You're right, though as you noted, the Passiacs were almost as well protected as the Canonicuses.
It was pretty rapidly established what level of protection was required against the guns in use in theatre (7" rifles etc.), and most Monitors had roughly 10" of armour (the extra 5" on Dictator was structural cast iron, and offered no real extra resistance to shot, and indeed would have generated massive spalling).
Can you give some backing for this? The 15 in Dahlgrens hit with roughly twice the force of the 68 pounder. Matching what you say about the 15 inchers' effects on Warrior's armor with what you say about the 68's effects on the heavier armor of the Canonicus seems difficult.
There were a lot of tests conducted in the 1860's. I first became aware of them from Parke's
British Battleships, but have since read the various reports of the Committee of Ordnance and Armour in the PRO.
One of the problems of the large 15" is it's size, it it simply too large, and has too large a surface area to successfully penetrate much (indeed, in combat it never fully penetrated more than 4" (2x2" layers) of very low quality iron, made a partial penetration once against 5" of similar, but was defeated by 6"). However, there is a lot of kinetic energy in the round, and it really could batter the insides without penetration.
The 68 OTOH has twice the KE/ presented surface area of a double charged 15" (but considerably less raw KE at shot range). In tests it would reliably penetrate 3" of single thickness high quality wrought iron. The 4" of armour was selected for the 1855 ironclads as this was proof against any Russian coastal battery.
Certainly the monitors' deck and side armor pose less of a target (and are better protected) than the Warrior's exposed bow and stern. And any hits on the deck will be at quite an angle.
True, but the deck is extremely weak, and any penetrations are very delitirious. Warrior's ends OTOH are mainly her accommodations etc., and have little impact on fighting value.
Are you saying that the steering is not of fighting value? The Achilles was specifically designed it's steering isn't completely unprotected like the Warrior, and when that proved unsatisfactory, the British moved to the Minotaur class. In any case, battles between Ironclads of this time period will be certainly be time consuming slugging matches.
Allow me to quote Brown's
Warrior to Dreadnought (pg 14):
"The lack of armour on the ends was less serious than many contemporaries believed, since the ends were well subdivided, and even if both ends flooded draught would only increase by 26in. The lack of protection to the steering gear was more serious, although it was a small target and not likely to be hit. However the unprotected broadside guns outside the armoured battery were a bad feature."
Brown has actually produced a very good book and looks and rarely considered features such as contemporary theories of wave stress etc. to explain some design features.
The move towards thicker armour over smaller areas was triggered not by US guns (the British experimental 13" Horsfall Gun of 1855 was a far more dangerous piece of ordnance than a Rodman, even a 20" Rodman), but by Armstrong and Whitworth (and the older Horsfall) demonstrating consistant penetration of some very heavy armour plates as early as 1861-2. The guns which emerged as the RN service pieces (fielded from 1864) were based on the Armstrong Shunt Gun with Whitworth rifling. The progress is amazing, in 1864 the fleet starts to rearm with a steel rifled gun capable of piercing thick armour at long ranges, and firing palliser shell (i.e. an armour piercing round with a bursting charge fuse to detonate after penetration). The idea of a "shotproof ship" is dead, it is accepted that guns with trump armour, all other things being equal.
Really? That's nice to know. I actually got the completed 1868 thing from the U.S. Department of the Navy and you would think they would have mentioned if completing in that context means something completely different from U.S. usage. And 4 weeks to fit masts, rigging, weapons seems incredibly fast. Canonicus took 8 months from launching to be ready for service and this was in the middle of a war with armored warships in critically short supply. Unless the British also use launch in a completely different way from the U.S.
Canonicus took so long because the US was critically short of materials, especially armour plate. A look at the ironclad programme shows the US could build hulls quicker than it would fit them out. Some languished for years.
Launching does mean something slightly different. For the British, the launched ships were fully cased, engined etc., with the major jobs being masting, arming and provisioning (mostly arming). For US Monitors, launched ships didn't actually have turrets ISTR.
Defence launched on 31 Dec 61, at the height of the Trent Crisis, she was scheduled to steam for the Lisbon* on 31 Jan 62, with the crew transferred directly from another vessel (Ariadne? I can't remember which offhand). Royal Oak was scheduled to steam for the Lisbon before the start of April (she was not cased when the crisis hit, and the dockyard moved to 24 hour working to finish her rapidly, as indeed happened generally).
* During the Trent Crisis the RN decided to concentrate their main battle force for America at Lisbon, including all available armoured frigates. In the event of war, this force would steam immediately for America, and would strike the USN at Hampton Roads.
