How close before being detected?

First let me say that I'm playing with the Grey Wolves mod. I encountered a convoy during the night and got 6 hits out of 6 fish. Howevver; I was only able to do this because of the map updates. I can't see anything in the pitch black until I'm just a few hundred meters away. Once the convoy had passed I popped up and engaged a dead in the water cargo ship with my deck gun. Again, I could barely see him until I was 400m away. Once I started shooting he turned on his spotlight and made it much easier to see. Unfortunately, it also made it easier for someone to see and shoot at me (not sure if it was the cargo or some other ship, but the nearest warship was over 3k away). So, if I can barely see these ships at those ranges, how close can I expect to get when surfaced before they see me?