To finalize my discussion with myself about fans to augment your own acceleration, I found one designed for a maximum of 300 watts, which is in the range that a FRC robot can legally put out. It puts out 4 newtons while spinning at some mind-boggling RPMs. It would sound really cool spooling up.
So you'd have a drive motor running continuously at near-maximum output for 4 newtons of forward force. Maybe I'm not looking in the right places (after all, R/C fans are designed for applications where they are moving through air quickly), but that seems like it'd be too little force for a hovercraft to be effective.
You could build very light if you wanted maximum acceleration, but you're still hauling a big trailer behind you and you'd lose pushing matches because your absolute pushing power would be lower (you'd lose your wheel traction in proportion to your weight loss).
A maximum-weight robot would get a 15% acceleration boost by using this little fan, assuming it would be a legal thing to do, you could rig a motor and gearbox to the fan, and the constant 300 watt drain wouldn't greatly impair other robot actions.
Here is the fan:
http://www.modelflight.com.au/rc_mod..._micro_fan.htm