OK. we're making progress. Just for discussion's sake, lets assume we're talking about an adaptation of a riverboat type engine. Riverboat engines were geared to turn wheels about 20 times per minute. Propeller drives usually ran about 40 times per minute. So the output of the high-pressure engine would be geared UP to 40 rpm max. From the descriptions of what happed when one of the Arkansas' engines stopped at top dead center, I think it is safe to assume we are talking about two completely separate engines, without cross-shafting, like a side-wheel riverboat. Prior to the war, an 800-1000 ton riverboat was a biggie. Because of her draft, Arkansas may have displaced somewhere in this range. From Ways, #4166, pg 344 we have some data on the engines of the New Falls City. She was a side-wheeler, built in 1858 and displaced 880 tons. Her engines had cylinders with 30 inch diameter and a 9 foot stroke driven by six boilers, each 46" x 28 feet. I've never seen any listing of her rated speed, but she ran Red River which had some pretty stiff currents, seasonally. Unusually for a river craft, Arkansas, is credited with a draft more typical for a sea-going vessel (she was meant to be one). The question is how large were the propellers? Because of the description of her hull form at the stern, it is likely that she could turn 8 or 9 footers. These would be fairly efficient. Mind you, we're getting power to the drive shaft through a gearing system of unknown efficiency. I think I can justify this assumption because the rebuilt ironclad at Columbus, GA could turn propellers 6 1/2 to 7 feet in diameter. Arkansas was supposed to be capable of about 8 mph. This is a tricky number, because riverboats were frequently described by their speed against a current. Obviously they were much faster downstream! To the best of my knowledge, Arkansas' career allowed her to work downstream during her battles. The exception would be when her moorings were changed frequently at Vicksburg in an attempt to dodge Union fire. Since the vessels she fought were also going downstream or moored, about the only way to get a sense of her true speed would be in comparison to the vessels she chased down the Yazoo. She had no problem overtaking the Carondelet, but the Tyler and Queen of the West outran her. Perhaps we have access to information on their rated speeds?