Yes, once I used six or more on a Liberty ship – and even then it didn't sink, so I gave up and went to hunt elsewhere.
But even with the Greywolves mod you will sink a significant number with just one torpedo.
In unmodded SH3, try aiming just under the front of the bridge. Use a magnetic pistol on C2s, an impact pistol on C3s.
I seem to remember tha tankers seemed to light up if you hit them just about anywhere amidships. And I've heard a good place to aim for is just in front of the smokestack in the rear.
Good hunting!
