A writer on a Facebook group has being doing his studies on accuracy and armour capabilities of both sides in WW1 and this is his findings: very interesting reading:
The Armour protection of British Battle Cruisers.
There are several factor that one needs to bear in mind when looking at the Armour of British and German Battlecruisers, and the issue and subject is not as easy, or as simple as Thick Armour Good, Thin Armour Bad. And British Battlecruisers were most certainly not the weakly armoured warships just waiting to explode.
First up is the design philosophy of British battlecruisers. They were designed around the notion of the Big Gun and Salvo firing under Director Control. The idea being that the British ships would carry a bigger gun that fired a heavier shell and carried further than any potential opponent. This was initially true when Invincible was launched, her 12" guns totally outclassed the 8", 9" and 10" guns of other nations Armoured Cruisers.
And even when the Germans replied with Von der Tann, the range advantage and weight of shell was still in favour of the British ship.
And its here that Ballistics play their part. The British tended to favour a gun that fired a heavy shell at a "low" velocity, using the weight of the shell to smash through armour.
The Germans preferred a gun that fired a light shell at a "high" velocity, using the speed of the shell to punch through anything it hit.
The trade off was that for the Germans the effective range of their guns (the range at which they would penetrate British Armour) was lesser than that of the British Guns. The lighter German shell looses velocity quicker and so also looses its ability to penetrate Armour quicker than the heavier British Shell.
Added to this that British Armour Plate, inch for inch was of better quality than that fitted to any other nations warship.
The British guns had a longer reach and with Director Control could hit further out that any other nations warship, added to that that British Battlecruisers tended to have a speed advantage over their adversaries in practical conditions a British battlecruiser could choose to fight at a range at which their guns carried and the guns of their Opponent didn't.
The Empirical evidence from the actions of the Great War show this quite starkly. Over the course of Jutland and Dogger Bank for example (and taking into account estimations for the various Battlecruisers sunk in action) the Germans were able to penetrate British 13" Armour once at a range of 10'500 yards (a hit on Warspite); British 9" Armour five times (Lion once, Princess Royal once, Queen Mary three times); British 7" plate three times; 6" plate seven or eight times.
In addition to this information it should be borne in mind that the maximum range at which British 9" plate was penetrated was 14'600 yards (the hit on Lion at 4pm), meanwhile British 12" shells were penetrating German 11" armour at 17'500 yards; British 13.5" shells were penetrating German 10" armour at nearly 19'000 yards, and German 9" Armour at nearly 20'000 yards.
Dogger Bank is another perfect example. The British battlecruisers had free reign to open fire at will on the German Battlecruisers and the Germans were unable to reply for 15 minutes because the British ships were out of range, even for Derfflinger with her 12" guns!
Also worth bearing in mind that the Dogger Bank action settled into an action fought at around 18'000 yards. Lion was able to penetrate Seydlitz's 1" upper armoured deck and penetrate the 9" Barbette of C turret at around 18'000 yards, where as a 12" hit on Lions main belt failed to penetrate her 9" armour, but did displace the Plate slightly. An issue that was rectified with extra support built in behind the Armoured Belt.
The Legend that British Battlecruisers had indifferent armour started with Jellicoe and Beatty and sadly does not hold form once its put under the microscope.