The sub I was tracking just dived question.
 

Hi all.
I am on the first mission, I have just arrived at the mission destination circle.
Have a surface contact about 15km (surfaced enemy sub). Have plotted an intercept course using marker and ruler etc.
I was still surfaced as the target was traveling at 11knots and to close with him I had to travel at flank speed of 16knots.
He just dived (I am assuming this as I lost contact with him) at around 6Km from interception point.
What should I do now?

Enemy subs with their limited AI are more of an eye candy rather then actual worthy opponents...
Go hunt some surface ships/warships instead...

Roger

I have just ordered a hydro search and picked up another contact.
I now have a blue line indicating its direction and I know it's a warship.
Does the color of the line (blue) indicate if it is friendly?
Should I ignore it (It is long range and moving away) and continue hunting merchant shipping?
This will be my first contact and I would prefer to practice on an unarmed vessel.
My mission is to:
Intercept and report Polish task force (secondary)
Destroy Polish task force + other shipping (primary)

Yes, the blue line indicates that they are friendly's, if you try and attack them, it will cause issues with the game. So don't attack friendly ships. (This also goes with neutral ships marked in green.)

On another note, I've actually managed to sink an enemy sub before. It was a huge pain to do it because it didn't show up on my screen when it dove down but i ended up sinking it by launching a salvo or torpedoes at a depth of 3, 3.5, 4 meters. I was determined to sink it. :Kaleun_Cheers:

Yesterday in that same mission I managed to sink a surfaced sub - was quite fun to see how the torpedo picked it apart completely. However, it was not counted in my stats as another sunk ship? A bug or do subs just not count?