What is the fin on a bottom of a ship called?

The very bottom of a ship is where you find the keel. The keel runs the full length of the ship along its centerline and is the backbone of the hull. From the keel all the frames run out and up. the hull is attached to the frames. Picture the keel as the backbone and the frames as the ribs. The hull is the watertight skin that allows the ship to float. Within the hull are compartments (rooms) and at the top of the hull is the main deck. The superstructure is built upon the main deck.