Maybe it's just a matter of not seeing any indication of damage, but it's been taking an inordinate amount of time and ammo to sink freighters now. The game doesn't seem to allow below-the-waterline hits so I aim for the hull. I see the health bar turn from bright green to dull green, but it almost never seems to actually begin flooding. In my most recent test, (against a ~6000t Empire Explorer, using AP), I put 30 shots into it, followed it for 20 minutes, put another 10 into it (no fires at any point), followed it for another 20 minutes, and only then did it begin to flood - though once it finally did begin to flood, it went relatively quickly in about 5 minutes, compared to half-sunk torpedo victims which can take 30+ minutes for that last bit of buoyancy to deplete.

Even against a derelict type 7b sub, it took 10 shots and 5 minutes to start flooding.

It feels like these sinking times are muich more appropriate for a realistic or semi-realistic setting