5000m leaves alot of room for error(at least for me) and if I'm not mistaken the torpedos will run that far and more. The rough seas will screw up the torpedos. Also, I always attack on the side with less coverage of the warships. I get to about 3000m for closer and send out a spread on the lead ship if possible. Heck, sometimes my torpedo will hit a vessel on the opposite side. Yesterday, I nailed a corvette on the other side of the convoy and about 6000m off from me
. Anyway, investigate were the least coverage is and use that to your advantage. If you are in a position that you "really have do not have a choice" find a new attack plan that gives you a choice.
If I were you, I would go over this convoy again and try some new tactics.