Start with that 8.5 HP and learn how to pick your way through big waves that come along. You have to watch each wave – can’t just relax and hit them any old way. Worst is big rolling waves going same direction you are going. If you aren’t careful, you will go over the first wave and poke the nose under the 2nd wave and then you’re swamped. If you see the motor is going to get dunked, shut it off ASAP to avoid possible major damage from going down running. When you get good at it’s handling tricks, I’d go as big as 15 HP. Best to only run it where there is a boat standing by to tow you in if you swamp it. On hydroplanes like that, I double the amount of oil in the gas because your motor is running at higher RPMs than it was designed for. You may also have to go one heat range colder on the spark plugs to keep the firing tips from self-destructing. Colder plugs will not make it go slower. The added oil is a common thing to do. Champion Outboard made a racing version of their 16 HP motor and recommended gas/ oil mix at a whopping 4 to 1. I built my first hydroplane when I was 14, and promptly got good at getting dunked motors running again.
Have fun!
Dave
