Hmm, what to do with a convoy.... depends on how you like to handle it and more importantly, what year... early in the war you might get away with repeated attacks, later on you might just get away in one piece...
My prefered strategy between 1939 and 1941 * when faced with a convoy where I'm roughly 90 degrees off the outer lane of ships at 1km or so is to shoot as follows:
1 eel, magnetic, fast at the biggest ship in the center row, usually a tanker or some such
1 eel, impact fast at the lead ship of the 2nd row
1 eel each, impact fast at the 2nd and 1st ship in the closest row, in that order
If you're lucky and you timed it well this will result in 4 sunk or crippled ships
also, always try to sink the tankers and the largest merchants first, use two eels if you want to make sure..
*note: from 1941 onwards I switch to all electrics, even at night