ASDIC — something the Americans and the rest of the world would come to know as SONAR — was a new and potentially effective system for detecting submerged U-boats, but from Kohl’s experience it didn’t yet possess a particularly useful range. He’d personally eluded an ASDIC-equipped corvette a month before that had closed to within three hundred metres, and U-1004 had still gone undetected despite being at only moderate depth. When rigged for silent running, U-1004 and her fellow Type-X U-boats were almost impossible to detect at ranges greater than two or three hundred meters, unless caught suicidally close to the surface.
Kohl turned the scope to the other destroyer he could see, this one further away and to the west of the convoy, and reported the range at 3,500 metres. It was quite overcast that afternoon over the North Sea, with occasional rain squalls and very choppy surface conditions, and lookouts on any enemy vessels would need to be very lucky to see the tiny wake of U-1004’s small, high-powered attack periscope. He returned his attention to the convoy itself: more than thirty warships of various sizes and classes, covering a dozen square kilometres of ocean in a long, drawn-out line-astern formation.
The fleet wasn’t ‘zig-zagging’ as the enemy’s merchantmen convoys were prone to while making their perilous journeys across the Atlantic, something that made targeting substantially easier for the U-boat commander. Even so, gaining a firing position was going to be difficult: the fleet passing across his bow was steaming at better than twenty knots, which was well in excess of U-1004’s top submerged speed of slightly more than 18½… and Kohl couldn’t use anything close to full speed if he wanted to remain undetected by the enemy’s passive hydrophone sensors.
It was several more minutes before the U-boat reached a workable firing position, the enemy fleet still unaware of any danger as it steamed southward toward The Channel and inevitable combat. There were few operational vessels in Dönitz’s infant U-boat fleet, as the construction of capital ships had taken precedence, and capital ships like battleships or carriers also took up far more space, manpower and resources. The small number of U-boats that were available to the Kriegsmarine however were the most modern and advanced in the world, and had already had an effect upon the course of the war that was way out of proportion to their number. Kohl, one of the more experienced of the U-boat service’s commanders, had alone already sunk twelve ships for more than 70,000 tons. Most of that tally had been in the older but nevertheless quite deadly Type-VII fleet boat, but his record had been good enough to accord him the honour of working up a new crew on just the fourth ship of the new Type-X class to come down the slipway.
The Type-X was quite literally a quantum leap forward in submarine technology. To begin with, she’d been designed from the outset as an underwater vessel, rather than a seagoing vessel that could submerge (as was the case with the submarines of the rest of the world’s navies). Until the moment the Type-X left the drawing board and entered service, all submarines had been designed with a shape more conducive to surface sailing; intended primarily to remain on the surface, only submerging in times of combat. As a result, all other submarines, while possessed of adequate capabilities while surfaced, were little better than poor performers when submerged.
The preceding German Type-VII was a good example of conventional submarine technology and better than most, and it’d been capable of just 18 knots surfaced and no more than seven when submerged. The choice of a surface-going layout also meant she was a comparatively noisy vessel when underwater (as were all conventional subs) due to an abundance of nooks and crannies around the hull and conning tower where water could catch and swirl to produce the deadly turbulence and cavitation that cried out like a thunder storm to an enemy’s hydrophones.
The Type-X changed all that, and had been designed from the outset as a vessel intended to spend its time beneath the surface of the ocean. She had a blunt, streamlined nose joined to an equally-featureless hull that was exceptionally ‘clean’ and devoid of protrusions or indentations throughout its entire length, stretching right back to the stern. Its tail was also a departure from usual practice: instead of twin propeller shafts mounted beneath the hull on either side of a single rudder, the stern tapered to a rounded tip, at the end of which was a single large, multi-bladed screw. Instead of a conventional rudder, the tail also sprouted four fins in a ‘+’ shape just ahead of the large propeller that provided the boat with a level of manoeuvrability much improved over other, less advanced models.
The new type of stern design did however precluded the firing of torpedoes to the rear, and all six of the 533mm (21-inch) torpedo tubes the vessel possessed were subsequently mounted forward in two vertical columns of three on either side of the nose. The vessel carried 24 torpedoes for her main armament, and the advanced, semi-automatic loading mechanisms for the six-metre-long ‘fish’ were efficient enough to reload all six tubes in the same time it would take any normal submarine crew to reload one. A secondary anti-aircraft armament consisted of a pair of twin turrets mounted at each end of the long, narrow conning tower. Those turrets each sported a pair of 30mm automatic cannon which, when not in use, could be depressed to point directly downward and were locked away inside sealed hatches along the fore and aft edges of the conning tower so as to leave no projection when submerged. The old standard of a heavy deck gun had been done away with entirely: not only did such a fitting create a great deal of noise and drag when underwater, but it also didn’t fit with the new, altered role of the U-boat. The Type-X was intended to spend its days on patrol entirely below the surface, and as such it would have no use for a deck gun at all.
The layout of her engines was also a massive departure for U-boat construction. Conventional designs usually comprised a set of main diesel engines (usually four, as was the case with the Type-VII) used as the primary source of propulsion when surfaced, and would also charge the comparatively small store of batteries used to power the secondary electric motors used when submerged. The Type-X instead used just one pair of far smaller diesel engines, and those engines were at no time directly connected to her single screw. They powered a 450-kilowatt electric generator that could either drive her more powerful primary electric motors (which were connected to her single screw) or charge the greatly increased store of lead-acid batteries packed into the pressure hull below the crew decks. The lower reliance on diesels meant far less fuel need be carried, and the increased battery power also meant the vessel had much greater endurance and speed when submerged — something that Kohl and his crew were putting to great use at that moment.
“Range to target two-thousand, five hundred,” Kohl noted softly from his position in the conning tower, his eyes never leaving the attack-periscope’s viewing port in the dim, red light of battle-stations. “Bearing fifteen degrees… heading one-eighty and steady… speed twenty… twenty-two knots.” As the XO passed on his observations to the appropriate crew stations, he carried out another 360° sweep, which came up clear of any threats.