Something i scribbled into MS Paint some years ago:

Come in from ahead, and at an oblique angle. Gives you overlapping targets, and you stay out of the lead Tin Can's way. Only problem is your showing your broadsides to the flanking escort. So when the lead escort crosses your bow, move in before the flanking escort reach's you. Try not to get any closer then 2200 yards from escorts if possible.

Flanking escort will be the problem, If you can get inside him, you've bought yourself more time. Or you can fire before he reach's your position if you don't think you can make the speed to get to the inside. If your forced to do that, you won't have much time to fire, but you'll still have the opportunity to make the shot if your focused. Either way the lead escort doesn't become a problem until later.

Whatever you do, don't do this.
EDIT:
What you may have to do (considering theres 6 escorts) is come in at an angle to the convoy, and then run a parrell course to the escort screen to minimize your profile. If you can pull that off, what will happen is you'll get to the inside, will have to make a sharp 90 degree turn, and fire. You won't have much time, so you'll ahve to put extra turns on your screws, which will make you rather noisy. So, once you've made it inside, your basically unzipping your fly, and dropping your pants the instant you start making your hard turn to port or starboard, so be quick with your shot, and start heading for the cellar before the fish hit.