Henry,

Even with multiple mortars the chance of a kill is still slim. And a few gunboats/monitors could likely do rather effective fire suppression on a McAllister mortar group if they posed a real threat.

In 7 hours that day of firing on the Passaic, the mortar scored a single hit. I've worked through some calcs on the deck area and the zones that might allow hull penetration. (This area comes in at only 37 to 42%--no wonder the one hit didn't penetrate fully, it was more likely to do just what it did.) Unless propulsion or steering are damaged, the boat will simply slip away if holed. The likelihood of scoring a second hit that holes the boat is poor.

The hit likelihood for the mortar appears to be no better than 1 in 84 shots. (7 hours continuous firing at 5 min/round.) And this is being generous since the same mortar had done other firing at gunboats, etc. that would have amounted to a further 10 hours or so. The chances of any single hit holing the boat appear to be about 40%.

So with five mortars firing the first hit could happen in about an hour and a half. Case I. There is about a 60% chance that it will be like the hit on the Passaic. If so, the boat goes on fighting and in another hour and a half is likely to suffer another hit if all five mortars can be kept in service at the same rate and accuracy (increasingly unlikely.)

Case II. The first hit holes the boat (perhaps a 40% probability.) It will probably take the commander less than 10 minutes to withdraw sufficiently to be out of effective range. During that time the five mortar crews can fire 10 more rounds. If the boat remained stationary their chance of a strike would be 12%. And the chance of holing it again would drop to an overall probability of about 5%. But the boat won't be stationary, it will be moving. The chance of a mortar hit on a moving boat will drop an order of magnitude. The overall probability of a second holing might be around 1%.

Case IIb. Where things get interesting is in the event of a hit against propulsion or steering. This is perhaps 25% of the vulnerable hull's deck area (10% overall hit probability.) Such a hit would require either the boat drift with the current or assistance from another vessel. This would increase the time under fire several fold and expose other boats. The far slower movement would improve the mortars' chances of scoring again, but still less than a stationary target...and with increasing range. Running aground within effective range of the mortars becomes a real concern. Unfortunately for the artillerists, the overall probability of this is still low.

The point is that, yes, multiple mortars could make it hot, but they would have no easy task. Once they drew the full attention of the monitors and gunboats, they would have serious difficulty keeping the full battery in service, and accuracy would trail off. This would then allow the monitors to take up station to engage the guns.

Equipping McAllister with five 10" seacoast mortars would have been a considerable expansion of a small set of works mounting only a few large guns. Doing this at other forts would also have been possible but none of these are trivial investments of limited CSA resources and labor: works, magazines, bombproof, crew, mortars, mortar beds, platforms, and ammo (about 500 rounds of 10" and up to 5,000 lbs of powder for a day's engagement.)

And although the defenders could not know it at the time, McAllister really didn't need a mortar battery sufficient to keep monitors at bay. It fulfilled its mission because of its earth and sand structure and its ability to stop wooden vessels from passing, not because its arms were effective at confronting monitors.