A little after ten o’clock on the night of 22 November 1939, sentries on the Essex shore at Shoeburyness watched a German aircraft drop something into the sea near the beach. In the dark it looked like a man on a parachute. The soldiers waded out after it, but the tide was coming in and drove them back.
It wasn’t a parachutist. It was the thing the Royal Navy had wanted for weeks: a German magnetic mine, sitting on English mud where someone could reach it. Within a day, two officers from the Navy’s mining school would have it open, using tools they’d had specially made in brass.
What they found inside helped Britain survive the first winter of the war.
The weapon that broke ships’ backs
The war at sea had started badly. The first ship mined in the war, the British steamer Magdepur, blew up and sank off the east coast on 10 September 1939. Over the autumn the losses kept coming, often with explosions nobody could explain. A prisoner of war said U-boats were laying magnetic mines.
A magnetic mine doesn’t need a ship to touch it. It lies on the seabed in shallow water, and a sensitive needle inside reacts when a big mass of iron passes overhead and distorts the local magnetic field. Then it fires. A moored contact mine usually tears a hole near the bow. A ground mine goes off under the middle of the ship, wrecking the machinery, splitting the plating and often breaking her back.
None of the Navy’s existing sweeps worked against it, and neither did the paravanes that protected ships from moored mines.
In November the Luftwaffe started laying mines from the air, which made things much worse. Mines dropped by aircraft could land in estuaries and channels that British mine barriers had been guarding against ships and submarines. Between 18 and 22 November, fifteen merchant ships were mined, among them the Dutch liner Simon Bolivar and the Japanese liner Terukuni Maru. The destroyer HMS Gipsy was sunk. The new cruiser HMS Belfast was badly damaged by a mine in the Firth of Forth.
Ships in three river estuaries were held in port. The official wartime account of the minesweepers described a real fear that “merchant traffic would be paralysed unless the remedy could be found.” The scientists at HMS Vernon, the Navy’s torpedo and mining establishment at Portsmouth, couldn’t design a defence without knowing exactly how the mine worked. For that they needed one intact.
Kit-bags on parachutes
On the night of 21 November, observers watched aircraft, thought to be Heinkel 115 seaplanes, drop mines into the Humber and into the estuaries of the Stour and the Thames. People said the mines looked like sailors’ kit-bags hanging under parachutes. Vernon sent officers to the east coast ports, but they found nothing.
Then came the report from Shoeburyness. The Army ran an experimental range there, and the object had come down on the wide mudflats of the Thames Estuary. The mudflats would dry out at low tide.
Shortly after midnight on 23 November, Lieutenant-Commander John Ouvry (J.G.D. Ouvry) of HMS Vernon was called to the Admiralty from the London hotel where he had been waiting for exactly this kind of summons. He was told low water would come at about 4 a.m. A car was ready to take him and a fellow Vernon officer, Lieutenant-Commander Roger Lewis (R.C. Lewis), to Essex. Their orders were to examine the mine and recover it “at all costs.”
Torchlight on the mud
At Shoeburyness they were met by a naval staff officer, a group of soldiers with lights, ropes and stakes, and two photographers. The night was dark and it was raining. Out in the river, a big crowd of ships lay at anchor, unable to sail until someone found out what was waiting in the channel. Earlier, a frozen-meat ship had set off a mine just by swinging round on her anchor as the tide turned.
A soldier led them out across the sand, splashing through the pools the ebbing tide had left. Eventually the torches picked out a black shape half-buried in the mud. The two officers went forward while the soldiers behind them lit the mine with an Aldis signalling lamp.
It was a cylinder about seven feet long, made of an aluminium alloy. It had tubular spokes on the nose and a hollow tail holding a heavy bronze spring for throwing out the parachute. Near the front were two worrying fittings. One looked like a hydrostatic valve. The other was a polished aluminium fitting held in by a screwed ring and sealed with black wax. Nobody could tell what it was.
That second fitting looked the most likely to contain the detonator, so Ouvry decided to start there. But there was a problem with the tools. If the mine had a magnetic trigger, an ordinary steel spanner might be enough to set it off.
.jpg){kind=link}
A rubbing on a signal pad and a brass spanner
Lewis solved it with the simplest equipment there was. He laid a sheet from a signal pad over the securing ring and took an impression of it, so a non-magnetic spanner could be made to fit. They took flash photographs of the mine from every side and measured it.
The tide was coming back in, so they decided to wait for noon, when the mine would be uncovered again. The soldiers lashed it down carefully with ropes and stakes. Walking back, the party found the parachute spread out on the sand. It was white silk, and it took eight men to drag it above the high-water mark.
At 6 a.m., just as the officers finished breakfast, word came that a second mine had been spotted about 300 yards from the first. They waded out to it, but it was already under water, so they waited for the tide again.
By early afternoon both mines were exposed. Brass tools had been made at Shoeburyness overnight, and a mine-recovery party from Vernon arrived under Chief Petty Officer Charles Baldwin with another set of non-magnetic tools. A lorry with a crane stood ready on the foreshore.
Two men walk out; two men watch
Ouvry set up the job so that a mistake wouldn’t be wasted. He and Baldwin would work on the first mine. Lewis and Able Seaman A.L. Vearncombe would stay back on the foreshore, far enough to be safe, and watch. Ouvry explained a fixed sequence of steps in advance so the observers could see and remember exactly what happened, “in case of a mistake on my part,” as he put it.
Ouvry and Baldwin emptied every piece of metal from their pockets and walked out across the sand. They had no way of knowing whether the Germans had built in a booby trap to fire the mine if anyone tried to take it apart. The official account compared them to men walking down a jungle path that might be an ambush.
They started with the aluminium fitting on top. The ring came off easily under the brass spanner. Ouvry lifted the fitting out very slowly, because he thought he might be holding either a detonator or some kind of magnetic needle. It was a detonator.
The second fuse
With what looked like the main danger removed, Ouvry waved Lewis and Vearncombe over to help turn the mine so they could reach the fittings buried underneath it. This was the moment when a hidden trap would have gone off. They rolled the half-ton cylinder over without incident.
Underneath was a second detonator, similar to the one in the German horned contact mine. If they had lifted the mine on the assumption it was safe after the first fuse, the story could have ended very differently. Now, in the words of the official history, “the fangs had now been drawn.”
“We felt on top of the world!” Ouvry wrote later.
Ouvry’s obituary in The Independent describes what else came out during those “sweaty hours”: the hydrostatic valve, two primers each with its own detonator, and the two parts the team had really come for. One was a delayed-action arming fuse. The other was the electro-magnetic unit that fired the mine when a steel hull passed overhead. That was the secret.
By 4 p.m. they were satisfied the mine was harmless. It was hoisted onto the lorry and sent to Portsmouth with its parachute. Lieutenant J.E.M. Glenny made the second mine safe as well.
Midnight at the Admiralty
Lewis went back to London that evening. At 11 p.m. he briefed a Board of Admiralty conference, and the First Lord of the Admiralty, Winston Churchill, heard the whole story. Churchill ordered work on the mine to continue day and night until there was an answer.
The mine reached Vernon the next day. It weighed 1,128 pounds and carried 660 pounds of explosive in the front. One of the experts who took it apart called its mechanism “a scientist’s paradise.” The Imperial War Museum, which still holds the Shoeburyness mine, identifies it as a Type GA air-dropped magnetic mine, known to the Germans as the LMA (Luftmine Type A), the same type that had wrecked Belfast.
The Germans thought they had a big lead. After the battle of the River Plate, Captain Hans Langsdorff of the Graf Spee reportedly told a captured British merchant captain that the magnetic mine was the secret weapon that would win the war, and that “the British navy has to start where we began eight years ago.”
He was wrong. The idea of a magnetic mine wasn’t new, and Vernon had been developing its own. What Britain had been missing was the details: how sensitive the German mine was and which magnetic polarity set it off. Shoeburyness provided them.
The King comes to Portsmouth
Ouvry and Lewis were awarded the Distinguished Service Order. Chief Petty Officer Baldwin and Able Seaman Vearncombe received the Distinguished Service Medal, and Glenny, who dealt with the second mine, the Distinguished Service Cross.
King George VI went to HMS Vernon in person on 19 December 1939 and presented the decorations in front of the ship’s company. The Independent describes them as the first naval honours of the war. The King was reportedly told that even Ouvry couldn’t get the Victoria Cross, because the work hadn’t been done “in the face of the enemy.” The mine itself had been the enemy, but the rules didn’t count that.
Degaussing: making steel ships hard for the mine to detect
With the mine understood, Britain had two ways to fight it: blow mines up before ships reached them, or make ships invisible to them. Vernon‘s scientists did both, and Commander Charles Goodeve of the Royal Canadian Naval Volunteer Reserve was central to it.
The mine worked because a steel hull concentrates the Earth’s magnetic field. In the northern hemisphere, that makes a ship look like a magnet with its north pole pointing down. If you add a small opposing field, the ship’s magnetic signature roughly cancels out and the mine’s needle doesn’t react. The Germans measured their trigger thresholds in gauss, so Goodeve called the process degaussing. The word stuck, and it’s still used today.
The first method, called “coiling,” ran a current-carrying cable around the hull. Early on this was often a girdle of wire fixed around the ship at about upper-deck level. At one stage Britain was using 1,200 miles of cable a week to equip ships. The girdle made it possible to send the new liner Queen Elizabeth secretly across the Atlantic to New York on her first voyage in 1940.
Coils were expensive, and there were thousands of small ships to protect, so Goodeve came up with a quicker fix called “wiping.” A heavy cable carrying a pulse of roughly 2,000 amps was drawn up the side of the ship from the waterline, leaving the steel with a small protective magnetic bias. The effect wore off and had to be repeated, but it was fast. During the Dunkirk evacuation, around 400 ships were wiped in four days.
The LL sweep: setting off mines on purpose
Destroying the mines took more trial and error. The first improvised sweeps included the “Bo’sun’s Nightmare,” a tangle of magnetised bars towed along near the seabed. There were also old colliers converted into mine destructor ships with huge electromagnets in their holds, and Wellington bombers fitted with a 48-foot magnetic ring and flown low over the water.
What finally worked was the LL sweep, or “Double L.” Two wooden-hulled trawlers each towed a floating electric cable, one long and one short, with electrodes at the ends. A generator and batteries sent pulses of about 3,000 amps through the cables for five seconds every minute, using seawater to complete the circuit. Each pulse created a magnetic field across a broad stretch of sea, strong enough to fool a mine into firing behind the ships, where it was harmless.
On 28 March 1940, the first four LL trawlers began work and quickly set off four mines in the Thames Estuary. The explosions shook the trawlers so hard that the seventy then being fitted out had to be strengthened. Even so, the official naval history concluded that “the conquest of the magnetic mine was now in sight.”
What Shoeburyness changed
German minelayers didn’t give up. They went on to use acoustic mines, then combined triggers, and each new type needed a new countermeasure. But the first big panic was over. In the winter of 1939, when it looked as if shipping into Britain could be choked off by a weapon nobody could see, two officers worked out the answer on a rainy mudflat with a rubbing on a signal pad and tools made of brass.
The Shoeburyness mine went on display in Vernon‘s museum, which the official account called “the most treasured ‘carcass’ of all.” It’s now in the collection of the Imperial War Museum.
Featured image: original animation drawn for aglena.com.
Further reading
The Gathering Storm (The Second World War, Vol. 1) by Winston S. Churchill
Churchill’s own account of the road to war and its first months, written by the man who sat in the Admiralty while the magnetic mines were sinking ships. His Nobel-winning history covers the 1939 mine crisis from the inside.
TurnOnSport Orienteering Compass
A cheap, popular baseplate compass that’s also a handy way to see the Shoeburyness problem for yourself. Bring a steel spanner close and watch the needle swing, then try a brass key and see that nothing happens.