I want to do an inboard set up and I'm fine with using a jetskis drive train. I likely won't reuse the engine though because I'm not a fan of 2stroke jetskis motors I think 20 foot will be ideal mabe even 22, ?
So for the motor what will you use?
A jetski motor/jet assembly runs extremely high rpm as compared to say an automotive engine. An issue with running rivers where there is a chance of ingesting a small rock is that the high rpm and a small hard object can result in a catastrophic instantaneous destruction of the impellor. I doubt that a single jetski pump motor combination will give you satisfactory performance. If you are considering a 20 - 22 foot boat, you should research the jet boat forums
to find anyone who has built a river boat in that size with a jetski powertrain set up.
Our normal build was a 21 foot, 1/4 inch bottom as boulder to hull interface interactions were anticipated, plywood floor covered with duradeck, a 350 cu. in. (300 plus HP )engine with a 212 Hamilton ahead of it, 75 gallons of fuel capacity, windshield and that power was adequate.
With a 20- 22 foot boat, you have room for the engine and room to walk around.
If your build is on a light weight Jon boat style hull, ie Tracker, Smokercraft etc, perhaps the jet will work, but a lot of work will be required to reinforce the bottom. Particularly in front of the intake as you need to provide
stability, to ensure that when you try to crush a rock that the hull can withstand the bump before it hits the actual intake. If you think that you can build a 21 foot boat and "beat the weight" parameter, you should do an analysis of what you will carry.
I tamped down a boulder on a remote river in the Pacific northwest, 120 miles from assistance, (not one of my better days, but in my defense - excuse it was obscured by silt laden water). We had to hire a K-Max, twin rotored helicopter, to get us off the river down to where we had another boat in the ocean that would tow us back to base. 3 guys, camping gear, additional fuel, stand up camper canvas, food, back up remote winch etc, came in around 4- 4,500 lbs. The helicopter, used for logging, had a scale on the tether.
A jet ski power train designed for maybe 1000- 1200 pounds would not have been adequate, though our trip was definitely overweight due to the purpose of the trip.
Instead, you can look at a two stage style pump if they are still available, Kem equipment built (not sure now) a 2 stage copy of a Hamilton, 772 and a V-6 marinized engine, ( they used to sell it as a package) would be a good fit for a light weight
combination and the V-6 saves , maybe 6 inches of longitudinal inside cockpit space.
Suggestion, look on the internet for jetski powered aluminum boats and focus on the weight of the hull, chine width and deadrise as these will be the prime design parameters with a low horsepower motor.
