More thoughts on batwings. Thealka was the last one to work the Big Sandy River, had a boiler explosion in 1920 (?). The spiders on the wheels appear to be cast iron, probably from an Ironton, Ohio foundry, and about eight feet across. I'd guess Thealka didn't have a framework under the deck, probably just 2-3" thick boards (oak?) laid cross-wise over the hull.
In the early boats, the sides of the hulls were single boards, looks about 2' by 12-16" and sixty to eighty feet long (big trees back then, and man-powered saws). Later boats used individual boards, since the big trees were gone and the sawmills couldn't process big boards. Most were oversized johnboats, without the deck of Thealka, and had barges for cargo tied to the side and forward of the wheels. The rudder was the barndoor type, similar to the kind used on the Murray river in Australia. I've only found one picture from the side, none from the stern, but there appears to be an open space behind the deck house.
I have not been able to determine what kind of boiler they used--I'd guess a vertical one, and the later ones used gasoline engines