Go for the big fat ones. A matter of arithmetic 10,000 tonnes divided by 2 torpedoes is 5,000 tons per torp. Two small freighters will be short of 5,000 tons.
What I often do is send 2 torps onto a fatty just to make sure my attack brings something.
Then I fire two torps at two separate biggies (Aim slightly forward of foremast) and try to sink them with one shot. If they all work, that's 30,000+ tonnes for a night's work.
By the time I fired four tubes it's time to get outta dodge.
The trick is not only sink ships but being able to brag about it later on in the pubs.
