In a recent "discussion" on an internet forum I was "informed" that the USN had dozens of Monitors and they'd sweep the seas of the British. My response was to post figures showing that at no point did the USN actually have more armoured ships available for service than the RN. One of the offshoots was I made a chart showing US ironclad strength (excluding the Mississippi flotilla, which were not really ironclads, although the fact this chart excludes the Milhawkee class of river monitors is debatable).
What is apparent is that the Passaic class Monitor are the absolute backbone of any defence of the US during the period when the RN might intervene (i.e. upto July 1863). Starting the early 1864 the USN ironclad force becomes much more powerful as the Canonicus and other 15" armed Monitors start to come into service, although about this time the RN starts putting ordnance into service capable of piercing Monitor turrets at extreme ranges.
The Passaic was an improved Monitor, although the design work was done before the US had actually fought the Monitor. 10 units were ordered at a contract price of ca. $400,000 each, now of which was reached. The cost per unit was more than double that of Britain 1854/5 ironclads (at a fixed £1 = $3 exchange, which in reality flutuated immensely) and the cost of 2 Passiacs would purchase a Defence or Hector class for the RN, and 3 Passiacs would purchase a Warrior or similar. A Passiac cost about the same as a two deck steam ship-of-the-line. They were not cheap vessels by any stretch of the imagination.
They were fairly large, 200 feet long, 46 feet in beam and had a draught of 11 ft 6. Builders measurement was 844 tons, placing them in the same size class as a sloop of war. This makes them far too large for much riverine work (they could not transit the St. Lawrence for example). The engines were the same Ericsson vibrating lever type used in Monitor, but pushing a larger vessel. At 320hp the Passiacs could approach 7 knots with clean bottoms, however after any period at sea due to their lack of coppering barnacles rapidly reduced their speed below 4 knots. Her turning circle is, at 350 yards, high for a monitor (Onondaga had a turning circle of 260 yards, and Dictator 230 yards, even the Russian copies had much tighter turning circles), and about the same as an RN ship of the line or 2nd class armoured frigate (the much finer 1st class armoured frigates often had much larger turning circles, Warrior's being measured at 760 yards at 12 kts, although a "mere" 608 yards at 5 knots).
Armament was designed to be 2x 15" Dahlgren guns. However, the lack of such ordnance meant only one per ship could be supplied, and they were armed with a 15" and a 11" for the most part. The guns could be reloaded in 7 minutes. Against armoured vessels only the 15" really matters, but against wood the 11" would have been effective. The real problem was that the 15" gun completely blocked the port, the gunners could not aim the piece, hence the entire turret was aimed using the 11" gun, and the 15" gunners simply fired when the 11" gunners fired. Since the guns have differing angles, this meanbt that shooting the 15" above 300 yards (point blank range) was an exercise in futility. I can only speculate how the gunners of Cammache (which had 2x 15") and those of the post-ACW Passaic (also with 2x 15") actually aimed their guns.
Armour was originally as Monitor (which had been uparmoured after Hampton Roads), 11 x 1" plates on the turret, 3-5 x 1" plates on the hull and a single 1" plate on the deck. This proved inadequate at Charleston in 1863, and all were uparmoured in places, notable armoured rings were placed around the turret bases as even the most minor damage in that area would jam the turret. The turret is unlikely to be pierced by any weapon at sea in 1863, although the Battle of Charleston showed that the guns could be knocked out by concussion very easily. The hull is much more vulnerable, especially against a vessel whose guns have reasonable height, since the deck would start to take strikes.
Whilst a useful ship, the Passaic is not a competitor for most European ironclads. The best matchup would be against an old Crimean War ironclad, which has similar speed and handling characteristics, although is heavier armed. I retain my doubts that the Passaic would be 50/50 against a wooden frigate.