How many Torpedos to sink t2 or other larger ship?

[H2OsKT]

Last night I had to restart my career after having collided with a destroyer in heavy fog. (The Destroyer won, all hands lost.)

I am back in the IIb boat with five fish. I went into Hartlepool, located a T2 tanker among other goodies. I already took out two coastal merchants on the way to this port, so all I had was three torps. I lined myself up in the harbor at about 400 meters, targeted midship and fired one torp. A few seconds later, I confirmed the hit, no fire or big explosion though. I re-aimed and fired number two towards the engine room. Dud torpedo, bounced off and detinated under the ship. I then crossed my fingers and fired tube three, my last torp at the same location and changed firing pistol to magnetic. Detonated on impact and I saw a nice little fire through the scope. I then waited and watched the boat liste to one side, but would not sink. I was so mad I lost my experienced crew earlier I decided to risk surfacing in port and unloaded all of my AA rounds on the tanker from about 200 meters. I saw lots of nice little fires, but nothing else. After a few minutes I had a trawler aproaching and another unknown warship steaming in. Went back below the surface and crept away to base with only 2 coastal merchants to brag about.

How many torps do the respective ships require to destroy? If more than one torp is needed, where should they be aimed for maximum success? Thank you for the responses in advance. :hmm: