Could be wrong, but whilst the noise generated by your sub may not (or may) increase from 0-10 knots your sonar will be affected.
Therefore you wont hear a contact until your closer.
So the only time I go over 5kts is when theres a torp after me or im trying to get somewhere. In which case sprint and drift is order of the day.
The "You have been counter detected" message is just the games way of telling you you screwed up. I guess its there so you dont play through a 3 hour mission only to find out you failed at the end.
