...and they're heavy tankers :). They can absorb some beating before they go down.

There is a certain % chance that the torpedo will detonate right under the ship's keel, in which case the rising gas bubble will magnify the shock damage and possible break the ship's back outright (modern torps with imaging sonars try to achieve this deliberately), but it is far from guaranteed.

Also check the message log for any "Torpedo XXX has malfunctioned" messages. These aren't quite WW2 Mk14s but they can still dud now and then.

Fun fact #1: The Russian 65-73 superheavy torpedo (the one that exploded inside the Kursk, incidentally) was developed to sink aircraft carriers and also very large merchant/tanker ships. They found out the same thing you are; these buggers take a while with normal-sized torps.

Fun fact #2: During the "Tanker Wars" in the Persian Gulf in the 80s, a popular trick for clearing a path through a minefield was to run a supertanker through it. They could take an inordinate amount of mine detonations before sinking. There's a photo out there of an almost-overflushed tanker passing through the Hormuz, sprinkled with flood holes from mines everywhere. Great stuff if you can find it.

noted.... thanks... enjoying it soo much.