We might need some more info to help you here, a lot depends on what mods you may be using. There is no way to turn off detection and some mods are set to be more difficult than others. You can search for threads that will tell you how to change the difficulty settings, but without a good challenge, the game will become quite boring IMO.
I assume by what you have said that you are positioned and sitting still waiting on the convoy. That's a good approach. I have two suggestions to try. Keep your scope as low in the water, and as short a period of time exposed as possible. If it is escorts that are detecting you, you may need to set up your position to enable a longer shot i.e., keep further away from their path. Again it depends on what mod you may have, but I use TMO and the escorts are tough. I usually find it neccessary to set up for a 3500 yard shot to remain undetected by them, and that's not always reliable, but usually works for me. If they do detect me, I have to decide whether to evade or fight. The fighting is usually more fun, and if that is the route I decide to take, it means a "down the throat" shot at the DD. A lot of fun, but risky. I have found about a 80% success rate with this procedure, and with some practice, so could you. Just set up for a zero degree shot and keep the DD in the crosshairs with small rudder adjustments and when he's about 800 yards from you let it fly. If you want more insurance, Fire one just to the left of his bow, and the other to the right. The advantage of the latter is, if he tries to evade, one of the fish will hit. Be aware, if you miss.......better dive quick ! Of course, that's when a save comes in handy, you can practice till you get it right.
