70 U-Boats Gone in 30 Days — The Secret Weapon That Turned the War

 

 

The sea was black that night — the kind of black that swallowed sound, light, and courage.
May 5th, 1943. 300 miles southeast of Greenland. A place where the cold gnawed at bone, and the horizon was nothing but shifting fog. Lieutenant Commander Robert Sherwood stood rigid on the bridge of HMS Tay, his gloved hands gripping the railing so tight his knuckles burned through the leather.

Below him, the convoy crept westward — 42 merchant ships, empty of cargo but heavy with anxiety. Their engines throbbed in unison, echoing through the water like a heartbeat heard from beneath the skin. They carried ballast, not goods. But they carried something far greater — the fragile hope that Britain would survive another month.

It was quiet. Too quiet. Sherwood hated quiet.

He leaned forward into the mist, straining to catch sight of the faint silhouettes ahead — the slow, lumbering freighters, grey shadows in a grey sea. He could almost sense their fear. Each man aboard those ships knew the truth: beneath them lurked death.

And death had propellers.

Four hundred feet below the surface, in a narrow steel tube painted black and stinking of diesel, a German sonar operator sat perfectly still. His headphones hummed faintly. He had been listening for hours, and then suddenly — there it was.

Thump… thump… thump.

Propellers. Dozens of them. The deep, throbbing rhythm of engines too large and too steady to be warships. He smiled faintly. Merchantmen. Easy prey.

He leaned toward his Kapitänleutnant, whispering in clipped German, “Multiple contacts. Estimate forty ships. Heavy tonnage.”

The captain’s lips curled. His periscope rose through the darkness like a serpent breaking the surface. Through it, he saw nothing but fog and night. But it didn’t matter. He knew what lay ahead. Somewhere in that invisible corridor of mist moved the greatest target a U-boat could ever dream of — a full Atlantic convoy.

And somewhere far above, on the bridge of HMS Tay, Robert Sherwood was thinking the same thing: They’re out there.

He could almost feel their presence, gliding silently beneath the swells. He’d fought them before — the U-boats, Hitler’s wolves of the Atlantic — and he’d seen what they could do. In a single night, they could turn the sea into a floating graveyard, a cemetery of burning oil and twisted steel.

But tonight would be different.
Tonight, the wolves would bleed.

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Two months earlier, the war at sea had looked lost.

It was March 1943, the darkest month of the Atlantic war. In just twenty days, 120 Allied ships were sent to the bottom. Nearly 700,000 tons of supplies gone. Only 12 U-boats destroyed. A kill ratio of ten to one — in Germany’s favor.

At that rate, Britain had three months to live.

Every loaf of bread, every gallon of fuel, every bullet came across that ocean. Lose the convoys, and Britain would starve before summer. There would be no D-Day, no return to Europe — only surrender.

Even Winston Churchill, who faced down Hitler himself with bulldog defiance, admitted later:

“The only thing that ever really frightened me during the war was the U-boat peril.”

He had reason to fear.

Under the cold, methodical command of Admiral Karl Dönitz, Germany’s Kriegsmarine had turned submarine warfare into science. Dozens of U-boats prowled the Atlantic in coordinated “Rudeltaktik” — wolfpack tactics. Fifteen to forty submarines would spread across a shipping lane like a net of steel.

One would detect a convoy through sonar, or by sighting its silhouettes against the horizon. It would radio headquarters. Within hours, the entire pack would converge.

They attacked at night — surfaced, sleek, faster than the merchant ships, invisible against the black water. Torpedoes hissed like serpents. Explosions rolled across the sea. Flames lit up the night, and the ocean became a canvas of burning oil.

By dawn, only wreckage remained.

Convoys HX-229 and SC-122 were the breaking point. Ninety merchant ships heading for Britain, loaded with oil, food, and ammunition. Dönitz unleashed three wolfpacks — 38 submarines — a steel phalanx waiting in the mid-Atlantic.

At dusk on March 16th, U-653 heard the hum of propellers through its hydrophones. By midnight, the convoy was surrounded. In the first hour alone, three ships exploded. By dawn, five more were gone. For three days, the sea screamed. Twenty-two ships sunk. 146,000 tons lost. Three hundred men dead.

And the Germans lost one boat.

The Royal Navy’s after-action report was blunt:

“It is questionable whether convoy defense remains viable as a strategy.”

Translation — We are losing the war.

But a few hundred miles away, in a basement below Derby House, Liverpool, a quiet revolution was taking shape.

Admiral Sir Max Horton, Commander-in-Chief of Western Approaches, stood before a wall-sized map of the Atlantic. Red pins marked sunken ships. Gray pins showed estimated U-boat positions. The ocean looked diseased — a spreading infection of death.

But Horton had something the Germans didn’t. Secrets.

Five of them.

Five invisible weapons that would turn the Atlantic from a killing ground into a trap.

He didn’t have more ships. He didn’t have more men.
He had information.

The first secret was HF/DF — “Huff-Duff,” high-frequency direction finding. The breakthrough wasn’t detecting submarines themselves — it was detecting their radio signals. Every time a U-boat reported a sighting, its Morse transmission gave away its bearing instantly.

A spike on a cathode-ray screen. A flash of light. The predator had spoken — and betrayed itself.

Two destroyers could triangulate the signal. Three could pinpoint it to a few miles. By April 1943, most Allied escorts carried Huff-Duff, and the Germans had no idea it existed.

The second secret came from the sky — 10-centimeter radar.

German radar detectors could only read long wavelengths. This new system operated on a band their sensors couldn’t even perceive. Suddenly, U-boats that surfaced at night — their favorite hunting ground — were ambushed by aircraft diving out of nowhere. To German commanders, it was black magic. They didn’t realize their “invisible” radar had simply gone blind.

The third secret was Ultra — the jewel of Bletchley Park.

The British codebreakers had cracked the Naval Enigma cipher. For the first time, the Allies could read Dönitz’s daily orders, often before the U-boat captains received them. Convoys were rerouted away from wolfpacks. Submarine rendezvous points became ambush zones. The hunters were being tracked without knowing it.

The fourth secret closed the last sanctuary of the German navy — the air gap.

For years, the mid-Atlantic had been beyond the reach of land-based aircraft. U-boats hunted freely there. But the arrival of the B-24 Liberator, modified for extreme range and armed with radar and depth charges, changed everything. Now, nowhere was safe.

And the fifth secret — perhaps the simplest, but most human — was TBS radio.
Voice communication between escort ships. No more delays, no more Morse lamps flashing across the waves. Orders could now be shouted over static in real time. For the first time, the convoy escort groups could fight as one organism — fluid, coordinated, fast.

Five technologies. Five revolutions. All ready by April 1943.

Now all that remained was to see if they worked together.

That test would come with Convoy ONS-5.

April 22nd, 1943. The convoy steamed west from Liverpool, bound for North America. Forty-two merchant ships in twelve columns, guarded by destroyers HMS Duncan and HMS Tay, and a screen of corvettes — Snowflake, Loosestrife, Pink, and Vidette.

Their mission was simple: get home alive.

But fate had other plans.

On April 28th, U-650 picked up the convoy’s noise through its hydrophones. The captain surfaced briefly and sent a sighting report. He didn’t know it yet, but the moment his operator tapped the Morse key, his position flashed across an Allied HF/DF screen like a flare.

On the bridge of Duncan, Commander Peter Gretton saw the spike. “There you are,” he muttered. “Got you.”

That single transmission would begin one of the largest battles in naval history — a 96-hour clash fought in fog, fire, and fear, where the outcome of the entire war would tilt.

Because by May 4th, the wolves had gathered — fifty U-boats closing in.

The largest pack ever assembled in the Atlantic.

And waiting for them was a convoy armed not with superior numbers… but with invisible eyes.

The sea would decide which side truly ruled the dark.

The fog arrived like a living thing.
May 4th, 1943 — North Atlantic, 300 miles southeast of Greenland. The world had vanished into a curtain of white. It wasn’t a simple mist — it was a choking, wet blindness that blurred the sea and sky into one shapeless void. Men couldn’t see twenty yards ahead. Even sound seemed smothered, swallowed by the fog’s thickness.

Lieutenant Commander Robert Sherwood, now in command of HMS Tay, stood motionless on the bridge, his voice low but sharp. “Keep your spacing! Engines steady!” His words crackled over TBS radio, reaching the other escorts spread like ghosts through the mist.

Somewhere behind him, the merchant ships groaned and moaned, their propellers turning slow and steady. They were shadows in the fog — forty-two massive silhouettes barely visible even from a hundred yards away.

And beneath that still water, the wolves had come.

Seventy U-boats. The largest concentration ever assembled in the North Atlantic. Admiral Karl Dönitz had ordered them to strike with everything they had. His orders were clear: Destroy ONS-5. Leave nothing afloat.

From his headquarters in occupied France, Dönitz believed this was the moment the tide would turn back. He was convinced his wolfpacks would shatter the convoys once more, driving the British into starvation and breaking the Allied bridge across the Atlantic.

But Dönitz didn’t know the rules of the hunt had changed.

In the control room of U-358, the German crew waited in silence, their faces pale beneath the red emergency lamps. The hydrophone operator listened, his eyes closed, one hand resting on the dials.
“Forty… maybe forty-five screws,” he whispered.
The captain leaned over the chart table. “Bearing?”
“West-northwest. They’re close.”

He smiled grimly. “Tell Berlin: target located.”

The radioman began tapping his Morse key — short bursts of coded signals flying into the ether.

But above, aboard HMS Loosestrife, a petty officer watching the HF/DF oscilloscope leaned toward his captain. “Spike, sir. Bearing zero-one-nine!”

In seconds, the convoy escorts had the submarine’s direction. In less than a minute, three ships had triangulated its position. The predator had spoken — and the prey had heard it.

Sherwood’s calm voice broke the radio silence: “Tay to all escorts, contact confirmed. Move to intercept.”

In the space of a breath, the convoy turned from prey into a pack of its own.

For hours, the fog thickened. The sea fell eerily calm, broken only by the thrum of engines. It was perfect hunting weather for submarines — but perfect detection weather for sonar.

Inside HMS Tay, the sonar operator sat hunched over his console, headphones clamped tight. The rhythmic ping of sound pulses echoed through the ship, bouncing off the ocean floor. Then suddenly, a second ping returned — faint, but distinct.

“Contact! Bearing two-six-five! Range eight hundred yards!”

Sherwood’s hand came down hard on the console. “Helm, hard to starboard! Depth charges — pattern Bravo!”

The ship swung sharply. Seconds later, the first charges rolled off the stern.

Whump. Whump. Whump.
Five explosions in rapid sequence.

The ship trembled underfoot. Columns of white water rose high into the fog, bursting upward like geysers. Oil slick bubbled to the surface.

“Contact lost!” the sonar man called. Then softer: “Regained… he’s going deep.”

Another barrage followed. This time, the sound that returned through the headphones wasn’t the dull echo of steel — it was the dull, hollow boom of an imploding hull.

“Confirmed kill,” the operator said quietly.

Sherwood exhaled for the first time in ten minutes. “Mark position. Next contact.”

But there was no time to rest.

All around the convoy, invisible predators moved closer. U-438, U-630, U-531 — names that would never reach shore again — circled, waiting for openings. The Germans coordinated over radio, unaware that every transmission betrayed them.

In Liverpool, the Western Approaches Command listened too. Using Ultra decrypts, Admiral Horton’s staff could see the wolfpack movements in real time. Every message from Dönitz’s headquarters was intercepted, decoded, and relayed to Sherwood within hours. The U-boats believed they were hunting blind prey. In truth, they were walking into a trap plotted from half an ocean away.

That night, the first torpedoes struck.

At 22:15, the British freighter Bristol City took a hit amidships. The explosion ripped through her hull, sending fire into the fog. Two minutes later, the Norwegian tanker Bonde erupted in a column of flame so tall it lit up the fog for miles. One witness aboard HMS Pink recalled:

“It was as if the sea itself was burning — the fog glowed orange, and you could hear the screams before you saw the fire.”

Then, just as suddenly, another torpedo found Harbury. Three ships in under five minutes.

The escorts didn’t wait for orders. Loosestrife and Snowflake sprinted toward the coordinates, their Type 271 10cm radar slicing clean through the fog. On their scopes, the blips were unmistakable — surfaced U-boats, still reloading torpedoes, thinking themselves invisible.

The British destroyers fired star shells — white phosphorus illuminating the fog like daylight. For the first time, the wolves saw their hunters.

HMS Snowflake opened fire with its forward gun. The shells tore through the fog, striking the U-boat’s conning tower. A second later, the submarine vanished beneath the surface. Depth charges followed — and then silence. Only the slick of oil rose back up.

By midnight, four U-boats were gone.

Below the surface, panic began to ripple through the German ranks. The radio traffic intercepted by the Allies that night told a story of disbelief:

“They see us before we see them!”
“Attacked out of fog — radar invisible!”
“No warning — cannot explain!”

They couldn’t know that their own technology had failed them. Their radar detectors, tuned for 60cm waves, were blind to the new 10cm Allied sets. They were being stalked by ghosts.

Inside U-650, the captain slammed a fist against the bulkhead. “This is madness,” he growled. “We have the range, we have the speed — how can they be finding us?”

But above him, the explanation was simple. Every radio transmission, every radar reflection, every attempt to reposition was being tracked by an ocean-wide web of Allied sensors. For the first time since 1939, the hunters had become the hunted.

The battle raged for twenty-four hours without pause. The fog turned from white to silver, from silver to red as fires spread across the sea. Allied aircraft — B-24 Liberators flying from Iceland — swept in and out of the clouds, dropping depth charges and sonobuoys.

One U-boat commander surfaced to recharge his batteries. He saw nothing but fog. Then a shadow appeared — a Liberator diving straight out of the clouds, engines screaming, depth charges already dropping. The sea erupted around him. He tried to dive, but the pressure crushed the hull before they could close the hatches.

At dawn, HMS Tay received the latest intercepts from Ultra. “Enemy withdrawing south.”

Sherwood lowered his binoculars, the fog still clinging to his face. “They’re breaking off,” he said quietly. “After all this, they’re running.”

No one spoke. The men were too tired to cheer.

By the end of that day, six U-boats were destroyed. At least a dozen more crippled. The convoy had lost thirteen merchant ships — but for the first time in the war, U-boat losses outnumbered Allied losses in a major engagement.

The wolfpack had been broken.

But the battle of May 5th was only the beginning.

The next three weeks would become known in German war diaries as “Der Schwarze Mai” — Black May.

And by the time it ended, seventy U-boats — nearly a quarter of Dönitz’s operational fleet — would be gone.

The fog that night eventually lifted, but what it revealed was worse than darkness. Debris. Oil. Lifeboats adrift. Survivors clinging to planks in icy water.

Sherwood stood on deck, staring at the gray horizon. “Get every man we can,” he said softly. “Friend or foe.”

In the freezing dawn, the British pulled survivors from both sides. German sailors, their faces pale and eyes hollow, were hauled aboard ships they had tried to kill. None spoke. None needed to.

The war beneath the waves had shifted. The wolfpacks were dying. And the Atlantic — once their kingdom — had become their grave.