Henry,

I am using Abbot's "Siege Artillery in the Campaigns against Richmond, with notes on the 15 inch gun..." (very long winded title) as my primary reference. The Univ. of Michigan has done a reprint, but it doesn't include the illustrations unfortunately. I'm also checking Roberts "Hand-Book of Artillery...with the Manual of Heavy Artillery..." another overly long name.

Abbot appears a strong proponent of mortars and concluded that a number of improvements should be made in their use. In a footnote on page 46 he states: "General Duane, Lieutenant Colonel of Engineers, informs me that a 10-inch mortar shell filled with sand and fired from Fort McAlister at one of our monitors at a distance of about a mile, just penetrated her deck."

I just found a more detailed reference to the event in "Undaunted: The History of Fort McAllister, Georgia" by William E. Christman. CSA Engineer Capt. McCrady related, "The firing of the 10-inch mortar (Captain Martin) was very accurate, all the shells falling near No. 1 [Passaic], and one filled with sand striking her deck and breaking to pieces." (Notes say ORA-Series I, 14: 220,221.) Christman goes on that "Unknown to the Georgians, Captain Martin's shell caused extensive damage to Passaic's armored deck. Captain Drayton surprisingly noted the damage sustained by his vessel from the mortar shot: 'The mortar shell which fell on deck over the bread room, would undoubtedly have gone through had it not struck on a beam, opening quite a hole through, and had it been loaded with powder instead of sand, might have set the vessel on fire. I have measured a piece of it, and it does not seem to have been larger than X-inch; this certainly does not say much for the strength of the deck, the injury to which has been so much more serious than that of the Montauk, that I must attribute it to a worst class of iron, unless heavier guns have been mounted since the attack made by Commander Worden.'" (Notes say ORN-Series I, 13: 718.)

This is interesting because it suggests that the 10-inch had some marginal capacity to puncture all the way through the vessel--if it struck in the right place or if the armor was defective. However, the USN seems to have taken the lesson to heart as the Kalamazoo and Casco classes were to have 3" decks vs. the Passaic's 1" deck. The original Monitor had 2" deck while the Canonicus class had 1.5"

It's also important to remember that this mortar battery had been in operation for 4 hours each on at least two previous occasions versus the anchored Montauk. As Drayton suggest it had been either unable to strike her, or unable to penetrate her deck. They had another opportunity to fire on the Montauk for an hour or two when she destroyed the Rattlesnake/Nashville. And in the engagement with the Passaic and her sisters they fired for another 8 hours. So in 18 hours of firing, the 10-inch land based mortar scored one potentially dangerous hit against a Passaic class monitor.

Five 10-inch mortars hardly seem sufficient in early '62. The practice from a vessel would have been less effective than a shore battery, and the crews would have lacked experience. Worse yet is the fact that approaching vessels would not have been stationary, they would be moving, making the firing solution at least an order of magnitude more difficult for high arc firing characteristic of mortars. So even in the case of unarmored gunboats, I doubt the mortars would have any chance against an aggressive naval officer.

5:1 with 10-inch mortars vs. even a single 1" deck Passaic class monitor seems like very poor odds to me. The 11 and 15-inch guns had a similarly slow firing rate (3 and 5 minutes respectively), but against mortar boats it would be higher than normal since they wouldn't need to rotate their turrets for loading vs. mortar boats alone. So even 5 to 1 in vessel count, the fire rate would be effectively 2:1. The big smoothbores could easily ricochet shoot into the mortar boats, and switch to grape inside half a mile. Their firing solution would be trivial--unlike firing at barbette guns where elevation had to be spot on. Even if a lucky mortar shot hit, the chances of penetrating the deck and causing chaos below were less than 50/50.

Within half a mile or so the mortar boats probably could not penetrate 1" iron deck because they would not have sufficient elevation (smaller powder charge and lower velocity.) They could switch to 60 degree elevation firing if they had practiced it, but accuracy would be greatly impaired.

A monitor or other vessel could close the 0.5 to 1.5 mile danger space in 20 minutes, even against a current. The mortar boats could get off perhaps 20 aimed shots in this time, and a monitor perhaps ten. (Another problem is that the stability of the mortar firing platform on the river would be impaired by any large shell ricochets, strikes, or bursts, or even the recoil from fellow boats.)

P.S. The "Undaunted..." title is available at Fort McAllister for $5 and I think you might enjoy it. Perhaps you can arrange for them to mail you a copy? Its main flaw is that it is bereft of maps of the fort emplacements and actions.