A rudder behind a ship acts as an airfoil or a wing; it produces lift and drag in a proper low. A cross section of a rudder (or a lifting surface) is such that at a rudder angle (or angle of attack of the low) a relative large force perpendicular to the low direction comes into existence.
Figure 4.3 shows the components of the force produced by the rudder. The rudder pro…le has been placed with an angle of attack ® in a homogeneous low with a constant velocity V . Practically, V is de…ned as the velocity of the fluid far before the rudder. For small angles of attack, ®, the total force P on the prole acts at about e t 0:25 ¢ c, provided that the span width s is large with respect to the chord length c. This force P can be decomposed in a lift force L perpendicular to the low and a drag force D in the direction of the low. The total force P can be decomposed in a normal force N and a tangential force T, too.
Rudder forces are made dimensionless by the stagnation pressure and the projected rudder area Ar: