They go straight down to a hundred and sixty meters. Destroyers drop depth charges on them for eight hours. Bischoff takes a nap. When he wakes up, depth charges are booming all over the place and everything is fine. It should be dark and stormy up there now: bad weather for Catalinas. He evades the destroyers by (in a nutshell) doing clever things he has learned the hard way. The U-boat is as thin as a knitting needle, and when you turn it directly toward or away from the source of a ping, it makes almost no reflection. All that’s required is a clear mental map of where you are with respect to the destroyers.