Perhaps I didn't write my question specific enough
Why didn't Kriegsmarine U-boats submerge at night for the purpose of listening for merchant shipping on the hydrophone. Surely you can hydro-hear a darkened ship/convoy further at night than you can see it?
Yes, once a hydro contact is made and rough position/course established, U-boat would surface and race at full speed intercept course for the hydro contact.
Approach and attack surfaced of course (before escort radar). Only spotting at 5-25 km range pre-approach until visual contact with the aid of hydrophone.