Attack convoys, your's best tactic.
 

My best tatic so far is laying at 25-35 meter in front of the convoy, waiting closed to their layer heading and praying for the escort to pass.

Then, moving back slow to periscope depth.

I do a quick look, try to find the right target, and get closed to it.

when at 1000-800 meters from the main target, i usually try to get another target in my tail. And launch to it from rear tubes.

When my torpedo is closing on rear target ( 2/3 of the course to it ), i launch on main target, then accelerate heading directly to the main target, then i go a little deeper.

The first explosion on rear target draw attention from escort, then explosion from the main target put them on troubles.

when i am exactly (or so) below my mnain target i slow down, and go back to periscope depht.



When i have expended my torpedo, i just try to get very close to a moving marchant, try to go below, in same speed same heading .... for a short lenght of time, then revert my heading to very slow speed, even stopping fully....

This tactics work quite good.