Rudder

The rudder is the primary means by which a ship is steered. It is a flat or streamlined movable surface, usually mounted at the stern behind the propeller, that pivots about a vertical axis called the rudder stock. When the rudder is turned to one side by the steering gear, the water flowing past it (greatly helped by the stream of water thrown back by the propeller) is deflected, and this produces a sideways force on the rudder near the stern. That force pushes the stern in the opposite direction, swinging the ship's head round and turning the vessel onto a new heading. The rudder therefore works most effectively when the ship is making way through the water and when the propeller is driving water across it.

Rudders are turned by the steering gear, a powerful hydraulic system that responds to orders from the bridge or the autopilot, and because steering is crucial to safety, ships are required by SOLAS to have main and auxiliary steering arrangements with duplicated components. There are several rudder designs, differing in how they are hung and balanced: a balanced rudder has part of its area forward of the stock so that the water pressure helps turn it and reduces the effort the steering gear must supply, while a spade rudder hangs from the stock without a lower support. The size and shape of the rudder are matched to the ship for good manoeuvrability.

The rudder is closely bound up with the ship's handling characteristics, how tightly it turns, how quickly it answers the helm, and how it behaves at slow speed when the flow across the rudder is weak. In confined waters and when berthing, skilful use of the rudder together with the engine, and often with tugs and thrusters, is what allows a large ship to be manoeuvred precisely. As the surface that directs the ship's course, the rudder is among the most fundamental components of any vessel.