I find it best to do a submerged attack in the centre on the convoy. The bigger ships are toward the center of the convoy, noise of reloads and manouvering is covered by the sounds of the convoy, as well as giving you more time to get away to a safe depth if they come after you as they have to weave their wy through the convoy to get to you.
Approach from ahead of the convoy on silent running, being slightly off center, so you have less chance of being picked up by the lead escort (if there is a heavy sea it will make it easier) Once past the escort and he's has drwan away by 1000 meters, you can then start to pick which ships are the ones you want to go for. Fire at the ships that are at the greatest range first working down to the closest. (buy using different speed settinf and or torpedos can help in making them arive around the same time) Try to shoot at no more than 1000 meters for your last targets as it will give them less time to react if the longer range targets get hit first.
By the time the torpedos hit and the escorts wake up, you will be into the convoy, making it harder for them to find you. Just make sure not to leave your periscope up for longer than needed as if a merchant spots it the escorts will be after you!
Take silent running off and start loading torpedos, while staying at slow speed. this way you can get two atack in before the convoy passes. Use the stern tube to take out crippled vessels / snooping escorts / targets of oppitunity if needed.
When you have passed throught the convoy, watch out for the rear escort, as he will be looking for you with a vengence.
Good luck and good hunting.
