Hey Alex. The black line represents the direction of the contact. If it is at long range you wont be able to see it. Put a mark on the map at the end of the line and observe it for a while and then put another mark at the end of the line. This will give you a general idea of the target's direction and range. Unless he is moving almost directly toward your position, you will probably have to surface and get in a better position. Plot a course to get around in front of him, submerge and wait for him to get in range. You may have to do this more than once to get in a good attacking position.
Of course this line on the map is an in game thing. In real life the hydrophone operator would give the captain the targets position, speed and direction. That is what the black line represents.
Hope this helps. Good hunting and before Jimbuna gets here,
Be More Agressive.