I've managed a few P/C's with one eel. The problem is how much random damage you can do with one torp and the amount of hit points. If you have the time to be patient they can sink from flooding, but you have to hit them somewhere vital to get anywhere.
I think bow hits make a big difference because they then seem to drive down under the waves as the bow compatrment fills with water and the rest of the boat gets pulled under.
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