Das Boot is an epic movie based of historical
fiction. The technical details, while accurate in many respects, aren't present beyond a certain point. What the movie was trying to portray was the jockeying between a destroyer, looking for an attack path that will place the sub in the blast radius of its pattern, and the sub trying to exploit the destroyer's baffles and disturbances caused by depth charges. As much as I love both the movie and the book, the stunts we pull in the sims are a bit more realistic...then again, the prospect of game over is great motivation.
To put this into perspective, the scene where U-96 encounters its first destroyer, Der Alte ranges the destroyer at 6000 meters with intent to fire a spread. Given the rot-gut weather at that point in the story, ranging that far out with any degree of accuracy would have been a wonder, but taking a shot would have been out of the question. It was too far away. Of course, the audience doesn't know this. However, it makes good drama and that is the point of the movie.
If you want some
real fun, learn a bit of German and compare the English subtitles to the actual spoken German dialogue. The scene where u-96 encounters Thomsen at sea is particularly egregious in the Original-Uncut version.