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Why Rudder Angle Limited Upto 35 degree only?

The answer is Rudder Stall.
  • Flow around the rudder is smooth at small angles of attack.
  • Lift increases-proportionally to the angle of attack (i.e rudder angle). Flow separates across the rudder at larger angles (>20'~30') causes the lift force to drop drastically
  • This drop in lift is accompanied by a drastic increase in drag
  • This is why practical limits(hard stops) are set on rudders ~35'.

Angle of attack: It is the angle made by the direction of the incoming fluid and the plane of the rudder. 

Critical angle of attack: Critical angle of attack is the angle of attack at which the fluid flow across the rudder separates. i.e. at critical angle, the fluid flow is such that the formation of eddies behind the face of rudder increase to such an extent that instead of guiding the fluid flow, the rudder tends to stop the fluid flow. 

Depending upon the drag, viscosity, blade shape and flow characteristic of the fluid, the critical angle varies for different rudder and fluid combination.
For marine rudder and water, the critical angle happens to be around 35 °. From the going, it is well understood that as the rudder reaches its critical angle of 35°, then instead of changing direction of the vessel, it will slow down the vessel, which is undesirable.

Make a note that critical angle is synonymous to stall angle. Thus if someone says that rudder reaches stall angle, it means that rudder reaches its critical angle.


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