Well in the same career i also sank a german merchant. It was too dark to see even at extremly close range and it was darkened near norwegeian waters (while the invasion lasted). I thought it was possible to be friednly but it was reported as unknown merchant so i thought noone will report friendly without declaring it.Apprently i was wrong and deck gunned it to death.Few days later almost the same happened near scapa flow with another german merchant (another report) this time it was day and before i fire my torps i took extra time on the flag, and yes it was again friendly.
