By the spring of 1942 the range of German submarines had been extended outward from the North Atlantic to much of the ocean area of the world. In the Arctic Ocean to the north U-boats attacked Allied supply lines to the Russian ports of Murmansk and Archangel. In the Mediterranean Sea they shared with Italian submarines the effort to cut the long and tenuous British supply line to its desert army in North Africa. Other U-boats preyed on Allied ships in the South Atlantic. Still others had penetrated the Indian Ocean, and at the far end of it at Penang on the Malay Coast, the Japanese permitted the construction of a U-boat base.
Most importantly, U-boats were operating in increasing strength off the United States, and the white sands of many East Coast beaches were blackened by oil from ships hit by torpedoes made in Germany.
Many American ships, unlike those of the war-wise British, were still sailing alone, providing targets without risk to U-boats. Even those ships which sailed with an escort of U.S. Navy destroyers were at a disadvantage because American officers and sailors lacked the experience of their Royal Navy counterparts, and their attacks were markedly less successful.
The result was "U-boat paradise." German losses were virtually nil, and their successes were enormous — more than 2 million tons of shipping sunk between January and June, 1942. Few indeed were the U-boats in American waters that did not find ample targets for all of their aale (though most navies called their torpedoes "fish," the Germans called theirs "eels").
The peak month of their success was June, 1942, when 700,000 tons of shipping was sunk. The Germans now had 290 submarines in operation — five times the number at the start of the war. Once again, as in World War I, U-boats made the outlook for the Allied war effort bleak. On June 19, General George C. Marshall, United States Army Chief of Staff, wrote a memorandum to Admiral Ernest J. King, the Chief of Naval Operations, which summed up the threat.
The losses by submarines off our Atlantic seaboard and in the Caribbean now threaten our entire war effort. The following statistics bearing on the subject have been brought to my attention: of the 74 ships allocated to the Army for July by the War Shipping Administration, 17 have already been sunk. Twenty-two per cent of the bauxite fleet has already been destroyed. Twenty per cent of the Puerto Rican fleet has been lost. Tanker sinkings have been 3.5 per cent per month of tonnage in use. We are all aware of the limited number of escort craft available, but has every conceivable improvised means been brought to bear on this situation? I am fearful that another month or two of this will so cripple our means of transport that we will be unable to bring sufficient men and planes to bear against the enemy in critical theatres to exercise a determining influence on the war.
Despite every American effort, nothing succeeded in keeping the number of U-boats off the coast of the United States from increasing. In July, 1942, there were 70 German submarines on hand for the 'American shooting season.' The number increased to 86 in August, to 100 in September, and to 105 in October.
In the last six months of that year U-boats sank 524 ships. November was their top month; in it 117 ships were sunk, 72 of them in convoy. It was clear to others besides General Marshall that German submarines might conceivably decide the war by preventing the might of the United States from being brought to bear in Europe.
Adding insult to injury, two U-boats surfaced with impunity at night near American East Coast beaches and put saboteurs ashore in small boats. One of these submarines, the U-202, landed a party near the town of Amagansett on the eastern end of Long Island. The other, the U-584, put a party ashore on a Florida beach south of Jacksonville. (Both groups of saboteurs were apprehended and captured before they could accomplish their missions of destruction.)
At the very height of the U-boats' success, though, the turning point was reached. While the loss of Allied tonnage continued to mount, a more subtle statistic developed. The tonnage per U-boat began to decline. No longer were German submarines able to pot luckless ships without interference. The U.S. Navy's antisubmarine effort was becoming a force to be reckoned with. U-boat losses, which totaled only three for the month of January 1942, had climbed to fifteen for the month of December, and the 1942 total of eighty-six German submarines sunk was more than twice that of 1941.
Even while German submarines were having their greatest success, U-boat captains began to report a puzzling occurrence. While surfaced at night to recharge batteries — something which they had always been able to do under the cover of darkness, with little fear of attack — they were now being surprised by Allied aircraft, which suddenly swooped down, illuminated the submarines with searchlights, and loosed bombs or depth charges.
The frequency and accuracy of such surprise attacks ruled out the possibility of chance. The attacking planes obviously knew exactly where the surfaced U-boats were, no matter how dark the night. An increasing number of submarines were falling victim to nighttime attacks.
It was not long before the Germans learned the cause of the problem. Allied scientists had devised an electronic detection apparatus called radar (for radio detection and ranging) which sent out electronic impulses. When these impulses struck an object such as a submarine, they were reflected back and picked up by the radar apparatus, giving its operator information about the object's direction and distance.
Using radar, antisubmarine aircraft were able to make their attacks without regard to conditions of visibility. In one month alone — September, 1942 — there were 120 occasions in which Allied aircraft, guided by radar, attacked U-boats at night.
In addition, as in World War I, Allied ships employed underwater listening devices to pick up the sounds of German U-boats at a distance. By 1942 scientists had contributed a new underwater-sound development. The British called their version asdic. The Americans called theirs sonar (from sound navigation and ranging).
Both emitted sound impulses which traveled through the water. On striking an object such as a submarine, the sound impulses were bounced back to the originating device. Mechanisms within the device then determined the distance and direction of the submerged object, and presented this information visually to the device's operator. Using asdic or sonar, it was now possible to beam a "searchlight" of sound under water to find submarines. Unlike the more precise radar, the operation of asdic and sonar was as much an art as a science, and its performance depended heavily on the vagaries of water conditions and on the skill of the individual operator. However, even with its limitations, it was a vitally important new tool in fighting submarines.
At the same time the invention of an American naval officer was demonstrated successfully. Called the "hedgehog," it enabled the attacking antisubmarine ship to fire missiles in a pattern in front of it from its forecastle. The advantage was that it permitted the attacking ship to fire at a submarine before getting so close that its sonar contact with the target was lost, as occurred in depth charge attacks.
Moreover, hedgehogs eliminated an even greater disadvantage of depth charges. Often the explosions from depth charges created such disturbance in the water around a submerged submarine that it was difficult and sometimes impossible for the attacking ship to regain sound contact. The advantage of hedgehogs was that they exploded only if they hit a submarine. If they missed, they dropped to the bottom without exploding, and the attacking ship found less difficulty in re-establishing contact for another attack on its submerged enemy.
Allied scientists developed another device which was to take a toll of U-boats. Called the magnetic anomaly detector — MAD for short — it could detect under water the disturbance in the earth's magnetic field caused by the presence of a large quantity of metal — such as the metal in a submarine. MAD scored its first success in the Strait of Gibraltar. Carried in a plane, it registered the presence of an underwater magnetic anomaly caused by the presence of the U-761, which was attacked and destroyed. The U-392 and the U-731 were also sunk in the strait after being detected in the same way.
To fight back against the growing arsenal of Allied antisubmarine weapons, the Germans developed countermeasures and weapons of their own. For protection against air attack U-boats were fitted out with 20mm and 37mm antiaircraft guns, and more than one attacking Allied aircraft — including a U.S. Navy blimp in the Straits of Florida — fell victim to their fire.
The Germans perfected a torpedo which incorporated two new developments. One development was a magnetic-action detonator which would explode the torpedo's warhead underneath the keel of a ship. It permitted one torpedo to do the work of three of the old type — such was the increase in destructiveness of an explosion that broke a ship's back.
The second new development was a guidance mechanism which would take the torpedo out in a straight line from the submarine and then send it coursing through a convoy in a planned pattern of curves until it struck a ship or stopped running.
Another innovation, devised to frustrate attacks on submerged submarines, was known as pillenwerfer ("pellet thrower"). This device, ejected from the stern of a submerged U-boat, created a sizable accumulation of small bubbles. To the probing sonar of attacking ships, the accumulation of bubbles gave an echo deceptively like that of the submarine itself. During the confusion over which echo was which, a harried U-boat could often escape.
To deceive Allied radar, the Germans developed a balloon known by the code name aphrodite. It gave the appearance of a submarine's conning tower on a radar scope. Submarines left aphrodites behind them at different places to confuse Allied radars.
Still another development was an acoustic torpedo, which made its way to a target by homing in on the sound of a ship's propellers. (To counteract the acoustic torpedo, the Allies eveloped a device called the foxer. Towed behind a ship, the foxer made more noise than the ship's propellers and thus attracted any approaching acoustic torpedoes toward itself and away from the ship.)
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