dont know if it has been pointed out already but i notice the majority of "not sinking altough 6 torps hit it" cases happen when hitting the target at the bow or astern.
This afternoon i tried to sink a merchant anchored at a jap harbour. Due to the fact it was moored in one of those little inlets i had to shoot its bow.
5 Torps (!) later i could see some sailors bring out sticks and marshmellows and they went on partying by the nice fire at the bow happily singing Dixie..... No sinking....not even a dent apart from a cosy campfire at the front mast.
And its not the first time..... God i hate marshmellows
