Here are some screenies from a quick test in the convoy attack mission from the submarine school..
I got opposite results to tater, the instant my torpedoes hit I got the target destroyed message.. weird!!
Nippon was hit at stern by only one torp and I got the message instantly!!! Even though it took her 15 minutes to go down..
The Biyo rolled onto her side and went down nicely.
When the first Nippon did go down, as she went down stern first there was a massive splinter explosion which I thought was a bit odd. But on the whole much improved.
I hit another Nippon and she started to go down stern first slowly, then about 5 minutes later her back broke and the bow went down quickly while the stern settled and sank slowly... nice... lots of fires etc..