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Based on the Great War U-Cruisers, this design is a possible U-boat for Germany in the 1935 timeframe.

Type III, German Cruiser Submarine laid down 1935

Displacement:
1,684 t light; 1,739 t standard; 1,894 t normal; 2,017 t full load

Dimensions: Length overall / water x beam x draught
280.51 ft / 278.87 ft x 26.25 ft x 19.69 ft (normal load)
85.50 m / 85.00 m x 8.00 m x 6.00 m

Armament:
2 - 5.91" / 150 mm guns in single mounts, 102.98lbs / 46.71kg shells, 1935 Model
Breech loading guns in deck mounts
on centreline ends, evenly spread
2 - 1.46" / 37.0 mm guns (1x2 guns), 1.55lbs / 0.70kg shells, 1935 Model
Dual purpose guns in deck mount
on centreline aft, all raised guns - superfiring
4 - 0.79" / 20.0 mm guns (2x2 guns), 0.24lbs / 0.11kg shells, 1935 Model
Machine guns in deck mounts
on centreline, all amidships, all raised mounts - superfiring
Weight of broadside 210 lbs / 95 kg
Shells per gun, main battery: 150
6 - 21.0" / 533.4 mm submerged torpedo tubes

Armour:

- Conning tower: 0.79" / 20 mm

Machinery:
Diesel Internal combustion generators plus batteries,
Electric motors, 2 shafts, 2,735 shp / 2,040 Kw = 16.00 kts
Range 8,000nm at 10.00 kts
Bunker at max displacement = 278 tons

Complement:
143 - 186

Cost:
£0.468 million / $1.871 million

Distribution of weights at normal displacement:
Armament: 26 tons, 1.4 %
Armour: 3 tons, 0.1 %
- Belts: 0 tons, 0.0 %
- Torpedo bulkhead: 0 tons, 0.0 %
- Armament: 0 tons, 0.0 %
- Armour Deck: 0 tons, 0.0 %
- Conning Tower: 3 tons, 0.1 %
Machinery: 78 tons, 4.1 %
Hull, fittings & equipment: 1,137 tons, 60.1 %
Fuel, ammunition & stores: 210 tons, 11.1 %
Miscellaneous weights: 440 tons, 23.2 %

Overall survivability and seakeeping ability:
Survivability (Non-critical penetrating hits needed to sink ship):
986 lbs / 447 Kg = 9.6 x 5.9 " / 150 mm shells or 0.6 torpedoes
Stability (Unstable if below 1.00): 0.95
Metacentric height 0.6 ft / 0.2 m
Roll period: 14.2 seconds
Steadiness - As gun platform (Average = 50 %): 0 %
- Recoil effect (Restricted arc if above 1.00): 0.00
Seaboat quality (Average = 1.00): 0.00

Hull form characteristics:
Hull has a flush deck
Block coefficient: 0.460
Length to Beam Ratio: 10.63 : 1
'Natural speed' for length: 16.70 kts
Power going to wave formation at top speed: 36 %
Trim (Max stability = 0, Max steadiness = 100): 50
Bow angle (Positive = bow angles forward): 10.00 degrees
Stern overhang: 1.64 ft / 0.50 m
Freeboard (% = measuring location as a percentage of overall length):
- Stem: 0.00 ft / 0.00 m
- Forecastle (20 %): 0.00 ft / 0.00 m
- Mid (50 %): 0.00 ft / 0.00 m
- Quarterdeck (15 %): 0.00 ft / 0.00 m
- Stern: 0.00 ft / 0.00 m
- Average freeboard: 0.00 ft / 0.00 m
Ship tends to be wet forward

Ship space, strength and comments:
Space - Hull below water (magazines/engines, low = better): 76.8 %
- Above water (accommodation/working, high = better): 0.0 %
Waterplane Area: 4,708 Square feet or 437 Square metres
Displacement factor (Displacement / loading): 234 %
Structure weight / hull surface area: 146 lbs/sq ft or 714 Kg/sq metre
Hull strength (Relative):
- Cross-sectional: 3.60
- Longitudinal: 2.54
- Overall: 2.77
Caution: Poor stability - excessive risk of capsizing
Hull space for machinery, storage, compartmentation is excellent
Room for accommodation and workspaces is extremely poor
Ship has quick, lively roll, not a steady gun platform
Caution: Lacks seaworthiness - very limited seakeeping ability

4 tubes are forward, 2 aft. 14 reload torpedoes forward, 6 aft.

40 tons of the miscellaneous tonnage are for 20 reload torpedoes.

Why 2x150mm guns?

You can't sink destroyers/escorts with that armament (the usual cruiser submarine req) and just having 1 gun means you can still sink merchant ships for quite a lot less fuss.

Why 2 guns? These subs are based on the WWI U-139 type boats, which were equipped that way.

The question still stands in point. 1x150mm is more than sufficient to sink a merchant ship.

Another question people should ask is; whose merchant ships is Germany going to be killing?

Depends on whom, if anyone, is at war with Germany.

Redundancy? If one fails there is still another.

The Philippines would be happy to allow the German naval attache in Manila to have a look at the AdF U-cruiser Trucha (2x2 115mm) if it is desired.

Quoted

Originally posted by Red Admiral
Another question people should ask is; whose merchant ships is Germany going to be killing?

Acctually I was thinking just that!

A revised version, using the new 12.8cm/45 gun in place of the 15cm gun (the 12.8cm shell has twice the bursting charge of the 15cm shell, increasing it's effectiveness against thin-skinned targets).

Type III, German Cruiser Submarine laid down 1935

Displacement:
2,000 t light; 2,068 t standard; 2,316 t normal; 2,514 t full load

Dimensions: Length overall / water x beam x draught
280.51 ft / 278.87 ft x 26.25 ft (Bulges 32.81 ft) x 19.69 ft (normal load)
85.50 m / 85.00 m x 8.00 m (Bulges 10.00 m) x 6.00 m

Armament:
2 - 5.04" / 128 mm guns in single mounts, 61.73lbs / 28.00kg shells, 1935 Model
Breech loading guns in deck mounts
on centreline ends, evenly spread
2 - 1.46" / 37.0 mm guns (1x2 guns), 1.55lbs / 0.70kg shells, 1935 Model
Dual purpose guns in deck mount
on centreline aft, all raised guns - superfiring
4 - 0.79" / 20.0 mm guns (2x2 guns), 0.24lbs / 0.11kg shells, 1935 Model
Machine guns in deck mounts
on centreline, all amidships, all raised mounts - superfiring
Weight of broadside 128 lbs / 58 kg
Shells per gun, main battery: 300
6 - 21.0" / 533.4 mm submerged torpedo tubes

Armour:

- Conning tower: 0.79" / 20 mm

Machinery:
Diesel Internal combustion generators plus batteries,
Electric motors, 2 shafts, 3,820 shp / 2,849 Kw = 17.00 kts
Range 12,000nm at 10.00 kts
Bunker at max displacement = 445 tons

Complement:
166 - 216

Cost:
£0.504 million / $2.017 million

Distribution of weights at normal displacement:
Armament: 17 tons, 0.7 %
Armour: 3 tons, 0.1 %
- Belts: 0 tons, 0.0 %
- Torpedo bulkhead: 0 tons, 0.0 %
- Armament: 0 tons, 0.0 %
- Armour Deck: 0 tons, 0.0 %
- Conning Tower: 3 tons, 0.1 %
Machinery: 109 tons, 4.7 %
Hull, fittings & equipment: 1,432 tons, 61.8 %
Fuel, ammunition & stores: 316 tons, 13.6 %
Miscellaneous weights: 440 tons, 19.0 %

Overall survivability and seakeeping ability:
Survivability (Non-critical penetrating hits needed to sink ship):
2,090 lbs / 948 Kg = 32.7 x 5.0 " / 128 mm shells or 1.2 torpedoes
Stability (Unstable if below 1.00): 1.16
Metacentric height 0.9 ft / 0.3 m
Roll period: 14.7 seconds
Steadiness - As gun platform (Average = 50 %): 0 %
- Recoil effect (Restricted arc if above 1.00): 0.00
Seaboat quality (Average = 1.00): 0.00

Hull form characteristics:
Hull has a flush deck
Block coefficient: 0.450
Length to Beam Ratio: 8.50 : 1
'Natural speed' for length: 16.70 kts
Power going to wave formation at top speed: 42 %
Trim (Max stability = 0, Max steadiness = 100): 50
Bow angle (Positive = bow angles forward): 30.00 degrees
Stern overhang: 1.64 ft / 0.50 m
Freeboard (% = measuring location as a percentage of overall length):
- Stem: 0.00 ft / 0.00 m
- Forecastle (20 %): 0.00 ft / 0.00 m
- Mid (50 %): 0.00 ft / 0.00 m
- Quarterdeck (15 %): 0.00 ft / 0.00 m
- Stern: 0.00 ft / 0.00 m
- Average freeboard: 0.00 ft / 0.00 m
Ship tends to be wet forward

Ship space, strength and comments:
Space - Hull below water (magazines/engines, low = better): 59.6 %
- Above water (accommodation/working, high = better): 0.0 %
Waterplane Area: 4,669 Square feet or 434 Square metres
Displacement factor (Displacement / loading): 292 %
Structure weight / hull surface area: 177 lbs/sq ft or 864 Kg/sq metre
Hull strength (Relative):
- Cross-sectional: 4.00
- Longitudinal: 2.62
- Overall: 2.91
Hull space for machinery, storage, compartmentation is excellent
Room for accommodation and workspaces is extremely poor
Ship has quick, lively roll, not a steady gun platform
Caution: Lacks seaworthiness - very limited seakeeping ability

4 tubes are forward, 2 aft. 14 reload torpedoes forward, 6 aft.

40 tons of the miscellaneous tonnage are for 20 reload torpedoes.