8 torps?!

[Esoteric Ops]

Hello

I am a casual player but I really enjoy sub sims. Here's the deal, I intercepted a convoy and decided to attack the largest merchant I could, a large transporter @ 10,000t with 28ft of draft. I have only had this game for a week and so as not to make the same mistake I made the day before (launching just 2 torps on a medium for it to just carry on, on its merry way,) I decided to launch 4 torps at this one, with the depth @ 14ft. Well it made nice fireworks but didn't sink, so I came back and fired 2 more torps aft, at this point 2 destroyers were right on my location so I had to fire and avoid straight away.

All 6 torps hit and it still didn't sink! I waited for a couple of hrs, slid away and came back for another hit, I only had 2 torps left. Well, there she was, the same transporter (still on fire I might add,) so I carefully positioned myself, got a perfect lock and fired off my last 2 torps, loads of explosions but it still carried on! I shadowed it for a number of hours just to see if it would sink but it didn't, 8 torps!! Can someone pls guess what I am doing wrong or is it normal for a large trans to survive that many direct hits?

Thx to any replies.