That is 'way too much prop for that motor, you will burn up the ESC and then the motor. The prop size is a bit big for the boat although it may work with the right motor. I suggest an outrunner motor, they have several advantages over an inrunner like the Traxxas:
1) They have more torque and can spin a larger prop, essential for decent airboat performance.
2) They are very efficient at relatively low rpm.
3) They are easier to dry out when they get wet (flip, spray, etc.) so they last longer.
I run a semi-scale catamaran-type airboat of similar size with a Scorpion SII 3026-1190 Kv outrunner on 4S and get speeds in the mid-30s on a 9x6 prop. Your flatty will not go quite as fast. Similar motors are available from the infamous HobbyKing for less $$$ but less quality too. Depends on what the customer wants to pay. Scorpions are near top of the line with great efficiency.
