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2 person jon boat: power to move the boat

I plan to build a light, strong, two man, 16' x 3' jon boat, to weigh out at approximately 80 to 100 lbs at most.

I will need to build reinforced rear deck area to support the motor, prop and cage.

For me speed is not a big issue as I'm going to be in marshes and small lakes/rivers.

What would be the smallest motor and prop combo that would move this boat at a 3-5 mph over lilly pads infested fresh water.

I'm a complete novice when it comes to thing air boat related so practical information is what I seek.

Manitoba, Canada

From a practical standpoint, 16x3 would be a canoe, not an airboat hull.
Any engine that would be operationally satisfactory would produce enough torque roll to turn it over.

Why so narrow? There must be another consideration that you've not shared.
As described, almost sounds more like submarine plans rather than that of a boat.
If it's only lily pads and veg, get a longtail mud motor kit and a 13 hp predator engine, put it on a jon boat. A lot quieter than a fan. If you build an airboat do it right and get a 7'+ wide hull and at least enough power to run dry ground. THAT is what an airboat is for!
For what you want it to do, a 13 hp Predator or similar would work fine. Mount a 28" to 36" propeller directly to crankshaft, I recommend 18 degree pitch three blade. Build a stand at height you need that mounts to floor and braced to transom. I would make it bolt in so it can be installed and removed as a unit. For steering mount the engine on a trailer spindle and hub. Another way to mount engine and prop is on transom like a typical outboard. The engine and prop will weigh less than a 15 hp two stroke outboard. You can weld a tiller handle to engine base plate or use a stick steer and cable setup. You don't need rudders to steer when engine and prop swivels. You have to have a throttle control with spring return. Keep your build as low as possible and seat height needs to be about the way the hull came. The higher the weight is the worse stability will be. I have a 1436 with this setup. At an idle it moves the boat just under one mph, full throttle on claim day on smooth water with just me in boat it will do 23 mph. I suggest adding flotation pods on transom, it went from hilariously squirrly to being out right fun after I added them. This boat is not for showing out and making a lot of noise, climbing hills, or running dry ground. It is great for getting under low branches and goes over partially submerged logs with ease. It gets me into the places others wish they could fish.