[UnderseaLcpl]

If it's DD hunting you want the best method is to let him do what he wants to and come perpendicular to you. You should be at 1/3 speed 0 rudder.
As he closes the last 400m or so flank ahead and hard rudder towards him.
The dd will attempt to follow you but at the 20-something kts he is doing he won't be able to and will swing his stern away.
You should dust off his beam at about 80m from the depth charge track.
Ahead slow full opposite rudder. The DD will make a wide circle at about 12kts directly across your bow 400-600 yds out
Prep a tube for impact/fast 2m and put your periscope crosshairs on 10 or 350 degrees, depending on which side he approaches from. When his bow crosses the line fire ( don't forget to open tube or you will likely miss astern)
If you are using g7e you need a 15 degree lead angle.
Since you have manual targeting off I guess you can just fire when the arrow is green.

This can also be done to set up a stern shot but will take too long to explain. Do it a few times to get the feel of the destroyer's behavior and the many variations of this technique will make themselves clear to you.

To date (June '40) I have sunk 14 destroyers and 2 ASW trawlers for their impudence. Even managed to tackle 3 at one time....alll went to the bottom.

One final note....
DO NOT try this on dd's with K-guns. That requires a head on approach with a great deal of precision either to land a down the throat shot or to slip in between tha gaps of the stern racks and the kguns, and you WILL take damage.
For them the best answer is stay away or Zaunkonig.

The destroyer's greatest weakness is thinking HE is the hunter....