A salvo, under keel, if possible, under oil bunker

Anything can be done in this way, with the minimum cost.

Also, it gives you good screenshots - ships with exploding oil bunker

i like to race ahead at range and pull up to get a good firing angle, submerging if its good weather to hide myself. get a nice angle and aim for just above the keel under the funnel, a c2 can be taken out with one eal this way. and a c3 with two (sometimes one)
if i'm at a bad angle or behind or infront i'll use a magnetic head and aim under the keel.
if the ship is under 2000 GRT i'll usually actually take her with the deck gun. 3-4 well placed close range shots under her waterline is usually enough.

sometimes i use magnetic heads to blow the screws off larger ships in convoys that way i can finish them later on when all the commotion has died down.
pete