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STEERING THE VESSEL

 

David Rivas

Although the propulsion of a vessel depends largely on the right choice of propeller, the main thing that comes to mind when we talk about steering is the rudder, since itīs 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:

 

  1. To generate the necessary maneuver management to start a vesselīs drop from one band to another.

  2. To overcome the resistance from the water pressure on the hull; this naturally tends to prevent the vesselīs 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 bladeīs center of pressure.

 

Under these considerations, we will obtain the desired effect of the rudder on the vesselīs maneuverability, since it is understood that these forces affect the vesselīs course.

 

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 Joësselīs formula :

Equivalents:

PT = Pressure force (Kg)

K = Constant for form of kinetic friction

A = Rudder surface area (m2)

S = Speed (knots)

Θ = Rudder steering angle in relation to the sea current speed (35° is customary since statistically, vesselsī 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 rudderīs 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

IT  = Blade rudder length


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