Hood. Battlecruiser. Thin armour. Too bad.
By Mike Creasy
What was a battlecruiser anyhow? Why didn’t they put on more armour? How can any big warship blow up and sink in seconds?
Battlecruiser was a term invented by Admiral Jackie Fisher, Commander-in-Chief of Britain’s Navy in WW1. Fisher was raised in the very early days of steam-powered warships, when battle tactics were still based on wind-powered fleet actions.
There’s a fundamental difference in the whole approach to battle between wind and steam. If movement is limited by wind (and your whole fleet can move only very slowly, if at all) you have to have your forces positioned well before the battle. And to do that, you need fast scouts out front, so you can know what the enemy is doing.
In the days of sail, slow battleships (6 to 10 knots) would stay close together in order to concentrate their fire. Fast frigates moving at maybe 10 or 12 knots would range ahead.
By 1900, battle fleets were steam powered and able to maintain about 20 knots. Common thinking was to have fast (25 – 30 knot) cruisers scouting ahead, with torpedo boats and torpedo boat destroyers further out. Britain had all of these in large numbers, in support of large numbers of dreadnoughts and pre-dreadnoughts. But other navies had built cruiser squadrons that were a match for British cruisers. The answer was to build bigger, faster cruisers and, in an exchange of rather stuffy memos between the Admiralty and the Director of Naval Construction, the concept of a battlecruiser was born.
The Hood class battlecruisers (4 were planned) were laid down in 1916 in response to several new German ships. Both the Hoods and the German ships were well armoured by the standards of the day, with the Germans generally having more armour and smaller main guns. HMS Hood’s original design called for very little horizontal armour, but this was changed during the extended (3 year) construction period.
As built, Hood had a 12” main (side) armour belt with a 7’ belt just above it. She had anywhere from 1” to 5” horizontal (deck) armour, and plenty of protection on turrets and conning tower. By comparison, the Revenge class battleships (Revenge, Ramilies, Royal Sovereign and Royal Oak) built in 1916-1919, had 13” main belts with 2” deck armour.
Hood carried the same guns as a battleship, she was very fast (32 kts), and she was bigger than any battleship of her time so…. battleship or battlecruiser?? Semantics, really.
Whatever she was, Hood was a major advance in capital ship design when she was launched in 1919. Her sister ships were cancelled as the politicians tightened the purse- strings after the end of the war, and Hood alone became the flagship of a new age of naval design as she cruised the world between wars.
Fast forward 20-odd years to the famous meeting with Bismarck. How could a capital ship – even a now old and outdated one - disappear from sight in the space of 7 minutes? The answer probably has two main elements: armour design and chemicals.
Armour design is always a big concern for warships. Add too much and it’ll be safe from any guns, but it won’t float. Hood’s armour had many weaknesses, but a recent analysis of the likely fatal shot concludes that Bismarck’s 38cm shell probably penetrated the 7” belt before detonating in or near one of the three aft magazines.
Her designers were well aware of this threat from German heavy guns. In a remarkable display of foresight, a 1920 analysis forecast the penetration of Hood’s 7” side armour, with the shell bursting inside a magazine.
In other words, the shell may not have entered through the upper deck as has been commonly thought for many years.
What’s even more interesting is what was in those magazines when the shell arrived – over 100 tons of cordite.
British WW2 cordite propellant was much different than American or German big gun powder. Cordite was a “double-base” propellant containing nitro-glycerine. It was known to be somewhat unstable, having been at the heart of at least three capital ship magazine explosions during the Battle of Jutland in WW1.
German and American propellants were “single-base” and much more stable.
When the Bismarck’s shell penetrated Hood’s armour and exploded, the double-base cordite began to burn. Inside the confined spaces of an armoured ship, it was like a slow-speed explosion as the hot gases sought room to expand. Light steel bulkheads inside the armour were knocked down. As pressures built up, the heavier engine room and boiler room bulkheads were blasted down, and some of the gases were vented upwards through large engine room vents on the boat deck to form the huge cloud of brownish smoke described by all survivors.
Hood was still moving at 28 knots when the gas ball found the only way out of the armoured cocoon that was the ship; down through her unarmoured bottom. Her back broke and about 300 feet of her length was blasted into little pieces……
If Hood’s magazines had been loaded with the American-style single base propellant instead of cordite there may have been no fire and even if there was, tests have shown that single-base powders tended to lose combustion quickly.
Would the end result have been different? Maybe. Maybe things would have turned out the same after a longer fight. Who can say?
In hindsight, it’s hard to understand why Britain persisted in using cordite propellants as long as it did. The WW2 cordite formulation probably caused more than one magazine explosion, and it was very hard on gunbarrels as well.
HMS Hood was the last battlecruiser built by any navy. She was completed at a cost of over 6 million pounds sterling, a staggering cost for a capital ship in those days, and she drew the ire of a future First Lord of the Admiralty (and future Prime Minister).
A fellow named Winston, who thought that battleships were a waste.
The War at Sea, Vol 1 and 2, Capt S.W Roskill, HMSO, 1956
Jane’s Fighting Ships of WW 2, Jane’s Publishing, 1946
Fighting Ships of WW 1 and WW 2, Phoebus Publishing, 1976