[UnderseaLcpl]

I disagree with previous assertions. DD's are easy to sink;
1) Go perpendicular to his course, as in he is close to 90 or 270 degrees in your periscope.
2) Let him get to around 400m, he should speed up and the asdic pings will start to increase in frequency rapidly.
3) Ahead flank full rudder in the direction he is coming from.
4) This should put you 70-80m from the first detonation that is in line with your boat.
5) Slow to 1-3 kts full rudder in the opposite direction.
6) The DD will come about at 9-12kts as he searches for you again, make sure your periscope is very low in calm seas or he will speed up.
7) prep a g7a for fast speed at 2m, fire angle 0
8) when the DD hits the 25-30 degree mark on your periscope go rudder amidships.
9) Train your periscope at 12 degrees. This takes some practice as you must subtract a couple of degrees for close range or oblique AOB (away)
Open the selected tube.
10) Just as his bow crosses the center of your periscope, fire.
11) Unless the torpedo is a dud or is (very rarely) spotted early you will hit him somewhere and that is almost always enough.

Even if he does not sink he is usually immobilized or reduced to a very slow speed and easily sunk or ignored. I have an 80-90% success rate for this tactic when pursued by 2 DD's or less. It gets tricky when 3 or more are up there as it is hard to keep track of everyone and the AI pathing leads to destroyers you thought were a safe distance away closing in. Also, the "Down the throat" attack is useful as well. I usually miss or get a dud from the extreme oblique angle but the DD will veer away giving you precious seconds to maneuver.

Closing argument; Maintain periscope depth and do not give up the initiative! When you go deep you give the initiative away. I would not recommend this unless pursued by 4 or more DD's in the immediate vicinity.

Remember, once the escorts are gone (given proper conditions) you can always end-around the convoy for another attack. Early in the war if the sea permits you can deck gun all of them to death.

Call me a fool but this works for me in stock and GWX 2.1

Thanks for reading!

Finally, once you reach the proper stage of the war, Zaunkonig II down the throat.
Takes the hassle out of destroyer-smashing.