Good shot Nebraska,
It's just physics. Plain and simple airfoils are way more effective than flat rudders. I had street sign flat rudders on my first airboat and they would flex to the point I couldnt break myself lose in the mud etc. The airfoils are much more rigid and will turn the boat when flatties wont. It's the difference between manual steering and power steering.
Properly ballanced airfoils are smoothe and easy to turn and make the boat feel smoothe to operate.
Scotty