Gary, I've considered the question you pose. Understand that what would be coming out of the muzzle of that Blakely probably wouldn't have been a shell, but a bolt. This would have been a forged chunk of metal dependent on kinetic forces alone to do it's work.

To be specific regarding the capabilty to of ordnance to breach the built-up rolled plate of a Passaic Class Monitor we have to consider where the hit would have taken place. I doubt that the turret would have been breached by a round, though the inner laminations of the turret would have spalled. This would have sent splinters through the turret to the detriment of any crew in it. The Passaic had eight to twelve inches of armor protecting the guns. The rotating mechanism of the turret, along with steering, would probably be severley affected. The most destructive hits, from the standpoint of sinking a Passaic Class vessel would have been a low-incidence (glancing) shot to her rear deck just aft of the turret. The combination of bulkhead and thin deck in that area, along with the machinery contained in it would have made a hit there as I described particularly effective, the hull would have cracked width-wise. A below the waterline hit with the round you mention would probably breech the armor belt along the waterline and would certainly cut through the thin hull.

The Laird Rams, the two ocean-going turreted cruisers constructed in England for the Confederacy, were to have been armed with Armstrong breech-loaders capable of launching a four hundred pound bolt. The threat the two vessels posed to the Federals were the cause of blockade of England being threatened to Lords Russell and Palmerston by foreign minister Francis Adams. Rather than the Blakely it was the Armstrong rifle that caused fear. The Laird Rams were never finished for the Confederacy, payment for the vessels was offered by the British government and accepted.

The most dangerous weapon available to the Confederacy in combatting the Monitor Menace was plunging fire in the form of Seacoast Mortars. I doubt any Monitor was hit on its deck by a large caliber Mortar. It would have been on the bottom in short order.