At the outbreak of World War II, the German navy and air force unleashed a hidden menace into the sea lanes that Britain relied upon for survival. They unleashed magnetic influence naval mines.
Such mines were a significant development in naval area denial technology—and their introduction sparked a blitzkrieg of British naval advances in exchange.
It’s an inventive way to sink ships. Steel warships create an invisible magnetic signature as they sail. Magnetic mines detonate when they detect this signature, even when they’re moored tens of meters underwater.
The results were catastrophic.
Underwater explosions broke ship keels and breached submarine hulls. After only a single mine detonated, the possibility of more mines discouraged movement in the area, and tied down manpower and material for mine-spotting and disposal.
The resulting underwater explosion inflicts ship-crippling damage and brings merchant shipping to a standstill while the navy clears the affected sea lanes.
The Germans and British navies first recognized the power of the naval mine as a defensive and offensive weapon during World War I.
During the war, contact mines emerged as a major weapon. Chris Henry’s 2005 book Depth Charge estimates that the British, German and American navies placed more than 215,000 mines in the seas during the four-year war.
Surface ships were the main means of delivery for contact mines. Sailors would roll a buoyant mine over the stern, after which a heavy “sinker” would drag the ordnance down and fix it to the sea floor. A cable tether then held the floating mine in place.
This “moored mine” would then wait for an unsuspecting ship to bump into it. Spiked like the mines in the Windows game Minesweeper, the chemical or mechanical triggers in these early sea mines detonated on collision with passing ships.
For the German navy, the mines kept the British blockade from closing in on the European continental coast. Even before the onset of trench warfare, German defensive coastal minefields caused significant attrition. They even sank the first battleship of the war, HMS Audacious.
For the British and American navies, mines were a potent anti-submarine defense. The British blockade largely contained the German surface fleet to port, but their submarines still roamed freely.
Three hundred fifty-one U-boats of the German navy destroyed nearly 13 million gross tons of allied and neutral shipping during the course of the war. This hit Britain particularly hard, as London relied heavily on support from its empire and the United States.
To keep Germany’s U-boats out of the Atlantic, the allies drew up plans for a massive North Sea minefield in the final months of the war. A predominantly American fleet laid over 70,000 Mark 6 mines in the waters that separate the British Orkney and Shetland islands from Norway.
Another minefield in the English Channel already diverted the German submarine fleet north, but the North Sea field promised to contain the German underwater menace.
Mines were the main cause of German submarine losses during the war, destroying somewhere between 48 and 55 U-boats. The North Sea barrage alone destroyed four U-boats, and may have knocked out a further four during the final three months of the war. The Royal Navy took great notice of this early on.
On the other hand, Germany learned that mines were indiscriminate. German-laid mines delivered the deadly blow to several of their lost U-boats. For future wars, the new German navy wanted to immunize themselves from their own mines. This required simultaneous technological advancement in both mines and their countermeasures.
Neither side were slow to get to work.
Interwar research push
The British research drive began before the war even ended. In December 1916, the Royal Navy created the Mining School at Portsmouth and tasked it with improving the abysmal performance of British Mark 6 mines against submerged vessels.
By August 1918, the Royal Navy Mining School had developed the first dip needle magnetic proximity trigger for seabed mines—the M sinker. The British deployed these 1,000-pound magnetic mines off the coast of Belgium in 1918, but the war ended before they could prove their worth.
The end of the war saw a concerted effort to clear Europe’s seas of mines. Work on magnetic influence mines stopped, and Britain’s naval scientists went back to the drawing board. There, they weathered budget cuts passed by a government which couldn’t fathom another major war in the coming decades.
After the armistice, the German navy lay scuttled on the seafloor at Scapa Flow, but the experience and lessons of the war lived on in its personnel. The navy’s mine engineers began working on magnetic influence mines in 1923. Two years later, Germany developed its first magnetic proximity trigger—the E-BIK, also a dip needle unit.
To understand what a “dip needle unit” is, you must first understand why ships are magnetic in the first place.
Modern warships are naturally prone to developing a magnetic signature thanks to ferromagnetism. The earth projects a magnetic field, like a giant bar magnet. As a ship sails, it cuts across the planet’s magnetic field, and the magnetic domains inside the steel of ship’s hulls align and magnetize the hull.
The E-BIK’s dip needle works on the same principle as a compass. Magnetic fluctuations caused by a passing ship attract one end of a calibrated needle. The other end touches the metal contacts of the detonator circuit—and explodes the device.
In contrast to Germany’s dip needles, British interwar research focused on induction units. The Royal Navy had researched magnetic induction from 1915 to detect submerged ships intruding into British harbors.
As a submarine passes over a cable laid across the seabed, its magnetic field and motion induce current in the cable, which a galvanometer then displays to a human operator.
The Royal Navy twinned these induction loops with remote-controlled minefields. Remotely triggered mines had been in service with the British navy since the 1870s, but until induction loops came along, they were practically useless against the invisible submarine threat.
The induction loop would prove to be an effective means of port defense throughout the 20th century. It also inspired British research on magnetic influence mines, which sought to replace the human element with a coiled rod that could directly trigger the mine.
As Europe descended into war, both sides had proven magnetic mines in their arsenal. Britain had introduced the M Mark 1—its first coiled rod induction mine—into service in 1939.
Britain’s induction mines were more reliable than the needle mines that made up Germany’s arsenal. Berlin also lacked the copper and nickel resources for large-scale production of induction units. But the Third Reich was by no means at a disadvantage.
Germany had a greater stockpile of mines and the ability to lay mines from the air. Britain did not have an air-deployable magnetic mine in service until after September 1939.
The Germans entered the war with the upper hand.
Germany’s head-start on the Royal Navy in magnetic influence mines was nearly disastrous for the British war effort. Almost immediately after the declaration of war, the German navy—now named the Kriegsmarine—began mining shipping lanes.
The initial efforts followed established methods. Surface ships dumped WWI-era moored contact mines along the routes that brought in Britain’s precious coal. U-boats crept into river estuaries like the Humber and Thames, and laid mines out of their torpedo tubes.
On Sept. 10, 1939, a week after British Prime Minister Neville Chamberlain declared war on Germany, German mines claimed their first victims—the merchant steamships Goodwood and Magdapur.
But among the obvious signs of contact mine damage, there was evidence of abnormal damage to injured ships that managed to crawl back to safe harbor.
Unlike contact mines which tore holes in the hulls they collided with, these crippled ships showed signs of enormous hull stresses. Cracked steel plating, burst rivets and cracks in the cast iron hinted at large but distant underwater explosions.
The Royal Navy’s mine experts were well aware of what kind of mines could be responsible—they had been working on magnetic and acoustic influence mines of their own for years. What they didn’t have was physical evidence of how to detect, neutralize or trick the mines before they brought Britain to its knees.
Minesweepers trawled Britain’s waters. They cut the cables holding moored mines to the seabed. When they floated to the surface, deckhands destroyed them with rifles. Other mines washed ashore, and disposal teams from H.M.S. Vernon swooped in to recover them—carefully ensuring that none of their members were carrying metal on them.
The Royal Navy examined over 200 mines in October and November 1939, all contact mines. They were not responsible for the mysterious broken keels returning to port.
The situation was becoming desperate.
During the same period, Britain lost more than 200,000 tons of shipping to German mines. Each time a ship struck a mine, the navy shut down the affected sea lane or port until they could establish a safe channel. Ships would wait out at sea for days at a time at the mercy of German submarine and air attack.
The breakthrough was the inter-service rivalries that thrust the Luftwaffe into the naval mining effort.
On Nov. 21, German He-59 aircraft dropped long, cigar-shaped canisters off the British eastern coast. The next night, an anti-aircraft machine gun team spotted one of the Heinkels over the Thames Estuary near Shoeburyness. They lit the sky up with bullets and the seaplane dropped its cargo and fled.
The Luftwaffe had dropped the keys to defeating their magnetic mines right into London’s lap. Two mines fell into the mudflats of the estuary, and a soldier watched one parachute descend all the way to the ground.
The Royal Navy called Lt. Cmdrs. John Ouvry and Roger Lewis from Vernon into action. The First Lord of the Admiralty, Winston Churchill, wanted the exposed ordnance recovered “at all costs.”
Ouvry’s team—alongside British Army explosive experts—waded out to the mine at night. In the light of assembled flashlights, Ouvry and Lewis noted two exposed fittings. One was a brass hydrostatic valve, and the other had an unknown function, but it appeared to be a polished aluminum fitting with a copper strip sticking out of it.
In the morning, Ouvry and a chief petty officer headed out to the mine to attempt to render it safe. They removed all the metal from their persons, to not inadvertently trigger an explosion. Lewis observed with an able seaman from a safe distance. If Ouvry didn’t make it, Lewis might at least learn from his mistakes.
He loosened his first fitting—the suspicious aluminum fitting he spotted the night before. From the contacts at the bottom of the fitting, it was clear to Ouvry that the component was a new kind of detonator.
He suspected he had removed the primary detonator, so he called Lewis’ team to help turn the mine over and inspect the underside. The four men carefully rolled the still-live mine over to reveal another two fittings.
Ouvry then unscrewed a plate opposite the brass fitting spotted the night before. Underneath, under a tightened threaded fitting, were several terminals on either side of a screw head. Ouvry separated and insulated the terminal wires, and then used a large non-magnetic screwdriver to pull out a second detonator which he recognized from previous German contact mines.
He unscrewed the brass hydrostatic valve opposite this second detonator, and found a long spring trailing into the device. At the bottom of the spring were rings of primer—designed to create an initial explosion, and then detonate a larger explosive charge in the body of the mine.
One more fitting remained in the underside. Ouvry unscrewed this large brass fitting and found five wires trailing into the device. He cut the wires and insulated the leads. They had removed all the fittings and the device was now safe.
The mine arrived in Portsmouth the next day, and the experts at Vernon began pulling it apart to find out how it worked—and more importantly, how they could defeat mines like it.
To be sure, the weapon had some clever engineering tricks.
The fittings kept the mine from activating until it hit either seawater or the ground. This allowed aircraft to carry and deploy the mines without having to worry about them detonating mid-air.
Seawater passed through hydrostatic valves. In one fitting, water pressed down onto a plate to push the primary charges around the detonator. In the other, water pressure started a 24-minute clock. After the clockwork wound down, the timer connected the battery to the trigger circuit, activating the device.
This armed the mine after it settled, and stabilized it to prevent accidental detonation by the dip needle.
Inside the body of the mine, suspended on a rubber diaphragm in front of the mine’s 661-pound payload, an aluminum dome housed the needle unit inside an aluminum gimbal. This stabilized the needle before the 24-minute countdown ended.
The mine also had a fail-safe mechanism to prevent the British from recovering it. The crew of the Heinkel should have removed the copper strip on the aluminum fitting that Ouvry removed first. This sensor would have activated seven seconds after the device hit solid ground. Any jolt after those seven seconds would have detonated the device, preventing its recovery.
Thankfully for Ouvry and the Allied war effort, the Luftwaffe crew had been in too much of a rush to escape anti-aircraft fire to activate this fail-safe.
The recovered mine now sits inside the light cruiser HMS Belfast, moored in London as a wartime naval museum. It is a fitting match—Belfast had its back broken by a German mine in November 1939.
On Jan. 20, 1940, in a live broadcast from London, Churchill sounded upbeat about the mine threat.
“The magnetic mine, and all the other mines with which the narrow waters, the approaches to this island, are strewn, do not present us with any problem which we deem insoluble,” he said.
Churchill had every reason to be optimistic—the Royal Navy had been hard at work in applying the knowledge gained from the “luftmine” of Shoeburyness.
In the five weeks after the recovery, HMS Vernon experimented with five different kinds of mine-sweeping technologies.
The first attempt was the simplest.
Two wooden ships trailed a cable between them with strong bar magnets suspended above the sea floor. This approach detonated its first mine on Nov. 30, but it was a flawed implementation of a sound concept.
The permanently-magnetic bar magnets latched onto each other in transit, and were difficult to separate. The cables they trailed from also needed frequent adjustment every time there was a change in the seabed depth.
The start of December saw the introduction of another solution. A non-magnetic, wooden-hulled ship towed a barge containing a strong electromagnet. This produced the magnetic signature of a 20,000-ton ship. It was a mixed success.
The electromagnet activated the mines, but the resulting explosion sank the barge that carried it. This might have been acceptable if the barge kept its distance, but the towed skid had a habit of drifting dangerously close to the tug.
The follow-up was the icebreaker of the mine-sweeping world—the 2,900-ton mine destructor ship. This ship carried a 500-ton forward-facing electromagnet. The electromagnetic field tripped its first mine on Jan. 4, 1940. At 50 yards, the blast showered the ship with seawater and caused minor damage throughout the vessel.
It was a qualified success—the ships accumulated damage with each explosion—but the Royal Navy were desperate enough to commission 10 more.
The Royal Air Force also contributed to the minesweeping boom with a modified Wellington bomber. The R.A.F. Gunnery Research Unit fitted the bomber with a 2.5-ton electromagnetic ring that could project a magnetic field far below the aircraft.
The 51-foot diameter balsa wood ring housed an aluminum coil powered by a Ford V-8 car engine. The R.A.F. called the new variant the Wellington Directional Wireless Installation—a deliberate misnomer designed to keep the aircraft’s true purpose secret.
On Jan. 8, the R.A.F. detonated their first mine from the air, but had to fly slow and close to sea level. Five days later, the Wellington destroyed its second mine—but did so at less than 35 feet above the waterline.
The explosion threw the robust plane up into the air with no serious damage. It was a successful proof of concept, and the R.A.F. commissioned three further DWIs. In June 1940, the Gunnery Research Unit deployed to Egypt, and the DWIs continued their work in the Suez Canal.
But these were all sideshows to the two longest-standing countermeasures—the “Double L sweep” and degaussing.
Canadian chemist Charles Goodeve was the brains behind the double longitudinal sweep. Similar to the earlier efforts with the towed cable of bar magnets, the Double L sweep consisted of a non-magnetic sweep towing two cables, one long and one short, each topped with an electrode.
As electricity passed between the two contacts, it created a magnetic pulse that could clear significant areas of seabed. With two ships positioned parallel at 200 yards apart, the setup cleared ten acres of sea at a time.
The “Double L” sweep claimed its first kill on Feb. 10, 1940. By March, the method cleared 74 mines, and by the end of June they had taken 300 mines out of commission. As the cheapest and safest way for the Royal Navy to secure its sea lanes from German mines, the Double L sweep became standard practice for the Allied navies.
To prevent undiscovered mines from wrecking any further damage on the Allied fleet, the Royal Navy also implemented “degaussing.” Goodeve reportedly coined the phrase from the name of the unit used to measure magnetic field strength, and as a rhyme of “delousing.”
The principle of degaussing was the complete opposite approach to the Wellington DWI. While aircraft projected a “north pole down” magnetic field to trigger mines, a degaussing ship fitted a copper cable along its outer hull—projecting a “north pole up” field that neutralized the ship’s magnetic signature.
The Royal Navy fitted a coil to HMS Manchester days after the recovery of the mine at Shoeburyness. Degaussing’s biggest boost came after the navy’s flagship, HMS Nelson, triggered a mine on Dec. 4.
Six days later, the Admiralty ordered the fitting of degaussing coils to all military and merchant fleet vessels, and requisitioned Britain’s strategic stock of copper to create the heavy gauge cables needed for the job.
The fleet couldn’t fit the cables fast enough, and many small coastal vessels that carried cargo from port to port lacked the power to degaus on the go. Goodeve had a solution to that too—“deperming.”
Instead of fitting ships with a cable of their own, the Royal Navy would use a high-powered cable to “wipe” the accumulated magnetic field from a ship. This procedure left ships demagnetized for up to six months at a time.
In five months, these two methods of degaussing resulted in 1,000 wiped ships and a further 2,000 ships carrying their own cables. This gave British shipping the ability to traverse mined sea lanes in relative safety.
Confident of the nation’s success, now-prime minister Churchill told parliament on Aug. 20, 1940 that the magnetic mine had been “effectively mastered.”
But the Germans had beaten them to it. The Germans used the towed bar magnets concept in their Seekuh sweeping technique. Electric cables eventually replaced this, much like the British Double L sweep. The mine destructor ship concept was also in place by 1938, two years before the RAF began dropping magnetic mines into German waters.
The Germans created their own electromagnetic minesweeping aircraft later in the war. In response to the R.A.F.’s post-1940 mining campaigns in Europe, the Luftwaffe fitted Junkers JU-52s and Blohm & Voss BV-138s with similar rings to the Wellington DWI.
German forward-thinking and the lessons of World War I prevented them falling prey to British mining campaigns. Despite this, Britain’s ability and luck in defeating Germany’s technological advantage is one of the greatest examples of wartime ingenuity to date.
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