This game dose have certan adictive qualities, especialy when you figure out how to aim torpidos so they hit just where you want them to. Or after your first brush with crush depth while Tommy distroyers are droping depth charges on you like they are candy but you still manage to keep from being crushed and manage to escape from the attack zone; that was nearly enuf to replace drinking for me, only nearly though

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If you are looking for a way to get better at attacking convoys short of going into a campain and trying to run them down on the high seas I recomend the single missions. Even in the stock game there are some that are very useful for practise on sneeking up on convoys or millitary task forces, much more dificult if you ask me, and seting your boat in an attack position, fireing and exiting the danger zone. The Grey Wolves Expantion mod makes many aspects of the game more realistic, and difficult, but over all a lot more fun and comes with a bunch of convoy attack single missions.
There are also some other mods that can help you be more leathal at sea, the one that I like the best is the Mai GUI mod. It takes some time to figure out how to use it but once you get the hang of it there is no going back if you ask me.