What made those ten months doubly frustrating was that by breaking codes other than Shark, BP knew a great deal about the U-boats: the commissioning of new craft and their trial runs in the Baltic, their expected performance, their armament, the experience levels of their commanders, their transfers to the west of France, even the times of their departures for active duty and their arrivals back in port. What was lacking was the most important information, in Hinsley's words, what happened "between the time they left harbour and the time they returned from patrol."
To make this long blackout period still more disastrous, B-Dienst was again gaining the upper hand in the contest of which antagonist was breaking the other's codes. The British Admiralty had switched to a new nonmachine code, but B-Dienst was readily reconstructing it. Donitz knew the schedules of the North Atlantic convoys and the courses they would take.
The Sigint seesaw had peremptorily swung back to the German side—at a time when Reich production was delivering increasing numbers of U-boats into Donitz's hands. In addition, he was receiving large "milk cow" supply submarines, each of which could deliver seven hundred tons of spare fuel and torpedoes, saving the U-boats the forty-six-hundred-mile round-trip back to their bases.
The consequences were muted for a while by the fierce winter of 1941-42, for its violent seas decreased the U-boats' effectiveness. When the weather moderated, however, Donitz and his commanders made up for lost time. They steadily increased their sinkings, while their losses of U-boats declined. The first half of 1942 resulted, again in Hinsley's words, in the U-boats' "greatest sustained period of success in the whole course of the war."
In view of the sudden inability of the convoys to steer clear of wolf packs coinciding with a change in code, the U-boat command might well have been tipped off that their earlier codes had been broken, if not for one momentous change in the war's course: the entry of the United States into the conflict. Four days after Pearl Harbor, Hitler had honored his Tripartite pledge to the Japanese by declaring war on the U.S. All that American shipping he had placed off-limits to the U-boats in order not to provoke the U.S. now became fair game.
As a result, instead of continuing to send his boats against the Atlantic convoys and finding them suspiciously vulnerable, Donitz directed his boats against the coasts of the U.S. and Canada. He did not have as many to dispatch as he would have liked. Many had to be diverted to the Mediterranean to help protect the supply lines to Rommel in North Africa. In addition, Hitler, still fearing a flank attack through Scandinavia, had ordered other U-boats to patrols in Norwegian waters. For attacks on U.S. shipping, the German admiral had only a "handful of boats, but they were the large, long-range Type IX craft now being supplied by German boatyards.
American unpreparedness enabled him to make the most of what he had. The U.S. could assemble few vessels suitable for escort duty, and the ones available were not assigned to convoy protection. The navy's head, Admiral Ernest King, did not approve of convoys; he preferred to go after the U-boats in open-sea hunts. These forays, however, proved fruitless. The U-boats simply hid out on the bottom until King's patrols had passed before rising to continue their slaughter of the busy traffic along the eastern coast. U.S. coastal cities, resisting the inconvenience and possible trade loss that would ensue from blackouts, kept their lights undimmed, providing the German prowlers neatly silhouetted targets. In just two weeks the U-boats sank twenty-five ships totaling more than two hundred thousand tons, a high percentage of them tankers, and continued the sinkings at roughly one a day. Crowds of watchers along the coasts witnessed the deadly pyrotechnics of exploding ships. Another "happy time" for the U-boats had begun.
If Dönitz's approach to sea warfare had a fault, it was in his belief that what mattered most was the tonnage sunk by his subs. Despite knowing that choking off British supplies in the North Atlantic was the real key to German victory, he was unable to resist the opportunities to pile up tonnage records elsewhere. His impressive statistics, it must be remembered, made for status-saving, job-preserving reports to Hitler. Yet while his U-boats were scoring easy points in U.S. waters, massive convoys were passing through to Britain almost unmolested.
Not until the summer of 1942 did the situation begin to change. By then Admiral King had given up the hunt missions and agreed to convoys. Coastal cities were blacked out. Plus, the navy established what it called its "bucket brigade." Tankers and merchant ships traveled up the coast in protected convoys by day and holed up in sheltered ports at night. The British helped by sending over escort corvettes and a squadron of RAF Coastal Command planes. Guided by visiting Britons, the U.S. Navy had begun setting up a Submarine Tracking Room similar to that of the Royal Navy. The happy time came to an end.
Dönitz rerouted his wolf packs to the North Atlantic. With few U-boat losses and strong inflows of new boats, his fleet had grown four times as large as when Shark had been introduced. Also he had found a chink in the Allies' defensive armor. This was the "Air Gap," a distance of three hundred miles between the extremity of air cover from Newfoundland and Iceland and that extending from the British Isles. In this gap he formed his boats into "picket lines." Aided by B-Dienst's decrypts, the pickets could detect approaching convoys and alert other subs to swarm in for the kill. Before Shark, when BP was breaking the naval Enigma, only one in ten convoys was sighted by the wolf packs. Now with BP blind, they found one of every three.
The carnage in the North Atlantic marked the second powerful German surge toward victory. In the first of the new round of convoy battles, eleven out of thirty-three ships went down. In the two months of September and October 1942, forty-three ships were sunk. By November the losses soared to 743,321 tons, the highest figure for any month in the entire war. During 1942 more than eight thousand merchant sailors were killed. Two of the ships sunk were carrying U.S. servicemen to England, adding to the lives lost.
The same grim story held true for convoys trying to deliver armaments and supplies to the Soviet Union. Grimmest of all was the fate of convoy PQ17, which set out from Iceland on June 27, 1942. The convoy was attacked in the Barents Sea by U-boats and aircraft. At that time the Tirpitz was still available as a raider. The convoy was given the misguided order to scatter. It didn't matter that the Tirpitz never got into the action. The U-boats and planes picked off the dispersed merchant ships one by one. Of the thirty-seven ships in the convoy, only thirteen reached Russian ports. As a result of the disaster, all convoys to Russia were suspended during the spring and summer of 1943.
The mounting destruction by the U-boats cast a pall of despair over news of the war that was otherwise turning in the Allies' favor. The British Eighth Army had defeated Rommel at El Alamein. The Allied landings in northwest Africa had surprised the Germans. The Germans' decision to occupy the whole of France had prompted the Vichy government to scuttle the French fleet at Toulon. But without Ultra's help, the situation in the North Atlantic was threatening to undo all the other triumphs.
Breaking Shark, Turing saw, required four-wheel bombes. Tabulating Machine Company engineers worked on their development, introducing some limited use of electronic tubes to speed their operation and race through the increased number of permutations introduced by the fourth rotor. Until these bombes could be delivered, Hut 8 could only wait. Luckily, an even harsher winter than the one preceding it slowed the U-boats in January and February 1943.
In March, however, they stormed back in force. During just the first twenty days, ninety-seven ships were sunk, with over half a million tons of supplies sent to the bottom. The official Admiralty verdict was that "the Germans never came so near to disrupting communications between the New World and the Old" as in those twenty days.
Behind the scenes, though, important changes were occurring. For one, the sheer productive might of the U.S. was tipping the scales. The Ships for Victory program was turning out standardized Liberty ships at a rate of three per day, enabling the Allies to produce more vessels than the Germans were sinking. T
he American merchant craft were faster, ensuring that convoys could move more swiftly. Escort patrol ships, aircraft carriers, destroyers and larger warships were being delivered by American shipyards. Recon aircraft with ever greater ranges were issuing from American factories in unprecedented numbers. Long-range American B-24 bombers were joining with British bombers to extend air cover over the convoys and close the Air Gap.
In addition, convoys were benefiting from British and American technology. Radiotelephones were installed to improve communications between ships and to coordinate their maneuvers. Escort vessels were equipped with their own direction-finding equipment to help them home in on lurking U-boats. Some larger freighters were fitted with airplane catapults, from which game pilots took off knowing they would, after their search-and-destroy missions, have to reach a land base or ditch near an Allied vessel in the hope of being picked up. Airborne radar and powerful new searchlights enabled Allied planes to detect and swoop in on U-boats traveling on the surface at night.
On December 13, 1942, the decisive change came. Shark was finally broken. This resulted from one last, all-important capture of German code materials and from the clever use Bletchley made of them.
The capture occurred in the Mediterranean, off the coast of Egypt. A British flying boat's crew sighted U-boat U-559 and alerted four destroyers to pursue it. Later that night, a depth charge forced the sub to the surface, virtually under the guns of the HMS Petard. The U-559's commander ordered his crew to abandon ship. Before escaping, the U-boat's engineer opened the sea cocks to scuttle the sub. But she remained afloat, her conning tower just visible above the waves. Four of the Petard's, crewmen either swam to the sub or jumped onto it from the deck of their ship—survivors' accounts vary. Three of the crew, Lieutenant Tony Fasson along with Colin Grazier and Ken Lacroix, clambered into the sub's interior while the fourth, young canteen assistant Tommy Brown, ran up and down the conning tower ladder in order to hand over to a whaleboat what the others could deliver to him. They grabbed the four-rotor Enigma from the radio room and an armful of charts and papers that Brown managed to transfer to the whaleboat alongside. At that moment the U-boat went under. Lacroix just managed to escape up the conning tower; Fasson and Grazier never made it.
Their sacrifice gave the Hut 8 team what it needed to crack Shark. In adding the fourth rotor, the Germans had taken into account that at times, in order to communicate with three-rotor machines, that rotor would have to be put in a neutral position—as, for example, when the U-boat had to communicate with a shore weather station. Among the papers the brave men of the Petard had delivered were the current editions of the three-rotor codebook for the Short Weather Cipher and the four-rotor U-boat key. The result was that when Shark was used for weather signals, the three-rotor bombes could be used to decipher the messages, and the remaining part of the day's key could be reconstructed by testing no more than twenty-six letters of the nonrotating fourth rotor. In the first hour after this breakthrough, a message revealed the positions of fifteen U-boats.
Those in Hut 8 felt both deep relief and huge elation. Pat Bing, then a teenaged typist, later recalled the excitement of finally being able to tap out German text on long strips of sticky tape, fasten the tapes to paper and send them by the compressed-air tubes the young women called "Spit and Suck" to Hut 4 for the interpreters and disseminators to work on. The deciphered Shark messages were, she said, "a great gift from the brainy boys' department."
Historian Patrick Beesly has recorded the impact of the conquest of Shark. The flood of decrypts and translated signals that poured into the Admiralty's Operational Intelligence Center, he noted, "made it possible, for the first time, for . . . the Submarine Tracking Room to build up a comprehensive and accurate picture of the whole operational U-boat fleet."
At this point the cryptologic war reached a stalemate. BP was reading Donitz's copious exchanges with his U-boats, but B-Dienst was reading the Admiralty's output. As Hinsley put it, "Between February and June 1943 the battle of the Atlantic hinged to no small extent on the changing fortunes of a continuing trial of cryptographic and cryptanalytic resourcefulness between the B-Dienst and the Allies."
There was one significant difference. From their decrypts, Bletchley's cryptanalysts gained unmistakable proof that the Germans were breaking the Royal Navy's main code, and so set in motion the changes necessary to provide a more secure system.
It took the Admiralty until June to make the changeover. In the interim, U-boat warfare rose to its savage climax. As an example, during four days in mid-March, Donitz's wolf packs hurled themselves against two intermingled convoys, HX229 and SCI22, whose course had been plotted by B-Dienst, and sank thirty-two of their ships plus a destroyer, with a loss of only one U-boat. It was the greatest U-boat success of the war. Again the Admiralty was reduced to despair, even to considering that "we should not be able to continue convoy as an effective system of defence."
April produced a standoff. Shark decrypts enabled the Admiralty to reroute threatened convoys, but B-Dienst decrypts informed Donitz how to counter the instructions and reposition his boats to the best advantage. So exhausted were his crews and their equipment by the March onslaught, however, that he could not maintain their previous level of sinkings. April's toll dropped to 277,000 tons.
Then came what those in U-boat command regarded as "Black May." Two calamities struck the U-boats. One was a sharp rise in their own losses: thirty-one boats were sunk during the month, and the total for the first six months of 1943 rose past one hundred. The second was a wavering in morale. Less experienced commanders exhibited a drop-off in zeal and a rise in caution compared with their predecessors. Donitz was driven to increasingly shrill denunciations of his crews for their failures to press home their attacks.
The turn of the tide was dramatized by the passage of convoy SCI30 in mid-May. Though attacked by a pack of U-boats, not a ship was sunk. By contrast, six U-boats were lost and others damaged.
The price was more than the German admiral, who had lost his own son in one of the downed boats, was willing to pay. On May 24 he sent out orders for his U-boats to withdraw from the North Atlantic and shift to less hazardous patrols southeast of the Azores.
Dönitz refused to concede, however. He later wrote, "Wolfpack operations against convoys in the North Atlantic . . . could only be resumed if we succeeded in radically increasing the fighting power of the U-boats." He was determined that in the autumn of 1944 he would launch a new campaign, using U-boats equipped with new developments from German science. The improvements included superior radar, better antiaircraft protection and more efficient acoustic torpedoes.
It was an abortive effort. By then the odds were stacked overwhelmingly against Dönitz. Long-range aircraft made the air cover for convoys complete, especially after Portugal permitted the Allies to occupy the Azores. Escort carriers were plentiful. New technological developments included "hedgehogs" that enabled destroyers to throw depth charges ahead of their course as well as behind. To counter the acoustic torpedoes, the Allies perfected the "Foxer," a device towed astern of the escort vessels; it attracted the torpedoes and caused them to explode harmlessly.
Above all, the cryptographic advantage had swung completely to the Allies. As Beesly expressed it, Dönitz "was now groping in the dark while our picture was so clear that convoys could be converted, Support Groups transferred, air cover increased or reduced in accordance with the daily or even hourly demands of a situation."
Few Allied ships were sunk, and too many U-boats were destroyed. On November 16 Dönitz ordered another withdrawal.
In the meantime, control of Shark had passed to American cryptanalysts. With a flood of super-high-speed bombes being produced by the National Cash Register Company in Dayton, Ohio, the Yanks were better equipped to deal in a timely fashion with the vast numbers of permutations that had to be run through to reach Shark Enigmas' settings. During the second half of 1943, National Cash delivered seventy-five bombes, more than the
British produced during the rest of the war. American analysts informed the Tracking Rooms in London and Washington.
Dönitz still would not give in. He pressed U-boat designers to apply new techniques in a series of Super U-boats. Informed by Shark decrypts, British and American commanders watched these new developments with grave misgivings. The new boats' streamlined hulls and quiet new electric motors allowed them to slip along underwater at speeds matching those of most Allied escort vessels. Most worrisome of all, they were equipped with snorkel devices that took in oxygen and recharged batteries as the boats traveled at hard-to-spot periscope depths, enabling the subs to stay submerged for up to ten days.
Dönitz planned his Super U-boat convoy battles for early 1945, but his ambitious plans were thwarted as Allied bombardments of U-boat assembly plants and bases caused delays. Then the finished boats revealed flaws that had to be corrected. Before he could deploy his new subs, the war ended.
The Battle of the Atlantic was the longest sustained conflict of the war, one that cost both sides heavily. The Allies lost 2,603 merchant ships and 175 naval vessels. The lives lost exceeded 40,000, including 26,000 civilians. German U-boat losses numbered 784, killing 28,000 crewmen—two-thirds of the total force. The casualty rate was the highest suffered by any service during the war.
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