Question About Ship Damage

[Mythix]

Hey, i have a question about damage in the game. In the single mission courageous, i landed two torpedo on the carrier. It did not sink. When retrying the mission i landed one torpedo only and it sunk completely. Im a bit confused on how that works. Is there a certain point on a ship i have to fire at to sink it? and if i dont is there a minimum amount of torps that need to hit to sink it? With reference to the illustrious fleet carrier.