Ballista

The ballista is huge crossbow type weapon that uses torsional force to propel generally smaller and lighter projectiles, for example darts or arrows, much farther and faster than a catapult or similar device. The ballista, which first appears in episode 301 of Avatar, "The Awakening", is an enormous device that is mounted on a rotating platform aboard a Fire Navy ship, similar to the catapults and trebuchet seen in earlier episodes. The weapon is mainly used to deliver a metal projectile to the hull of the enemy's vessel at a point below the water line. Once the hull is punctured, the projectile opens up to prevent itself from exiting the vessel.