I have a 6hp Yamaha on my 18 Tiderunner and it pushes me along just fine. If you can afford a 9.9 then go for it. If you can only afford to get a 6 for now, then I certainly would'nt stress about it because it will get the job done. My cousin runs a 6hp Johnson on his 17 Arima and I swear that it has a 1000 hrs on it without any problems. My 2 cents anyway. Jack