The Shoeburyness mine

Late in the evening of 22 November 1939 German aircraft dropped mines by parachute into the mouth of the Thames. One of them fell not into the water but onto the flats off Shoeburyness, east of Southend-on-Sea, where at low tide it lay on the exposed mud. The next day, 23 November, Lieutenant Commander John Ouvry of the mine school HMS Vernon in Portsmouth disarmed it at the risk of his life, and the mine was taken to the Admiralty’s laboratories. For the first time the British had in their hands a complete German magnetic mine, the weapon that for two months had been sinking ships off their coasts and against which they had no effective defence. Within a few days they had established how its firing unit worked. That knowledge made it possible to develop the magnetic sweep and the demagnetisation of the hulls of warships and merchant ships, known as degaussing.

The whole episode is often placed under 22 November. The mine was indeed dropped on the evening of 22 November, but it was found during the night and disarmed on 23 November; that is the date given by Stephen Roskill’s official British history of the war at sea.

The mine crisis

The Germans had been laying magnetic mines off the British coast since the first days of September. These ground mines, lying on the bottom of shallow channels, exploded beneath a ship without touching it, when the steel hull altered the earth’s magnetic field above them. British minesweepers could sweep only moored mines. The Admiralty knew it was dealing with magnetic mines, but did not know their “firing rule”: it did not know what change in the field the firing unit responded to, in which direction and of what strength. Without that, it could not build a sweep that would detonate the mines from a safe distance. A special staff under Rear Admiral William Wake-Walker had been working on the problem with the scientists of HMS Vernon since the autumn, and on 20 October a magnetic mine was detonated for the first time with a makeshift sweep. Without knowing the principle of the firing unit, however, it was impossible to build a sweep that could be relied upon.

In November the crisis deepened. According to Roskill, in that month mines sank 27 merchant ships totalling some 121,000 GRT and the destroyer HMS Blanche, and damaged many others, among them the cruiser HMS Belfast, which struck a mine in the Firth of Forth on 21 November. Nore Command, responsible for the Thames Estuary, struggled to lay out and mark safe channels. By mid-month only one of the three deep-water channels into the Port of London was open, and it seemed that the Germans might stop shipping to London altogether. On 21 November the navigation lights in the Thames Estuary were extinguished and night navigation was forbidden west of a line between the Downs anchorage and Orford Ness.

From 20 November aircraft were laying mines as well. These were Heinkel He 115 seaplanes of the coastal air units, formally part of the Luftwaffe but working for the navy, flying among other places from the base on the island of Sylt. They dropped mines by parachute at night into the narrow channels at the mouth of the Thames and off the ports of eastern England. According to Roskill, air-dropped mines were less dangerous than might have appeared, because in the darkness the crews could not establish their position precisely. That imprecision decided what happened at Shoeburyness.

A night on the mudflats

Shoeburyness, an old garrison town absorbed into Southend, lay on the north shore of the Thames Estuary. It housed the British Army’s artillery proving ground, and off the shore stretched broad flats uncovered at every low tide. According to accounts, on the evening of 22 November observers on the shore spotted an aircraft and large objects descending by parachute. The news quickly reached the Admiralty. Two mine specialists from HMS Vernon were summoned, Lieutenant Commanders John Ouvry and Roger Lewis. Ouvry had been one of the designers of the British magnetic mine, so he knew better than anyone what to expect. According to the accounts of both officers, they drove to the Thames Estuary that same night, and towards morning, at low tide and by the light of torches, they found on the mud, several hundred metres from the shore, a large cylinder with a parachute. A little further off lay a second mine.

The mine was about two metres long and weighed several hundred kilograms. The British later designated this type GA. First the mine had to be photographed and the fittings for the firing units measured, and then they had to wait for the next low tide, because the flood tide covered the flats. The greatest worry was that the mine might have an anti-handling device or respond to metal: so both officers removed everything made of iron and steel, down to buttons and buckles. In the workshops of the Shoeburyness proving ground keys and tools of non-magnetic brass were made to the measurements taken.

Disarming the mine

In the afternoon of 23 November, at the next low tide, Ouvry and Chief Petty Officer Charles Baldwin approached the mine. Lewis and Able Seaman Archibald Vearncombe stayed at a safe distance. According to the participants’ accounts, Ouvry called out each successive step to them, so that if the mine exploded the next team would know what not to do. First he unscrewed the fitting containing the detonator from the casing. When the mine was rolled over, it turned out to have other fittings as well; in one of them sat a second detonator, which was also removed. Only then was the mine considered safe. Lewis and Vearncombe, joined by Dr Albert Wood, a scientist from HMS Vernon’s mine design department, then helped to take it apart, and the parts were carried to Portsmouth.

Winston Churchill, First Lord of the Admiralty since September 1939, followed the struggle against the magnetic mine personally and pressed for quick results. In his memoirs he described how several dozen officers and officials gathered at the Admiralty to hear Ouvry and Lewis’s account. On 19 December 1939, on the parade ground of HMS Vernon, King George VI decorated Ouvry and Lewis with the Distinguished Service Order (DSO), and Baldwin and Vearncombe with the Distinguished Service Medal (DSM). Lieutenant J. E. M. Glenny also received the Distinguished Service Cross (DSC), awarded for disarming mines. According to the Minewarfare and Clearance Diving Officers’ Association (MCDOA), these were the Royal Navy’s first decorations of the war.

The secret of the firing unit

In the laboratories of HMS Vernon it emerged that the German firing unit worked differently from the British one. It responded to a change in the strength of the vertical component of the magnetic field, not to the rate of that change, and only in one direction. A ship built in the northern hemisphere carried permanent magnetism in her hull with the north pole pointing downwards, and it was this, superimposed on the earth’s field, that set off the firing unit. Knowing this “firing rule” and the sensitivity of the unit, the Admiralty could at last design countermeasures on a sound basis.

It is worth adding that the principle of German magnetic firing units, and the possibility of defending against them by generating a suitable magnetic field, had already been described in the Oslo Report, which had reached the British at the beginning of November. The report, however, dealt with torpedoes and gave no details, and in London it was treated with suspicion. Only the Shoeburyness mine provided hard data.

Degaussing, sweeps and Wellingtons

The countermeasures went in two directions. The first was to detonate mines from a safe distance. The most effective proved to be the LL (double-L) sweep: two wooden boats towed long floating cables through which powerful pulses of current were passed. The current flowing through the seawater between the electrodes generated a magnetic field that fired the mines over a whole stretch of water behind the boats. LL sweeps entered service in the first months of 1940. Before that, makeshift means were used: ships with huge electromagnets in the bows, such as HMS Borde, which were often damaged by the explosions themselves, and towed sledges carrying magnets. The air force converted several Vickers Wellington bombers by fitting beneath them an aluminium ring some 15 metres in diameter, through which flowed current from a generator on board. For secrecy they were called DWI (Directional Wireless Installation). Flying low over the sea, they set off the mines beneath them. According to Roskill, however, they swept only a narrow strip that could not be marked, although later they proved especially useful in clearing mines from the Suez Canal.

The second direction was to rid ships of the field that fired the mines. The term degaussing was coined by the Canadian Charles Goodeve, a chemist from University College London who was working at HMS Vernon as a naval reserve officer. The name came from the gauss, the unit of magnetic field strength named after the German mathematician Carl Friedrich Gauss, in which the Germans expressed the sensitivity of their firing units. Cables were run around the hulls of larger warships and merchant ships, carrying current from the ships’ generators to cancel out the hull’s field. Among the first vessels so fitted were the aircraft carrier HMS Ark Royal and the liner Queen Mary. For smaller ships, which lacked sufficiently powerful generators, so-called wiping, also Goodeve’s idea, was enough: a cable was drawn along the ship’s side and a pulse of current of about 2,000 amperes was passed through it, which altered the magnetisation of the hull for a time. Degaussing thousands of ships was an undertaking on the scale of arming the whole merchant fleet; it was precisely to this that Roskill compared it. Enormous quantities of cable had to be produced, and degaussing and checking stations had to be organised in ports all over the world.

Significance

The Shoeburyness mine was one of the turning points of the first months of the war at sea. Until 23 November the British had been groping for a means of countering the new weapon. After that day, as Roskill wrote, “the defeat of the magnetic mine was in sight”, although achieving it still required a great industrial effort. A shortage of mines on the German side also helped: in December the Germans laid fewer of them because their stocks were running out, and by the time they had increased production the British already had sweeps and degaussed ships. By the spring of 1940 the magnetic mine had ceased to be a threat capable of paralysing the Port of London. At the end of May 1940, during the evacuation from Dunkirk, some 400 ships and boats that sailed through mined waters were “wiped” within four days.

The mine war nevertheless went on until the end of the war. The Germans introduced new firing units one after another: acoustic ones, responding to the noise of propellers, and from 1944 pressure ones, responding to the drop in water pressure beneath a moving ship, against which for a long time there was no sweep. Each time the British had to capture a new type of mine, disarm it and discover how it worked. This work cost the lives of many mine disposal men: on 6 August 1940 a German mine fitted with an anti-handling booby trap exploded in a workshop at HMS Vernon, killing five men. At the same time, throughout the war the RAF dropped its own air-laid mines in German shipping lanes, from the Bay of Biscay to the Baltic. Degaussing became a permanent part of the building and running of ships and remains so to this day. The narrow, daily swept channels along the east coast of England, along which convoys carried coal to London, remained throughout the war one of the less well-known but important fronts of the Battle of the Atlantic.