If you listen to the escorts, a lot of them intermittently switch off their engines to passively listen for subs (they don't have active sonar - the 1500-2000m crescent thing), so if you're going hell for leather under water they're going to hear you. If you keep your scope up for too long in good weather - bingo!
You have to get into firing postion in front, facing directly the convoy (gives less cross-sectional area for active sonar) and sit tight at ~300 ft. As the 1st merchant/escort passes, head up to scope depth (takes ages) and turn 90 degrees to convoy, or keep it straight to fire at a gyro angle=90.
Once you fired your torps back to max depth, and move away from your firing position (tell-tale torp wake), and continue through the back of the convoy @ 1 Knot.
Worked every time for me.
