Summer 1940.
Italy invades Greece to get some more valued territory. Athens is attacked from Albania and amphibiously. Germany moves its units around to get ready for Russia. They also attack the British beachhead in Brussels while it is weak. Germans also finish off Yugoslavia by attacking Belgrade.
Now that the channel isn’t blocked, the German u-boats come out. 1 sub to each convoy the British have.
To attack a convoy, a sub merely needs to enter a space that the convoy travels through as indicated by the zigzag lines (I used Battle of the Bulge markers for this, but making your own straight markers would be better.) If there are more than 1 convoy in the sea space that the sub is attacking, then the sub needs to choose which convoy it will attack.
Planes and ships can try to intercept the German ships, but the British have no ASW ships or planes in port to do this, the cruisers, battleships and empty carrier stay in port as they are not equipped to attack subs. Britain needs destroyers badly (some are being built now in the construction boxes but are some turns away still).
Submarine warfare is only 1 round. Attacker rolls for each sub he has attacking a convoy. 3 or less and the defender loses a transport for each hit. Defender rolls for each destroyer or other ASW unit he has. 3 or less is a hit and each hit removes a sub. Attack is over.