If you are close to a slow moving target the slow speed will make less noise and wake. I use this setting 90% of the time.
How many torps it takes is based on several factors including how accurate the shot, where you hit the ship, which torp you use, the size of the ship, how low you hit it, if you have any crew ability power-ups.
I've taken out large ships with one torp consistantly. Many people here complain about small ships walking away after 5 hits. As your skill improves, the number of torps you "waste" will go down.
Limited batteries is very realistic based on history. At high speed you only get about 15 minutes on full charge, but at 2 knots, you can run for 48+ hours (depending on which sub and the skill of your crew.)
-Pv-
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