hello guys, am building a small rc boat. I don't know what capacity servo motor should I use to turn the rudder.
Please help, my boat size is 20".

Thank you

9grm servos would do, also it depends how big the rudder is.

Minimum 5 Kg Servo (torque) Use a standard Servo

Load on rudder in water is much, much higher than in air

No arguments ... listen to experts. ( Sanjai Ji)