George- Just returned the 2007 edition of "Send a Gunboat" taken out through ILL. UCF has this text on their shelves.

On pages 30,31 there is mention of conversion of dockyard lighters into mortar vessels, 22 of which were completed for use in the Baltic with 17 commited to action. No doubt it is no coincidence that the number of vessels matches that of the schooner fleet assembled for reduction of the forts on the lower Mississippi under D.D. Porter, as well as the caliber of the shells thrown. The loss of the records regarding naval ordnance, lost to thievery, in the National Archives might have mentioned an assessment of the Crimean naval campaign in the planning of operations by the West Gulf Squadron. Thank You for the reference to this worthwhile reference, though I believe the writeup on the operations could have been more complete as far as sailing times to station and what conditions at the bases were at the time. Interesting to note that the British Mortar vessels had their ordnance mounted on a rotating table.