Henry,

One must take note of the probability in each step, rather than skipping to the end. First, in the drawings I've seen so far it appears that a considerable area fraction of the deck would be backed with beams. This in effect reduces the effective hit profile (my estimate is more than 30%, probably 50% effective considering other features.) http://www.navsource.org/archives/01/Passaic1l.jpg http://www.navsource.org/archives/01/Passaic1n.jpg Second, penetration through the unreinforced zones would not necessarily be fatal to the boat--even torpedo hits below the waterline were not necessarily catastrophic, as the Montauk demonstrated. Third, Drayton states a concern about powder filled shell causing a fire. Yet powder would be substantially lighter than sand, reducing the penetrating capacity. The shell would most likely shatter even if it produced some penetration, making it more of a fire hazard rather than a sinking hazard.

As I was pointing out, for penetration with the relatively small caliber 10" mortar, solid shot (preferably wrought iron) had the greatest potential. Yet this was not likely to be fatal even if it went clear through--the best case. There would still be time to disengage and save the boat.

If the target is penetration of the deck with mortar fire then the preference from most to least would be: wrought iron shot, cast iron shot, sand filled shell, powder filled shell. The 10" was at best marginal for the job.

To be a substantial threat during a ~4 hour engagement it appears that one would need at least five 10" mortars. Even then, the chances of a hit vs. a stationary monitor are in the range of 50%. And the chances of a hit that took a monitor out of action would likely be cut in half again. Furthermore, being taken out of action doesn't require sinking, as the Montauk had proven when holed by a torpedo after sinking the CSS Nashville/Rattlesnake. Taken altogether, the chance of a sunken vessel drops another order of magnitude (e.g. fatal hit = hit * 0.5 * 0.5 * 0.5.)

Were 10" mortars a threat to stationary vessels? Yes. However, it would require a number of 10" mortars and many projectiles to give the threat real teeth. This has to be weighed against other options and likely response. For example, mortars could not hope to have any significant impact on the *passage* of wooden or ironclad vessels, while direct fire artillery could. (And of course the most dangerous weapons for ironclads were mines.)

Another factor is that I'm not aware of the monitors actually firing against the mortar emplacement. Doing so would likely be effective in reducing mortar accuracy with indirect fire. Large shells would damage berms and the necessary pointing aids. It wouldn't be necessary to directly knock the mortars out of action (although displaced earth would likely knock one or two out for the engagement.) Instead, seriously affecting any mortars' aim would be the mission if the mortar fire became problematic.

Mortars were important tools, but they were anything but sure bets. There were no easy answers.