I couldn't begin to add-up all the hours spent in thinking, drawing, research, and even designing and building my own steam engine, all because of wanting to build a sternwheel boat, (I'm not past that, I'm just too old to do it now. lol).
Do a search for "Atkin boat plans", he designed 2>3 sternwheel boats in the size range you're considering.
Go on YouTube, lots of sternwheel boats.
Split wheels? Up here in the PNW we've a rich history of sternwheel boats and from all the books I've read and research I've learned that split wheels were quite rare, two main reasons, but with IC engines instead of steam one of those reasons is moot.
The reason left is cost.
You'll find that there is no need whatsoever to try and add mechanical complexity, you'll have more than enough of that
without even trying.
Unless you absolutely require the Nth degree of turning area reduction split wheels on a sternwheeler don't add much.
If the rudder arrangement is proper, (the Atkin boats were not,) you'll have little trouble being able to "back-and-fill" in a tight area.
When I said "proper", you need rudders both ahead of and behind the wheel, (usually three, with split wheels you need at least four,) and you should have a way to operate them independently, (inner and outer steering wheels,) and have simple cable/rope operated rudder position indicators.
Propulsion? Hydraulics are a nightmare and noisy beyond belief, everybody will wear earplugs. Listening to a couple of little gas engines screaming away running hydraulic pumps will ruin any peaceful enjoyment.
Use mechanical drive, don't use worm gears, with an IC engine you need LOTS of reduction. That brings me to a point I almost forgot: Welding, learn how to do it, you're going to have to fab and weld all kinds of things.
Anyway, you want a fairly good-sized engine that will be able to produce some torque at an easy speed, partly to reduce the need for heavy/complicated/power robbing reduction stages. A relatively easy way to get part of your reduction is using a vehicle differential with welded spider gears, (in the old days we called that a Lincoln Locker,) but weight and its distribution is a constant reality in a small sternwheel boat, you need to use a pick-up truck unit so you can get 4:11, 4:56, 4:89 gearing.
And then you cut off the axle housings and all the brake stuff and use pillow blocks outboard to support the axles, now you've got a reduction unit with a U-joint input that you can connect up a transmission/engine to that and have industrial V-belts/pulleys, (or chain/sprockets,) at each end of the axles to run out to the wheel shaft.
If you can stand the weight, just get rid of the brake stuff, you'd also save on pillow blocks, mounts, and having to fab inner oil seals at the differntial.
Multiple chain reductions are fairly lightweight but require bearing blocks, shafts, supports, and GUARDS so no one gets an arm cut off, also chain is expensive and a maintenance hog.
Start looking around at old industrial drives, even old rusty farm equipment, stuff that was used to run conveyor systems and mow fields, you got to get inventive if you don't have a fortune to blow.
The wheel? There is only one way to get propulsive efficiency out of a sternwheel, DIAMETER, as much as you dare.
You need it big enough so that you can have lots of floats without them being too close to one another.
Only in a fairly small section of their travel are they doing much work to move the boat, much of their effort is either pushing water down at some angle or lifting water up, neither of which add productive thrust., a larger diameter helps to alleviate some of that.
Me? I would forget about split wheels unless I was going to use a sound-enclosed marine generator set and electric drive.
But 15>20 hp motors are not cheap, neither are speed controllers. And keeping water out and away from high voltages and currents is not cheap or easy at all.
Well, for now I'm tired of typing, but look at the Atkin boats, look at some YouTube, you'll get some ideas to run thru your mind.
