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Although the propulsion of a vessel depends largely on the right choice of the propeller, the main thing that comes to mind when we talk about steering is the rudder, since it's extremely difficult to stay on course or maneuver the ship without this device. Thus, based on our experience, we could list at least two main functions of the rudder:
- To generate the necessary maneuver management to start a vessel's drop from one band to another.
- To overcome the resistance from the water pressure on the hull; this naturally tends to prevent the vessel's maneuverability.
The forces that act upon the rudder should be controlled, in order to steer the vessel;
 generally, two different action forces take part: the frictional force in direction of the blade
(which we consider insignificant for the rudder calculation), and the force of the normal pressure that is perpendicular to the blade, of which the point of application is called the
blade's center of pressure.
Under these considerations, we will obtain the desired effect of the rudder on the vessel's maneuverability, since it is understood that these forces affect the vessel's course.
The rudder, without a doubt, is one of the vital elements of the ship. Classification Societies usually allow a special chapter for design and construction. In the drawing-board phase
they develop theories about loads on the rudder structure classified as follows:
- Transverse force: The one applied to the rudder by water pressure.
- Flexor moment: force applied to the rudder itself when turning the ship.
- Dynamic forces: the stress delivered to the rudder by wave movement.
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Operating forces on the structure of a simple rudder
The rudders are classified according to their structure, their type of assembly and their distribution of shovel with respect to their spin axis, these types occur generally:
- Without compensating.
- Compensated.
- Simple rudder.
- Compensated in the superior support of the helmet.
- Semi compensated in two supports.
- Compensated constant.
- Semi compensated, two supports.
- Semi compensated, one support.
In order to develop our rudder calculation, we need to know the pressure it is under and its torsional moment; one way to do this is by using Joëssel formula:
Equivalents:
- PT = Pressure force (kg)
- K = Constant for form of kinetic friction
- A = Rudder surface area (m2)
- V = Speed (knots)
- Θ = Rudder steering angle in relation to the sea current speed (35° is customary since statistically, vessels' turns are less efficient while turning
at a wider rudder angle).
Once we know the pressure that has to be controlled with our rudder, we could experiment with the length of the rudder's blade for our design, by using the following formula:
Equivalents:
- dT = Distance from the center of pressure to the front edge of the rudder blade.
- lT = Blade rudder length.
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