Aspect Ratio
> The rudder area is usually relative to the area of the immersed metal plane.
> The ratio of the depth to width of a rudder is known as the aspect ratio and its value is generally 2.
> Higher aspect ratio reduces the stern torque considerably so, in large vessels a high aspect ratio is used
Aspect Ratio = (Depth of Rudder / Width of Rudder)
The force acting on rudder depends on 4 factors;:
> The form of rudder
> Area of the rudder
> The angle of helm
> The speed of the ship
Force acting on the rudder, F = k A V²
where,
k = Constant depending upon the shape of the rudder, water density, rudder angle. It varies from 570 to 610.
A = Area of rudder in m²
V = Speed of ship in m/s
Area of rudder is 1/60 to 1/70 th of the immersed middle plane area of the vessel. Faster vessels require less rudder area.