Types of Rudders on Ships: Balanced, Semi-Balanced and More

Ship rudders are grouped in two ways. By how the blade is balanced around its stock, they are unbalanced, balanced or semi-balanced. By how the blade is supported, they include spade rudders, hung only from the stock, and horn rudders, carried partly on a fixed horn. Flap rudders, active (Pleuger) rudders and the Voith Schneider propeller cover less conventional steering arrangements.

How designers choose a rudder

The right rudder for a ship depends on its hull shape, speed, propeller design and stern structure, and on how water flows off the hull and propeller at the stern.

The choice is made in stages. At the concept stage, designers and naval architects only estimate rough dimensions for the rudder and propeller. During preliminary design those dimensions are fixed more closely, and it becomes clear which rudder type suits the hull and propeller.

Position matters as much as type. The rudder has to sit in the propeller's outflow so that the accelerated water gives it enough force to turn the ship. Small changes to the rudder's type, size or position can noticeably change how the ship answers the helm and how tightly it turns, so designers refine it carefully.

Balanced rudders

In a balanced rudder, part of the blade area lies forward of the rudder stock, the shaft that turns the rudder. When the rudder is put over, water pressure on that forward part pushes against the pressure on the area behind the stock.

The reason is torque. The force of the water acts at the rudder's centre of pressure, which sits some way back from the leading edge. Placing the stock close to that point means the steering gear needs far less torque to turn the rudder than if the stock were at the leading edge, so the steering gear can be smaller and uses less power.

Many balanced rudders are spade rudders. A spade rudder is attached only at the top, to the rudder stock, with no support at the bottom, which leaves the flow around the blade clean but puts heavy bending loads on the stock.

Unbalanced rudders

In an unbalanced rudder, the stock runs along the leading edge of the blade, so all of the area is behind it. Turning it takes much more torque than turning a balanced rudder. The blade is usually hung on pintles along the sternpost, which support it along its height. Unbalanced rudders are uncommon on modern merchant ships.

Semi-balanced rudders

A semi-balanced rudder combines the two. Its upper part sits behind a fixed horn that projects down from the hull, and only the lower part of the blade extends forward of the stock. The unbalanced upper section is supported by the horn, while the balanced lower section reduces the torque needed to turn the rudder. Most large merchant ships use a semi-balanced horn rudder.

Horn rudders come in different horn depths. A deeper horn supports more of the blade, which changes how the rudder responds and how much torque it needs.

Flap rudders

Aircraft wings use flaps to change the effective shape of the wing and produce more lift at low speed. A flap rudder applies the same idea. A hinged flap on its trailing edge deflects further than the main blade when the rudder is put over, which produces more side force for the same rudder angle. The flap is linked to the main blade, so it moves automatically as the rudder turns.

Active rudders (Pleuger rudders)

Large ships often struggle to manoeuvre in confined basins at low speed, when the main propeller gives little useful flow over the rudder. An active rudder, often called a Pleuger rudder after its maker, solves this with a small auxiliary propeller driven by an electric motor in a pod on the rudder blade. Because the pod turns with the rudder, its thrust can be pointed in the direction the ship needs to go, giving steering at very low speed.

The auxiliary propeller is meant for low-speed manoeuvring; at normal sea speed the rudder works like a conventional one.

The Voith Schneider propeller

The Voith Schneider propeller is a propulsion system that steers the ship as well, so a vessel fitted with it does not need a rudder.

It looks nothing like a screw propeller. A row of vertical, blade-shaped foils hangs below a rotor disc set flush with the bottom of the hull. The disc turns in a horizontal plane, carrying the blades around in a circle.

Thrust comes from the blades' angle. As the rotor turns, each blade's angle of attack changes through the cycle, and adjusting that pattern changes both the direction and the strength of the thrust. Because the thrust can point in any direction, the propeller acts as a combined propulsion and control system.