Merchant chases during storms

Hello,

What would be the best way to hunt lone merchants in bad weather ?

Generally, I can easily chase them using the hydrophone, and after some time even manage to see and ID them - but that's only for a second or two, since sighting them is only by chance, as in heavy rain/fog I can pass by them on surface at 500 meters and not notice them.
Last time it took me 4 torps to hit a small merchant - after I drew his course, and sat out where he should pass me, two torps hit him, but didn't explode - because I had forgot that my distance to him was less than 300 m (but really, at ranges more than 300 m I couldn't see him well, and get a firing solution), the third was a miss, and only at fourth torp I managed to hit him, since he continued in a straight line.

Now I have another case like this - I have trailed a C2 for around 36 hours, hoping for the weather (15 m/s, heavy fog, heavy rain) to improve. And I have only two torpedoes left, so I can't afford any risky shots, they need to sink the ship. I surfaced right next to him recently, and now he seems to zig-zag - at least from what I can hear on the hydro, haven't tried to close so much again.

What should I do ?

Playing latest RuB, realism 93% (using weapon officer assistance). Year is 1940, and the ship is near the english east coast.