[11Bravo]

I am new to this game, but I had a similar experience. Made contact with a single merchant by sonar. Tracked it for ever by sonar while zigzagging across the ocean to spot it. Finally spot it and watch it for many 3:15's to get its course. Maneuver around it to avoid being spotted and get into perfect daylight attack position, submerged, 600m off its course. Wait forever for it to arrive. Shoot. Ship vanishes before torpedoes reach it.

I suspect that ships spawn and run on certain waypoints, and then eventually disappear from the game. My ship must have reached its last waypoint while the torps were heading in. At least that's what I told BDU.