are you playing with any mod. Becose the mods modify the struture of damage. then (realy i dont know how far is this true becose you cannot know wich is the type of cargo) depending of the petrol, gasoil, airplane fuel or balast (water) is more easy or complex to sunk one of this ships.
in my experience tankers are more suitables of impact explotion than keel shoots, mybe by flloding moddels or maybe i was shooting ballast cargo.
http://www.subsim.com/radioroom/showthread.php?t=157483