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Why rudder movement confined at 35 degree ?

The normal force acting on rudder area is resolved to lift force and drag force. The lift force is responsible for steering the vessel, where the drag force tends to retard the ship as its direction is opposite to that of
ship's motion .But beyond 40 degree of rudder angle, the drag force increases rapidly and lift force drops rapidly. So beyond 40 degree of rudder angle, this result in the stalling of the ship and the ship loses speed and does not respond to steering.Therefore, rudder angle is normally restricted to within 35 degree to port and starboard to avoid stalling effect of rudder and to maintain high lift to drag force ratio.
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