BY 8:25 A.M., SUNDAY, FAIR ISLE WAS VISIBLE 3 SEA MILES ahead, to starboard, but Schwieger could not yet make out the “Mainland” of the Orkneys archipelago, off the northernmost tip of Scotland, which by now he expected to see off his port side. The Mainland was the largest island in the Orkneys and had the highest elevation.
On this third day of the cruise, there was new tension in the boat. U-20 was about to leave “Bright Hans” behind and enter the closely watched waters of the North Atlantic, north of Scotland, in the vicinity of the big British base at Scapa Flow. Schwieger could not have been surprised, therefore, when just after logging his location, he spotted two destroyers in the distance, moving with a deliberation that suggested they were on patrol.
He ordered a fast dive, climbed down into the conning tower, and closed the hatch behind him.
SIMPLE ENOUGH in concept, diving was in fact a complex and perilous process that took time and left a U-boat exposed to attack. With a well-trained crew, a submarine of U-20’s class could descend from a fully surfaced condition to a level deep enough to clear the hulls of the largest ships in as little as seventy-five seconds. In a crisis, however, each of those seconds could seem very long.
Certain older boats needed from two and a half minutes to as many as five. Their crews nicknamed them “suicide boats.” While diving, a U-boat was at its most vulnerable, subject to ramming by warships, and to gunfire from long distances away. Penetration by a single shell would prevent a U-boat from submerging, thus eliminating its one advantage and its sole means of escape.
The men controlling U-20’s bow and stern hydroplanes—horizontal rudders—
now adjusted them for maximum dive, bow planes down, stern planes up. To submerge, a submarine did not simply fill its dive tanks with water and sink. As the boat moved forward under power, water flowed over the planes in the same way that air passes over the wings and flaps of an aircraft, driving the boat below the surface. Seawater would be pumped into the tanks only to the degree necessary to achieve a particular depth. Finding this point took skill, for it varied from day to day, even moment to moment, as sea conditions changed and the weight of the boat steadily declined. The firing of a torpedo made a U-boat suddenly 3,000 pounds lighter. Even the consumption of food diminished the boat’s weight by a perceptible amount. The boxes and crates in which food was stored went overboard; the supply of fresh water, a significant source of weight, fell daily.
The buoyancy of seawater changed in accord with shifts in temperature and salinity. In the Baltic, boats descended much more readily than in the more heavily salted waters of the North Sea. A submarine passing the mouth of a river might suddenly find itself dropping because of the outrush of fresh water, like an airplane passing through an air pocket. Changes in water temperature due to current and depth also affected buoyancy. A miscalculation could cause catastrophe. A submarine might bob unexpectedly to the surface within view of a destroyer.
Bad weather further complicated things. High waves could prevent the hydroplanes from digging fully into the sea. Commander Paul Koenig recalled one terrifying morning when, after surfacing into a storm, he spotted the smoke plume of a nearby destroyer and ordered an emergency dive. The men in the control room below opened vents to admit water into the tanks at both sides of the bow to reduce buoyancy. The boat stayed on the surface. Koenig watched through one of the tiny windows in the conning tower with increasing anxiety as each new wave lifted the bow into the air.
Koenig ordered the hydroplanes tilted to their maximum angles and called for full speed ahead, hoping the acceleration would increase the downward drive of the planes. Still the boat stayed on the surface, rising and falling in the waves.
At last the planes dug in, and the boat began to descend. But now a new problem presented itself. The boat plummeted downward at an angle so steep that Koenig had to grasp the periscope eyepiece to keep from falling. The
“manometer,” which registered depth, showed a startling rate of descent. Then came an impact. Men were propelled forward, along with everything else in the boat that wasn’t bolted in place.
There was silence. The face of the manometer cast a reddish light through the control room. One officer broke the tension. “Well, we seem to have arrived,” he
said.
The boat stood at a steep angle, about 36 degrees. The stern oscillated up and down. The engines continued running, “raving at intervals in a way that made the whole boat roar from stem to stern,” Koenig wrote. The chief engineer was first to grasp what was happening. He ordered full stop.
Koenig understood. The submarine’s bow was lodged in the seabed, which here, according to his charts, was 31 meters below the surface, about 100 feet.
His boat was twice that in length. With the action of the waves, the stern at intervals protruded from the surface, and the propellers spun in open air, stirring a geyser of foam visible a long way off. Koenig feared—expected—that at any moment a shell from the destroyer would come crashing through the hull.
Now, with the problem defined, Koenig directed the crew to fill the stern diving tanks and blow water from the bow. Gradually the submarine rose and righted but stayed safely submerged. Koenig ordered full speed and away.
IN DIVING, timing was crucial. As U-20 began its descent, its engineers shut down the diesel engines and engaged the electric. All vents and exhaust ports leading to the exterior of the hull had to be closed and hatches sealed. Once this was done, Schwieger gave the order to begin admitting water to the dive tanks. Air was forced out through valves at the top, and seawater entered through valves below. Suction engines helped draw the water in. To speed the process, Schwieger sent a contingent of men into the bow.
Just as U-20 neared its cruising depth, Schwieger ordered air pumped back into the tanks to halt the boat’s descent. The crew always knew when this point was reached because the pumps would engage with an angry growl.
In the control room, the helmsmen maintained depth by adjusting the hydroplanes. To ascend to periscope depth, they maneuvered only with the planes, not by filling the dive tanks with air. This allowed more precision and reduced the possibility that the boat might unexpectedly surface. While submerged, a U-boat had to keep moving at all times, held trim and steady by the hydroplanes. The only exception was when a boat was in shallow waters, where it could sit on the bottom. In deep seas like the North Atlantic, this was impossible, for the pressures at the seabed would crush a boat’s hull. The constant forward motion caused a problem. When the periscope was up, it produced a wake on the surface, a white feather of water visible for miles.
As U-20 descended, all activity ceased for a few moments, save for those tasks that made no noise. As always, the crew listened for leaks and monitored internal air pressure.
Then came the moment crews found so thrilling, when the boat was fully submerged and moving forward through the sea in a way unlike that of any other vessel, not grinding through waves like surface ships, but gliding, like a bird in air.
A sightless bird, however. The windows in the conning tower afforded a view only of things near at hand and in any case were usually covered by steel shutters. Traveling like this required a great deal of confidence, because Schwieger now had no way of knowing what lay ahead. In this day before sonar, a submarine traveled utterly blind, trusting entirely in the accuracy of sea charts.
One great fear of all U-boat men was that a half-sunk derelict or an uncharted rock might lie in their path.
SHORTLY AFTER NOON on Sunday, Schwieger gave orders to ascend. Now came “the blind moment,” as commanders called it, that dismayingly long interval just before the periscope broke the surface. Everyone listened carefully for the sounds of ships transmitted through the hull—the rush of water past a prow, the thrum of propellers. There was no other way to tell what lay above. As Schwieger stared through the eyepiece, the water became brighter and more clear. These seconds were, according to one commander, “some of the most nerve-racking that a man can endure.”
The biggest fear for Schwieger and fellow commanders was that the periscope would emerge within close range of a destroyer, or worse, in the destroyer’s path. One U-boat surfaced so close to a ship that the vessel’s black hull filled the lens. The commander at first thought he was looking at a particularly dark storm cloud.
The instant Schwieger’s periscope cleared the surface, he made a fast 360- degree sweep of the surrounding seascape. He saw nothing of concern. Here a U-boat had a significant advantage over surface ships. Schwieger could see the smoke coming from the funnels of steamships at a great distance, but the lookouts aboard those ships had to be much closer to spot him.
Schwieger gave the order to bring the submarine fully to the surface. Now, in addition to using the hydroplanes, the crew adjusted the air-water mix in the dive tanks to increase buoyancy. Within U-20, the crew heard a roar as compressed air was blasted into the tanks to force seawater out. Sometimes a commander chose to come all the way up, exposing the boat’s deck; at other times he ran
“awash,” with just the conning tower above the surface, imparting a sensation akin to walking on water.
U-20 EMERGED, but now Schwieger found a situation very different than his initial view through the periscope had led him to expect. The sea ahead was swarming with British patrol vessels—six of them—strung in a line between Fair Isle and the northernmost island of the Orkneys, North Ronaldsay, whose lighthouse was familiar to any mariner traversing these waters.
And behind him, Schwieger spotted two more destroyers. He had seen these earlier in the day but believed he had outdistanced them. In his log he wrote,
“they heave in sight again, course towards ‘U20’; one of the patrol boats turns towards us.”