Revealing mistake: In one scene just before the attack on a big Hunter the young captain (Jürgen Prochnow) is sitting with his charts. Beside him he has got a glass of beer standing. The sub lurches heavily right and left in the hard weather, but the surface of the liquid stays the same.

Das Boot (1981)
1 revealing mistake
Directed by: Wolfgang Petersen
Starring: Jurgen Prochnow, Herbert Gronemeyer, Hubertus Bengsch, Klaus Wennemann
Continuity mistake: At the end of the first attack by the destroyer - where she misses the U-boat and throws her bombs further away - the boatswain makes 22, then 23 strokes on the slate of the Control Centre,thus counting the number of depth charges used against the boat. A few seconds later, in a shot from another angle, with the slate in full view, we see only 22 strokes. (01:03:30)
Captain: April fool. Practice makes perfect.
Trivia: The movie is based on actual events and some of the characters have existed in real life. One of them, Pilgrim, took his own life after the book on which the movie is based was released.
Question: I know practically nothing about submarines, so this seemed a bit strange to me. When there's that huge storm, why do they keep staying at the surface, only going down for an hour at a time?
Answer: Because that's what the U-boat was designed to do. Unlike modern nuclear boats, they didn't have the capacity to stay submerged for long periods - basically just as long as the air lasted; there wasn't atmosphere control equipment. They would 'snorkel' near the surface, recharge the batteries with the diesel engines, and then dive for a few hours at most before having to come back up and repeat the process.
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Answer: U-boats also travelled much faster on the surface...about 18 knots, vs only 8 knots or so beneath the surface.