Re: Yamaha 9.9hp 4 - Stroke Upgrade to 15hp??
I have a 12 ft. fiberglass boat with a 9.9 4 stroke Yamaha (2010). The boat weighs over 250 pounds and is rated for a max 14.5 hp. I weigh 265 pounds and that 9.9 will pick me, my 55 pound 6 year old and the boat up, plane it and do nearly 20 mph, so I disagree with the any planing boat will be suspect, comment. Although I'm not aware of how to increase the hp, I would suggest to you, the following...The 15 has a larger displacement than the 9.9. Overall displacement (along with stroke) generally translates into torque. Although you may extract 15 hp out of your 9.9, you will be unlikely to get the torque a 15 will produce, because there is about a 150 cc difference in displacement. Torque is what you feel on the throttle. The end result will mostly add up to a wasted effort. I stand to be corrected on this, but unless a 9.9 was just a detuned 15 (identical powerhead with a restricted carb), it's probably not worth the effort. What some guys do (and I don't advocate this) is to get a 15 and put a 9.9 sticker kit on it. As long as you respect the spirit of the law by not opening it up on a restricted lake, you wont draw any attention to yourself and you'll have the versatility of the extra power on other lakes. Your other, probably cheaper and lawful option is to buy yourself a lighter boat for your 9.9. Good luck.
