Thanks for sharing your thoughts. Spalling alone could have been more devasting as it splinters would rip apart the gun crews. I suppose it's almost like the modern hollow charge anti-tank round spewing hot gases inside a tank. It would hurt.

Indirect fire against a ship is pretty difficult. Fort Jackson & Fort St. Philip (Mississippi River below New Orleans) were supposed to have been bombarded by David Porter's mortar boats. Porter thought they were deadly accurate and successful. Farragut's men in the crow's nest said they missed - all the time. Considering how Porter's men couldn't hit a stationary target, how well could the Corn-feds have fared against a moving monitor? However, if the Corn-feds were fortunate to have hit a monitor, it probably would have been very devasting. Deck armor was relatively light (and greasing it down really doesn't help deflect an ball that drops atop of you) and even the monitor turrets weren't armored on the top. As you've mentioned, a hit probably would have plunged through and it would be devasting.