When the rudder is turned to a helm angle, the water flowing past the hull strikes the rudder at an angle of attack. The rudder acts as a hydrofoil: the flow over the two faces of the rudder is different, producing a pressure difference (higher pressure on the face towards the flow, lower pressure on the back face). This pressure difference produces a lift force normal to the plane of the rudder (the rudder normal force), which acts at the centre of effort. The normal force is approximately proportional to the rudder area, the square of the speed, and the sine of the helm angle: Fn = k A v^2 sin(alpha). This force, acting at the rudder, produces the turning moment on the ship and also a heeling moment.

The centre of effort is the point on the rudder where the resultant of the normal (lift) force and the drag force acts. It is the point through which the total hydrodynamic force on the rudder is considered to act, and it is used to determine the turning moment and the torque on the rudder stock. The centre of effort is located at a certain distance from the rudder stock axis, and its position determines the balance of the rudder.

As the helm angle increases, the normal force increases and the centre of effort moves. At small angles the centre of effort is near the leading edge; as the angle increases, the pressure distribution changes and the centre of effort moves towards the trailing edge (and slightly towards the leading edge for a balanced rudder). The movement of the centre of effort affects the torque on the rudder stock and the balance of the rudder.

A balanced rudder is one in which part of the rudder area is forward of the rudder stock axis, so that the centre of effort is close to (or slightly ahead of) the stock axis. This reduces the torque required to turn the rudder, because the hydrodynamic force acts close to the stock, so the steering gear can be smaller and the rudder is easier to operate. The benefits are: reduced steering gear size and power, reduced torque on the stock, and easier, more responsive steering. The balance is such that the rudder is stable (tends to return to the centreline) and does not require excessive force to hold.

The rudder normal force acts at the rudder, which is below the waterline and at the stern. The force has a horizontal component that produces the turning moment, and because the force acts at a point below the centre of gravity (and at the stern), it produces a heeling moment about the longitudinal axis. The heeling moment = Fn x (vertical distance of the rudder centre below the centre of lateral resistance). This heeling moment heels the ship towards the side of the rudder (the ship heels outboard when turning). The angle of heel is given by tan(theta) = heeling moment/(Delta x GM). Sketch: the rudder force acting at the stern below the waterline, producing a heeling moment that heels the ship.