There are a couple of preliminary things to check; are the radios or servos from different manufacturers? Sometimes the wires are different colors, and you put it in backwards without realizing it. If you are using a 3 channel receiver, make sure the servo is in the right spot. You know that the throttle servo works, change the rudder servo to that slot, and see if it works with the stick or trigger. If the rudder servo is digital, make sure that it is getting enough juice from the battery. If the rudder servo is still not working, the motor may be toast. Depending on the type and brand will determine whether it is worth rebuilding or replacing. You may want to take the casing off of the receiver that came with the boat and check to see if there is corrosion on the board. Also check the pins to make sure that they are all in good shape. I never take a chance with a reciever, if it doesnt work correctly, or glitches, I get rid of it. Many people coat their receivers to help fight off water and corrosion in case they get wet, I believe it is called Aeroplate. A tight radio box, good tape, and seals, along with a balloon are your best first line of defense. With a boat that size and power, a failsafe (if you don't already have one) is a must. It will prevent you looking all over for your boat if it gets away, and better yet it, will protect people, wildlife, and property around you. Also, if you are not running with an FM radio, be very carfeful, an AM radio is more prone to interference due to engine spark, even with a resistor spark plug.