Blackett's War (18 page)

British officials were divided over whether to make this new secret weapon known to rival powers. Political leaders thought disclosure would help peace efforts by convincing other navies that submarines were obsolete, and thus to accept British proposals to restrict their numbers if not abolish them outright. The Admiralty’s director of intelligence pointed out the logical inconsistency in this thinking: either submarines were a threat to Britain’s great surface fleets or they were not, and if the government truly believed that asdic had rendered the threat impotent then there was no need to care whether other naval powers kept building submarines; indeed, “if
this country really had an antidote then we would let other nations waste their resources on submarines.” In the end the policy of keeping asdic under wraps as long as possible prevailed; the very word remained an official secret until 1929.
³

That did not, however, prevent the German navy from learning of the technology, and Dönitz more cannily played the game in reverse. “According to the English Press, England apparently believes herself equal to the U-boat danger on the grounds of her detection apparatus,” he observed in early 1939. “Our goal must be
under all circumstances to leave England in this belief
.”
⁴
In any event, the British naval staff was far more worried about Hitler’s newly constructed fleet of surface raiders, built around the two fast “pocket” battleships
Deutschland
and
Admiral Graf Spee
. These were warships of a new and intimidating design, powered by diesel engines that delivered a formidable cruising range of 21,500 miles and a speed of 28 knots, armed with six 11-inch guns, and they consumed an inordinate share of the British navy’s planning and attention. In the judgment of the Royal Navy’s official historian Stephen Roskill, it was probably the greatest mistake of the war that the Admiralty made.

IN TRUTH,
Hitler’s senior naval officers were deeply worried themselves.

On August 15, 1939, Dönitz received a message that a “submarine officers’ reunion” would be held four days later. It was a coded instruction for his U-boat force to put to sea at once and take up war stations around England. Two weeks later, in the early morning hours of Sunday, August 27, Hitler held a bizarre conversation at the Reich Chancellery with a Swedish businessman, Birger Dahlerus. A friend of the Luftwaffe chief Hermann Göring, Dahlerus had become an unofficial emissary between Germany and Britain in the final days leading to Germany’s invasion of Poland. Shuttling back and forth between Berlin and London, he saw himself as someone who understood both sides and could avert war where the professional diplomats had failed. Dahlerus had lived in England, and tried to convince Göring that Britain would this time stand by her commitments. Likely sincere though hopelessly naive (he would later testify as a character witness for Göring at Nuremberg), Dahlerus was in reality little better than a dupe of the Nazi government’s effort to drive a wedge between Britain and Poland.

This time he came bearing a letter from Lord Halifax, the British foreign secretary and the man in Chamberlain’s government whom Hitler had
quite accurately picked out as the most likely to still be eager to grasp a “peaceful solution”—meaning a sellout of Poland that once again would let Hitler gobble up a neighboring territory without having to fight for it. Hitler, awakened by Göring at midnight to receive the self-appointed envoy, proceeded to deliver a twenty-minute harangue, pacing the floor, testifying to his sincere efforts to come to an understanding with the British and their incomprehensible refusal to grasp his proffered hand of friendship. Then, suddenly, the Führer stopped in the middle of the room and stood there staring, “his behavior that of a completely abnormal person,” Dahlerus thought. Hitler started almost chanting an incantation of what he would do if there should be a war:
“Ich U-boote bauen, U-boote bauen, U-boote, U-boote, U-boote, U-boote!”
(“I will build U-boats, build U-boats, U-boats, U-boats, U-boats, U-boats!”) Dahlerus glanced nervously at Göring to see how he was reacting to this display of the Führer’s phantasmic temper, “but he did not turn a hair.”
⁵

Despite these threats, Hitler was profoundly equivocal about the U-boat arm. He had early on secured the loyalty of the Kriegsmarine’s officer corps with the lavish promise of an expansive naval construction program, and the navy quickly became the most ardently pro-Nazi of the military services in Germany. Under the latest construction program—the Ziel (“Target”) Plan, or Z Plan, which Hitler approved in January 1939—the navy was to grow by 1945 to a powerful fleet of 10 battleships, 4 aircraft carriers, 3 battle cruisers, 3 pocket battleships, 5 heavy cruisers, 48 light cruisers, and 68 destroyers.
⁶
It was a force that would, as in the last war, directly seek to challenge the Royal Navy’s command of the North Sea, and even waters beyond. Both Raeder and Hitler rejected an alternative proposal to concentrate on quickly building up a large force of raiders, both U-boats and pocket battleships, to strike at British commerce.

The emphasis on conventional, big-ship sea power was the dominant German naval policy throughout the Nazi buildup, so much so that Dönitz had at first despaired upon being assigned to the U-boat force again: “I saw myself pushed into a siding,” he said, when he learned in July 1934 that he was being moved from command of the cruiser
Emden
to the submarine arm.
⁷
Although the Z Plan also called for 249 submarines, the reality on the eve of the war was far short of anything approaching that figure. There were a total of only 57 U-boats in service on September 1, 1939, only 26 of those the oceangoing types capable of operating in the open waters of the Atlantic.

The outbreak of war found the rest of the German navy equally short of the ambitious goals of the Z Plan, with only 2 battle cruisers, 3 pocket battleships, 3 heavy and 5 light cruisers, and no main battleships or aircraft carriers at all completed. A gloomy assessment by Raeder noted that the navy “was in no way very adequately equipped for the great struggle with Great Britain”—a war, he pointedly added, “which according to the Führer’s previous assertions, we had no need to expect before about 1944.” In the short time since 1935, the submarine arm had become a “well-trained and suitably organized” force. But, Raeder continued,

Dönitz concurred, adding his own pessimistic appraisal. “We cannot expect the number of U-boats now on operation to be more than a petty annoyance to British commerce,” he wrote. Of the 26 boats suitable for operation in the Atlantic, 3 were in the Baltic and 5 were not yet ready for active duty or were undergoing trials, leaving a total of 18 that had been able to take up stations for the fight against Britain as of September 1. He had sent every available boat to sea to be in a position to deliver the strongest possible opening shot at the onset of hostilities, but in a sustained fight the number on station at any time would quickly fall to about one third of the total available force, to allow for repair and resupply. Even the currently planned construction of new U-boats over the next six years would yield a force of less than half the number of operational boats that Dönitz calculated would be necessary to achieve a decisive result against the enemy’s merchant shipping.
⁹

That magic number was 300, Dönitz said, and Germany’s failure ever to produce an operational fleet of oceangoing U-boats of that size would subsequently be cited by Dönitz himself as the chief reason he lost the Battle of the Atlantic. Many historians since have bought that assertion; scarcely a book written about the U-boat war does not cite the figure of 300 boats as the force Germany required. In fact, the number was almost purely arbitrary, based on no real analysis at all; Dönitz probably chose it as the largest figure he thought he could sell to Raeder and Hitler at the time. Commanders from time immemorial have sought to cover in advance the possibility of
their failure by complaining of inadequacies in the men, arms, and supplies provided them. And Dönitz was if anything more optimistic than his superiors on the naval staff, some of whom argued that it would be irresponsible to send the U-boats to sea at all against such one-sided odds. Dönitz countered in memorandum after memorandum that he was confident the U-boats could hold their own.

Some of his confidence—though surprisingly little—stemmed from improvements in U-boat designs and technology that had taken place since the end of the last war. The new boats were certainly more solidly constructed, with welded components replacing rivets. The horsepower of their engines had doubled, increasing the top surface speed of the oceangoing boats from about 13 knots to 17 or 18. A system allowing both propeller shafts to be driven by one of the two engines at a time when cruising at low speed allowed for much more efficient fuel use.
¹⁰
But overall the design of the boats had changed remarkably little. The basic shape, functional layout, and sea performance of the U-boats that entered the war in 1939 were hardly different from those of the fleet surrendered at Harwich at the end of the last war.

Potentially far more significant were innovations in torpedoes. The new Type G7e was a masterpiece of German engineering. It was powered by a quiet electric motor running off batteries and gave off none of the telltale stream of air bubbles of the standard models, which were propelled by steam engines driven by an expanding vapor mixture of seawater injected into burning air and gas. The German U-boat designers had also invented a closed system that fired the torpedo without sending a blast of compressed air into the water with it; the new design used an air-driven piston to push the seawater and torpedo out of its tube.

The real change, though, was the tactics Dönitz had been honing to counter both convoys and asdic. Asdic could only detect submerged objects. Dönitz was contemplating night surfaced attacks, relying on the concealment of darkness and the U-boat’s low silhouette to get within torpedo range of his targets. That would render asdic simply irrelevant. As for convoys, they were effective for a simple reason of arithmetic: it is not thirty times easier to find thirty ships together than it is to find one ship. If a U-boat did chance upon a convoy it usually could fire only one or two torpedoes at most before fleeing. But if the attack could somehow be continued to sink a significant percentage of ships in a convoy once it was located, the advantage of traveling in a convoy in the first place would be negated. Dönitz had worked
on this problem and was convinced he had the solution: a concentration of targets would be met by a concentration of attackers. He devised a plan for deploying U-boats in loose groups along the Atlantic sea-lanes; when a U-boat spotted an enemy merchantman it would transmit a report by radio to the other boats in his group and to U-boat headquarters, which would in turn order additional groups to the area. Meanwhile, the first U-boat would continue to shadow and keep touch with the convoy just at the limit of visibility until nightfall, when the assembled group would move in for the kill. Dönitz tried out the concept in a naval staff war game in early 1939 and in exercises at sea that spring in the Bay of Biscay, and however contrived the conditions of the experiments he was sure they had proved his ideas. “The simple principle of fighting a convoy of several steamers with several U-boats as well, is correct,” he wrote after the exercise.
¹¹

The key to making it work was a secure communication system linking the boats at sea with BdU—Befehlshaber der U-Boote, the commander of U-boats. The German military had such a system: since the 1920s it had been using a modified version of a commercial device called the Enigma to encode its radio traffic. The Enigma was a small box containing a typewriter keyboard and a set of lamps labeled with the letters of the alphabet. Pressing a key completed a circuit that sent an electric signal on a meandering path through three wired rotors to light up a corresponding letter; the rotors advanced their position with each successive keystroke, cycling roughly like a mechanical odometer, so that a new scrambler pattern was employed for every successive letter of the message. The radio operator wrote down the coded version of his message produced by the Enigma, then transmitted it by hand over his radio in Morse code. The operator at the other end reversed the procedure, typing the received message into his Enigma machine to rescramble it back to the original text.

Even if a machine fell into enemy hands the system retained its frightful impenetrability. By choosing a different starting position of the rotors, rearranging the left-to-right order of the three rotors as they were inserted into the machine, and, in later models, by plugging a set of jumper cables into various jacks, literally trillions of different encipherment sequences could be generated. The German navy, since May 1, 1937, employed a particularly secure procedure for letting the intended recipient know which of those trillions of settings had been used for a particular message; it involved sending at the start of the message a separately encoded eight-letter code group that could only be deciphered using a printed table distributed once
a month to users of the system. These external code tables added an extra layer of security to the navy’s Enigma codes that made it all but impervious to mathematical methods of attack by any would-be code breakers.

Overall, Dönitz was supremely confident that he had in his U-boats a weapon that could not only win battles, but win the war. “It is clear that the attack on English sea-communications alone,” he wrote in a memorandum to the naval staff on May 23, 1939, “can have war-decisive effect in a naval war against England.”
¹²
Dönitz calculated that if he could sink 700,000 tons of shipping a month, he could destroy the merchant ships that Britain depended upon for her survival faster than her shipyards could replace them.