Germany mines British waters

In the first days of September 1939 German submarines began secretly laying mines off the coasts of Britain. On the east coast of England the work was done by small coastal boats, and in the English Channel by the ocean-going U-26, which had been ordered to mine the approaches to the naval base at Portland. Most of these mines were a new weapon: magnetic mines lying on the seabed, which exploded beneath a passing ship without touching it. British minesweepers, designed to deal with moored mines, could not sweep them. In November 1939 mines sank more ships off the British coast than U-boat torpedoes did in the Atlantic, and for several weeks they threatened all shipping to London.

Many chronologies state that on 8 September the Germans began the systematic mining of British waters by mining the harbour at Portland. This needs two qualifications. U-26 did not enter the harbour; it laid its mines by the Shambles bank, on the approaches to Portland and to Weymouth Bay. The date of the operation varies between sources: some accounts give 8 September, while the German naval war chronology by Jürgen Rohwer and Gerhard Hümmelchen and the records of German U-boat losses give 10 September. Nor did the minelaying begin at Portland: on the east coast U-boats had been laying mines since 4 September.

German mines in 1939

The Kriegsmarine entered the war with a large stock of mines. Its depots held more than 20,000 contact mines, moored below the surface and detonated on contact with a hull. There was also a new weapon on which the Germans had been working since the 1930s: the ground mine with a magnetic pistol. It lay on the bottom of shallow waters and exploded when a steel ship passed over it, altering the vertical component of the earth’s magnetic field. The explosion beneath the hull broke the ship’s keel. Submarines could lay such mines through their torpedo tubes. According to the British official history, at the outbreak of war the Germans had only about 1,500 magnetic mines, and production in the first months was small.

The British knew how magnetic mines worked. They had themselves laid a few such mines at the end of the First World War, and in 1939 they had a type of their own ready for production. In July 1939 the first magnetic sweep was tried out in the Solent. What they did not know was what kind of field the German mine responded to, and without that they could not build an effective sweep.

International law covered only contact mines. The Hague Convention of 1907 permitted them to be laid in enemy waters but imposed restrictions intended to protect neutral shipping. From the start of the war the Germans declared an extensive mined area in the North Sea in front of their own bases, the so-called Westwall barrage, which cruisers and destroyers had been laying since 3 September. The mines off the British coast, by contrast, were laid secretly and without warning on the routes of merchant shipping, including neutral shipping. The British official history held that it was the manner of their use, not the type of mine itself, that broke the laws of war.

The east coast

The first mines off the British coast were laid by small Type II submarines, which had been patrolling the North Sea since the end of August. During the night of 4 September U-13 mined the waters off Orford Ness in Suffolk, and on 5 September U-15 the waters off Flamborough Head in Yorkshire. Over the following days U-16 and U-17 laid mines off Hartlepool and in the Downs anchorage. The first results came on 10 September: off Flamborough Head the collier Goodwood sank with the loss of one man, and that afternoon off Orford Ness the Brocklebank Line freighter Magdapur went down, killing six seamen. These were the first British ships sunk by mines in this war.

At first the Admiralty did not know what was sinking the ships: mines, submarines or aerial torpedoes. It soon began to suspect ground mines. Certainty drew closer on 16 September, when the liner City of Paris was damaged by an underwater explosion that did not breach her hull. Damage of that kind was left by an explosion at some distance beneath a ship, not by contact with a moored mine.

U-26 off Portland

The riskiest task fell to U-26, one of the two large Type IA submarines. The boat was not a success: she had poor stability and dived slowly, so before the war she had served mainly for training and propaganda. Commanded by Lieutenant Commander Klaus Ewerth, she sailed from Wilhelmshaven on 29 August with mines and six torpedoes on board. The naval high command wanted Portland mined: it was an important Royal Navy base, home of the anti-submarine school, and also a port that could be used to ship the British Expeditionary Force to France. The commander of U-boats, Karl Dönitz, considered the operation too dangerous, precisely because the waters around Portland were the best guarded by British ships fitted with asdic.

Dönitz’s fears proved justified. From 4 September Ewerth twice tried to approach Portland and each time had to turn back before the patrols. At the third attempt he laid his mines by the Shambles bank, east of Portland Bill, in the fairway leading to Portland and Weymouth. He then withdrew into deeper water, pursued by British warships that prevented him from making radio contact. The Admiralty announced that it had sunk a German minelaying submarine. For several days U-boat command feared that U-26 had gone down in shallow water, from which the British might recover her cipher machine. According to some accounts it was this affair that led to the cipher keys being changed and to boats laying mines off the British coast being forbidden to carry an Enigma. Ewerth, however, got out into the Atlantic, reported his mission accomplished and returned to Wilhelmshaven on 26 September.

The Portland minefield killed no one, but it sank three ships of neutral states totalling some 17,400 gross register tons. On the evening of 15 September the Belgian motor ship Alex van Opstal, sailing from New York to Antwerp and directed by the British to Weymouth for contraband control, struck a mine. The explosion broke the ship in two. Everyone on board survived, though several were injured. On 7 October the Dutch freighter Binnendijk sank on the same barrage, and on 22 November the Greek Elena R. The irony was that neutral ships ran onto these mines because British contraband control had ordered them to put in at Weymouth.

U-32 carried out a similar operation in the Bristol Channel on 17 September. Two ships were damaged on her mines.

The British response

The Royal Navy laid mines too. During the night of 9–10 September two destroyers converted into minelayers, HMS Esk and HMS Express, laid mines in the Heligoland Bight on the likely routes of German warships. More important was the barrage in the Strait of Dover, laid on 11–16 September by the minelayers HMS Adventure and HMS Plover together with converted train ferries. According to Rohwer and Hümmelchen’s chronology it comprised some 3,000 mines. It was meant to protect the movement of troops to France and to close the U-boats’ shortest route to the Atlantic. In October three German submarines were lost on it: U-12, U-40 and U-16. After these losses Dönitz practically stopped sending boats through the English Channel, and the route to the ocean became the long passage round Scotland.

Against magnetic mines the British were almost helpless in the autumn of 1939. Minesweepers could sweep only moored mines, while the ground mines lay in the shallow waters of the Thames estuary, off Harwich, in the mouth of the Humber and in the east-coast fairways used since 6 September by the first convoys. The Admiralty set up a special staff under Rear Admiral William Wake-Walker which, together with the mining school HMS Vernon at Portsmouth, was to find a counter to the new weapon as quickly as possible.

The mine crisis of autumn 1939

In the following weeks the Germans widened the campaign. From mid-October mines were laid at night by German destroyers that came right up to the English coast, and from mid-November also by seaplanes dropping mines by parachute. By the end of October mines had sunk 19 British ships totalling some 59,000 gross register tons. November was worse still: 27 merchant ships of some 121,000 tons went down, as did the destroyer HMS Blanche. On 18 November the Dutch liner Simon Bolivar sank off Harwich with the loss, according to various sources, of between 84 and more than a hundred lives. On 21 November the new cruiser HMS Belfast was badly damaged by a mine in the Firth of Forth, and on 4 December the battleship HMS Nelson in Loch Ewe. By mid-November only one of the three deep-water channels into the Port of London was open, and the Admiralty feared that the Germans would bring traffic on the Thames to a complete standstill.

The breakthrough came during the night of 22–23 November 1939. A German aircraft dropped a magnetic mine onto the mudflats at Shoeburyness in the Thames estuary, where at low tide it lay exposed on the mud. Lieutenant Commander John Ouvry of HMS Vernon defused it at the risk of his life. It turned out that the German pistol responded to a change in the vertical component of the magnetic field caused by a ship built in the northern hemisphere. This knowledge made it possible to develop two countermeasures quickly. One was the electric sweep, with which pairs of wooden vessels detonated mines astern of them. The other was the degaussing of hulls: cables were run round the hulls of warships and merchant ships, carrying a current that cancelled out the ship’s magnetic field. Vickers Wellington aircraft fitted with a large electromagnetic ring, which set off mines from the air, were also tried. Degaussing thousands of ships was an undertaking on the scale of re-equipping the entire merchant fleet and took many months. In December, however, the Germans began to run short of magnetic mines, and in the winter of 1939–1940 the worst of the danger passed.

Significance

The mining campaign of autumn 1939 was the Kriegsmarine’s first serious success in the war against Britain and the only moment of the Phoney War in which the Germans managed to take the British by surprise technically. Mines proved a more efficient weapon than torpedoes. According to the British official history, the damage done by magnetic mines was out of all proportion to the roughly 470 mines of this type that the Germans laid in the first three months of the war. But the Germans ran out of mines, and by the time they had increased production the British already had their countermeasures.

The mine war did not end in 1940. The Germans introduced further types of pistol, acoustic and pressure, and the British further sweeps and methods of protection. The air forces of both sides dropped mines until the end of the war, and in 1940–1945 British aircraft mined German shipping lanes from the Bay of Biscay to the Baltic. The experience of 1939 led the Admiralty to begin degaussing all warships and merchant ships early, and degaussing ranges were set up in ports all over the world. The mining campaign also showed that Britain’s coastal waters were a front as important as the ocean: throughout the war the east-coast convoys carrying coal to London sailed along narrow fairways that were swept every day.