Map Lines
 

I know there is red and black ones that come out from your position, and fade away. What is the difference in color? I thought red meant enemy, but I found out today that isn't true.

Oh, and how can I possibly intercept what is on the other end if I don't know anything at all about it and can't see it? Do you guys usually ignore these?

these lines represent your sound contacts and you only get them underwater. I always thought the colour did represent friend or foe, but this doesn't matter to me anyway, as all realism mods I have used in the past make all these lines black anyway.
I assume you're not using Improved Uboat or Real Uboat? It's totally unrealistic to be able to identify the hostility of a ship by sound anyway.

Basically these lines are useful to tell you where ships are when you're below periscope depth. They will keep updating every so often, and if you keep marking the end of the line you can see which direction ships are going.

OK but what do the 2 colors each represent exactly?

I believe the red lines are for warship contacts & the black are for merchant contacts.

I have them turned off.

The length of the lines indicate the distance to the vessel, comparing it against the distance scale on the map gives me an accurate depiction of exactly how far it is.

Knowing whether a ship is a warship or a merchant from its sound isn't completely unrealistic. Most warships have two screws and generally run faster than a merchant, which generally have just one screw, and these differences sound different in the water. The game could be merely simulating what a good, trained sonarman ought to be able to detect. I think the game does take this a little to the extreme though, so maybe the sonarman is a little too good in a lot of cases.

I know in today's navy they have these sound differences down so exact, that every individual merchant and warship has its own sound signature that can be distinguished just by hearing it run in the water, allowing a sonarman to know not only know what type of ship it is but exactly what ship it is down to its name, all by listening to its sound running in the water. This is mostly done by computer though.