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.”
LUSITANIA