Actually, the motor wasn't running at it's topspeed. Reason was, the ESC (speedcontroller) wasn't calibrated in the right way. It was very difficult to calibrate the ESC again, using the PS3 controller. (the ESC seemed not to react).
Therefore, our idea was to controll the hovercraft with a radio transmitter. We removed the Arduino part and calibrated the ESC again this way en tested the hovercraft. This is what it looked like:
Hope you enjoy it!
PS: should I make an step by step instructable?
EDIT: I've made one, find it here: http://www.instructables.com/id/How-to-make-a-RC-Hovercraft/