Running fully submerged it is hard to catch up with even the slowest mechant so I run decks awash (or even fully surfaced if I am feeling brave) and use the diesals.
Just latch onto the merchants course and aim to catch up slowly making fine adjustments to your couse so you are eventually running on exactly the same course as they are directly behind.
If I can ID the merchant I go for a magnetic shot, if not I set it for 1m and impact. No need to mess with the TDC or put any other data in, just fire with the Persicope/UZO at 0 degree gyro so long as you are directly behind. I sometime do this from 2000m no problem, just got to line it up right and take your time.
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O/B U-69, 2nd Flotilla
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