Damn tanker wont sink after 5 torpedo hits???

[DesertFalcon]

so I started the convoy training mission in sub school or whatever. I find the convoy, kill or wound the protecting destroyers, and then go to work on the first tanker I see. I thought, having gotten rid of the escorts (most fled instantly after they got hit once) I could leisurely wipe out the helpless convoy....

so I hit this tanker once, twice, but it doesnt even stop moving. I watch it for awhile, get irritated waiting for him to sink, and pop a couple caps in him with my deck gun (the big one). Third times a charm, right? so I blow him another huge hole in his side. STILL WONT SINK?? 2 more shots with my aft tubes continue to produce the same result. At this point I have zero torps left and deck gun is exausted, yet this damn tanker sails on, not even on fire. can somebody explain this ridiculousness??

TL DR - I torpedoed a tanker 5 times and shot him with my deck gun, and he sailed on alive.