GWX is very dynamic to play and the number of small units you sink appears to have no effect in decreasing these units in the game in my experience so far.
You should practice attacking convoys at night and on the surface at that stage of the war as the boats did. It takes a little practice to work the depth for decks awash (approx 7 meters) and then approach this way. I come in at standard speed most times.
There is no radar on the escorts yet, and you are nearly invisable to them. Your accuracy will improve the closer you can get in.
