Also take into account early war torpedoes are steam driven and leave a trail of bubbles on the surface. If a ships spots this it will attempt evasive action weather a course diversion or speeding up or both.
You could try adding a few degrees to the torpedo angle to allow for this 1 or 2 degrees should be enough..
with undefended merchants you can afford to get closer to them before you fire. I am no expert with manual targeting as I let the WO do work most of the time. just hope my little bit of advice helps
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