Very interesting protection scheme. I was wondering if sts-200 would have his designers re-invent the protected cruiser, and they haven't - quite. But that 4" armoured deck is clearly the main protective feature with everything above it deemed expendable.
Still worth noting just what an achievement these ships would be. We haven't had confirmation of the armament, but it's looking like 3x3 15" and the ships only make sense if they're fast. Of course, these ships are effectively about 25,500 tons standard after the pre-planned "refits" - which makes them bigger than Lion and not far behind Queen Elizabeth. But still - Howe (Hood's better armoured sister) has the same 11" belt, maybe half-an-inch more deck armour, only 8x15", 31.5kt - on 10,000 tons more.
They have done this with an approach that an American all-or-nothing evangelist might find slightly extreme. Compared to Howe, the upper belt is gone, the end belts are gone, the main belt is both short and shallow (and as RelativeGalaxy7 noted, virtually underwater at full load) and this is covered for by carrying the armoured deck lower. The citadel covers the magazines and machinery and nothing else - to the point where I wonder how she'd float with flooded ends. In addition to reducing the turret/barbette armour, nothing has been said so far about a conning tower, and I'm betting the secondary armament is both light and carried in unprotected mountings on the upper deck. It's an incredibly tight design, optimised to the limit to stay (just) within the bounds of the Treaty while combining the heaviest possible armament with (barely) acceptable protection and speed.
And will it work? Well, they'll shock the world when they come out, but I'm not so sure how viable they will be long term (putting a proper 1930s AA armament on them looks like it would need a full rebuild). And of course, being 30,000-ton battlecruisers in all but name, they will come with an appropriate price tag.
That's an excellent analysis - of both the good and the bad points.
I did think about a true 'protected' cruiser, using sloped deck edges instead of any belt at all, but your next point about buoyancy illustrates why I didn't go that way; it could have its unprotected hull riddled just around the waterline, causing loss of stability.
As you and
@RelativeGalaxy7 noted, the citadel is very deeply submerged - so there's very little armoured buoyance reserve (I haven't mentioned it yet in the story, but the citadel is 425' long, on a 728' ship). Floating with ends riddled is going to depend on still having buoyancy above the armour deck.
To my mind, a greater concern is stability - i.e. capsize - as ships rarely actually had their ends riddled. That's why I wanted the deck and belt as far out to the side of the ship as possible at and above the waterline, and is also the reason for the high double hull, splinter protection and inboard bulkhead that extends to the upper deck from the TPS.
Of course a nice 12" upper belt would be better, but this isn't a 40000-ton ship.
Secondary armament will indeed be light ... as built, very light, and there is no armoured conning tower (more to come there).
Adding yet more weight, particularly high up is always going to be an issue on such a tight design. I suspect the designers would feint if they could foresee the need for a pair of HACS, heavy DP secondaries, lots of Bofors and heavy antennae ... all mounted as high as possible!
As to price - you've really hit the nail on the head there. It's well and good talking about the low cost in 'Treaty tons', but if a proper battleship costs, say £6.5M in the story, I doubt there'd be any change from £5.5M for one of these. It's a smaller ship, but a highly engineered one and is still full of expensive guns, equipment and machinery.
That's why they must be able to engage battleships, and preferably continue to make at least some sense in a world with a less restrictive treaty ... and more to come on that topic too.
