Prize warfare pop-up screen example; these will only be visible when you get close to neutral & unarmed merchants. Also, they will only appear when you are surfaced and using the bridge camera.
That way, no cheating by using free camera to see if a ship can be torpedoed.
Also, there will always be a chance it's a Q-ship, so be careful.
There will be lots of different cargoes depending on the ship - I may adjust the screen but this is the basic idea.
WWI French magazine illustration of a Q-ship attacking a U-Boat - surprising the German crew!
