The Battle of the Atlantic: why Britain almost lost to Hitler's U-boats

Lieutenant Fritz-Julius Lemp sounded the klaxon for battle stations, as U-30 dipped below the surface of the Atlantic to attack what he thought was “an armed merchant cruiser” travelling at 16 knots. When the U-boat was 1,600 yards from its target, Lemp “called out his firing intervals for a spread of four torpedoes”. Back came the message: “Torpedoes loose!”

At 7.43pm, the Athenia shook with a mighty explosion. The ship listed to port, and diners screamed in confusion as tables, chairs and cutlery went flying. In the melee, husbands were separated from wives, mothers from children and sisters from brothers. The crew rushed to lower the 26 lifeboats, as a few hundred yards away the crew of U-30 roared their delight. They had struck the first submarine blow for the fatherland, sinking an armed cruiser less than 12 hours after the declaration of war.

But as Lemp ordered the U-boat’s withdrawal, the wireless picked up a distress signal. It wasn’t an armed cruiser slipping beneath the waves: it was a passenger ship. “What a mess,” muttered a shocked Lemp.“What a mess.”

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A few hours before 117 passengers and crew went down with the Athenia, Winston Churchill had been appointed First Lord of the Admiralty in the war cabinet formed by Prime Minister Neville Chamberlain. One of Churchill’s first calls was to Admiral Dudley Pound, the First Sea Lord, who provided a thorough briefing on the state of the Royal Navy. Unfortunately, Pound was the head of a service dangerously naive to the threat posed by U-boats. In the late 1930s, senior figures at the Admiralty thought that the errors made during the U-boat campaign of World War I had convinced Germany that submarine warfare was not of strategic benefit.

There was also the Second London Naval Treaty, signed by 30 countries in 1936 (including Germany), which outlawed ‘unrestricted warfare’ at sea. For these reasons, the Admiralty believed the naval battles of World War II would be between surface vessels.

Winston Churchill's first crisis

The sinking of the Athenia presented Churchill with his first crisis. Informed that the Germans had 60 U-boats (they had in fact 46 with 100 more in production), he introduced the convoy system – whereby merchant ships sailed en masse with naval escorts – but there was a problem: the Admiralty’s complacency in the interwar years had resulted in a lack of suitable vessels. Some ships would just have to take their chances crossing the Atlantic.

The dismay felt by Churchill at the sinking of the Athenia was replicated by Adolf Hitler. He was well aware of the mistakes made during World War I, when U-boat attacks on American vessels had precipitated that country’s entry into the war. The Führer didn’t want to provoke US President Franklin Roosevelt and cause the past to repeat.

“Until further orders, no hostile action will be taken against passenger liners, even when sailing under escort,” he ordered, a command that frustrated Rear Admiral Karl Dönitz, who was in charge of the U-boat fleet.

It wasn’t Dönitz’s only irritation. A U-boat commander in the first war, he was convinced that submarines were the most potent weapon in the German navy, and the best means to defeat the country’s Allied adversaries.

With 70 per cent of its food supply imported, Britain could ill afford to lose so many ships. There were also the precious metals required for the war effort: tin, lead, iron ore, aluminium, copper and zinc – all now resting on the ocean bed

This strategy wasn’t shared by Erich Raeder, head of the navy, who believed surface vessels were the most effective way to fight the British at sea. A pessimist compared with the more positive Dönitz, Raeder baulked at his subordinate’s demand for a fleet of 300 U-boats, despite the damage they inflicted.

Between September and December 1939, the U-boats sank 110 merchant vessels, the consequences of which were soon felt in Britain. With 70 per cent of its food supply imported, Britain could ill afford to lose so many ships. There were also the precious metals required for the war effort: tin, lead, iron ore, aluminium, copper and zinc – all now resting on the ocean bed.

This situation wasn’t what the Royal Navy had envisaged in the buildup to war, and nor was the U-boat’s ‘wolfpack’ tactic of attacking at night and on the surface. Attached to the hulls of British surface ships were the much-heralded ASDIC (Anti-Submarine Detection Investigation Committee) detection devices, which could locate a U-boat with a ‘ping’ from an echo. But ASDIC only worked if the U-boats were underwater.

Hitler senses success

The early success of the U-boat attacks gave Hitler a glimpse of what they might achieve. He relaxed the rules of engagement so that now his submarines could attack any vessel that was using its wireless. What Hitler didn’t do, however, was increase U-boat production – instead being persuaded by Hermann Göring, head of the Luftwaffe, to prioritise aircraft manufacture.

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In August 1940 alone, 56 merchant ships were sunk by U-boats. But there was some good news for Churchill at the beginning of September, when he persuaded Roosevelt to sign the Destroyers for Bases Agreement, whereby the US transferred 50 of its old destroyers to Britain in exchange for land rights on British possessions.

Now convoys would have better protection as they made the hazardous crossing. Or that was the theory. On 13 September 1940, the SS City of Benares sailed from Liverpool, bound for Canada. Among the 407 crew and passengers were 90 children, whose parents wanted them out of harm’s way as the Luftwaffe intensified the Blitz. There were 20 vessels in total in the convoy, escorted by one destroyer and two armed sloops.

The sinking of the SS City of Benares

Four days out of England, the City of Benares was hit by a U-boat torpedo just as 11-year-old Fred Steels climbed into bed. As water rushed into the cabin, he rushed out. “When I got out of the cabin, there was a huge hole in the deck,” Steels later recalled. “A dirty great seaman grabbed me and another boy and threw us into a lifeboat.”

In the panic to abandon the rapidly sinking vessel, many passengers were thrown into the water beside poorly launched lifeboats. Steels was one of the lucky ones. His lifeboat, No 12, was in the capable hands of Fourth Officer Ronald Cooper, who had under his command 32 Indian crew, four British and six boys with their two chaperones. “We were frightened, but we tried not to show it,” remembered Steels.

Most survivors were picked up within 12 hours, but lifeboat No 12 had drifted out of sight. When dawn broke, Cooper handed out sardines, biscuits and water to the occupants of the boat and led them in a series of songs. Soon, morale began to waver in the face of mountainous seas. “We went through three storms – great Atlantic gales,” said Steels.

After a week adrift on the Atlantic, there was no longer any singing, nor much in the way of food. Cold, wet and hungry, the survivors lay huddled in the boat. On day eight they were spotted by an RAF flying boat, and within hours Steels was on board a Royal Navy destroyer. In his report, Cooper remarked that the boys had “behaved splendidly”.

In all, 260 people drowned in the sinking of the City of Benares, including 77 of the 90 children. The incident caused worldwide outrage, but the government and the Admiralty knew it had to take some of the blame: it was evident that U-boats were operating further west than previously thought, and the ‘mid-Atlantic gap’ – the section of the ocean that was beyond the range of RAF Coastal Command aircraft – had to be closed. That meant increased protection for convoys from destroyers.

Fifty-nine ships were sunk in September 1940 and 63 in October, which, combined with the 56 vessels lost in August, meant that in three months 700,000 tons of supplies had disappeared beneath the waves

Nevertheless, the U-boats continued to take a heavy toll on the Atlantic convoys: 59 ships were sunk in September 1940 and 63 in October, which, combined with the 56 vessels lost in August, meant that in three months 700,000 tons of supplies had disappeared beneath the waves. The figures were even worse in early 1941, with nearly one million tons lost in February and March. Churchill knew such losses were unsustainable. And then: a glimmer of hope for the Prime Minister. On 17 March 17 1941, U-100 was sunk by the destroyer HMS Vanoc, which had been guided to its prey by a seaborne radar device, a first in the Battle of the Atlantic.

That wasn’t the only weapon in the Admiralty’s armoury. In the same month the Vanoc sunk U-100, a British destroyer seized an armed German trawler close to the Norwegian Lofoten Islands. A search of the vessel unearthed documents containing codes. They were sent to Bletchley Park, a Victorian mansion in Buckinghamshire that housed a team of brilliant codebreakers.

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The discovery of Enigma machine

Among the documents found on the trawler was one that provided the settings for the typewriter-like German cipher machine Enigma. The previous year, a young mathematician called Alan Turing - benefiting from information provided by Polish codebreakers - had not only deduced how Enigma functioned, but also created what he called his ‘bombe’: a machine that deciphered the Enigma codes to reveal their contents.

The codes seized in Norway were used to decrypt several German naval messages, and a few weeks later Britain got lucky again, at the same time that Lieutenant Fritz-Julius Lemp’s fortune ran out: in May 1941, Lemp drowned when the submarine he was commanding, U-110, was attacked and forced to the surface of the Atlantic 300 miles west of Ireland.

On board was found an intact Enigma machine, the first to fall into British hands. The discovery wasn’t as significant as the Admiralty had hoped, though, and for the most part Ultra - the codeword for the Bletchley Park intelligence - furnished the Navy with limited advantages during the long years of the Battle of the Atlantic.

In January 1942, a fourth ‘rotor’ was installed in the German naval Enigma, and it took the brains at Bletchley Park ten months to crack the new codes. In addition, Germany’s own naval intelligence, B-Dienst, was skilled in decrypting Royal Naval messages, so in effect the two departments cancelled each other out.

Three and a half years after the Battle of the Atlantic had begun, the conflict was nearing its climax. The winter of 1943 was especially brutal for the Merchant Navy, both on account of the weather and U-boat activity. A total of 38 vessels were sunk in January and 63 in February 1943, but in March the tally reached crisis point for the Allies.

That month, three U-boat wolf packs ambushed two convoys, SC122 and HX229, as they crossed the Atlantic, sending 141,000 tons of shipping and more than 300 merchant seamen to the ocean bed. As the Admiralty reflected a few months later, “the Germans never came so near to disrupting communications between the New World and the Old as in the firstb 20 days of March 1943”.

That they didn’t was because, in the nick of time, the Allies brought into service the technology required to defeat their foe. The first was HF/DF, nicknamed ‘huff-duff’, a high-frequency radio direction finder fitted to many escort vessels. It was able to detect the smallest signal sent from one U-boat to another, as well as messages sent between a submarine pack and their U-boat HQ in France. Once detected, the submarine appeared as a point of light on a fluorescent screen, and a fast escort vessel set off on the bearing to attack it.

Also deployed was a new 10cm radar system, used in warships and the air, based on the system of coastal radar stations that had proved so effective in the battle of Britain. The radar had been in use for a while, but German U-boat commanders believed they had negated its effect with the installation of a radar detection system known as the Metox 600, which warned when an enemy aircraft was within 60 miles.

The crew of U-333 discovered otherwise in March, when they were attacked at night by a Wellington bomber that “just jumped out of the darkness”. The submarine escaped with minor damage, but Dönitz was alarmed when he heard of the attack.

Not only did the Allies have groundbreaking technology at their disposal, but Britain’s war cabinet had belatedly recognised the importance of aircraft support for convoys. In March 1943, it allocated 39 VLR (Very Long Range) Liberators to Coastal Command, with dozens more to follow. Dönitz could only look on in envy, deprived of similar aerial resources.

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The consequences were to quickly prove catastrophic for his fleet of U-boats. On 1 May, two convoys, HX237 and SC129, sailed from New York for Liverpool. Forty U-boats were sent to sink the 72 freighters, stacked with supplies for Britain, but managed to destroy only five; in the deadly game of marine cat and mouse, the convoy’s escort located and sank the same number of U-boats.

In total, 56 U-boats were sunk in April and May 1943, and among the hundreds drowned was Dönitz’s own son, Peter, killed on his first patrol. The Battle of the Atlantic would continue, but with the technology and aircraft resources now at the Allies’ disposal, the outcome was guaranteed. So, too, was Britain’s vital supply line to North America. The cost to both sides was enormous, as was the courage of sailors and submariners, whose grave was the grey, pitiless Atlantic.

Of the 38,000 men who served in the U-boats, only 8,000 survived the war; no other branch of any service in any nation that participated in the war suffered such a casualty rate. One of the very few commanders to survive was Peter-Erich Cremer, who returned to Germany desolate. “Most of my comrades were no longer alive; the years of my youth had gone,” he reflected. “All around me was emptiness.”

War on the water elsewhere: the Mediterranean and the Pacific

As well as the Atlantic, two other watery theatres witnessed intense submarine battles during World War II. The first was in the Mediterranean, where a submarine campaign was waged from September 1941 to September 1944, inflicting a heavy toll on both German and British submarines. Of the 60 U-boats that entered the Med through the Strait of Gibraltar in 1941, only one returned, but they inflicted great damage on the Allied convoys endeavouring to supply the besieged island of Malta!and British troops in North Africa.

The Royal Navy’s 10th Submarine Flotilla, based in Malta from 1941 onwards, was tasked with protecting the convoys the U-boats were attacking, as well as trying to disrupt Axis supplies to their own forces in North Africa. US submarines achieved notable success in the Pacific as they sought to avenge the Japanese attack on Pearl Harbor in December 1941. Operating from that same Hawaiian base, as well as Fremantle and Brisbane in Australia, American submarines were in continual action until the surrender of Japan in August 1945.

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Overall, US submarines sank 1,314 enemy ships in the Pacific, a total of 5.3 million tons of shipping, representing 55 per cent of Japan’s maritime losses. The price, though, was heavy, and of the 16,000 American submariners who served, 375 offcers and 3,131 enlisted men were lost, along with 52 submarines. Nonetheless, this still represented the lowest casualty rate of any combatant submarine service in World War II. Arguably the most famous American ace was Richard O’Kane, who sank 33 enemy ships while in command of USS Tang. In addition, the Tang rescued numerous American aircrew who had bailed out into the ocean.

This content first appeared in the April 2019 issue of BBC History Revealed