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If you want faster you need more HP period you can not equate speed with engine performance. Get the engine running proper WOT rpm and the speed is what it is. You can definitely try a prop with less pitch bur doubt you will gain much speed but you will increase the service life of the engine by not lugging it down. You can put a go cart motor on a drag racer but dont expect to win any races. A nice healthy 15hp will get your boat on a nice plane that 6hp is nothing more than a kicker.