When will rudders be most effective?

• A. When going ahead (flow of water, prop wash)
• B. When backing down
• C. When at stop
• D. When turning

Answer: (A)

Rudder effectiveness comes from deflecting the water that is flowing past the hull. The force on the rudder—and thus the turning moment it provides—grows with the speed of that flow. When the vessel moves ahead, propeller wash and hull motion drive a strong flow over the rudder, giving the largest turning force. Backing down changes the flow direction and weakens the flow over the rudder, making steering far less effective. At a stop there’s essentially no water movement over the rudder, so steerage is minimal. Turning while moving forward is still effective, but not as strong as when there is full forward flow from the propwash; the maximum effectiveness is achieved when going ahead.