The J/CB/1921 is a faster version of the battleship

Apparently, the J/BB/1921 is under-protected, and the J/CB/1921 uses that same protection scheme with a 33-knot speed for a reasonable battlecruiser design. The armament and protection are identical, but the battlcruiser has greater dimensions and a higher freeboard. An interesting phenomenon was that the seakeeping improved as the beam was increased. In many smaller ships, decreasing the beam improves seakeeping. The resulting ship is rather lightly protected, but seems to be more capable than I would have expected for the size. This is the Springsharp report:

J/CB/1921, Japan Battlecruiser laid down 1921

Displacement:
 57,727 t light; 60,303 t standard; 64,800 t normal; 68,398 t full load

Dimensions: Length overall / water x beam x draught
 914.97 ft / 912.00 ft x 116.00 ft x 34.00 ft (normal load)
 278.88 m / 277.98 m x 35.36 m  x 10.36 m

Armament:
      8 - 18.00" / 457 mm guns (4x2 guns), 2,916.00lbs / 1,322.68kg shells, 1921 Model
   Breech loading guns in turrets (on barbettes)
   on centreline ends, evenly spread, 2 raised mounts - superfiring
      16 - 5.50" / 140 mm guns in single mounts, 83.19lbs / 37.73kg shells, 1921 Model
   Quick firing guns in casemate mounts 
   on side, all amidships, 8 raised mounts - superfiring
      8 - 4.70" / 119 mm guns in single mounts, 51.91lbs / 23.55kg shells, 1921 Model
   Anti-aircraft guns in deck mounts with hoists
   on side, evenly spread, 6 raised mounts
      24 - 2.24" / 57.0 mm guns in single mounts, 5.65lbs / 2.56kg shells, 1921 Model
   Anti-aircraft guns in deck mounts 
   on side, evenly spread, 16 raised mounts
 Weight of broadside 25,210 lbs / 11,435 kg
 Shells per gun, main battery: 90

Armour:
   - Belts:  Width (max) Length (avg)  Height (avg)
 Main: 13.0" / 330 mm 570.00 ft / 173.74 m 9.00 ft / 2.74 m
 Ends: Unarmoured
 Upper: 10.0" / 254 mm 570.00 ft / 173.74 m 8.00 ft / 2.44 m
   Main Belt covers 96 % of normal length
   Main belt does not fully cover magazines and engineering spaces

   - Torpedo Bulkhead:
  4.00" / 102 mm 570.00 ft / 173.74 m 30.00 ft / 9.14 m

   - Gun armour: Face (max) Other gunhouse (avg) Barbette/hoist (max)
 Main: 13.0" / 330 mm 10.0" / 254 mm  13.0" / 330 mm
 2nd: 5.00" / 127 mm 5.00" / 127 mm  5.00" / 127 mm

   - Armour deck: 5.00" / 127 mm, Conning tower: 13.00" / 330 mm

Machinery:
 Oil fired boilers, steam turbines, 
 Geared drive, 4 shafts, 279,915 shp / 208,816 Kw = 33.00 kts
 Range 10,000nm at 15.00 kts
 Bunker at max displacement = 8,094 tons

Complement:
 2,029 - 2,639

Cost:
 £15.316 million / $61.263 million

Distribution of weights at normal displacement:
 Armament: 3,151 tons, 4.9 %
 Armour: 19,928 tons, 30.8 %
    - Belts: 4,845 tons, 7.5 %
    - Torpedo bulkhead: 2,531 tons, 3.9 %
    - Armament: 4,738 tons, 7.3 %
    - Armour Deck: 7,363 tons, 11.4 %
    - Conning Tower: 452 tons, 0.7 %
 Machinery: 9,639 tons, 14.9 %
 Hull, fittings & equipment: 25,009 tons, 38.6 %
 Fuel, ammunition & stores: 7,073 tons, 10.9 %
 Miscellaneous weights: 0 tons, 0.0 %

Overall survivability and seakeeping ability:
 Survivability (Non-critical penetrating hits needed to sink ship):
   80,058 lbs / 36,314 Kg = 27.5 x 18.0 " / 457 mm shells or 11.2 torpedoes
 Stability (Unstable if below 1.00): 1.14
 Metacentric height 8.0 ft / 2.4 m
 Roll period: 17.3 seconds
 Steadiness - As gun platform (Average = 50 %): 53 %
   - Recoil effect (Restricted arc if above 1.00): 0.70
 Seaboat quality  (Average = 1.00): 1.05

Hull form characteristics:
 Hull has rise forward of midbreak
 Block coefficient: 0.631
 Length to Beam Ratio: 7.86 : 1
 'Natural speed' for length: 30.20 kts
 Power going to wave formation at top speed: 54 %
 Trim (Max stability = 0, Max steadiness = 100): 50
 Bow angle (Positive = bow angles forward): 5.00 degrees
 Stern overhang: -2.00 ft / -0.61 m
 Freeboard (% = measuring location as a percentage of overall length):
    - Stem:  34.00 ft / 10.36 m
    - Forecastle (20 %): 30.00 ft / 9.14 m
    - Mid (67 %):  30.00 ft / 9.14 m (21.00 ft / 6.40 m aft of break)
    - Quarterdeck (15 %): 21.00 ft / 6.40 m
    - Stern:  21.00 ft / 6.40 m
    - Average freeboard: 27.35 ft / 8.34 m

Ship space, strength and comments:
 Space - Hull below water (magazines/engines, low = better): 102.5 %
  - Above water (accommodation/working, high = better): 205.6 %
 Waterplane Area: 79,603 Square feet or 7,395 Square metres
 Displacement factor (Displacement / loading): 101 %
 Structure weight / hull surface area: 225 lbs/sq ft or 1,099 Kg/sq metre
 Hull strength (Relative):
  - Cross-sectional: 0.99
  - Longitudinal: 1.14
  - Overall: 1.00
 Hull space for machinery, storage, compartmentation is adequate
 Room for accommodation and workspaces is excellent