Dönitz, bless him, is trying to build new U-boats that can stay submerged all the time. But he has to beg for every ton of steel and for the services of every engineer. In the meantime there is this stopgap measure, the Schnorkel, which is just plumbing: a pipe that sticks up out of the water and enables you to run on diesel power, just beneath the surface. Even the Schnorkel will show up on radar, but less brilliantly. Every time U-691 surfaces for more than an hour, Holz is up there working on the Schnorkel, welding new bits on, grinding old bits off, wrapping it in rubber or some other stuff that he hopes will absorb the radar. The engineers who installed the Schnorkel in Lorient six months ago wouldn’t recognize it now because it has evolved, like shrews evolving into tigers. If Bischoff can just get U-691 back to a safe port, others can learn from Holz’s innovations, and the few U-boats that haven’t been sunk can derive some benefit from the experiment.