What do the dashed circles mean?

[sharkbit]

Been playing for less than a week and I've been playing the tutorials and experimenting with some different approaches and getting the feel for the game. And having a blast.

I started the convoy attack tutorial, sunk a couple of ships with my first salvo. I came up astern of the convoy after I had reloaded and wanted to try an end around to get back in front of the convoy and maybe make a night surface attack. I plot a course to skirt the convoy at about 4000m and work my way around.
I'm on the surface and I'm messing around on the nav map, I notice that when I click on a ship, there is a dashed circle around the ship in addition to the information about the ship. The escorts have 2 different dashed arcs around them-one completely around them and the other partially around it, at a shorter range.

So my question is-What are these dashed circles that I'm seeing?

My guess is they are the visual range from each ship, and the second arc on the escorts is the asdic range, but before I could experiment, the game decided that 2 hours of play was enough and I got the "SIlent Hunter 3 has encountered a problem and must shut down..." message.

Of course, my theory leads to another question-

If this is a visual range indicator, is it the max range in ideal conditions(daylight, clear skies) or does it vary with conditions.

After dark, I was just in the inner circle of the trailing escort and nothing happened, but soon after, the game quit so I wasn't able to see what would happen.