Thanks again for vigilance and effort LemonA and Pisces. However you got me confused now - is the method really workable at all or is it all wrong?
If you have the time it would be great if you posted a step by step on the method to make it clear how this should be done
And guntherprien if the convoy zigs one needs to follow the convoy for some time to make an estimation on what the real direction is. If the enemy does not have radar it might be easier to just stalk the convoy in a way that the closest ships are just visible.