Well I haven’t even read the Alpha3 rules yet, and they sound confusing.
But until Alpha3, it was fairly simple. During the collect income phase, both players are supposed to check for convoy disruptions (page 22, Europe manual).
I thought the rules on page 22 were very clear… What don’t you understand?
Say you are the UK. When collecting income you need to check all of the coastal territories that you own. If there are any enemy warships in sea zones bordering your territories, then you need to check for a convoy symbol in the zone. If there is a symbol, then you count the number of surface warships and deduct one IPC from your income for each enemy surface warship in convoy zones bordering your income producing territories. Deduct two for each enemy submarine. You cannot lose more than the value of the territory being raided, so if there was a German destroyer and 2 German carriers in Z119 at the end of your turn, you would deduct 2 IPC’s from your income total, because you are losing all the income for Scotland, which is worth 2. Normally a destroyer and 2 carriers would disrupt 3 IPC’s worth of income, but it is capped at 2 because Scotland is only worth 2.