Like others have said, by 1943 things get rough.
What I find is that after mid-1943 about your only hope for hitting a convoy is to switch to steaming torpedoes and make a single attack from stand-off range. You'll be shooting from the side of the convoy from outside the flank escort's detection range. FAT pattern-running torpedoes are good but at the range you will be firing at they won't have much juice left in them for much pattern running.
The trick is to fire your bow tubes and dive for the red line, undetected, silent running at less than 100 RPM on the screws. Your stern tubes should be full of homing torpedoes and not used except in an emergency.
Once you are detected you are in deep doo-doo. Convoys by this time often have 6 or more escorts, and they will call up support from nearby if it is available. Sitting still does nothing in the game. Nor does bottoming the sub. They will pound you into mud. Once detected, your only choice is to come to periscope depth and try some trick torpedo shooting to kill the escorts. Homing torpedoes can be very helpful here but they don't always work and you have to be careful they don't swing back around and nail your own boat. If you run out of torpedoes while they are hunting you it's basically all over. If they hear you they will sit on top of you until you run out of air, even if they run out of depth charges.
The days of sailing into the midst of a convoy and shooting and reloading are over.
Oh, and stay out of shallow water.
Steve