Why does it take so many torpedo's

Why does it usually take so many torpedos to sink one ship?
I have set the torpedos depth to 6.2 meters for large tankers,5.8 meters for merchants and 2.5-3 meteres for small tankers.

It's not very often that I can sink a ship with just one torpedo.
On most sinkings for a large merchant I have to use 2 or 3 torpedos and for a large tanker it has taken 4 torpedoes.

I have tried to line up with the stern of any ship & take out their props,do this for as many ships as I can,then surface at a distance of 1-3 miles then shell the ships,but this is not always possible when the convoys hang around or the escorts hover around or the distance stretches over many miles.

Is it possible to set the torpedos to have a more realistic effect or even better one shot = one kill?

Thanks in advance