From captain logs of U-575
http://www.u575.de/1Feindfahrt.html
Dark night without moon.
Visible contact to destroyer in 4000m distance.
Shot torpedo at merchant in 3000m distance.
(So visible range is definitely > 4000m)
http://www.u575.de/3Feindfahrt.html
Good visibility, about 7sm, bright moon night.
EDIT: I made a mistake here. It seems he always gives distances in meters and speeds in sm.
So the value 7sm must be a speed, either of own or enemy.??
http://www.u575.de/8Feindfahrt.html
Very bad visibility at night: 200-500m