How to hold the boat at 0 knots at 12 meters periscope depth.
 

1.Filling the ballast tanks.
2. Fill the negative tank with 0,04 m³ of water.
3. Level the boat to depths of 12 meters with the depth rudders.
4. Do not ask for help from the Def.
With this method, I was able to hold the boat for nearly 1 hour at 12 meters depth by zero knots speed. Maybe this is nothing new but the new single player might be interested.

By the way.
It would be nice if the curser in the recognition manuel gets a darker color. By now it is very hard to see it, because nearly white like the background of the recognition manuel.