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Charlie, as long as the 8.5 (and tank) don’t bring the waterline up so high that the lake trades places with the air inside your boat, that should be plenty.
As a 15 year old, I built a similar sized MiniMost-based boat that was much lighter yet stronger and started off with a Merc KH7 (about 1/2 the weight of your motor and tank), which at full throttle had only the fin and the anti-cavitation plate in the water. It was scary as heck to manouever. I’d planned to find a Mark 20H and rat it up, but a bad experience convinced me to to stop after throwing a Mark 25 on it and going out on a windy day and catching air under the bow. Seeing the boat fly upside down over my head cured me of overpowering that small craft.Although the skeleton of the boat was strong enough to prevent the transom from detaching, I underestimated how bad the weight distribution was.
Moral of the story is, put safety first and talk to people with experience. As you learn how your boat handles at 15 mph, you could consider upgrading the motor.
I have run my mini max with everything from 3 hp to 15. Speeds ranged from 10 mph to around 33 mph. All fun. Yes, a very wet ride, don’t wear clothes you don’t want to get wet. I prefer a motor with the tank attached, not much room for the remote tank as you see in the pic with my Evinrude 15, tank is between my elbows.
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!
DaveWow, a Lark on a Minimax. I thought I was crazy!! After the Minimax, I built an 8′ tunnel hull of my own design, that was five feet WIDE and had more free board than the Minimax, with non-trip chines. It may have made a navel architect cringe but it solved all the lessons of the Minimax and was actually a much better, safer boat. I put a manual start, Super Quiet Johnson 40 on that, after a lot of experimentation with a 7.5 KE4 and an 18. I Only used the 40 a few times because it was both insane and not as fast as I would have expected, because of prop limitations. Still, I SMOKED a Mark 58 Merc on a hydro with my Johnson, because the race was from a standing start, where the big old, 55 gallon drum pistons of the Johnson excelled. If a boat could have four wheel drive, that rig had it! But I digress…..I thought I was crazy! Knowing what I know, why the guy with a Lark on a Minimax didn’t die, is a miracle!
Long live American manufacturing!
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