You can always have a range to target, it's just less accurate and sometimes really annoying, but i don't think it's a big problem in the end.
If you can't identify a ship just get closer, i mean REALLY closer if your navigation skills are good it should be easy. In the same time try to measure its speed and bearing and plot an attack later. Or if you are patient enough wait till daylight visibility should be better.
Broad daylight and calm seas are obviously better to learn but attacks in stormy weather are possible too
The main problem is the heavy fog heavy rain weather with visibility <300m. The only solution i've ever found is to get behind the ship exactly in his 180° and launch a magnetic triggered torpedo which most of the time blow its propellers and makes it a 'sitting duck'.
Manual targeting is much easier than it seems so hold on!
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