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.