I've been doing a little research on the engagement at St. Charles and its aftermath. There is at least one contemporary news report that states Mound City was aground after the boiler hit and that several Confederate soldiers could actually touch her hull while standing on the bank. I've never seen anything to suggest that either the Confederates or Union forces considered her sunk. Confederate reports frequently refer to her as "blown up" for obvious reasons. One point that is not often made about this particular class of vessel was their inherent vulnerability. They were only ironed forward and about half way along the casement. The armor was pretty light and their displacement relatively small. As both sides began going to heavy rifled guns as the war progressed, this type of vessel became more and more vulnerable. Take a look at what happened to Carondelet (a sister ship) when she engaged the Arkansas in the Yazoo River. Arkansas was dubbed "a box of guns" by her Captain and she had little problem at close ranges in heavily damaging the Union ironclad. They just weren't the same class of vessel, even allowing for the makeshift rail iron on the Arkansas' casement. Both Union and Confederate non-Monitor ironclads were vulnerable to plunging fire from batteries that could "shoot down" onto the decks and casements. This sounds like what happed at St. Charles. Mound City and her sisters were useful against concentrations of CSA field artillery along the rivers. If you take a look at her class of river ironclads, they took a beating on many occasions. The key to their utility was that the Union Navy had the resources to salvage and/or repair them promptly. In some cases of other classes like the Indianola it didn't seem worth the trouble.