Henry,

Mortars had some potential versus ironclads, but they needed to be numerous and large and they couldn't do the same job that the equivalent count of heavy rifled artillery could do.

The big problem is that mortars simply were not very accurate at range. The psychological threat they posed greatly exceeded their practical threat. They were excellent at harassing forces otherwise protected by works, keeping men in bomb-proofs day and night, and impeding siege operations.

However, the key to their success-indirect fire control-was in its infancy. Add to that physical aiming of mortars was tricky at best, fiddling with stakes, sight lines, plummet lines, and trying to find the natural line of the gun. The range was determined by the powder charge and very rough adjustments to elevation--not at all precise. Couple this with a mobile target and the chances of a hit become extremely slim. The fire rate was only about 5 rounds/hr continuous. The CSA had a lot of trouble with the mortar beds as well, so commanders might have been looking at only about 50% availability during a sustained action.

The number of mortars and shots required to make a hit would be large. Abbot discussing the Dutch Gap canal operation estimated that the CSA should have employed at least twenty 8" and 10" mortars to stop the work. At Fort Fisher he notes that thirty 13" mortars could have provided effective fire and prevented the monitors from anchoring. This would work for the 1 and 1.5" armor decks then in use. However, the 10" was insufficient--one sand-filled mortar shell struck a monitor's deck at Fort McAllister and just penetrated. (To be effective it needs to punch on through.)

Abbot makes a convincing case that 13" mortars would have been sufficient to keep the monitors from *anchoring* close and blasting away. However, this also requires crews as well trained as his crack siege artillerists, superior indirect fire control, etc. Thirty 7" Brooke rifles might have had a similar or greater effect.

By comparison, the rifles did connect because they were much more inherently accurate and precise, even if they didn't tend to knock the vessels out with single blows. They didn't suffer the additional handicaps of indirect fire. A mortar can't do much to stop passage of a moving vessel, a rifle can, at least making it costly. So while a large number of mortars might handle the stationary case, so would a large number of rifles, but the rifles could also handle the mobile case.

There is another problem: the 13" mortars were cast hollow using the Rodman cooling method. This was something Anderson refused to adopt until it was too late (calling it a "Yankee catchpenny.")

This is also a case of needing both technologies. The CSA needed the heavy rifles for its ironclads; heavy mortars couldn't do that job.

The roles of the two weapons in coast/harbor/river defense were complimentary.