Further to my message posting, I wish to add that the volume titled THE CONFEDERACY: A GUIDE TO THE ARCHIVES OF THE GOVERNMENT OF THE CONFEDERATE STATES OF AMERICA, by Henry Putney Beers, originally published 1968, republished 1986, pages 350-351 indicates that original plans of the CSS VIRGINIA (MERRIMAC) are at the Confederate Museum, and other plans are at the Virginia Historical Society and the Mariners' Museum. The Mariners' Museum also has the plans of John L. Porter of an iron warship whose construction was begun in England, but never completed. The Smithsonian Institution's Division of Transportation has a large collection of photocopies of plans and other items relating to Confederate Naval vessels.
Some plans are also in the ORN.