Side-force coefficient

The side-force coefficient is the non-dimensional lateral hydrodynamic force on a rudder, keel, fin, or hull, defined by F_y = 0.5 rho V^2 A C_y with A the appendage planform area, the lateral analogue of the lift coefficient for a vertically oriented foil. On a rudder it grows nearly linearly with helm angle until stall near 35 degrees for a typical aspect ratio, where flow separates and the side force falls off. It is the quantity that turns the ship: side force times its lever about the center of gravity is the turning moment. Effective aspect ratio governs the slope, so a rudder spanning the full skeg develops more side force per degree than a short, isolated foil.