Black May (10 page)

Outside Lorient harbor, at Kernevel, Dönitz established his headquarters, BdU, in a requisitioned chateau. Kptlt. Fritz-Julius Lemp, later of the ill-fated U-110, was the first commander to put in at Lorient, in U—30, on 7 July 1940. With establishment of the Biscay bases in close proximity to Allied shipping, not only that which moved eastwest but also north-south convoys and independent shipping to and from Gibraltar and Freetown, and with replenishment and repair facilities readily available to the operational fleet, Dönitz effectively multiplied both his force and their time at sea by a factor of three.

In these circumstances, a year later Dönitz was able finally to institute full-scale a tactic that he had first conceived while commander of the submarine UB-68 in the Mediterranean in 1918. Instead of individual operations against convoys and independents, he envisioned group operations led by a flotilla commander employing the new technology of radio telegraphy. Twenty-one years later, in May 1939, when a Rear Admiral in Hitler’s Kriegsmarine and Führer der Uboote (FdU), Dönitz tested his concept in sea exercises, which proved the feasibility of radio-controlled mass attacks, what he called
die Rudeltaktik
(pack tactics, later known to the British as wolfpack operations).’
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In the original theory, pack attacks would be directed by a Type IXB command boat at sea, but soon after war operations began, Dönitz abandoned that plan and established the principle of shore direction in its stead: At headquarters, it was thought, where contact could be maintained with every boat, and where intelligence was readily available, Dönitz and his operations staff could best command the formation of U-boat patrol lines (or “rakes”) that would stand at right angles to convoy courses, with average spacings of 8 to 15 miles between boats, and vector them toward known or suspected targets.

First attempts at this battle management system in October and November 1939 were unsuccessful, in part because the U-boats were too
few in number to form effective packs. Subsequent efforts in 1940 were little better in practice and result. By spring 1941, however, when the number of boats had increased and the high degree of coordination required by the management plan was more precisely fixed in the minds of U-boat Commanders, Donitz launched the
Rudeltaktik
in earnest. The plan called for the first boat on a line that sighted a convoy to report its position, course, and speed to BdU by radio. That boat thereafter would not normally attack the convoy but rather shadow it, constantly updating its position and course as the contact-keeper.

From BdU, radio orders would direct the other boats in line to converge on the convoy and attack it in concentration at night on the surface, when the U-boats could take advantage of the darkness, of their low nap-of-the-water silhouettes, and of their high speed under diesel engine power. The strength of the
Rudeltaktik
was that it enabled U-boats to fall upon a convoy all together like a pack of wolves on a flock of sheep. Its weakness was that the high-frequency radio signals of both the shadowing boat and of BdU were vulnerable to interception, to radio direction-fixing, and, after May—June 1941, to decryption. But it was the system that Donitz would stick to like paste up to and through the climactic events of May 1943.

Typically, a transatlantic convoy crossed 3,000 miles at sea in nineteen to twenty days westward, fifteen to eighteen eastward. The average number of ships sailing in each convoy by May 1940 was forty-six, and the forties range remained the rule until 1943, when the optimum convoy size was raised in number. Until May 1941, the U-boats operated east of 40° W longitude in the Western Approaches north and south of Ireland. It was possible for the RN to give adequate surface escort to that distance from bases in Britain, Northern Ireland, and Iceland.

When in May 1941, however, the U-boats moved farther to the west outside the escorts’ radius of action and sank nine ships for 54,451 GRT from Convoy HX.126 in qu AJ (40"52’ to 4i°36'W longitude), on the 20th/21st, the Admiralty decided that the time had come to provide complete transatlantic—what came to be called “end-to-end”—escort. Here the Royal Canadian Navy (RCN), then in the middle of a huge expansion, played a central role. An RCN escort group out of St. John’s,
Newfoundland, covered HX convoys as far as 35°W, where they were relieved by a British group from Iceland, which in turn was relieved by a U.K.-based group for the final leg from 20°W to the North Channel. The number of RN escorts rose from 108 in June 1941 to 134 in November. The number of RCN escorts in the western Atlantic similarly increased.
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Also, beginning in September, U.S. Navy (hereafter USN) destroyers escorted Britain-bound convoys as far as a midocean meeting point (MOMP), or “chop” (Change of Operational Control) line, south of Iceland, where RN escorts took the guard. And by August the Germans were remarking on a higher efficiency of escorts on both the transatlantic and Gibraltar runs: “Not only had the numbers of escort vessels and aircraft risen, but their methods of keeping the U-boats at a distance had improved. Where a few months ago one U-boat had been adequate for shadowing, a whole group was now required.”
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The Admiralty introduced oilers to enable the escorts, particularly short-legged destroyers whose fuel bunkers had been designed for close-in warfare in the North Sea and English Channel, to extend their range and stay with their convoys. Also, as earlier noted, in what historian J. David Brown has called one of the most important decisions taken by the Admiralty at this time, on 18 June 1941 the Trade Division required all ships with speeds of 13–15 knots, which previously had sailed independently, to form up with slower-moving convoys; only those ships whose speed exceeded 15 knots would be routed independently thereafter on the expectation that at that and higher speeds, those ships could evade U-boats, whose maximum sustained speeds were in the range of 17–18 knots. Whereas in the second quarter of the year 120 ships proceeding independently were lost to U-boats, during the second half of 1941 only 49 independents went down, because fewer independents were being sailed.
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All the while, the U-boats were not going unscathed. By 31 December 1941 sixty-seven boats had been sunk in the year, forty-five by RN ships, one by an RCN vessel, three by Allied submarines, five by mines, four (and another shared) by RAF aircraft, and the remainder by miscellaneous causes. Where in 1940, when there was a monthly average of 13.5 U-boats at sea, there was a yearly total of 22 U-boat casualties, for a
monthly average loss of 1.8 percent, in 1941, when there was a monthly average of 25.5 boats at sea, there was a total of 67 casualties, for a monthly average loss of 5.5 percent. During the last quarter of 1941, one Atlantic boat was lost for every three ships in convoy sunk—an insupportably poor exchange rate. By any measure, at year’s end the Allies were ruling the convoy lanes. The improvement in the performance of surface escorts was due to higher experience levels, better skill in the use of detection gear and weapons, tighter training programs, and closer coordination by voice radio with RAF Coastal Command aircraft—though it must be said that that Command had not yet come into its own as an anti-submarine force.

The principal weapon of the surface escort against a submerged U-boat was the depth charge, a canister of high explosive, first Amatol, then Minol, and finally, in the summer of 1942, Torpex—though Minol fillings continued in use through 1943. Torpex was a high-explosive mix of Cyclonite, TNT, and aluminum flakes. The standard Mark VII “heavy” D/C weighed 250 pounds, sank at 16 feet per second, and, when detonated by a preset-to-depth hydrostatic fuse, caused the formation of a spherically shaped gas bubble with an initial temperature of about 3,000° Centigrade and, on the periphery of the bubble, a pressure pulse, or shock wave, equivalent to 50,000 atmospheres. The pressure pulse rose to peak level within a few milliseconds and moved through the water at the velocity of sound, its energy dissipating directly with the square of the distance traveled. Because of the greater density and incompressibility of water, the shock wave traveled farther and faster underwater than it did in an above-water explosion. Such a wave was considered able to fracture a U-boat’s hull if initiated within 20–26 feet of it and to cause damage within 50 feet.

Surface evidence of the shock wave was given by a breaking of the water’s top layer and a “dome” of spray. Evidence of the expanding gas bubble was provided by the familiar towering “plume” that broke through later.
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An escort vessel both “fired” and “dropped” D/Cs, though the verb
fired
was normally used for both. Firing was done laterally by mortar “throwers,” two to each side, port and starboard. The drops came by gravity off racks, or rails, at the stern. In 1941 and 1942
escort Captains used the throwers and rails in various combinations, usually in salvos of five, but by 1943 experience had shown that the most effective D/C attack was made in a “10 Pattern,” with four fired and six dropped, five set to explode above the target and five below. In order, the forward throwers fired first, then the aft throwers, after which six D/Cs were rolled off the stern. The resulting pattern formed a fore-to-aft oval.
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In December 1942 a Mark X “One Ton” D/C was distributed, usually one to a ship. Filled with a 2,000-pound charge, and theoretically the equivalent of a 10 Pattern, the Mark X was designed to reach deepdiving boats. It was housed in a ten-foot-long cylinder and fired by cordite charge from a torpedo tube. Escorts had still other weapons available to them, including 4-inch deck guns and Oerlikon anti-aircraft guns that, on several occasions, at maximum depression, were used with effect against surfaced boats. A weapon of last resort was ramming and, though this action usually brought damage to the escort’s bows, requiring seven to eight weeks to repair, by May 1943 about twenty-four boats had been dispatched by this means.
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Detection of a submerged U-boat, whose principal advantage was underwater stealth, was made possible by the development between the wars of a sound-ranging device called asdic (an acronym that grew out of Anti-Submarine Division, the Admiralty department that initiated the system). An asdic apparatus, housed in a dome on the underside of a vessel’s hull, sent out sound waves in pulses that, when they struck an undersea object such as a U-boat, returned a pulse echo that gave the object’s bearing and range, though not (before 1943) its depth. When the pulses bounced off a U-boat’s hull they emitted within the boat a loud, piercing
PING-ping!
A similar system in the U.S. Navy was called Sonar (Sound Navigation and Ranging), a name later adopted by the RN.

Early wartime Types 123 through 129 had serious limitations, including short range of effectiveness, about 1,300 yards (1.2 kilometers) in good sea conditions, and inutility above vessel speed of around 15 knots. Range and accuracy improved in 1942 with the introduction of
the Types 144/145 series, and depth determination became possible with the Type 147 in 1943, but upgrading in the fleet was slow and erratic: in 1944 58 percent of British corvettes still mounted old equipment. Another failing of early asdic was its inability to hold a target during the final 200 yards of an attack approach, when a U-boat could use the blind zone for evasive maneuvers. This problem would be met later by the development of thrown-ahead weapons such as the “Hedgehog,” a bank of twenty-four contact-fused projectiles with Torpex warheads equivalent to 50 pounds of TNT fired from mortar spigots to form ahead of the attacking ship a circle 120 feet across or an ellipse 120 feet by 140 feet. The Hedgehog had teething troubles well into the summer of 1943.

Again, the early asdic series could not track very deep-diving boats. Diving beneath the D/C spread—a submerged boat could alter depth at the rate of one and a half feet per second—was, of course, another defense of boats that were being “pinged.” Numerous efforts were made by the British to determine the maximum diving depth of the VIIC, the most numerous U-boat type. In 1942 RN submariners tested the captured U-570 (renamed H.M.S.
Graph
) and found that her thick hull survived the atmospheres of pressure at 200 meters (656 feet). The deepest measurement taken by asdic during the war was 238 meters (780 feet). Günter Hessler, who commanded U—107 until November 1941, then served on Dönitz’s BdU staff, wrote after the war that by the summer of 1942 new Type VIICs had been strengthened to a standard of 200 meters, “which in practice meant that in an emergency they could go down to 300 metres without harm.”
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From interrogations of U-boat prisoners the British learned that most Commanders considered 200 meters the deepest safe depth when under attack, but that “good evidence” indicated that in the summer of 1943 one boat involuntarily dived to 340 meters (1,115 feet) without breaking up.
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Interesting is the finding that as late as early 1943 the maximum depth setting for British D/Cs was 550 feet. In June of that year both the D/C and asdic recorder settings would be readjusted to 750 feet.
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Another defense against asdic was discovered by U-boats that operated
in warm waters, such as those around Freetown, where temperature gradients and heavy density layers refracted the asdic sound beam. Boats in those climes frequently carried Bathy thermographs and thermometers to measure density and temperature. Efforts were made for a time to cover U-boat hulls with sound-absorbing (anechoic) coatings, such as layers of synthetic rubber, but these tended to separate from the hulls underwater. After 1943, the
Pillenwerfer
decoy that mimicked a U-boat’s asdic signature came into use. Called by the British SBT, for “submarine bubble target,” the decoy consisted of a canister containing metallic calcium-zinc “pills” that on contact with sea water created hydrogen bubbles. These bubbles returned an echo to asdic pulses that represented the dimensions of a U-boat. The canister was ejected through a six-inch-diameter tube (”
Rohr 6”
in Type VIICs) that projected through the pressure hull in the maneuvering room. The device remained suspended at a depth of about 30 meters (98 feet), and its bubble screen lasted for 15–20 minutes. Because it did not move laterally, the asdic operator was usually not fooled by it. More sophisticated decoys that did move were introduced later in the war.
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