When raining with heavy fog as it says in your weather report, visibility will be down to about 300 meters. What you want to do in this situation is get a fix on his bearing and heading with your sound gear. Plot it on your map then surface and run at full speed along that plotted course. You may have to submerge to take new readings to refine your plot. Keep this up till you get visual. Run right up along side him then turn 90 deg away and bring your stern tube to bear then fire off a fish. It doesnt matter that he sees you too as he wont be able to manuver enough at that close range. Bear in mind that if this is an armed merchant or a destroyer you will get your ass handed to you.
