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The point with any smoothbore is, the shot looses velocity very quickly. Hence, any capability of armor penetration is at short range. The RN tried the 15" Dahlgren against the Warrior target, and it failed to penetrate at ranges over 500 yds.

Just to give you some idea: I don't know the MV of a 11" Dahlgren, but if it was 1300 f/s (not unrealistic) it might be able to penetrate about 6" of wrought iron at point blanc. At 1000 yds, velocity of the shot would be down to about 960 f/s, and penetration down to less than 4" of iron. Performance against armor drops off that quickly with round shot.

Also, because the Dahlgren was relatively low MV, the British 68 pdr would be superior in penetration at ranges up to about 500 yds, because its higher MV (1580 f/s). With their superior wrought iron armor, British ironclads would have superior protection and superior penetration at ranges up to 500 yds. Beyond 500 yds, penetration would be unlikely anyways, so the RN would have the advantage at any realistic fighting range.

Next, the Dahlgrens would fire once per 5 minutes max, so a one turret monitor would fire 24 shots per hour at most, a twin turret monitor 48. Warrior would fire 13x30 68 pdr rounds (with a one round per 2 minutes rof) in the same time, that is some 400 rounds per hour with superior penetration power in the same time frame. Or, one of the Crimean war batteries would fire 7 or 8x30 68 pdr shot per hour, a 5 time advantage in rof over a 2 turret monitor.

I am sorry, US monitors suck and would be crushed by any RN ironclad.

Cheers,
Wijnand

"Why not compare her to a more equal ship?"

That's why I referred to the Crimean war batteries, built in 1855, of about 1800 tons and superior to most if not all USN ironclads and monitors, with the probable exception of New Ironsides.
BTW, the US rifles also had relatively moderate velocity and were marginal armor piercers.
BTW II: most Yanks don't object to the greater displacement and cost of USN ships of later ages compared to their foreign contemporaries, for example the fact that the Washington was twice as expensive than KGV. In that case the argument runs: well, the US could afford them. In 1860, the RN could afford Warriors and they were vastly superior to anything else.

Cheers,
Wijnand