How did German U-boats attack merchant vessels during World War II? Did they have to surface or could they launch torpedoes from underwater? They could fire from both the surface and periscope depth. When a convoy was first detected by a U-Boat, they would contact the base giving them the position and course of the convoy. This first sighting submarine would then shadow the convoy. Meanwhile, headquarters would send out a signal to all the submarines that could attack the convoy. It was then the