How Allied Forces Used Code and Hunches To Turn the Tables on German U-Boats
The May 24, 1943, wireless message, encrypted and transmitted from U-boat headquarters outside Berlin, was intercepted at a time when the Allies at best held a tenuous grasp on Germany’s naval Enigma cipher system. Yet almost miraculously, experts at Bletchley Park, Britain’s code and cipher facility northwest of London, promptly cracked it.
Grand Admiral Karl Dönitz, former U-boat fleet commander and, since January, Germany’s overall navy chief, had ordered 16 submarines, then prowling North Atlantic convoy routes, to proceed to “naval grid square 87 of the large square West of TT.” As customary, the decrypted message was teleprinted to the Royal Navy’s Submarine Tracking Room and to F-21, the U.S. Navy equivalent.
The message’s top-secret content, what the Allies called “special intelligence” or “Ultra”—both meaning intelligence derived from solving high- grade codes and ciphers—reached Commander Rodger Winn and Lieutenant Kenneth Knowles. Winn of the Royal Navy and Knowles of the U.S. Navy were both middle- aged reservists—so-called “civilians in uniform.” Save for wartime manpower exigencies, neither would be serving, even in shore billets. But now Winn, stationed in London, and Knowles in Washington, D.C., had to unravel what amounted to a puzzle-within-a-puzzle—one with life-and-death consequences.
German naval charts were overlaid with a system of lettered and numbered grids. U-boats maneuvered according to grid positions, not latitude and longitude. Culling and cross-checking various sources had enabled Allied submarine trackers to piece together much of the Atlantic Ocean grid system. But the Germans greatly complicated matters, using two letter “substitution codes” that changed frequently. In this case what did the grid reference “TT” stand for? Failure, or even delay, in unlocking the U-boats’ precise destination—and intentions—could spell disaster in the Atlantic.
With Allied navy regulars needed at sea, filling backroom intelligence slots with reservists was the only option. But harnessing the disparate skills of attorneys, bankers, journalists, linguists, and mathematicians produced unanticipated benefits. Unburdened by chain-of-command rigidity, outsiders could, as one British observer quipped, “resist…cooking raw intelligence to make their masters’ favourite dishes.”
Thirty-seven-year-old Rodger Winn, a successful London barrister, was a case in point. Childhood polio had left him short in stature with a twisted spine and pronounced limp. Rejected for traditional military service, Winn instead became a civilian volunteer. Winn’s legal acumen and German language fluency landed him in the Submarine Tracking Room, a small but vital component of the Royal Navy Operational Intelligence Center (OIC). Winn proved a quick study, earned a reserve commander’s commission, and displaced his ailing Royal Navy boss.
Tracking enemy submarines depended on signals intelligence, which in turn hinged on intercepting U-boat wireless radio transmissions. Much has been made of Bletchley Park’s ability to solve Enigma, but Ultra decrypts were often not readily at hand. Whenever the Germans refined Enigma, Bletchley’s experts often “went blind,” as they put it, for a period. And decrypting itself took time—hours, often days, and sometimes weeks. Too often, in fact, decrypted Ultra information could be so outdated as to be useless. Not least, Royal Navy brass, determined not to tip their hand to the Germans, insisted that Ultra be used discreetly—with what today is called “plausible deniability.”
A more reliable sub-tracking tool than Enigma decrypts was Radio Direction Finding, a technology that determined the direction, though not the range, of wireless transmissions. Radio Direction Finding effectiveness required amassing enough trans-missions to perform “resection,” i.e. using the known location of one U-boat to estimate the locations of others. Fortunately for the Allies, Dönitz made it easier for them by micromanaging wolfpack operations, sometimes sending and receiving hundreds of wireless messages daily.
The hub of Rodger Winn’s Submarine Tracking Room, housed in London’s northwest suburbs, was an immense square table across which sprawled a chart of the Atlantic Ocean peppered with tiny colored pins—red pins for confirmed U-boat fixes, white for sometimes questionable visual sightings, blue for resected locations. Winn and his three dozen staffers worked around the clock, confined in a smoky subterranean room they dubbed “Lenin’s Tomb.”
British intelligence officer Rodger Winn (right) and his American counterpart Kenneth Knowles (left) raced to anticipate the Kriegsmarine’s next moves in the Atlantic—efforts that resulted in U-118’s sinking in June 1943.
With outward calm, Winn and key depu-ties sifted through scraps of intelligence, eyed pin movements, and estimated courses, speeds, and distances. Like a chess master, he proved uncanny in anticipating enemy tac- tics and devising brilliant countermoves. Winn, in fact, became so adept at “evasive routing” (altering convoy paths to avoid U-boats) that higher-ups routinely rubber-stamped his recommendations.
Winn’s American counterpart—at first his acolyte—was 38-year-old Kenneth A. Knowles, a bespectacled and reclusive navy reservist. Knowles’s intellect earned him a slot at the U.S. Naval Academy. He excelled academically but fellow midshipman pegged him as “quiet and unassuming,” and as someone who made friends “rather slowly”—unpromising traits if he hoped to climb the ranks.
For a decade after graduation, Knowles followed a normal progression of ship and shore billets. But then, in 1938, still just a lieutenant, Knowles was medically discharged with vision and hearing problems. Though ousted from his regular navy career, he maintained service ties as the civilian editor for Our Navy , a professional journal with offices near New York’s Brooklyn Navy Yard. Reactivated when war broke out, Knowles nonetheless had to settle for teaching landlocked Navy Reserve Officer Training Corps prospects at the University of Texas.
Fortunately, during one early shipboard tour, Knowles had caught the eye of Francis “Frog” Low, now a key aide to U.S. Navy Commander-in-Chief Admiral Ernest J. King. Captain Low recalled Knowles’s talent for reducing mountains of information to clear, concise, and actionable recommendations. Thus, in late June 1942, when King decided to establish the U.S. Navy’s own Sub-marine Tracking Room, Knowles was plucked from obscurity and sent overseas to understudy Rodger Winn.
Similar in age, Britain’s Winn and America’s Knowles had both bucked medical headwinds to gain consequential wartime stature. But, with Winn ranked higher and already well-reputed, Knowles’s job was to learn from the master. Admitted to Winn’s tracking room, Knowles observed how intelligence estimates were converted to what Winn called “useful fictions”: educated guesses about U-boat positions, movements, and intentions. Gaining insight into Ultra, Knowles also witnessed how Winn and his bosses used it sparingly.
After returning to the United States in December 1942, Knowles set up his own smaller-scale Submarine Tracking Room—designated F-21—in the Main Navy Building on Washington, D.C.’s National Mall. Though now an ocean apart, Knowles fostered what he considered a “close personal relationship” with Winn. Meanwhile, however, Allied naval brass squabbled over collaboration in the Battle of the Atlantic.
While Britain’s First Sea Lord Dudley Pound doggedly argued for a single unified command under Royal Navy aegis, the purportedly Anglophobic Ernest King just as stubbornly demurred. Not until March 1943, with U-boat savagery cresting, was a compromise forged. In simplest terms, Commonwealth (British and Canadian) navies assumed responsibility for the North Atlantic convoy routes while the U.S. Navy guarded the more southerly Central Atlantic passages.
King coupled this new arrangement with an audacious new gambit. “It is arranged,” he announced to fellow American service chiefs on May 1, 1943, “to set up immediately…an antisubmarine command to be known as the Tenth Fleet.” Ostensibly, Tenth Fleet would be a “fleet without ships.” But, under King’s direct authority (delegated to newly promoted Rear Admiral Francis Low), Tenth Fleet could, on the fly, organize and dispatch sea and air task groups to reinforce Central Atlantic convoy routes.
Tenth Fleet’s formation coincided with a crisis point for Dönitz. Before the month of May 1943 was out, over three dozen U-boats would be lost to Allied attacks, most in the North Atlantic. A once favorable exchange ratio had all but evaporated: now, instead of sacrificing one U-boat for every 100,000 tons of Allied shipping sunk, the rate was one U-boat for every 10,000 tons.
If ever there was reason for Dönitz to withdraw from the North Atlantic, this was it. That said, North Atlantic convoy routes remained the Kriegsmarine’s favored hunting ground. It was where Dönitz’s U-boats strove to sink Allied shipping faster than it could be replaced—what he called Tonnagekrieg (“integral tonnage strategy”). The strategy was now doubtlessly faltering, but what countermoves might the always wily Dönitz have up his sleeve?
Germany exacted a small measure of payback on May 29, 1944, when U-549’s torpedos hit escort carrier USS Block Island (shown) and destroyer escort USS Barr, inflicting more than three dozen casualties.
Though it’s impossible now to reconstruct Winn’s and Knowles’s thought processes as each puzzled over what “TT” stood for, multiple factors came into play. First, because Winn had been reading Ultra messages for 14 months, Knowles for just 5, Knowles had good reason to defer to Winn’s acumen. Moreover, the continuing volume of North Atlantic U-boat message traffic suggested that Dönitz was still committed to this arena.
But there were also reasons for the pair to reach different conclusions. The reconfiguration of Battle of the Atlantic responsibilities kept each man keenly attuned to the dangers in his respective realm. Moreover, differences were emerging over just how—cautiously or aggressively—to exploit Ultra. As Knowles remarked decades later: “We were more aggressive in our use of Ultra intelligence than the British and got more mileage from it.”
After deliberation, Rodger Winn projected that the 16 U-boats were bound for a grid area “about 200 miles east of Nova Scotia.” Knowles’s estimate, meanwhile, was grid square “CD 87,” an area 54 sea miles by 54 sea miles “600 miles southwest of the Azores”—a difference of about 1,000 miles.
Now, perhaps for the first time, the acolyte challenged the master. And for good reason. In truth the May 24 signal exposed Karl Dönitz’s deep concern over what even he acknowledged were “unbearable losses.” His immediate response was to distance North Atlantic U-boats from Allied airfields in Nova Scotia and Iceland. And Dönitz was also being pressured to redress Axis setbacks in North Africa by attacking Allied merchant convoys to and from Gibraltar.
Accordingly, the 16 U-boats were indeed bound for CD 87, there to form a wolfpack designated Group Trutz (Defiance). Simultaneously, as a ruse, three U-boats stayed behind, transmitting wireless messages meant to convince the enemy that nothing had changed. The deception apparently worked; as of June 5, OIC still estimated that 20 U-boats lay between Newfoundland and Greenland.
But meanwhile, well to the southeast, Group Trutz was angling to pounce on three Allied convoys: “GUS.7A,” 46 merchant vessels with 10 combat escorts westbound from Oran, Algeria; “UGS 9,” 79 merchant vessels with 14 combat escorts eastbound from New York City; and, also eastbound, “Flight 10,” 20 infantry landing craft accompanied by a re-fueling tanker. With GUS.7A set to transit grid CD 87 first, Ken Knowles deftly recommended—and convoy routers promptly agreed—to steer GUS.7A well south of Trutz.
Prudent “evasive routing.” Only now, Tenth Fleet also went on offense.
Just that April, First Sea Lord Dudley Pound and Ernest King had exchanged testy cables about using Ultra. “We should not risk what is so valuable to us,” Pound tartly chided. King’s riposte: “We are not deriving fullest value from it.”
Underlying King’s impatience were potent advances in antisubmarine warfare capabilities. Foremost was Tenth Fleet’s growing arsenal of escort aircraft carriers. Nicknamed “jeep carriers” or “baby flattops,” the small aircraft carriers were converted merchant vessels with flight decks capable of accommodating 15 to 20 single-engine aircraft. Merchant convoys could now be accompanied by their own air cover.
That March, USS Bogue , one of the first escort carriers, was temporarily loaned to the Royal Navy. Unfortunately, conditions in the North Atlantic made it nearly impossible to conduct flight operations. Bogue ’s aircraft—9 Grumman Avenger torpedo bombers and 12 Grumman F4F Wildcat fighters—lamented a journalist, were “held prisoner in their lashings by the fogs and gales, the freezing rain and sleet and spray.”
Two months later, Bogue , protected by U.S. Navy destroyers, was operating in more favorable weather and explicitly tasked with hunting down U-boats. The ship also carried HF/DF (High Frequency/Direction Finding, called “Huff- Duff”), a compact version of Radio Direction Finding. Near twilight on May 22, after chasing down a Huff-Duff bearing, Avengers scored Bogue ’s first U-boat kill, strafing and bombing U-569 until its survivors scuttled the boat and jumped overboard.
Next, on May 30, Bogue was dispatched to F-21’s best estimate of Trutz ’s current locale. Late on the afternoon of June 4, aided again by Huff-Duff, Bogue Avengers attacked three U-boats, sinking none but damaging two. Then, early on June 5, an Avenger-Wildcat team sank U-217 , effectively clearing the path for convoys UGS 9 and Flight 10. Admiral King’s impatience, Tenth Fleet’s capabilities, and Ken Knowles’s growing reputation set the stage for a new offensive priority.
Tenth Fleet sank U-boats over a broad swath of the Central Atlantic.
Shortly after Knowles established F-21, an unattributed report titled “Offensive Action against U-boats” reached King. It proposed attacking “milk cows”: large refueling and reprovisioning vessels—also called “U-tankers”—that were both vital and scarce (altogether, 10 had been built). “The loss of several,” said the report “would decrease [U-boat] effectiveness out of all proportion.” Scheduling refueling rendezvous required advanced planning—offering the Americans time enough to act on decrypted Ultra intelligence. Asserted the report’s author: “We are now able to determine up to a week or ten days in advance the exact position of their fueling area.”
The prospect intrigued Knowles. Combing logs of U-boat message traffic, he learned that most refueling rendezvous were set up well beyond the reach of land-based aircraft. The proof of concept came on June 12. Using Enigma decrypts supplemented by Huff-Duff bearings, eight Bogue aircraft ganged up on U-118 , expending 14 depth charges and 5,000 rounds of machine-gun ammunition to sink the submarine and capture 17 survivors.
U-118 was a provisional U-tanker, a minelayer pressed into refueling duty because land-based British aircraft were likewise sinking purpose-built milk cows as they transited through the Bay of Biscay off the western coast of France—a chokepoint swarming with Royal Navy antisubmarine forces.
Knowles, now a valued resource, met regularly with escort carrier task group commanders before they went to sea. Most couldn’t access Ultra; but, as Knowles noted, with F-21 information “more right than wrong…they listened very carefully.” Indeed, as one commander phrased it, Knowles “was a soothsayer…absolutely uncanny in his predictions…. We based our operations on [F-21 estimates] completely.”
Leveraging Knowles’s estimates, Tenth Fleet leaned into stalking the milk cow herd. About noon on July 13, the pilots of a Wildcat and an Avenger from jeep carrier USS Core spotted U-487 on the surface. The milk cow was en route to rendezvous with eight fuel-starved U-boats, but its crew was lazing on deck, some even toying with a floating bale of cotton. During the ensuing melee, the Wildcat was downed, and its pilot killed, but six more Core aircraft—two Wildcats and four Avengers—bombed and strafed U-487 until it sank steeply by the bow.
U-117 , another provisional U-tanker, came next. Early on August 7, alerted by Ultra information, escort carrier USS Card launched an Avenger which caught U-117 attempting to refuel U-66. Both U-boats fired flak to chase the Avenger out of range, but four more aircraft rushed in. Two were Avengers equipped with “FIDOs,” new acoustic homing torpedoes designed to sniff out and pursue spinning U-boat propellers. FIDOs were dropped as both subs dived. One FIDO ran astray but the other obliterated U-117.
Less than two months later, on October 4, Card aviators again struck paydirt. Robert L. Stearns (an Avenger pilot who, while assigned to Bogue , received a Navy Cross for his role in sinking U-118 ) spotted U-tanker U-460 as it fueled three U-boats. U-460 ’s customers promptly pulled the plug, but 460 ’s skipper unwisely tried to fight back.
Eschewing “misguided courage,” Stearns bided his time. “In a case like this,” he reasoned in his after-action report, “let [the U-boat skipper] get his guns hot” and “wait for the fighters to work it over.” Only when U-460 began to dive did Stearns finally unleash his FIDO. “About 25 seconds later, a well- defined shock-wave [was] followed…by a large amount of oil and debris.” Stearns had earned another Navy Cross; Dönitz had lost another milk cow.
By now, Tenth Fleet’s war on U-tankers was causing consternation within Britain’s Admiralty, which feared the loss of Ultra. Admittedly, Knowles reflected years later, “we skated on some pretty thin ice.” Indeed, after one of the U-tanker sinkings, Winn caustically advised Knowles that his prescience was just “too true to be good.”
This inter-Ally dispute boiled over in mid-March 1944 when USS Block Island aviators sank two U-boats (neither of them U-tankers) believed to be approaching a fueling rendezvous. Afterwards, First Sea Lord Andrew Cunningham (Dudley Pound’s successor) vigorously urged Admiral King to refrain from attacking refueling operations lest it prompt “major changes” in Enigma. King adamantly refused to “conform,” arguing that constraining U.S. Navy hunter- killers would only sow more suspicion.
Indeed, with Dönitz’s original U-tanker fleet now culled down to two, Tenth Fleet intensified the milk cow campaign. Just after dawn on April 26, 1944, again aided by Ultra and Huff-Duff, an Avenger from escort carrier USS Croatan spotted U-488 at a refueling site midway between Brazil and West Africa. This time, it was Croatan ’s five destroyer escorts that cornered the prey and destroyed it by employing barrages of “Hedgehogs”: projectiles fired like mortar rounds that, unlike depth charges, exploded only on contact.
Captain Aurelius B. Vosseller (left) with the USS Bogue’s scoreboard as of October 1944. The two Japanese kills were likely sunk en route to Germany.
A little over a month later, German submariners got their only—and ultimately pyrrhic— measure of revenge. After dark on May 29, while steaming off Portugal’s Madeira Islands roughly 300 miles west of Morocco, escort carrier USS Block Island edged into the crosshairs of U-549 , an attack boat bound for Argentina.
U-549 ’storpedoes dealt mortal blows to Block Island and severely damaged destroyer escort USS Barr. While Barr ’scasualties—28 dead or wounded—were heavy, Block Island’ s losses were proportionately minor: 6 sailors from its crew of 957 plus 4 Wildcat aviators forced to ditch and never found. Meanwhile, Hedgehogs from USS Eugene E. Elmore made quick work of U-549.
If payback for the loss of Block Island was swift, milk cow extinction was not very far behind. On June 3, when signals intelligence revealed that U-490 , the newest and last surviving milk cow, was en route to the Far East, Croatan ’s task group was put on its tail.
Before dawn of June 11, USS Frost —one of Croatan ’s destroyer escorts—detected, attacked, and damaged U-490 with Hedgehog rounds. U-490 ’s skipper, confident in the boat’s pressure reinforced hull, dived to nearly 1,000 feet, well beyond effective depth charge range. There the Germans hoped to outwait the seemingly endless onslaught. Fearing that enemy sonar would detect the squeals of a dozen guinea pigs—brought along by U-490 ’s doctor for experiments—the skipper ordered them killed.
After midnight, with the siege apparently over, U-490 surfaced, only to be pinned in the glare of searchlights from three destroyer escorts. “SOS please take our crew,” U-490 ’s skipper signaled as he scuttled the craft and its 60 survivors jumped overboard.
Despite unresolved tensions about how best to exploit Ultra, Rodger Winn and Kenneth Knowles achieved a remarkable record. In the estimation of U.S. Naval War college professor David Kohnen, their clandestine expertise contributed to “the ‘liquid-ation’ of more than 93 Axis submarines,” including “54 to U.S. Navy hunter-killer groups.” After the war, Knowles’s boss Francis Low rued the lack of recognition they received: “The Battle of the Atlantic was…a battle of wits in which intelligence played the major role. Unfortunately this…is fully understood only by a relatively small group because of [its] highly classified nature.”
Both Winn and Knowles ended the war as naval reserve captains. Afterwards, Rodger Winn returned to the law, becoming a Lord Justice before his death in 1968. For his part, Kenneth Knowles, who died in 1986, transitioned to the National Security Agency, an organization so wrapped in secrecy that its very existence wasn’t revealed until 1975.