines that operated in the Atlantic sank 109 ships of 593,864 tons.Rohwer. The Italians were also successful with their use of 'human torpedo' miniature underwater chariots, which disabled several British ships at Gibraltar.
The early U-boat operations from the French bases were spectacularly successful. This was the heyday of the great U-boat aces like Günther Prien of U-47, Otto Kretschmer of U-99, Joachim Schepke of U-100, Engelbert Endrass of U-46, Viktor Oehrn of U-37 and Heinrich Bleichrodt of U-48. The U-boat crews became heroes at home in Germany and referred to this as the 'happy time'. From June until October 1940, over 270 Allied ships were sunk: this period was referred to by U-boat crews as "Die Glückliche Zeit", the Happy Time.
The biggest challenge for the U-boats was to find the convoys in the vastness of the ocean. The Germans had a handful of very long range Focke-Wulf 200 Kondor aircraft based at Bordeaux and Stavanger which were used for reconnaissance, but being essentially a converted civilian airliner, this was a stop-gap solution. Due to ongoing friction between the Luftwaffe and Kriegsmarine, the primary source of convoy sightings was the U-boats themselves. Since a submarine's bridge is very close to the water, their range of visual detection was quite limited.
Instead of attacking the Allied convoys singly, the German U-boats were encouraged to work in packs coordinated centrally by radio. German codebreaking efforts had succeeded in decyphering the British Merchant Marine code book, allowing the Germans to estimate where and when convoys could be expected. The boats spread out into a long patrol line that bisected the path of the Allied convoy routes. Once in position, the crew scanned the horizon with binoculars looking for ship's masts or smoke, or used hydrophones to pick up the propeller noises of the convoys. When one boat sighted a convoy, it would report the sighting to U-boat headquarters before tracking it and waiting for other boats to come up, typically at night. Instead of being faced by a single submarine, the convoy escorts had to cope with a group of up to half a dozen U-boats attacking simultaneously. The most daring commanders, like Otto Kretschmer, penetrated the convoy’s escort screen and attacked from within the columns of merchantmen in the convoy. The escort vessels, which were too few in number and often lacking in endurance, had no answer to multiple submarines attacking on the surface at night as their ASDIC detection apparatus only worked well against underwater targets. Early British marine radar, working in the metric bands, lacked target discrimination and range.
Pack tactics were first used successfully in September and October 1940, to devastating effect in a series of convoy battles. On September 21, Convoy HX-72 of 42 merchantmen was attacked by a pack of four U-boats, losing eleven ships sunk and two damaged over two nights. In October, SC-7, with a weak escort of two sloops and two corvettes, was overwhelmed, losing 59% of its ships. The battle for HX-79 in the following days was in many ways worse for the escorts than that for SC-7. The loss of a quarter of the convoy without any loss to the U-boats despite a strong escort of two destroyers, four corvettes, three trawlers and a minesweeper demonstrated the effectiveness of the German tactics against the inadequate British anti-submarine technology of the time. Finally on December 1, seven German U-boats and three Italian submarines caught Convoy HX-90, sinking 10 ships and damaging three others. The success of pack tactics against these convoys encouraged Admiral Dönitz to adopt the wolf pack as his primary tactic.
Nor were the U-boats the only threat to the convoys. Following some early experience in support of the war at sea during Operation Weserübung, the Luftwaffe contributed small numbers of aircraft to the Battle of the Atlantic from 1940 onwards. These were primarily long-range reconnaissance planes, first with Focke-Wulf 200, and later Junkers 290 maritime patrol aircraft. At first, the Focke-Wulf aircraft were very successful, claiming 365,000 tons of shipping in early 1941. These planes were few in number, however, and were also directly under Luftwaffe control; the pilots had little specialized training for anti-shipping warfare.
==Great surface raiders==
Despite these successes, the U-boat was still not recognized as the primary threat to the North Atlantic convoys. With the exception of men like Dönitz, most naval officers on both sides regarded surface warships as the ultimate commerce destroyers.
For the first half of 1940, there were no German surface raiders in the Atlantic because the German Fleet had been concentrated for the invasion of Norway, and the sole pocket battleship raider, the Admiral Graf Spee, had been stopped at the Battle of the River Plate by an inferior and outgunned British squadron. But from the summer of 1940 a small steady stream of warships and armed merchant raiders set sail from Germany for the Atlantic.
The power of a battleship against a convoy was demonstrated by the fate of Convoy HX-84 which was found by the German pocket battleship Admiral Scheer on 5 November 1940. The Admiral Scheer quickly sank five ships and damaged several others as the convoy scattered. Only the sacrifice of the escorting Armed Merchant Cruiser HMS Jervis Bay and failing light allowed the rest of the convoy to escape. The British now suspended the North Atlantic convoys and the Home Fleet put to sea to try to intercept the Scheer. The search failed as the Scheer had disappeared into the South Atlantic. She reappeared in the Indian Ocean the following month.
Other German surface raiders now began to make their presence felt. On Christmas Day, 1940, the cruiser Admiral Hipper attacked the troop convoy WS-5A, but was driven off by the escorting cruisers.Roskill, p. 291-292. Hipper had more success two months later, on February 12, 1941, when she found the unescorted Convoy SLS-64 of 19 ships and sank seven of them.Roskill, p. 372. In January, 1941, the formidable (and fast) German battlecruisers Scharnhorst and Gneisenau, which outgunned any Allied ship that could catch them, had put to sea from Germany to raid the shipping lanes in Operation Berlin. With so many German raiders at large in the Atlantic, the British were forced to provide battleship escorts to as many convoys as possible. This twice saved convoys from slaughter by the German battlecruisers. In February, the presence of the old battleship HMS Ramillies deterred an attack on Convoy HX-106. A month later, Convoy SL-67 was saved by the presence of the WW1 battleship HMS Malaya.
In May, the Germans mounted the most ambitious raid of all: Operation Rheinübung. The new battleship Bismarck and the cruiser Prinz Eugen put to sea to attack the convoys. Forewarned by intelligence, a British squadron intercepted the raiders off Iceland. But the resulting Battle of the Denmark Strait was a propaganda disaster for the British, with the loss of the battlecruiser HMS Hood. But, thanks to a disabling torpedo hit on her rudders, the Bismarck was caught and sunk by the Home Fleet three days later. Her sinking marked the end of the warship raids.
The Channel Dash, the return of the Scharnhorst, Gneisenau and Prinz Eugen to Germany in February 1942, although an embarrassment for the British, marked the end of the German surface threat in the Atlantic. The loss of the Bismarck, Arctic convoys and the perceived invasion threat to Norway had persuaded Hitler to withdraw. Hitler, an Austrian by birth, had little Naval expertise and was unwilling to risk his heavily-promoted capital ships.
War had come too early for the German Plan Z naval expansion plan to be close to completion. The concept of battleships powerful enough to destroy any convoy escort, with accompanying ships able to annihilate the convoy, was never achieved. But although the number of ships the warship raiders sank was relatively small when compared with the losses to U-boats, mines and aircraft, their raids severely disrupted the Allied convoy system, seriously reducing British imports.
The German battlecruiser tactics of retreat when challenged stand in marked contrast to the British 'Nelsonian' tactics of closing immediately with the enemy, no matter what the imbalance in firepower was. If the Kriegsmarine had stood and fought when they had good odds, the Battle of the River Plate would have been a German victory, Convoys HX-34, HX-106 and SL-67 would have been annihilated, and the British would have likely lost the Prince of Wales as well as the Hood in the Battle of the Denmark Strait. The cause seems to be a mixture of dread of the reputation of the Royal Navy, mixed with the strategic desire to preserve the few German capital ships afloat. A lack of tactical experience is also evident - individual German battle units were in many ways superior to older Royal Navy units, and could have prevailed in set battles if handled correctly.
==Escorts strike back (March 1941 – May 1941)==
The disastrous convoy battles of October 1940 forced a change in British tactics. The most important of these was the introduction of permanent escort groups to improve the co-ordination and effectiveness of ships and men in battle. British efforts were helped by a gradual increase in the number of escort vessels available as the old ex-American destroyers and the new British- and Canadian-built Flower class corvettes were now coming into service in numbers. Many of these ships became part of the huge expansion of the Royal Canadian Navy, which grew from a handful of destroyers at the outbreak of war to take an increasing share of convoy escort duty. Others of the new ships were manned by Free French, Norwegian and Dutch crews, but these were a tiny minority of the total number, and directly under British command. By 1941 American public opinion had begun to swing against Germany, but the war was still essentially Great Britain and the Empire against Germany.
Initially, the new escort groups consisted of two or three destroyers and half a dozen corvettes. Since two or three of the group would usually be in dock repairing weather or battle damage, the groups typically sailed with about six ships. The training of the escorts also improved as the realities of the battle became obvious. A new base was set up at Tobermory in the Hebrides to prepare the new escort ships and their crews for the demands of battle under the strict regime of Vice Admiral Gilbert O. Stephenson.Roskill, p. 358-359.
In February 1941, the Admiralty moved the headquarters of Western Approaches Command from Plymouth to Liverpool, where much closer contact with, and control of, the Atlantic convoys was possible. Greater co-operation with supporting aircraft was also achieved. In April, the Admiralty took over operational control of Coastal Command aircraft. At a tactical level, new short-wave radar sets that could detect surfaced U-boats and were suitable for both small ships and aircraft began to arrive during 1941.
The impact of these changes first began to be felt in the convoy battles during the spring of 1941. In early March, Prien in U 47 failed to return from patrol. Two weeks later, in the battle of Convoy HX-112, the newly formed 3rd Escort Group of five destroyers and two corvettes held off the U-boat pack. U 100 was detected by the primitive radar on the destroyer Vanoc, rammed and sunk. Shortly afterwards the U 99 was also caught and sunk, its crew captured. Dönitz had lost his three leading aces: Kretschmer, Prien and Schepke.
Dönitz now moved his wolf packs further west, in order to catch the convoys before the anti-submarine escort joined. This new strategy was rewarded at the beginning of April when the pack found Convoy SC-26 before its anti-submarine escort had joined. Ten ships were sunk, but another U-boat was lost.
On May 9, the British destroyer HMS Bulldog captured U-110 and recovered a complete, intact Enigma Machine. Combined with a couple of other captures, this was a vital breakthrough for the Allied code-breaking efforts. The machine was taken to Bletchley Park, where it was used to help break the German codes. This, and the genius of men like Flower and Turing would give Britain the ability to read German naval signals for much of the remainder of the campaign, and, incidentally, provide the impetus for the development of the first programmable electronic device, the Colossus computer.
==Field of battle widens (June 1941 – December 1941)==
In June 1941, the British decided to provide convoy escort for the full length of the North Atlantic crossing. To this end, the Admiralty on 23 May asked the Royal Canadian Navy to assume the responsibility for protecting convoys in the western zone and to establish the base for its escort force at St. John's, Newfoundland. On 13 June 1941 Commodore L.W. Murray, Royal Canadian Navy, assumed his post as Commodore Commanding Newfoundland Escort Force, under the overall authority of the Commander in Chief, Western Approaches, at Liverpool. Six Canadian destroyers and 17 corvettes, reinforced by seven destroyers, three sloops and five corvettes of the Royal Navy, were assembled for duty in the force, which escorted the convoys from Canadian ports to Newfoundland and then on to a meeting point south of Iceland, where the British escort groups took over.
By 1941 the United States was taking an increasing part in the war, despite its nominal neutrality. In April 1941 President Roosevelt extended the 'Pan-American Security Zone' east almost as far as Iceland. British forces had occupied Iceland when Denmark fell to the Germans in 1940, the US was persuaded to provide forces to relieve British troops on the island. American warships began escorting Allied convoys in the western Atlantic as far as Iceland, and had several hostile encounters with U-boats.
At the same time, the British were working on a number of technical developments which would address the German submarine superiority. It is interesting to note that, though these were British inventions, the critical technology was provided freely to the US, who then re-named and manufactured them. In many cases this has resulted in the misconception that these were American developments.
Firstly, new depth charges were developed that fired to the side of the destroyers rather than simply dropping them over the stern as the destroyer passed over. The asdic contact was lost directly underneath the boat, and the U-boats often used this to escape. In addition, depth charges were fired in patterns, to 'box' the enemy in with explosions. The shockwaves would then destroy the U-boat by crushing it in the middle of these explosions.
===Catapult Aircraft Merchantmen===
Aircraft ranges were also improving all the time, but the Atlantic was far too large to be covered completely at the time. A stop-gap measure was instituted by fitting ramps to the front of some of the cargo ships known as Catapult Aircraft Merchantmen (CAM ships), armed with a lone expendable Hurricane fighter aircraft. When a German plane approached, the fighter was fired off the end of the ramp with a large rocket to shoot down or drive off the German aircraft, the pilot ditching in the water and being picked up by one of the escort ships if land was too far away. Eight combat launches were made, resulting in the destruction of six Axis aircraft for the loss of one Allied pilot.
German aircraft had been gradually driven out of the campaign by the growing strength of RAF Coastal Command and the introduction of first CAM ships.
===High-Frequency Direction-Finding===
One of the most important developments was that of ship-borne direction-finding radio equipment, known as HF/DF (High-Frequency Direction-Finding) or Huff-Duff, which was gradually fitted to the larger escort ships. HF/DF let an operator see the direction of a broadcast, even if the messages they were sending could not be read. Since the Wolf pack tactics relied on U-boats surfacing to report the position of a convoy, there was a steady stream of messages to intercept. A destroyer could then run down the direction of the signal to attack the U-boat, or at least force it to submerge, preventing a coherent attack on the convoy. When two ships fitted with HF/DF were present with a convoy, the exact position of the U-boat could be triangulated. The British also made extensive use of shore HF/DF stations, so they could keep convoys updated with positions of U-Boats around them at all times.
The radio technology behind land based HF/DF was well understood by both sides. However useful shipborne HF/DF appeared too difficult because of severe distortion to the received radio wavefront due to reflections from the ship's superstructure. However, a solution to this problem was proposed by the Polish engineer Waclaw Struszynski whilst working at the Admiralty Signal Establishment in England. Struszynski also led the team which developed the practical system, and these systems were installed on Royal Navy ships in 1942. The Germans continued to believe that useful HF/DF could not be achieved at sea, and U-Boat radio operators continued to use HF radio. The British system used an oscilloscope-based indicator which almost instantly fixed the position of the shortest HF message. With this there was hardly any need to triangulate - the escort could just run down the precise bearing provided and use radar for final positioning. Many U-Boats attacks were suppressed and submarines sunk in this way - a good example of the great difference technology made in this battle.
===Enigma cipher===
A major factor in the success of the British during the second half of 1941, and throughout the rest of the campaign, was the cracking of the Naval Enigma machine cipher. The wolf pack tactics relied on radio communications, based on the assumption that the Enigma cipher could not be broken and that short signal messages could not be pinpointed with enough accuracy to endanger the signalling U-boat. Both assumptions were wrong. Throughout the summer and autumn of 1941, a combination of reading Enigma messages and radio direction finding enabled the British to plot the positions of the U-boat patrol lines, allowing the convoys to be routed to evade them.
But this infusion of strength to the Allied side had to be set against the growing numbers of U-boats now coming into service. The German Type VIIC submarine started reaching the Atlantic in large numbers in 1941; eventually 585 of them would be delivered. Although the Allies generally succeeded in defending the convoys through the summer and autumn of 1941, they were not sinking U-boats in anything like sufficient numbers. The Flower corvette escorts could detect and defend, but they were not fast enough to go on the attack.
In October 1941, Hitler ordered Dönitz to move many of the U-boats into the Mediterranean, to support German operations in that theatre. The resulting concentration near Gibraltar resulted in a series of battles around the Gibraltar and Sierra Leone convoys. In December 1941, Convoy HG-76 sailed, escorted by the 36th Escort Group of two sloops and six corvettes under Captain Frederic John Walker, reinforced by the first of the new escort carriers HMS Audacity and three destroyers from Gibraltar. The convoy was immediately intercepted by the waiting U-boat pack, resulting in a brutal battle. Walker was a tactical innovator, his ships were highly trained and the presence of an escort carrier meant that the U-boats were frequently sighted and forced to dive before they could get close to the convoy. Over the next five days, five U-boats were sunk (four by Walker's group) despite the loss of the Audacity after two days. The British lost the Audacity, a destroyer and just two merchant ships. The battle was the first clear Allied convoy victory in the campaign.
Through dogged effort, the Allies slowly gained the upper hand through until the end of 1941. Although Allied warships failed to sink U-boats in large numbers, most convoys evaded attack completely. Shipping losses were high, but manageable.
== Operation Drumbeat (January 1942 – June 1942)==
The attack on Pearl Harbor and the subsequent German declaration of war on the United States had an immediate effect on the campaign. Dönitz promptly planned to attack shipping off the American East Coast. Dönitz had only 12 boats of the Type IX class that were able to make the long trip to the U.S. East Coast, and half of them had been removed by Hitler’s order to counter British forces in the Mediterranean. One of the remainder was under repair, leaving only five boats to set out for the U.S. on the so-called Operation Drumbeat (Paukenschlag).
The U.S., having no direct experience of modern naval war on its own shores, did not employ shore-side black-outs. The U-boats simply stood off the shore of the eastern sea-board and picked off ships as they were silhouetted against the lights of the cities. Also, the U.S. Commander-in-Chief of the Atlantic Fleet, Admiral Ernest King, who hated the British, initially rejected the Royal Navy's calls for a coastal blackout or a convoy system. King has been criticized for this decision, but his defenders argue that the United States destroyer fleet was limited (partly because of the sale of 50 old destroyers to Britain earlier in the war), and King claimed that it was far more important that the destroyers protect Allied troop transports than shipping. This does not explain the refusal to require coastal black-outs, or to respond to any advice the Royal Navy provided. No troop transports were lost, but merchant ships sailing in U.S. waters were left exposed and suffered greatly. Britain eventually had to build coastal escorts and provide them for free to the U.S. in a 'reverse Lend Lease', since King was unwilling to make any provision himself.
The first boats started shooting on January 13, 1942, and by the time they left for France on February 6 they had sunk 156,939 tonnes of shipping without loss. The first batch of Type IXs had been replaced by Type VIIs and IXs refuelling at sea from Type XIV Milk Cows tankers and had sunk 397 ships totalling over 2 million tons. At the same time, not a single troop transport was lost. In 1943, the United States launched over 11 million tons of merchant shipping; that number declined in the latter war years, as priorities moved elsewhere.
In May, King (promoted to Commander-in-Chief U. S. Fleet and the Chief of Naval Operations) was forced to institute a convoy system. This quickly led to the loss of seven U-boats. But the U.S. did not have enough ships to cover all the holes, and the U-boats continued to operate freely during the Battle of the Caribbean and throughout the Gulf of Mexico (where they effectively closed several U.S. ports) until July, when the British-loaned escorts began arriving. The institution of an interlocking convoy system on the American coast and in the Caribbean Sea in mid-1942 resulted in an immediate drop in attacks in those areas. Attention shifted back to the Atlantic convoys. For the Allies, the situation was serious but not critical throughout much of 1942.
Operation Drumbeat had one other effect. It was so successful that Dönitz’s policy of economic war was seen even by Hitler to be the only effective use of the U-boat, and he was given complete command to use them as he saw fit. Meanwhile, Dönitz’s commander Raeder was dismissed as a result of a disastrous Battle of the Barents Sea in which two German heavy cruisers were beaten off by half a dozen Royal Navy destroyers. Dönitz was eventually made Grand Admiral of the fleet, and all building priorities turned to the U-boats.
==Battle returns to mid-Atlantic (July 1942 – February 1943)==
With the U.S. quickly arranging convoys, ship losses to the U-boats quickly dropped, and Dönitz realized his boats were better used elsewhere. On July 19, 1942, he ordered the last U-boats to withdraw from the United States Atlantic coast, and by the end of July 1942 he shifted his attention back to the North Atlantic.
There were enough U-boats spread across the Atlantic to allow several wolf packs to attack several different convoy routes. Often as many as 10 to 15 boats would attack in one or two waves, following the convoys by day and attacking at night. Losses quickly increased, and in October 1942 56 ships of over 258,000 tonnes were sunk in the ‘air gap’ between Greenland and Iceland that was still free of the ever-increasing Allied air patrols.
On November 19 1942, Admiral Noble was replaced as Commander-in-Chief of Western Approaches Command by Admiral Sir Max Horton. Horton used the growing number of escorts that were becoming available to Western Approaches Command to organize "support groups" that were used to reinforce convoys that came under attack. Unlike the regular escort groups, the support groups were not directly responsible for the safety of any particular convoy. This lack of responsibility gave them much greater tactical flexibility, allowing the support groups to detach ships to hunt submarines spotted by reconnaissance or picked up by high-frequency direction finding (HF/DF). In situations where the regular escorts would have had to return to their convoy, the support groups were able to persist in hunting a submarine for many hours. One tactic used by Captain Walker was to sit on top of a U-boat and wait until its air ran out and it was forced to the surface.
===Hedgehog===
By late 1942, the British had developed a new weapon, and warships were being fitted with the Hedgehog anti-submarine mortar which fired twenty-four contact-fused bombs directly 'ahead' of the attacking ship. Unlike depth charges, which exploded at certain set depths 'behind' the attacking warship disturbing the water and making it hard to keep track of the target, Hedgehog charges only exploded if they hit a U-boat. This meant that a U-boat could be continuously tracked and attacked until it was sunk. The Hedgehog was a particularly effective weapon, raising the percentage of kills from 7% of attacks to nearer 25%. When one of the Hedgehog charges exploded, it set off the others which increased the weapon's effectiveness.
===Leigh Light===
Detection by radar-equipped aircraft could suppress U-boat activity over a wide area, but an aircraft attack would only be successful with good visibility. U-boats were quite safe from aircraft at night, since the deployment of an illuminating flare gave adequate warning of an attack.
The introduction by the British of the Leigh Light in June 1942 was a significant factor in the North Atlantic struggle. It was a powerful searchlight that was automatically aligned with the airborne radar to illuminate targets suddenly while in the final stages of an attack run. This let British aircraft attack U-boats recharging batteries on the surface at night, forcing German submarine skippers to switch to daytime recharges.
The U-Boat commanders who survived reported a particular fear of this weapon system since the hum of an aircraft was inaudible at night above the noise of the boat. The aircraft acquired the submarine using centimetric radar which was undetectable with the typical U-Boat equipment, then lined up on an attack run. When metric radar was used, the set would automatically lower the radar power during the approach so that the submarine would not think it was being tracked. With a mile or so to go the searchlight would automatically come on, immediately and accurately illuminating the target from the sky, which had about 5 seconds warning before it was hit with a stick of depth-charges. A drop in Allied shipping losses from 600,000 to 200,000 tonnes per month was attributed to this ingenious device.
===Metox receiver===
By August 1942 U-boats were being fitted with radar detectors to enable them to avoid the sudden ambushes which a radar-equipped aircraft or corvette might spring. The first such receiver, named the Metox after its French developer, was capable of picking up the metric radar bands used by the early radars. This not only enabled U-boats to avoid detection by Canadian and US escorts, which were equipped with obsolete radar sets, but allowed them to track convoys where these sets were in use.
== Climax of the campaign (March 1943 – May 1943, "Black May")==
The winter weather provided a short respite from the fighting in January and February 1943, but in the spring of 1943 convoy battles started up again with the same ferocity. By the spring of 1943, there were so many U-boats on patrol in the North Atlantic that it was difficult for the convoys to evade detection, resulting in a succession of vicious convoy battles. In March another 260,000 tonnes were sunk and the escorts were heavily defeated in the battles of convoys SC-122 and HX-229.
The supply situation in Britain was such that there was talk of being unable to continue the war effort, with supplies of fuel being particularly low. It appeared that Dönitz was winning the war. And yet the next two months would see a complete reversal of fortunes.
The turning point was the battle centered around the slow Convoy ONS-5 (April–May 1943), when a convoy of 43 merchantmen escorted by two destroyers, a frigate and corvettes was attacked by a pack of 30 U-boats. Although 13 merchant ships were sunk, the U-boats were detected by HF/DF, six U-boats were sunk by the escorts or Allied aircraft. Despite a storm which scattered the convoy – the merchantmen reached the protection of land-based air cover causing Admiral Dönitz to call off the attack.
In April, losses of U-boats shot up while their kills of ships fell dramatically. By May, wolf packs were no longer organized and that month was to become known as Black May for the U-Boat. 
===Convergence of technologies===
The Battle of the Atlantic was won by the Allies in two months. There was no single reason for this, but what had changed was a sudden convergence of technologies, combined with an increase in Allied resources. 
The mid-Atlantic gap that had been unreachable by aircraft was closed by long-range B-24 Liberator aircraft. By spring 1943 the British had developed an effective sea-scanning centimetric radar small enough to be carried on patrol aircraft armed with airborne depth charges. Centimetric radar greatly improved detection and nullified the German Metox radar warning equipment. Further air cover was provided by the introduction of merchant aircraft carrier or MAC ships and later the growing numbers of American-built escort carriers. Flying Grumman Wildcats primarily, they sailed in the convoys and provided the much needed air cover and patrols all the way across the Atlantic. 
The larger numbers of escorts became available, both as a result of American building programmes and the release of escorts that had been tied up in the North African landings during November and December 1942. In particular, destroyer escorts (similar British ships were known as frigates) were designed, which could be build more economically than expensive fleet destroyers and were also more seaworthy than corvettes. There would not only be sufficient numbers of escorts to securely protect convoys, they could also form hunter-killer groups (often centered around escort carriers) to aggressively hunt U-boats.
The continual breaking of the German naval Enigma enabled the Allied convoys to evade the wolf packs while British support groups and American hunter-killer groups were able to hunt U-boats that approached the convoys or whose positions were revealed by Enigma decrypts.
Also crucial was the US shipbuilding production, as they could literally build merchant ships at a faster rate than the U-boats could sink them, thus enabling them to win the "tonnage war of attrition".
Allied air forces developed tactics and technology to make the Bay of Biscay, the main route for French based U-boats, very dangerous. The introduction of the Leigh Light enabled accurate attacks on U-boats re-charging their batteries on the surface at night. The Luftwaffe responded by providing fighter cover for U-boats exiting into and returning from the Atlantic and for returning blockade runners. Still, with intelligence coming from resistance personnel in the ports themselves, the last few miles to and from port proved hazardarous to many U-Boats.
==Final years (June 1943 - May 1945)==
The Germans had lost the technological race. Over the next two years, large numbers of U-boats were sunk, usually with all hands. With the battle won, supplies started to pour into England for the eventual invasion of France. This was clear even to the Germans, who became desperate to restart the battle.
Several attempts were made to salvage the Type VII force. Notable among these attempts were the fitting of massively improved anti-aircraft batteries, radar detectors, and finally the addition of the Schnorchel (snorkel) device to allow them to run underwater off their diesel engines to avoid radar. None of these were truly effective however, and by 1943 Allied air power was so strong that the U-boats were being attacked right in the Bay of Biscay as they left port.
From 1943 onwards, German He 177 bombers with Henschel Hs 293 guided glider bombs were sometimes used for attacks on convoys, claiming a number of successes. Lack of air superiority prevented them being a major threat to the Royal Navy.
Development of torpedoes also improved, with weapons which ran a pre-programmed course criss-crossing the convoy path. Perhaps the most dangerous was the development of the 'German Navy Acoustic Torpedo' or 'GNAT', which would home on the propellor noise of a target. This was very effective when first used, but within three days the Canadians had developed a practical countermeasure - a bunch of old pipes towed behind the ship which rattled and distracted the torpedo.
Late in the war, the Germans introduced the "Elektroboot" series, the Type XXI U-boat and a short range Type XXIII U-boat, finalized in January 1943 but production only commencing in 1944-1945. When underwater the Type XXI managed to run at 17 knots, faster than a Type VII running full out on the surface and almost as fast as the ships attacking her. But mass production of the new types didn't get going until 1944 and only one combat patrol was carried out by a Type XXI before the war ended, making no contact with the enemy.
The last submarine battle of the war took place on May 5, 1945. Commander in Chief of Submarines (Befehlshaber der Unterseeboote) Karl Dönitz ordered all U-boats to cease offensive operations and return to their bases, commending them "you have fought like lions"! U-853, lying in wait off Point Judith, Rhode Island, did not receive that order. Soon after, her torpedo struck SS Black Point, a collier underway for Boston, Massachusetts. Within 15 minutes, Black Point had capsized in 95 feet of water, the last US-flagged merchant ship sunk in World War II. Twelve men died, while 34 crew members were rescued. One of the rescuing ships, SS Kamen, sent a report of the torpedoing that was picked up by four American warships:
USS Ericsson (DD-440), 
USS Amick (DE-168), 
USS Atherton (DE-169), and 
USS Moberly (PF-63) 
discovered U-853 bottomed in 18 fathoms (33 m), and dropped more than a hundred depth charges through the night. The next morning, May 6, two blimps from Lakehurst, New Jersey, K-16 and K-58, joined the attack, locating oil slicks and marking suspected locations with smoke and dye markers. K-16 also attacked with 7.2-inch rocket bombs. Finally, planking, life rafts, a chart tabletop, clothing, and an officer's cap floated to the surface, indicating the destruction, with all hands, 55 officers and men, of the second to last U-boat sunk during World War II.
==Outcomes==
The Germans failed to strangle the flow of strategic supplies to Britain, and that failure resulted in the massive build-up of troops and supplies needed for the Normandy landings. The defeat of the U-boat campaign was a necessary precursor for the re-supply of Britain, and the build-up of a huge concentration of Allied forces that ensured Germany's defeat.
Victory was achieved at a huge cost: between 1939 and 1945, 3,500 Allied ships were sunk (gross tonnage 14.5 million) at a cost of 783 German U-boats.
Allies	Germans</tr>	30,248 merchant sailors	28,000 sailors</tr>	3,500 merchant vessels	783 submarines sunk</tr>	175 warships	</tr>
==See also==
* Timeline of the Second Battle of the Atlantic
* Battle of the Atlantic (1914-1918) for the First World War submarine campaign
* Arctic Convoys of World War II for the convoys to Russia
* Aces of the Deep, the ten most successful U-boat commanders of the war
* Black May (1943)
* The Norwegian Shipping and Trade Mission - Nortraship
* Das Boot - highly regarded for its realistic portrayal of World War II submarine warfare
* Silent Hunter III - Computer simulation of the Battle of the Atlantic
==Notes==
==References==
*Macintyre, Donald U-boat Killer (Avon Publications, 1956)
*Rohwer, Jürgen Die italienischen U-Boote in der Schlacht im Atlantik 1940-43
*Roskill, S.W. The War at Sea. Four volumes. (London: HMSO 1954-61)
==Further reading==
===Official histories===
* Behrens, C.B.A. Merchant Shipping and the Demands of War London: HMSO)
* Morison, S.E. The Two Ocean War and History of United States Naval Operation in World War II in 15 Volumes. Volume I The Battle of the Atlantic and volume X The Atlantic Battle Won deal with the Battle of the Atlantic
* Schull, Joseph. The Far Distant Ships
===Biographies===
* Cremer, Peter. U-333
* Dönitz, Karl. Ten Years And Twenty Days
* Gretton, Peter. Convoy Escort Commander (London). Autobiography of a former escort group commander
* Macintyre, Donald. U-boat Killer (London). Autobiography of another former escort group commander
* Rayner, Denys, Escort: The Battle of the Atlantic (London: William Kimber 1955)
* Robertson, Terence. The Golden Horseshoe (London). Biography of the top German U-boat ace, Otto Kretschmer
* Robertson, Terence. Walker R.N. (London 1955). Biography of the leading British escort group commander, Frederick John Walker
* Werner, Herbert A. Iron Coffins: The account of a surviving U-boat captain with historical and technical details
===General histories of the campaign===
* Blair, Clay. Hitler's U-boat War. Two volumes. Comprehensive history of the campaign
* Fairbank, David. Bitter Ocean: The Battle of the Atlantic, 1939-1945
* Gannon, Michael. Operation Drumbeat (Harper and Row)
* Macintyre, Donald. The Battle of the Atlantic (London 1961). Excellent single volume history by one of the British Escort Group commanders
* Rohwer, Dr. Jürgen. The Critical Convoy Battles of March 1943 (London: Ian Allan 1977). ISBN 0-7110-0749-7. A thorough and lucid analysis of the defeat of the U-boats
* Williams, Andrew, The Battle of the Atlantic: Hitler's Gray Wolves of the Sea and the Allies' Desperate Struggle to Defeat Them
* Woodman, Richard. The Real Cruel Sea; The Merchant Navy in the Battle of the Atlantic, 1939-1943 (London 2004) 
==External links==
*Battle of the Atlantic exhibition at Merseyside Maritime Museum
*U.S. Merchant Shipping and the British Import Crisis
*German U-Boats and Battle of the Atlantic
*Foreign Flagged Ships of the US War Administration sunk/damaged World War II
*Website for HMCS Sackville - the last remaining WWII corvette
*The Royal Naval Patrol Service - protecting convoys from U-boats and clearing enemy mines during World War II
*Sub.net ITALIA: Italian Submarines in Battle of Atlantic Pages
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