[Gaijin]

Haven't used it myself for a long time, but from what I remember, it is activated automatically when you surface. You need to go to sonar/radar menu and tell your man to switch it off.

If you don't, you're going to start being very popular with the RAF, from about 20km out of port onwards, as well as being target number 1 for any DD's in your vicinity.

(Yes, in terms of giving away your location, the emissions are pretty much the equivalent of having a neon sign on your bridge).

I don't bother equipping it, but that's just a personal choice.