Don't listen to the above reprobates. Disable the vessel with a shot to the screw and rudder. Then if they do not have an armed guard aboard (be careful!) board the vessel and pillage their refer (immersion calls for me to raid my refer for a turkey leg, or a roast beef on rye with horseradish). Only after a good refer raid do you move off and sink em'.
(In the early years, I do not look to kill unarmed merchant seaman who are non-combatants. That will change when theere ships are armed and in escorted convoys and can do harm to my boat and crew. Try to play by history as it really unfolded, you will enjoy the sim better than arcade style.)
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DID, no cheats, no shortcuts, no non-historic equipment deviations. Boat and crew safety is integral with my immersion style! Follow the historic events during your patrol:
http://www.uboat.net/today.html