Me and just the boat it will hit 32 mph on glass days. 2 guys and gear takes it down to about 20 to 22 mph.Click to see full answer. In this regard, how fast does a 9hp boat motor go?Assuming this is a light 14′ utility boat and a standard 9.9, Mercury would recommend a 8.5 to 9.5 pitch prop for the OP’s application and predicts a top speed around 20 mph.Also Know, how fast will a 4hp boat motor go? 7 miles per hour Similarly one may ask, how fast will a 6hp boat motor go? 400lb with a 15hp will run 24 mph.. for a 6 hp to reach 35 mph total weight is 135 lbs(boat motor fuel passengers).How fast will a 10 hp motor go?It completely depends on what sort of vehicle it is powering, and the grade you are climbing. To give you an upper limit, the current IHPVA record is 89 mph, 144 km/h. This is with an average power of about 1/2 horsepower (350–400 watts average).