The Yamato's sister ship
Shinano (which had been converted into an aircraft carrier) was sunk by an American submarine with only four torpedoes.
Apparently there was a weak spot in the Yamato class armor at the joint between the torpedo blister and the ship. The sub captain who sank
Shinano set his torpedoes for minimum depth, so all of his torpedoes hit this weak spot. In addition, the ship was still slightly unfinished, so her watertight compartments leaked badly and her damage control systems failed. All that together meant that four torpedoes were enough to bring down the biggest ship in the world.
Unfortunately for the sub captain, the Japanese had been so good at keeping
Shinano a secret that nobody believed him when he claimed to have sunk a huge carrier with a clipper bow. It was only after the war that American intelligence found out about the third Yamato class ship.
http://en.wikipedia.org/wiki/Japanes...arrier_Shinano
A good book about the sinking, written by the sub captain:
http://www.amazon.com/Sea-Assault-Jo...993608&sr=8-12