I think that they were high pressure engines. The bore and stroke is consistent with many of the bigger riverboat engines. The cited maximum number of revolutions for the propellers is interesting. Most side and stern wheelers had wheels that rotated in the 20-24 rpms maximum range. Propellers turned much faster, but I'll have to do some digging to see if I can come up with a rule of thumb for this. To drive the props at the speed your newspaper article states could require the output from the engines to be geared UP by a factor of 4. The length of stroke quoted indicates that these engines were intended to operate in a limited space. Some of the larger riverboat engines had strokes up to twelve feet. Here is something to compare. The side wheeler New Falls City of Red River fame was rated as displacing 880 tons. She had six boilers 46 inches by 28 foot long. Her engines were 30 inch bore with a 9 foot stroke. I've never seen an official estimate of the Arkansas tonnage, but she had a deep hull and I'm just guessing displaced 800-900 tons. The wetted area of the submerged portion of her hull would have had more drag than a typical riverboat hull, but the two aft propellers and her hull shape should have kept the boundary layer along her hull attached pretty well, so estimates that she could make up to 8 knots or better may be correct. I think the real limit on her propulsion would have been the diameter of her boilers. The tension (and bursting force) on the boilers is as follows: tension = pressure x the radius. One badly understood part of high pressure steam engines were the requirements for longer boilers with a smaller radius for safety reasons. the longer horizontal boilers had a lower thermal efficiency, but were safer in terms of bursting strength. Not all iron was of the same quality and workmanship varied. I think that part of the problem with her engines may not have been the engines themselves, but a mismatch in the gearing between the engines and the propeller drive shafts. Even today, gear design can be an art. If the alignment of the stroke arms to the gearbox was off, vibration would work on the connecting bolts. Vibration back through the shafts from the propellers could also have been a factor. Steam installations usually had large flywheels with heavy mass to deal with such matters, but we don't know just how they chose to drive the shafts. They cut a lot of corners on this vessel. She didn't have firebrick insulation between the funnel and the hull structure, as a result she radiated heat in the living and gun spaces. Summer temperatures in the hold were at least 130 degrees plus. There is one question I'd like cleared up. A typical riverboat would have a series of parallel boilers feeding steam pressure to main steamlines feeding two separate single cylinder engines. Arkansas is stated to have had two engines. But a seagoing vessel of the same era, having two engines could mean a single engine with two cylinders and a single output shaft. Either system can be rigged to drive two separate propellers. With clutches and the right gearbox, the twin cylinder engine could have differentially reversing props - a feature the Arkansas is claimed to possess. I just have a problem believing that they threw in such a complex system in such a short time. I think they stayed with riverboat practice with separate cylinders. It is fairly easy to reverse a riverboat engine, they maneuvered by differential revs on side wheels all the time. Another issue is whether these were new engines - citing the foundry implies this, but that firm might also have had a contract to provide the machinery to link existing engines to the driveshafts and props. Or just refurbish older equipment for that matter. There is a fair chance that she didn't have new boilers either. And I've never seen a citation on how many boilers she had installed. Used and recycled steamboat machinery was a roaring business when the vessels themselves had an average life of 4-6 years. Memphis would have had several firms selling used machinery. The manufacturers of steam engines tended to be up north for new equipment, but almost any decent riverport had the capacity to pull used machinery. Remember, the engine/boiler decks were the most accessible on riverboats. Only military applications required the boilers to be dropped into the hold. You keep hoping that a diary from the Chief Engineer of the vessel (George City) will pop up to clarify some of these issues.