What was the main target of German submarines?
German submarines targeted Allied shipping. Any merchant ship that carried fuel, stores, weapons and ammunition or any other goods that supported Allied military activity were targets. If you could take away fuel for tanks, you could idle those tanks. Take away aircraft fuel and planes sit on the ground. Deny an adversary ammunition and you will not be fired upon, but you've gotten the picture by now.
Main weapons of German subs were torpedoes and a cannon on the foredeck. If the target was entirely unarmed and the conditions good the sub would have surfaced and then used the gun. Poorer conditions, or an armed target forcing the sub to remain hidden would have caused the torpedoes to be used instead.
They had difficulty in combating merchant convoys to and from their overseas allied nations, as German submarines were a huge threat. Their main tactic was the use of depth-charges, which even if they did not hit the submarine directly, created an overpressure effect which could rupture a sub's hull. They also relied heavily upon heavily-armed naval escorts which had the capacity to fire ship-borne torpedoes, as well as upon their own submarines to track and…