how to attack in heavy fog?

Hi all!

I am a noob who is in command of a class VII (Kiel) in June 1940. These are the "happy days", I am starting to get convoys (as opposed to lone merchants) though you can tell things are slightly tougher than before. I must say that I am playing at the easy level!! (I think it is 29% realism). I think one of the ways in which the game tries to make thing a bit harder (or rather not so boring) is in the sea conditions: you have rough seas and you cannot use the deck gun, which reduces your kills.

I am now in a patrol with awful sea conditions: rain, fog and 200m visibility. My question is how you can sink ships in these conditions: you can detects ships easy with the hydrophones but you then surface and see nothing until you actually bump into the merchant! So then... what can you do??

Any advice appreciated. Thanks!