The rudder is used to steer the ship. The turning action is largely dependent on the area of the rudder. The required area of the rudder varies with different type of vessels since desired maneuvering ability differs considerably and the general ship design may imposed restriction.
A rudder allows the ship to turn, simple plates have been superseded by plates welded to cast or fabricated frame.
Rudders are hollow and so provide for some buoyancy . In order to minimise the risk of corrosion internal surfaces are provided with a protective coating and some are even filled with foam.
A drain plug is provided to allow for the drainage of water , enable internal inspection to be made using fiber optic device and even allow for the limited application of a protective coating. Plates are welded to the frames internally in order to provide flush fitting , the final closing plate must be welded externally. A means of lifting is provided taking the form of a tube as close to the center of gravity as possible. Rudders are tested to a pressure head 2.4m above the top of the rudder.
In practice 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. High aspect ratio is used in large vessels, where depth is not a constraint. Higher aspect ratio reduces the astern torque considerably.
Aspect Ratio = (Depth of Rudder / Width of Rudder)
The force on the rudder depend on:
- Area of the rudder
- The form of rudder
- The speed of the ship
- The angle of helm
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.
Rudder effectiveness can be improved by:
- Rudder arrangement in the propeller stream
- Increasing the rudder area,
- Better rudder type (e.g. spade rudder instead of semi-balanced rudder, high lift profiles or flap rudders),
- Steering gear which allows larger rudder angles than the customary 35°,
- Shorter rudder steering time (more powerful hydraulic pumps in steering gear).
Types of Rudder
-
Balanced rudder
The concept of a balanced rudder involved the shifting of the centre of rotation of the rudder to point approximately 1/2 of the way from the forward end to the after end. This meant that when the rudder was turned, the flow of water past the ship actively worked on the forward third to increase the angle of deflection, whereas the same flow acted on the after 1/2 to reduce the angle.
- When 30% to 40% of the area is forward of the turning axis there is no torque on the rudder stock at certain angles.
- At some angle of rudder, it is balanced. i.e., torque is zero, to keep rudder at that angle.
- Axis of rotation lies between 0.2 L and 0.37 L.
- Semi-balanced rudder
- A rudder with a small part of its area, less than 20%, forward of the turning axis.
- At no angle rudder is balanced.
- Axis of rotation lies less than 0.2 L.
- Unbalanced rudder
- A rudder with all of its area aft of the turning axis.
- At no angle rudder is balanced.
- Axis of rotation is the leading edge.
