18hp will be plenty. Seeing as how few human beings can manage even 1hp, think of a 450lb man generating 20hp. How strong would he be? How fast could he run?
Look at all the new V8 engines that drop cylinders at highway speeds. The new Hemi in the Dodge trucks is one I know of. It makes about 350HP peak at about 5000RPM's so at highway speed, about 1700RPM's it might make 125HP but more likely below 100HP, then it takes half the cylinders away so maybe 50-60HP tops moving a 6000lb truck at 55-60MPH. I think 18HP shall be plenty to move a combined weight of say 700lbs for the vehicle and two passengers.
This reminds me of high school when my friends dad bought a computer program that showed him what different setups would do to effect the e.t. on his mustang drag car. After entering his information the only suggestion the program had for him to lower his times was a lighter driver! (dude weighed about 350!)
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It's "Dr. _paralyzed_" actually. Not like with a PhD, but Doctor like in Dr. Pepper.