Invisible threat

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.