As an engineer I know the importance of the correct use of terminology, and one of my pet peeves is the casual - and incorrect - use of words which don't mean what the user thinks they mean. Half the time when some unlearned person uses the word "battlecruiser" the subject is a proper battleship. The other half it is an actual cruiser, or some other class not a battlecruiser. Contrarily, most of the time when someone unfamiliar with proper use of military vessel terminology refers to a true battlecruiser, they use another word entirely. It's enough to drive a person to sink (the offender's battleship).
Part of the reason for this confusion is that relatively few battlecruisers were built and they only operated for a few decades. This was primarily due to two factors; the development of technology and naval tactics in the Twenties and Thirties, and politics. The Washington Naval Treaty of 1921 set limits on the sizes of ships of various classes, as well as the size of armaments such ships could carry. The London Naval Treaty of 1930 further split some of these classes, and included a formal distinction between light cruiser, cruiser and heavy cruiser. Note that there is some overlap between battlecruiser and heavy cruiser, because they were designed for similar tasks. However, neither is a true battleship, and the heavy cruiser is generally smaller and more lightly armed and armored than a battlecruiser.
Also, as time progressed new ships for a class grew, in both size and capability. A late-built heavy cruiser is larger and much more formidable than a battleship built at the beginning of the Twentieth Century, besides being faster. However, except for the fuzzy line between them and heavy cruisers, the distinction between what is and what isn't a battlecruiser is quite clear. It all boils down to the reason behind the concept.
The basic idea of the battlecruiser is simple; provide a ship with more punch than a cruiser and more speed than a true battleship; speed enough for cruiser task force operations. Battlecruisers are less weakened battleships than they are strengthened cruisers. As originally proposed, these ships would be used with other cruisers for scouting, commerce raiding, engaging enemy cruiser task forces and making hit-and-run flank attacks to harass an enemy force in large engagements. They were supposed to use their superior speed to avoid being challenged by true battleships.
Achieving this speed required starting with something the size of a battleship, but with fewer main guns, smaller main guns, less armor protection, less range, or some combination. This to make room in a battleship-sized hull for enough machinery to push the huge vessel through the water at cruiser speeds.
The first to put the idea into extensive practice were the British, but many other nations with large navies also built their own versions. As a rule, the British gave their battlecruisers the same guns as true battleships, gaining speed by sacrificing armor and perhaps number of guns. The German battlecruisers, meanwhile, generally had true battleship armor and smaller-caliber guns, but many of them. Both types of craft achieved the goal of being faster than the equivalent battleships, and both had heavier guns and armor than typically found on cruisers of the day. The high turn of speed let the battlecruisers select their engagements, either avoiding true battleships or making slashing attacks which provided the latter with little opportunity to respond during large operations. Meanwhile, their heavier guns provided a significant advantage over ordinary cruisers.
People who today discredit the entire concept of the battlecruiser ignore the fact that the type of ship was very successful if used as originally intended. When a battlecruiser squadron went against a conventional cruiser squadron, the latter lost.
HMS Hood was the last of the British battlecruisers built for the Great War, not even being completed in time for that conflict. Once the British Navy realized the war would end before she was likely to launch her construction was slowed and her three sister ships were cancelled. In large part this was due to the expectation of making design changes based on what was learned by the "failure" of the British battlecruisers at Jutland.
However, the main problem at Jutland was that while British battlecruisers were made to run away from superior opposition, British captains weren't. British battlecruisers had the size and guns of a true battleship, but couldn't take the same punishment. At Jutland battlecruisers fought with true battleships in the British line against German battleships. Three were consequently destroyed in very dramatic fashion, while the true battleships were much less affected.
The Hood was modified following this operation, both during initial construction and subsequent refits (on her final voyage she carried a number of civilian workers who were finishing the work of her last refit) but she still blew up very quickly and dramatically in one of the first major naval engagements of the Second World War, the Battle of Denmark Straight. To put it bluntly, she was overmatched, largely for political reasons. Additionally, the true lessons of Jutland simply hadn't been learned by the Admiralty.
There is more than the thickness and toughness of the armor to consider when evaluating how well a ship is protected. Even battleships only put heavy armor where it will protect vital areas; magazines, engine rooms and such. Other parts of the ship are less protected. Also, the actual armor is within the hull, and has a shape only vaguely determined by the exterior shape of the ship. The internal armor layers are angled with respect to the expected path of incoming shells, which greatly increases the effective thickness. These angled walls of interior armor may also be intended to direct plunging fire - shells coming in at close to vertical - back out of the ship.
As the Hood was slowly brought to completion her armor was upgraded. Claims were made when she was launched in 1918 that she was now a full battleship, though that only worked if you considered older battleships. While the added protection was substantial, the improved portions covered less of her volume than was true on an actual battleship. She was still fast, but because the added armor was not fully compensated for by weight reduction in other areas she was overweight her entire career.
Worst of all, there was little improvement in training those who tended the enormous appetites of the main guns.
She should have done better at the Denmark Straights. The reason she didn't was the same reason those earlier battlecruisers blew up at Jutland. To increase rate of fire safety measures in ammunition handling were bypassed*. This left substantial amounts of propellant powder with a direct path of ignition from a hit in the right spot. And the Germans found the right spot.
The US got a late start on building battlecruisers. Indeed, most of those she started or planned during the Great War were never completed. The Lexington and Saratoga were instead converted to aircraft carriers during construction. Interestingly, these Lexington class ships if completed as originally planned would have been closer to the later concept of the fast battleship than to the battlecruisers of other nations during the period. (Isn't it just like Americans to not only want it all, but to get it?)
One supposed distinguishing factor between the classifications of battleship and battlecruiser is that the former can resist the effects of guns similar to those it carries, at least in the "citadel" where armor protection is concentrated, so that a duel between battleships becomes a slugging match. This is known as a balanced design. However, the resistance to shells depends more on the state of the delicate balance between offense and defense which exists at the time a ship is designed than it does on overall design philosophy. The Iowa class battleships had excellent armor protection, but would have been considered under-armored against an opponent with guns equivalent to their own. However, the Iowas were true battleships. They just happened to have what were probably the best (though not the biggest) large naval guns ever deployed. Those guns were very accurate, had very good range and the armor piercing shells they fired were very good at their job.
Digression: Note that the Iowa ships were not only fast and very economical steamers, but highly maneuverable. At least twice an escorting ship didn't believe the signal of an Iowa class battleship in regard to the degree of turn about to be executed, and wound up getting cut off, much to the smaller ship's disadvantage. There's a reason battleships always have the right of way...
As the Second World War progressed, the job of the battlecruiser was increasingly split between the heavy cruiser and the fast battleship, with the Iowas being excellent examples of the latter. Such vessels were made possible by a combination of a better hull form - which offered less drag - and new steam boilers and turbine engines which provided more power in less volume with less weight. (Note that the Iowas also had excellent safety measures for keeping flame from a hit away from exposed powder. Something which may have saved the Iowa herself when a faulty powder charge destroyed the inside of one of her turrets, late in her career.) With true battleships now as fast as cruisers, there was simply no need for the battlecruiser. If your cruiser task force needed a heavier punch, simply add a heavy cruiser or, for really deciding the issue, a fast battleship.
The battlecruiser as originally conceived made good tactical sense. It just happened that they were the best tool for the job for only a relatively short time.
*This is a guess, but one shared by many who have studied the sinking of the Hood.
This document is Copyright 2007 Rodford Edmiston Smith. Anyone wishing to repost it must have permission from the author, who can be reached at: stickmaker@usa.net