18 sub stacks, not heavy subs, setting them on convoy raiding on the celtic sea region and one or two on the north sea. By '39 at war's start i have about 2 or 3 stacks ready to pound the brits, which drains their tickets savagely. By '40 and '41 their convoys are dead, and it is a problem compounded by the fact i conquer Gibraltar after France to make their convoys take a longer route as well as maintain sub production constantly. You can also dabble in convoy bombing with the french ports, but subs in sufficient numbers do the trick. Its a good investment.
Forget about the heavy subs, they are trash.