RE: 1/3 scale Hydroplane
How long is the arm off of your rudder? in other words how many inches from the center pivot of the rudder to where the servo connects to it? i have a hunch its kind of on the small side and might be giving the steering servo (whatever your using) and harder time than it needs,
in a case like this you realy want to max out the servos throw side to side with the desireable rudder throw side to side, it well create a much more effective setup.
in other words you dont have to be using half of the servos availible travel to move the rudder fully, full servo travel to full rudder.
a few of the people above me have good ideas for servos to use, or along the lines of beefy robot servos. servocity.com has great large scale options.
dont forget a failsafe in that beast eather, CAN be dangerous!
Very very very cool, thanks for posting it.