In 1941 you should have no problem with a frontal-quadrant attack.
My ideal attack pattern is to plot an intercept directly in front of a convoy, then angle my course towards the outside of the convoy. I turn away from the convoy as required to keep my boat at least 1500 meters away from the lead escort. You should be at silent running throughout the approach. This means instructing your engineer to set silent running but also means running no faster than ahead slow. Sometimes, if I am scooting quite close by the lead escort I will even order all stop and coast while he goes by.
If I'm not able to position myself directly in front of the convoy I will try and approach from the side such that I am beside the lead escort but will pass well in front of the lead flank escort. You are basically trying to thread the needle avoiding detection between those two escorts. It is easiest to let the convoy run you over and only have to dodge the lead escort. This can be tricky though as the lead escort often weaves back and forth in front of the convoy. If he happens to be moving towards you while you are trying to slide past him, bad things can happen.
Note that this approach obviously only works if the convoy has not already been alerted to your presence through a visual or radar contact. Once they start zig-zagging your will not be able to make a silent approach - they are alerted and they will find you.
Once you get into 1943 it is generally not a good idea to try to get inside the convoy. At that point your only real hope is long range shots from 5000+ meters with pattern-running torpedoes and hope your Hail Mary shots hit something while you dive and avoid detection. You must avoid detection at all costs by that time of the war or you are finished. Also I recommend not making any attack against any target where ASW vessels are within range without having 100% hull integrity so that you can dive to your maximum depth.
Once you get homing torpedoes you can once again get in close to the convoys if you are good enough to pick off the escorts with the magic torpedoes.
Steve