Have you tried sitting even deeper? Say 250ft or so. I've crept right into convoys, under the very noses of the escorts at 250ft (or more, depending on sub class), silent running and 2 knots.
I also try to intercept and wait to come in from the flanks. It seems sitting right in front of the path of the convoy puts you in a more likely position to be detected. I like to sit 3000-5000 yards off their base course, wait for the lead escort to pass, then move in. Of course, if I've screwed up my plot of their base course, or they've changed course, it all falls apart
