Second Battle of the Atlantic

From Academic Kids

The Second Battle of the Atlantic was the longest continuous military campaign of World War II, running from 1939 right through to the defeat of Nazi Germany in 1945, and was at its height from mid-1940 through to about the end of 1943. The naval battle pitted Nazi Germany's U-boats against convoys from North America to the United Kingdom, protected mainly by the British and Canadian navies and air forces, which were later aided by United States forces. Although many ships were sunk, the Allies gradually gained the upper hand.


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 materiel per week in order to be able to survive and fight on against Nazi Germany. In essence, the Battle of the Atlantic was the Allied struggle to maintain and the Axis struggle to cut off the shipping that enabled Britain to survive.

The mining threat

The U-boat fleet, which was to dominate so much of the battle of the Atlantic, was very small at the beginning of the war and much of the early action by German forces involved mining convoy routes and ports around Britain.

Initially, contact mines were employed, which meant that a ship had to physically strike one of the mines in order to detonate it. Contact mines are usually suspended on the end of a cable just below the surface of the water and laid by ship or submarine. By the beginning of World War II most nations had also developed mines that could be dropped from aircraft, making it practicable to lay them in enemy harbours (although they simply floated on the surface). The use of dredging and nets was effective against this type of mine, but nonetheless this process was time-consuming and involved the closing of harbors while it was completed.

Into this arena came a new mine threat. Most contact mines leave holes in ship's hulls, but some ships surviving mine blasts were limping back to port with buckled plates, popped rivets, and broken backs. This appeared to be due to a new type of mine that was detonating at a distance from the ships, and doing the damage with the shockwave of the explosion. Often ships that had successfully run the gauntlet of the Atlantic crossing were destroyed entering freshly mineswept harbors on Britain's coast. More shipping was now being lost than could be replaced, and Churchill ordered that the recovery, intact, of one of these new mines was to be given highest priority.

Then the British experienced a stroke of luck in November 1939. A German mine was dropped from an aircraft laying mines onto mud flats in the Thames estuary, well above the waterline. As if this was not sufficiently good fortune, the land happened to belong to the army, and a base, including men and workshops, was close at hand. Experts were quickly dispatched from London to investigate the mine. They had some idea by this time that the mines used magnetic sensors, so they had everyone remove all metal, including their buttons, and made new tools out of non-magnetic brass. They then disarmed the mine and rushed it to labs at Portsmouth, where scientists discovered a new type of arming mechanism inside.

The arming mechanism had a sensitivity level that could be set, and the units on the scale were milligauss. Gauss is a measurement for the strength of a magnetic field, and so they knew why it went off before coming into contact with the ship. Using the detector from the mine, they were able to study the effect of a ship passing over it. A ship or any large ferrous object passing through the earth's magnetic field will concentrate the field at that point. The detector from the mine measured this effect, and was designed to go off at the mid-point of the ship passing overhead.

From this crucial data, methods were developed to clear the mines. Early methods included the use of large electromagnets dragged behind ships, or on the undersides of low-flying aircraft (a number of older bombers like the Vickers Wellington were used for this purpose). However both of these methods had the disadvantage of "sweeping" only a small strip at a time. A better solution was found in the form of electrical cables dragged behind ships, passing a large current through the seawater. This induced a huge magnetic field and swept the entire area between the two ships. The older methods continued to be used in smaller areas; the Suez Canal continued to be swept by aircraft, for instance.

While these methods were useful for clearing mines from local ports, they were of little or no use for enemy controlled areas. These were typically visited by warships only, and the majority of the fleet then underwent a massive degaussing process, where their magnetic fields were reduced to such a degree that it was no longer "noticed" by the mines. This started in late 1939, and by 1940 British warships were largely immune for the few months at a time until they once again built up a field. Many of the boats that sailed to Dunkirk were degaussed in a marathon four-day effort by hard-pressed degaussing stations.

The Germans had also developed a pressure-activated mine and planned to deploy it as well, but they saved it for later use when it became clear the British had beaten the magnetic system.

The 'Happy Time'

Prior to the war the admiral of the U-boats, Karl Dönitz, had advocated a system known as the Rudel or wolf pack, in which teams of U-boats would gang up on convoys and simply overwhelm the defending warships accompanying them. He also developed a theory of destroying an enemy fleet, not by attacking their ships directly, but by cutting off their supplies so they could not be used — an economic war. In order to be effective he 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.

However the U-boat was still looked upon by much of the naval world as a poor-man's weapon, and deliberately hunting merchant ships to be used only by cowards. This was true in the Kriegsmarine as well, and the Grand Admiral, Erich Raeder, successfully lobbied for monies to be spent on large capital ships instead. These were of even more dubious use considering the huge British fleet facing them, and even Raeder himself suggested they would be wiped out very quickly in the event of war.

Thus the U-boat fleet started the war consisting mainly of the short range Type II which was useful primarily for mine-laying and operations in and around the British coastal areas. They had neither the range nor the supplies to operate far from land, and as a result the RAF was able to counter the U-boats to some degree with standing patrols by Coastal Command aircraft. Early operations of aircraft against the U-boats were somewhat comical, but the crews gained experience quickly and the Western Approaches were soon cleared of the threat.

Meanwhile Royal Navy destroyers were being equipped with increasingly powerful sonar systems (known to the RN as ASDIC) and were able to block the exits into the North Sea and the Channel with some success. ASDIC was unable to find U-boats on the surface where they spent the vast majority of their time, but with aircraft cover forcing them underwater, running to the Atlantic could be a somewhat dangerous operation.

Atlantic operations

However with the fall of France the Kriegsmarine gained direct access to the Atlantic ocean. Huge fortified concrete ports for the U-boats were built, which resisted any successful bombing throughout the course of the war. Most of the U-boat fleet was moved to these bases where they also had excellent air cover, making it much harder for both the RAF and RN to do anything about it.

In addition the new Type VIIc design started arriving in large numbers in 1940. The VII was much more powerful than the Type II it replaced, including both a rapid-fire 88mm deck gun and four forward torpedo tubes. It also was much larger than the Type II, and could spend long times at sea, well away from land. Earlier VIIa and VIIb's had already reached service in small numbers, but the c was put into full production and eventually 585 of them would be delivered.

The Type VII dramatically increased the problems for the British. The boats would operate long distances from shore, meaning that they were well out of the range of land-based aircraft to harass them. The only counter was the Royal Navy's ships, but there were far too few of them to cover the vast amount of the sea that these boats operated in.

The RN had yet to institute the policy of convoys, primarily because it slows all of the boats down to the speed of the slowest member. The few Type VII's already delivered were able to escape into the Atlantic at night and then wait for ships to pass. They would then run on the surface and hunt down the scattered cargo ships with their gun. The early operations were spectacularly successful, and the U-boat crews were heroes to the people in the motherland. The crews referred to this as the 'happy time'.

The RN quickly introduced a convoy system which allowed them to concentrate their warships near the one place the U-boats were guaranteed to be found – the convoys. This had some effect, but not what they had hoped. The speed of these newer boats compared to their WWI counterparts meant that they could often run to the front of the convoy, wait for the convoy to sail into torpedo range underwater, fire a salvo, and leave long before the escorts could get to them.

However the German effort also had problems of its own. Their torpedoes continued to fail with an alarming rate, and the director in charge of their development continued to claim it was the crews' fault. Eventually this came to a head when one U-boat ace shot three perfect hits into the side of the HMS Ark Royal, only to watch all three explode harmlessly far away from the ship's side. Scenes like this continued until the matter was finally taken to hand in April 1940, although it wasn't until early 1942 that the problems were completely addressed.

Another issue was finding the convoys in a very large ocean. The Germans had nowhere near the number of patrol boats or tracking stations needed to make accurate fixes from shore. Instead they had a handful of very long range aircraft (namely the Fw 200 Condor) used as spotters. To this they added codebreaking efforts, which eventually succeeded in breaking the British Merchant Marine code book, allowing them to time the convoys as they left North America from Halifax and Sydney, Nova Scotia.

But the primary source of tracking was the U-boats themselves. They were strung out in lines across the North Atlantic waiting for a passing convoy. When spotting one, they would radio the position back to Kriegsmarine headquarters, where a furious effort would break out to vector other U-boats onto the attack. As the numbers of U-boats and the proficiency of the headquarters grew, they were eventually able to consistently form the wolfpacks that Dönitz wanted.

At the same time a number of technical developments looked set to aid the Allies. Firstly, new depth charges were developed that fired in front of the destroyers rather than simply dropping them over the side as the destroyer passed over. The sonar 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. A device used to achieve this was called Hedgehog, the name deriving from the firing spindles. This fired twelve contact-fused bombs at precisely timed and angled trajectories to hit a point in front of the ship.

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 CAMs, armed with an obsolete plane such as the Hawker Hurricane. When a German spotter plane approached, the fighter was fired off the end of the ramp with a large RATO rocket, eventually ditching in the water to allow the pilot to be picked up by one of the escort ships.

One of the most significant developments was improved direction-finding radio equipment. A new design enabled the operator to instantly see the direction of a broadcast. Since U-boats had to surface to radio, they gave their positions away as soon as they radioed in the position of a convoy. A destroyer could then engage the U-boat, preventing a coherent attack on the convoy.

In general the Royal Navy slowly gained the upper hand through until the end of 1941. Although they were doing limited damage to the U-boats themselves, they were managing to keep them from the convoys to an increasing degree. Shipping losses were high, but manageable.

Operation Drumbeat (second happy time)

The U-boat crews called this the second happy time.

Missing image
Table showing total Allied and neutral losses in GRT during WW2, the North Atlantic is clearly on top.

This changed when the US joined the war, by declaring war against Japan after the surprise attack at Pearl Harbor. Germany then declared war on the US and promptly attacked US shipping.

Dönitz had only 12 boats of the Type IX class that were able to make the long trip to the US east coast, and half of them were removed by Hitler's direct command to counter British forces. One of those was under repair, leaving only five ships to set out for the US on the so-called Operation Drumbeat (Paukenschlag). What followed is considered by many to be one of the most victorious naval campaigns since the Battle of Trafalgar.

The US, 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. Worse, the U.S. Commander-in-Chief of the Atlantic Fleet, Admiral Ernest King, rejected the RN's calls for a convoy system out of hand. King has been criticized for this decision, but his defenders argue that the United States destroyer fleet was limited and King believed that it is far more important that the destroyers protect Allied troop transports than shipping. This decision effectively left the U-Boats free to do as they pleased.

The first boats started shooting on January 13th, 1942, and by the time they left for France on February 6th they had sunk 156,939 tonnes of shipping without loss. After six months of this the statistics were equally grim. The first batch of Type IX's had been replaced by Type VII's and IX's refueling at sea from modified Type XIV Milk Cows (themselves modified Type IX's) and had sunk 397 ships totalling over 2 million tons. At the same time, not a single troop transport was lost.

It wasn't until May that King (by now promoted to Commander-in-Chief U. S. Fleet and the Chief of Naval Operations) instituted a convoy system. This quickly led to the loss of seven U-boats. But the US did not have enough ships to cover all the holes, and the U-boats continued to operate freely in the Caribbean and the Gulf of Mexico (where they effectively closed several US ports) until July.

Operation Drumbeat did have 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 being demoted as a result of a disastrous operation in the Barentz Sea in which several German cruisers had been beaten off by a small number of RN destroyers. Dönitz was eventually made Grand Admiral of the fleet, and all building priorities turned to the U-boats.

The Leigh Light

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 used in conjunction with airborne radar to allow allied aircraft to accurately target U-boats recharging batteries on the surface at night. A drop in allied shipping losses from 600,000 to 200,000 tonnes per month was attributed to this simple device.

Final stages

With the US 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, where the battle would enter its final terrible phase.

By this point there were more than enough U-boats spread across the Atlantic to allow several wolfpacks to attack the same convoy. In most cases 10 to 15 boats would attack in one or two waves, following the convoys by day and attacking at night. Losses quickly started ramping up, and in October 56 ships of over 258,000 tonnes were sunk in the limited area between Greenland and Ireland that was still free of the ever-increasing allied air patrols.

Operations died down over the winter, but in the spring they started up again with the same ferocity. In March another 260,000 tonnes were sunk, and the supply situation in England was such that there was talk of being unable to continue the war effort. Supplies of fuel were particularly low.

It appeared that Dönitz was winning the war. And yet March was the end of the battle. In April, losses of U-boats shot up while their kills of ships fell dramatically. By May the wolfpack was no longer. The Battle of the Atlantic was won by the Allies in two months.

There was no one reason for this, but a number of them that conspired to all take effect at almost the same time. The result was a huge blow from which Dönitz was unable to recover. The four major changes were largely technological.

Among these was the introduction of an effective sea-scanning radar small enough to be carried on the patrol aircraft. Although they had long been known to be able to detect a surfaced boat from many miles, the aircraft themselves had limited range. This changed with the improved supply of the very long range Shorts Sunderland and B-24 Liberator aircraft, which could cover much larger areas of the ocean.

But even they couldn't cover it all. The remaining holes were closed by the introduction of the Merchant aircraft carrier or MAC ship and then the small escort carrier. Flying Grumman Avengers primarily, they formed into the same convoys and provided air cover and patrol all the way across the Atlantic.

In addition the British introduced the new River class frigates, built with a single purpose in mind — killing U-boats. They were much quicker than previous frigates, better armed, and had a better radar system and ASDIC. Formed into hunter-killer groups (one of the major tactical reasons for the victory) by the new commander of the Western Approaches, they would sail far from the convoys in small groups, making it almost impossible for the wolfpacks to form up under their constant sneak attacks.

But by far the single biggest element to the victory was the cracking of the Enigma cipher machine, combined with German tactics that were formed with the certainty that the cipher could not be broken. This had been a running battle between upgrades to the machine and Polish and British cryptological efforts, dating back (in the Polish case, successfully) to well before the war. By 1943 the machine had lost the race, and increasing amounts of German naval radio traffic were being read. The Royal Navy knew where the packs were forming and sent in the hunter-killer groups to destroy them. In this they were aided by the German Navy's insistence on directing U-boat operations in detail.

The efforts were so successful that it is a wonder the Kriegsmarine didn't realize that this was happening. It appears that they seemed to have some idea, but repeated questions by Dönitz sent to German intelligence services always resulted in claims that there was no way the cipher could be broken. One would think that simply looking at the statistics would be enough – U-boat losses dropped every time a new version of the cipher was introduced – but time lags, luck, pigheadedness, and astounding efforts on the British part kept this from ever becoming clear.

In the next months the vast majority of the U-boat fleet would be sunk, typically with all hands.

Last gasps

With the battle won, US 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 Schnorkel 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.


The last, and most impressive, attempt to re-open the battle is the stuff of legend. Since even before the war the rocket designer Hellmuth Walter had been advocating the use of hydrogen peroxide (known as Perhydrol) as a fuel. His engines were to become famous for their use in rocket powered aircraft — notably the Me 163 Komet — but most of his early efforts were spent on systems for submarine propulsion.

In these cases the hydrogen peroxide was reduced chemically and the resulting gases used to spin a turbine at about 20,000rpm, which was then geared to a propeller. This allowed the submarine to run underwater at all times, as there was no need for air to run the engines. However the system also used up tremendous amounts of fuel, and any boat based on the design would either have to be absolutely huge, or have terribly limited range.

Thus the system saw only limited development even though a prototype was running in 1940. But when problems with the existing U-Boat designs became evident in 1942, the work was stepped up. Eventually two engineers came up with a simple solution to the problem.

Instead of running the submarine 100% on the perhydrol, they used it strictly for bursts of speed when needed. Most of the operations would then be carried out as with a normal boat, using a diesel engine to charge batteries. However while a conventional design would use the diesel as the primary engine and the batteries for short periods of underwater power, in this case the boat would run almost all the time on batteries in a low-speed cruise, turning on the perhydrol during attacks. The diesel was now dedicated entirely to charging the batteries, which it needed only three hours to do.

The perhydrol design suffered from several design flaws which were not fixed before the end of the war. As an intermediate solution, the perhydrol propulsion system was dropped in favour of a conventional diesel/electric solution, but retaining the streamlined hull-shape. The battery capacity was increased significantly along with fuel stores, and the boat was designed to operate underwater for long periods.

The result was 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. After the war tests carried out by the US Navy on two captured Type XXIs showed they could outrun some ASW ships by going in the direction of heavy seas. For most of the trip it ran silent underwater on batteries, surfacing only at night, and then only to Schnorkel depth. Weapons were likewise upgraded, with automated systems allowing the torpedo tubes to be reloaded in less than 1/4 the time, firing homing torpedoes that would attack on their own. Even the interior was improved: it was much larger and fitted with everything from showers to a meat refrigerator for long patrols.

The design was to be produced in two versions, primarily the Type XXI, and smaller numbers of the smaller Type XXIII. Both were much larger and more difficult to build than the existing designs, the Type XXI taking some 18 months. Work didn't really get started until 1944, and only small numbers were ever built. In their few uses, the designs proved invincible, trivially avoiding attacking ships and never even being seen by the patrol aircraft.


The Germans undoubtedly failed to strangle the flow of strategic supplies to Britain and this ultimately led to the Normandy landings and the defeat of Germany.

Winning the battle was however achieved with huge losses; 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
30,000 merchant sailors 28,000 sailors
3,500 merchant vessels 783 submarines sunk
175 warships  

See also

Further reading

The Battle of the Atlantic: Hitler's Gray Wolves of the Sea and the Allies' Desperate Struggle to Defeat Them, by Andrew Williams.

External links

fr:Bataille de l'atlantique (1940)

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