North Atlantic, October 18th, 1940, somewhere in the northwestern approaches, 0200, the convoy is called SC7.
35 ships, slow, averaging 7 knots, laden with steel, grain, and timber bound for Britain.
The escort force consists of two vessels, the sloop Scarborough and the Corvette HMS Bluebell.
Two warships for 35 freighters crossing 2,000 mi of open ocean. By dawn on October 19th, 20 of those 35 ships are gone.
Six U-boats did it. Six submarines working the surface at night, penetrating the convoy columns at close range, firing and reloading, firing again.
The German Navy logs it as the single most productive two-night period of the entire war to that point.
The U-boat arm is averaging 10 ships sunk per submarine at sea during October 1940, a figure that has never been matched before or since in submarine warfare.
The men doing this are Otto Kretschmer, Joachim Schepke, Heinrich Liebe, Karl Heinz Möhle, and Günther Prien von Forstner.
They are operating on the surface at night inside the convoy columns. The question Allied Command cannot yet answer is not how to find them.
It is why every defense they have built assumes the submarines will stay outside. Before the summer of 1940, the prevailing doctrine on both sides of the Atlantic held that submarine warfare had been effectively solved.
The instrument of that solution was ASDIC, an acronym for the Anti-Submarine Detection Investigation Committee.
Developed by the Royal Navy through the 1930s, ASDIC worked by projecting a pulse of sound from a surface ship’s hull and timing the returning echo off a submerged submarine.
By 1939, the Royal Navy had fitted ASDIC to approximately 165 escort vessels. Admiral Dudley Pound, First Sea Lord, told the War Cabinet in September 1939 that the U-boat threat was, quote, “Nothing like so serious as it was in the last war.”
He was right about one thing, ASDIC worked. Against a submerged U-boat, the critical word was submerged.
A surfaced U-boat, trimmed low, running on diesel engines, its pressure hull wash produced almost no ASDIC return.
It was acoustically almost invisible. The Royal Navy knew this. What it did not fully account for was that a surfaced U-boat in darkness was also nearly invisible to the human eye and could make 17 knots, fast enough to outrun most convoy escorts and fast enough to reposition between attacks.
German naval command had understood this vulnerability as early as 1935 when Dönitz, then commanding the Weddigen Flotilla, ran surface attack exercises in the Baltic.
His conclusion, recorded formally in the Kriegsmarine’s internal tactical assessments, was that wolf pack attacks at night on the surface would defeat ASDIC comprehensively.
He requested a fleet of 300 operational U-boats to execute the strategy. He was given 57 by September 1939.
The Royal Navy’s counter-strategy throughout 1939 and into mid-1940 was built on exactly the wrong model.
Escorts were trained to hunt submerged contacts. Search patterns, depth charge procedures, and patrol doctrines were all predicated on the assumption that the attacking submarine was beneath the surface.
When night surface attacks began in earnest in July 1940, following the German occupation of French Atlantic ports, which cut U-boat transit time dramatically and extended their operational range westward, the escorts were not fighting the war they had prepared for.
Between July and October 1940, U-boats sank 144 Allied merchant ships. During the same period, the Allies sank four U-boats in the Atlantic.
No senior naval officer called this impossible. They called it a temporary crisis of material, not enough escorts, not enough aircraft.
The solution, it was agreed, was more ships, more planes, more ASDIC sets. The actual solution was something else entirely, and it came from watching one man work.
Otto Kretschmer was born in Heiden Silesia in 1912. He joined the German Navy in 1930, served on surface vessels, and transferred to the U-boat arm in 1936.
By the time the war began, he had already completed one pre-war patrol and was considered, in the assessment of U-boat Command, a technically precise, rather than instinctively aggressive, commander.
What distinguished him was not his nerve. It was his arithmetic. In his early patrols on U-23, operating in the shallow waters of the North Sea and off the Scottish coast, Kretschmer developed a practice he called Einschuss Einschiff, one shot, one ship.
Where most U-boat commanders fired torpedo salvos of two or three to ensure a hit, Kretschmer fired single shots.
His hit rate was higher than any commander in the Kriegsmarine. He was conserving torpedoes, not out of caution, but out of a precise calculation.
More torpedoes available meant more ships sunk per patrol. That same arithmetic, applied to the problem of convoy attack, produced a conclusion his peers were not making.
A submarine outside a convoy firing into the columns was shooting at extreme range with restricted angles.
A submarine inside the columns, on the surface in darkness, among the merchant ships themselves, was firing at ranges sometimes under 600 yd with perfect beam shots.
The escorts were outside. The gaps were inside. He had worked this out in the North Sea by 1939.
He simply waited until the operational theater made it viable. When it did, in the northwestern approaches in the autumn of 1940, he executed it with a consistency that made his patrol records read less like combat logs and more like geometry.
The idea was not purely Kretschmer’s alone, and this is important. Dönitz had theorized surface night attacks since the mid-1930s.
Several commanders, among them Joachim Schepke and Heinrich Liebe, were practicing surface approaches by the summer of 1940.
But there was a critical difference between approaching from outside the convoy and penetrating to the interior of the columns, and that difference was treated, within the U-boat arm, as the line between acceptable risk and recklessness.
The resistance was structural, not personal. Standard U-boat doctrine required that once a convoy was contacted and reported, the shadowing U-boat maintain position and home in other boats by radio beacon before attacking.
Penetrating the convoy interior meant losing the ability to disengage, losing contact with coordinating submarines, and placing the U-boat in a position where it could be rammed by the merchant ships it was attacking.
The doctrine existed because the wolf pack concept required coordination, and coordination required that individual commanders subordinate their own attack opportunities to the group’s.
Kretschmer ignored the interior penetration doctrine on the night of October 18th, 19th, 1940 during the attack on convoy HX79.
He brought U-99 inside the columns. He fired 11 torpedoes. Seven ships went down, five of them sunk, two damaged.
He reloaded a forward tube inside the convoy while merchant ships passed within a few hundred yards on both sides.
The moment that nearly ended everything was simple and mechanical. A torpedo malfunction on the fourth shot left U-99 with a flooded forward tube in the middle of a panicking convoy.
For approximately 40 minutes, Kretschmer held position, surfaced, while his crew cleared the tube manually.
He did not dive. He could not dive safely from inside the columns without risking collision.
He waited. When the tube cleared, he fired again. The proof was not a controlled exercise, it was the patrol record itself, and it accumulated across three months in the autumn of 1940 with a specificity that made denial impossible.
In September 1940, Kretschmer and U-99 sank seven ships totaling 51,333 tons on a single patrol.
In October, operating against convoys SC7 and HX79 in the northwestern approaches, he sank six ships of 30,085 tons in two nights, the HX79 attack being one of the most concentrated single night sinkings of the war, with 12 ships lost to two U-boats.
By November 1940, Kretschmer’s cumulative total had passed 200,000 gross tons, a figure that no submarine commander of any nation had approached in any previous war.
He had achieved it with fewer torpedoes per ship sunk than any other active commander.
The Allied response revealed exactly where the consensus had broken down. The first successful counterattack against night surface penetration tactics did not occur until November 21st, 1940, when HMS Rhododendron, stationed astern of convoy OB 244, as a rescue vessel, sighted a surfaced U-boat at 1,500 yd and gained asdic contact when it dived.
Metallic wreckage and oil reached the surface within the hour. One counterattack in 4 months of sustained surface penetration operations.
If this operational history is new to you, the companion episode on Allied radar development, and specifically how centimetric radar changed the surface equation overnight, covers the technical details that most surveys leave out.
The complication that nearly changed the calculus was weather. Heavy winter conditions in November and December 1940 reduced U-boat operational tempo significantly.
Allied command interpreted the falling loss rates down to 150,000 tons in November as partial evidence that their countermeasures were working.
They were not. The U-boats had been slowed by the North Atlantic in winter, not by the escorts.
When conditions improved in February 1941, the attacks resumed at full intensity. March 16th-17th, 1941, North Atlantic, convoy HX 112, U-99, Kapitanleutnant Otto Kretschmer commanding.
HX 112 is an eastbound convoy, 41 ships escorted by the fifth escort group, five destroyers and two corvettes under Commander Donald Macintyre aboard HMS Walker.
This is the best escorted convoy Kretschmer has yet faced. On the evening of March 16th, U-100, commanded by Joachim Schepke, makes contact and reports.
By nightfall, both U-99 and U-100 are in position. Schepke attacks from outside the columns he has detected.
Kretschmer takes U-99 inside. He fires his first torpedo at 22:42. The tanker Vanessa, 5,728 tons, takes the hit and begins to burn.
He repositions within the columns. Fires again. Irene Maria, 2,595 tons, gone. Reposition. J.B. White, 6,999 tons, hit.
French Comte, 6,069 tons, hit. Corsham, 3,386 tons, hit. Five ships inside the convoy. The escorts are hunting outside the columns in the dark, in the direction from which they expect the threat to come.
At 0100 on March 17th, Kretschmer signals U-boat command, “Torpedoes exhausted.” He is preparing to withdraw.
He never reaches France. The full tactical breakdown of what Macintyre’s escort group did next, the specific radar contact, the exact depth charge pattern, and the 35-minute surface pursuit that followed is the subject of the next episode, available now.
At 0337, HMS Vanoc’s newly fitted Type 271 centimetric radar, one of the first sets deployed operationally, picks up U-100 on the surface at 1,000 yd.
Vanoc rams her. Schepke is killed on the bridge. 38 of his crew go down with U-100.
17 minutes later, HMS Walker gains asdic contact on U-99. Kretschmer, out of torpedoes, out of options, gives the order to scuttle.
39 of his crew reach the surface. Kretschmer is pulled from the water and taken prisoner.
The three top U-boat commanders by tonnage sunk, Prien lost 11 days earlier when U-47 was destroyed by HMS Wolverine attacking convoy OB 293, Schepke, and Kretschmer are all gone within 11 days of each other in March 1941.
The human arithmetic of HX 112, five Allied merchant ships sunk with their crews, two U-boats destroyed, 38 German sailors dead.
Kretschmer and 38 of his crew survive as prisoners of war. He spends the rest of the war in a Canadian POW camp.
He never commands another submarine. Kretschmer’s final tally at the time of his capture was 44 ships sunk and one warship damaged, approximately 273,000 gross tons.
It remains the highest confirmed tonnage record of any submarine commander in the Second World War, and it has never been exceeded in any conflict since.
The tactics he refined, surface penetration of convoy columns at night, were adopted formally into Kriegsmarine doctrine and remained the standard approach until centimetric radar and continuous air cover made surfaced operation in the North Atlantic untenable by mid-1943.
By that point, the U-boat arm had lost 783 submarines and approximately 30,000 men, roughly 75% of all who served.
In 1955, Kretschmer joined the newly formed West German Bundeswehr Navy. He retired in 1970 as a flotilla admiral, a rear admiral equivalent, and spent his final years writing and corresponding with former Allied officers who had hunted him.
He died in Bavaria in 1998. Here is the fact that reframes everything you just heard.
When HMS Walker’s asdic operator gained contact on U-99 in the early hours of March 17th, 1941, Commander Macintyre initially assessed it as a non-submarine contact, debris from U-100, which had just been rammed.
He very nearly called off the attack. The officer who insisted the contact was valid and pressed for the depth charge run was a junior lieutenant whose name does not appear prominently in any published account of the battle.
Kretschmer, the most successful submarine commander in history, was captured not because the system finally worked.
He was captured because one unnamed officer on one ship trusted his instrument over his superior’s judgment.
The system had almost let him go again.
