[Friedl9te]

Of course, only in submerged situations its of use to switch to Schleichfahrt.
Taxiing in the harbour, does not need Schleichfahrt at all.
Surfaced, and attacking during the night, the hydrophones should not detect the sub, because of their listening angle. (But the radar in late war does instead)

You are right, it is not necessary, to switch off schleichfahrt, trying to escape dc`s. Even the electric driven screws at higher speeds are loud enough and can be heard easily by the hydrophones of the destroyers from the hunter-killer-group.