The broad blade attached to the stern of a boat which, when moved from side to side, causes the boat to steer. Sternwheel river steamers invariably are equipped with more than one, sometimes as many as four or five, and rivermen refer to them plurally as "the rudders". Sidewheel vessels in the good old steamboat days usually had a single rudder but not always, some being equipped with "double rudders". A monkey rudder is one found aft of the wheel stream on a paddlewheel boat.