What mods are you using? (If any) Most ships can and do go down with 1 torpedo, but it depends on how long you're willing to wait. I've spent hours waiting for a single ship to sink, and most eventually go under.
Also, are you going for keel shots or just pumping torpedoes into the side? A single shot under the keel can instantly snap even the larger ships in half, depending on where you aim. Or, if you're more patient and have excellent timing/luck, a single torpedo aimed at the front of the ship (Right on the bow) will cause massive flooding in the forward areas, as the ship's momentum keeps the water flowing in.
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