Part of Plan Z was the construction of three O-class battlecruisers. The design originated from a proposal to increase the armament of the proposed P-class to 380mm guns.The O-class was designed for commerce raiding and their specifications show this. By mid-1939 all work on the P-class had stopped and was redirected into the O-class.
Displacement 31,142 tons
Full load displacement 35,367 tons
Waterline length 246.0m
Waterline beam 30.0m
Draft (design) 8.0m
Draft (full load) 8.8m
Armament six 38cm/52 SK C/34 paired
six 15cm/48 Tbts KC/36T paired
eight 10.5cm/65 SK C/33 paired (AA)
eight 3.7cm/L83 SK C/30 paired (AA)
twenty 2cm/65 C/30 in five quads (AA)
twelve 533mm torpedo tupes in four fixed triple mounts
four Arado-196 floatplanes
Speed 33.5 knots at designed displacement
Shaft horsepower 176,000
Endurance 14,000 nautical miles at 19 knots
Protection 90mm upper belt
190mm main side belt
30 mm upper deck
60mm armor deck
Let's start with protection. In comparison to the numbers listed above for the O-class, the HMS Hood had a a side belt of 203mm, a 127mm upper belt, an upper deck of 32mm, and a lower deck of 48mm. The O-class had lighter armor than the Hood. On 24 May 1941 the Hood was sunk by a single hit from the Bismarck which penetrated to her magazines, which detonated. The loss of the Hood directly confirmed the insanity of lightly armored capital ships engaging hostile capitals ships. The protection scheme of the O-class was obsolete the second it was designed. The lack of armor may have saved weight, leading to a greater speed, but the cost in staying power was tremendous. For further examples of lightly armored battlecruisers being lost to magazine explosions, look no further than the Indefatigable, Queen Mary and Invincible. All lost to magazine explosions at Jutland. The O-class would have been dead the moment it engaged a battleship.
The explosion of the Queen Mary at Jutland
Moving on to the armament, this is a spot where the O-class wasn't as bad. The O-class was armed with 6 38cm guns. Though while this armament would have been excellent for sinking merchant vessels, in an engagement with hostile battleships the O-class would have been heavily outgunned. However, the O-class had the advantage in speed over almost every class of battleship that served in WWII (the Iowa-class being the exception). Assuming a hostile battlecrusier had been able to catch an O-class, the gunnery duel would have been almost equal. For comparison the Repulse-class battlecruisers carried six BL 15in MK 1 guns, and the French battleship Dunkerque carried 8 330mm guns. As to the secondary anti-aircraft armament, the O-class would have had very light defenses against air attacks. Only 36 guns of varying calibers. This was light even for German vessels. The Bismarck carried 16 10.5cm AA guns, 16 3.7cm guns, and 20 2cm guns in comparison.
The Tirpitz firing her main battery
The power plant for the O-class were eight 24 cylinder MAN diesel engines (V type), arranged four to a shaft. The diesels generated 13750 horsepower each continuously, and had a maximum rating of 15000 horsepower each. The centerline shaft (the O-class would have had three propellers) was driven by a steam turbine and four high temperature and pressure boilers. The operating temperature for the boilers would have been 460 degrees celsius (and could be superheated to 960 degrees celsius), and the operating pressure would have been 55kg per square cm. The steam plant was rated at 55,000 horsepower normally, and 60,000 at maximum. The use of mixture of diesel-steam powerplant allowed a relatively low fuel consumption rate, and an endurance of 14,000nm to 15,000nm at 19 knots. Though, this range would only have been attained with only the diesels in operation.
The method in which these ships were to be used, was commerce raiding. It was planned that a task force made up of 3 H-class battleships, an aircraft carrier, with cruisers and destroyer as escorts, would engage convoy escorts. Allowing the battlecruisers to strike the undefended merchant vessels. How this proposed task force would break out of the North Sea, or the English Channel has not been shown. Had such a force attempted a breakout, the British would have undoubtedly sortied the Home Fleet to engage the force. Any O-class vessel attached would have almost assuredly been lost in an such an engagement. It must be remembered that the British were able to keep the Geramn High Seas Fleet bottled up in the North Sea for the duration of WWI. The location of Germany would have precluded the effective use of an O-class battlecruiser as a commerce raider simply because of the gauntlet that would have had to be run to reach the British convoys.
The O-class is an interesting mental exercise, in what a Bismarck with no armor would look like. However, in combat the glaring lack of armor would have doomed them to penetrating hits to magazine and machinery spaces. The armament was passable, but lacking in the AA role. The mission that was planned for the O-class was, while feasible, do not take into account the location of Germany or the possibility of an encounter with better armored and/or armed opposition. The O-class as designed was a floating coffin, nothing more, nothing less.
*Garzke, William and Dulin, Robert. Battleships: Axis and Neutral Battleships in WWII.
Painting at top by Richard Allison.