I don't have an answer to your question, but the subject of Virginia coal is treated in some detail in a book I am just finishing, Ironworker to the Confederacy by Charles Dew.
The operators of Tredegar Iron Works in Richmond had quite a problem insuring an adequate supply of coal and raw pig iron for cannon production, so they had to go out and buy coal pits and iron furnaces in an effort to smooth out the supply chain. The author of the book is a professor of history, so there are tons of footnotes and references on these subjects.
(Ooops. Guses I misunderstood the question. You are talking about the coal-carrying capacity of the naval vessel Virginia 1?)