I just did an experiment to see what it would take to drive a battlecruiser at 31 knots in 1905

I was determined to see what would be needed to drive a battlecruiser at 31 knots in 1905. I found out that the cost was unacceptable, due to the machinery limitations of the day. I suspected that I could greatly increase armor, however. I found out that the best that I could do, using an "all or nothing" scheme was a 9in armor basis. To do that requires a huge ship (in 1905). If anyone would have built ships to the design, they would be super-ships. There are fast, well-armed, and moderately well protected, for what they are.

GB/CB/1905 design study C1, Great Britain Battlecruiser laid down 1905

Displacement:
	40,301 t light; 41,617 t standard; 47,056 t normal; 51,408 t full load

Dimensions: Length overall / water x beam x draught
	794.00 ft / 794.00 ft x 102.00 ft x 31.00 ft (normal load)
	242.01 m / 242.01 m x 31.09 m  x 9.45 m

Armament:
      8 - 12.00" / 305 mm guns (3 mounts), 864.00lbs / 391.90kg shells, 1905 Model
	  Breech loading guns in turrets (on barbettes)
	  on centreline ends, majority forward, all raised mounts - superfiring
      12 - 6.00" / 152 mm guns (9 mounts), 108.00lbs / 48.99kg shells, 1905 Model
	  Quick firing guns in deck mounts 
	  on side, evenly spread, all raised mounts
	Weight of broadside 8,208 lbs / 3,723 kg
	Shells per gun, main battery: 80

Armour:
   - Belts:		Width (max)	Length (avg)		Height (avg)
	Main:	9.00" / 229 mm	500.00 ft / 152.40 m	16.00 ft / 4.88 m
	Ends:	Unarmoured
	  Main Belt covers 97 % of normal length
	  Main belt does not fully cover magazines and engineering spaces

   - Torpedo Bulkhead:
		1.50" / 38 mm	500.00 ft / 152.40 m	23.00 ft / 7.01 m

   - Gun armour:	Face (max)	Other gunhouse (avg)	Barbette/hoist (max)
	Main:	9.00" / 229 mm	5.00" / 127 mm		9.00" / 229 mm
	2nd:	2.00" / 51 mm	      -			      -

   - Armour deck: 2.50" / 64 mm

Machinery:
	Oil fired boilers, steam turbines, 
	Direct drive, 4 shafts, 190,809 shp / 142,344 Kw = 31.00 kts
	Range 7,500nm at 15.00 kts
	Bunker at max displacement = 9,792 tons

Complement:
	1,596 - 2,076

Cost:
	£4.296 million / $17.184 million

Distribution of weights at normal displacement:
	Armament: 1,026 tons, 2.2 %
	Armour: 8,891 tons, 18.9 %
	   - Belts: 3,111 tons, 6.6 %
	   - Torpedo bulkhead: 638 tons, 1.4 %
	   - Armament: 2,256 tons, 4.8 %
	   - Armour Deck: 2,885 tons, 6.1 %
	   - Conning Tower: 0 tons, 0.0 %
	Machinery: 14,455 tons, 30.7 %
	Hull, fittings & equipment: 15,929 tons, 33.9 %
	Fuel, ammunition & stores: 6,755 tons, 14.4 %
	Miscellaneous weights: 0 tons, 0.0 %

Overall survivability and seakeeping ability:
	Survivability (Non-critical penetrating hits needed to sink ship):
	  42,281 lbs / 19,178 Kg = 48.9 x 12.0 " / 305 mm shells or 4.2 torpedoes
	Stability (Unstable if below 1.00): 1.26
	Metacentric height 7.8 ft / 2.4 m
	Roll period: 15.3 seconds
	Steadiness	- As gun platform (Average = 50 %): 65 %
			- Recoil effect (Restricted arc if above 1.00): 0.45
	Seaboat quality  (Average = 1.00): 1.29

Hull form characteristics:
	Hull has raised forecastle, rise forward of midbreak
	Block coefficient: 0.656
	Length to Beam Ratio: 7.78 : 1
	'Natural speed' for length: 28.18 kts
	Power going to wave formation at top speed: 55 %
	Trim (Max stability = 0, Max steadiness = 100): 50
	Bow angle (Positive = bow angles forward): -2.00 degrees
	Stern overhang: -2.00 ft / -0.61 m
	Freeboard (% = measuring location as a percentage of overall length):
	   - Stem:		37.00 ft / 11.28 m
	   - Forecastle (20 %):	34.00 ft / 10.36 m (33.00 ft / 10.06 m aft of break)
	   - Mid (73 %):		32.00 ft / 9.75 m (22.00 ft / 6.71 m aft of break)
	   - Quarterdeck (15 %):	22.00 ft / 6.71 m
	   - Stern:		22.00 ft / 6.71 m
	   - Average freeboard:	30.21 ft / 9.21 m

Ship space, strength and comments:
	Space	- Hull below water (magazines/engines, low = better): 144.1 %
		- Above water (accommodation/working, high = better): 227.5 %
	Waterplane Area: 62,383 Square feet or 5,796 Square metres
	Displacement factor (Displacement / loading): 102 %
	Structure weight / hull surface area: 174 lbs/sq ft or 848 Kg/sq metre
	Hull strength (Relative):
		- Cross-sectional: 0.97
		- Longitudinal: 1.31
		- Overall: 1.00
	Hull space for machinery, storage, compartmentation is cramped
	Room for accommodation and workspaces is excellent
	Good seaboat, rides out heavy weather easily