Why does this work so well?: the GB/FG/1913

I just tried my friend Cliff's "frigate gunboat" design: GB/FG/1913 in Springsharp, and was amazed at how easy it was to make work, and just how good it was, without any substantial changes. His original specs were:

The specs were:
Displacement: 1200 tons legend
Dimensions: 315ft x 30ft x 10ft
Armament: 5-4in/50 QF, 2-21in TT
SHP: 25,000
speed: 32 knots
Fuel: 300 tons oil

and the Springsharp report is as follows:

GB/FG/1913, Great Britain Frigate Gunboat laid down 1913 (Engine 1921)

Displacement:
 1,048 t light; 1,095 t standard; 1,204 t normal; 1,292 t full load

Dimensions: Length overall / water x beam x draught
 315.77 ft / 315.00 ft x 30.00 ft x 10.00 ft (normal load)
 96.25 m / 96.01 m x 9.14 m  x 3.05 m

Armament:
      5 - 4.00" / 102 mm guns in single mounts, 32.00lbs / 14.51kg shells, 1913 Model
   Quick firing guns in deck mounts with hoists
   on centreline ends, majority forward, all raised mounts - superfiring
 Weight of broadside 160 lbs / 73 kg
 Shells per gun, main battery: 250
 2 - 21.0" / 533.4 mm above water torpedoes

Machinery:
 Oil fired boilers, steam turbines, 
 Direct drive, 2 shafts, 29,572 shp / 22,060 Kw = 32.00 kts
 Range 4,500nm at 12.00 kts
 Bunker at max displacement = 197 tons

Complement:
 101 - 132

Cost:
 £0.157 million / $0.628 million

Distribution of weights at normal displacement:
 Armament: 20 tons, 1.7 %
 Machinery: 645 tons, 53.6 %
 Hull, fittings & equipment: 383 tons, 31.8 %
 Fuel, ammunition & stores: 156 tons, 12.9 %
 Miscellaneous weights: 0 tons, 0.0 %

Overall survivability and seakeeping ability:
 Survivability (Non-critical penetrating hits needed to sink ship):
   312 lbs / 141 Kg = 9.7 x 4.0 " / 102 mm shells or 0.2 torpedoes
 Stability (Unstable if below 1.00): 1.37
 Metacentric height 1.4 ft / 0.4 m
 Roll period: 10.6 seconds
 Steadiness - As gun platform (Average = 50 %): 55 %
   - Recoil effect (Restricted arc if above 1.00): 0.42
 Seaboat quality  (Average = 1.00): 1.11

Hull form characteristics:
 Hull has rise forward of midbreak
 Block coefficient: 0.446
 Length to Beam Ratio: 10.50 : 1
 'Natural speed' for length: 17.75 kts
 Power going to wave formation at top speed: 68 %
 Trim (Max stability = 0, Max steadiness = 100): 50
 Bow angle (Positive = bow angles forward): 2.00 degrees
 Stern overhang: 0.00 ft / 0.00 m
 Freeboard (% = measuring location as a percentage of overall length):
    - Stem:  22.00 ft / 6.71 m
    - Forecastle (32 %): 19.00 ft / 5.79 m
    - Mid (32 %):  19.00 ft / 5.79 m (12.00 ft / 3.66 m aft of break)
    - Quarterdeck (15 %): 12.00 ft / 3.66 m
    - Stern:  12.00 ft / 3.66 m
    - Average freeboard: 14.62 ft / 4.46 m

Ship space, strength and comments:
 Space - Hull below water (magazines/engines, low = better): 186.4 %
  - Above water (accommodation/working, high = better): 116.1 %
 Waterplane Area: 6,008 Square feet or 558 Square metres
 Displacement factor (Displacement / loading): 55 %
 Structure weight / hull surface area: 30 lbs/sq ft or 144 Kg/sq metre
 Hull strength (Relative):
  - Cross-sectional: 0.48
  - Longitudinal: 2.44
  - Overall: 0.57
 Hull space for machinery, storage, compartmentation is cramped
 Room for accommodation and workspaces is adequate