[Hylarion]

I almost always kill medium to big merchant ships with one torpedo (either magnetic under keel, or impact at the bow). Bust most of the time, the small ships are a real pain to down with torpedoes (even with magnetic). This week-end, I followed a coastal merchant for nearly 3 hours after a bow impact hit (15 m/s winds so no gun, and no more torpedoes left) and it never sank, although the bow was more often under than above the water.
On the other hand, I've seen several times small merchants break in two parts with 5 to 10 gun shells under the waterline around the middle of the ship...
Go figure.