Capital-ship status can cover different kinds of vessel: aircraft carriers, ballistic-missile submarines, and amphibious ships project power in different ways. Battleships and large cruisers likewise served different purposes before the First World War. Speed, firepower, and armour could not all be maximized with the machinery and infrastructure then available. A navy therefore had to decide whether a large ship would principally be a powerful cruiser or a line-of-battle unit. Combining both roles required a better machinery power-to-weight ratio or an exceptionally large hull.
Chapter · 2:43-5:24
Why battlecruisers were a compromise
Battlecruisers emerged because early twentieth-century technology could not yet combine a cruiser's speed with a battleship's protection in one practical hull.