Max detection range
 

What is a maximum detection range for eneny ship to spot my u-boat in nice day (visibility unlimited) by binoculars or something?
I'm asking cos I'm looking for safe distance to shadow the convoy in feb 1940 in day light.

Or in with file I can find this values.

I'm sure someone will know this exactly, however I can say that whenever I've been on the surface and I spot a ship they are never more than 8 or 9 klicks away and they never seem to see me first. So I would guess that you are safe at 10 klicks.

Huge caveat: all my experience is early in the war, before they have radar.

Also depends on the type of ship. Battleships sometimes escort convoys and they can often see you (and accurately shoot) at your maximum visual detection range.