A rudder is an appendage used to steer a vessel. The rudder redirects the flow of water that flows past the hull and causes the ship to yaw and turn. The rudder is essentially the only control surface on conventional shaft-driven ships.
There are many different types of rudders, throughout history the rudder has evolved to better suit the needs and capabilities of the ship it is to be installed on. Early vessels were equipped with what is known as a quarter oar, where the ships direction was changed by using oars on either side of the stern. this evolved into the tiller or outboard rudder, and later the submerged or inboard rudder. Rudders can be as simple as as a flat plate attached to the hull via a hinge, but to be more accurate rudders are now often shaped to maximize efficiency over the surface and are shaped to generate lift, such as by using NACA profiles, much like that of a common airplane wing. The "hinge" or mounting system is known as the rudder stock or rudder post.