Rudder

A rudder is a primary control surface used to steer a ship, boat, submarine, hovercraft, airship, or other vehicle that moves through a fluid medium. On an airplane, the rudder is used primarily to counter adverse yaw and p-factor and is not the primary control used to turn the airplane. A rudder operates by redirecting the fluid past the hull or fuselage, thus imparting a turning or yawing motion to the craft.

In basic form, a rudder is a flat plane or sheet of material attached with hinges to the craft's stern, tail, or afterend.
Often rudders are shaped to minimize hydrodynamic or aerodynamic drag.
On simple watercraft, a tiller—essentially, a stick or pole acting as a lever arm—may be attached to the top of the rudder to allow it to be turned by a helmsman.

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Source: Wikipedia · Image: Wikimedia Commons · CC BY 2.5