A rudder is a device used to steer a ship, boat, submarine, hovercraft, aircraft, or other conveyance that moves through a medium (generally air or water). On an aircraft 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 after end. Often rudders are shaped so as to minimize hydrodynamic or aerodynamic drag.
There are two rudders so that when the boat heels the leeward one is pointing down and still functional.
Basically, the same reason scows, catamarans, and the really wide-stern ocean boats have two rudders.