Speed, power density, and capital-ship types

A thicker armor figure can conceal reduced coverage, a distinction that may help explain Hood's vulnerability. Early-1920s machinery made a fully protected ship above 30 knots difficult because propulsion consumed so much displacement and internal volume. By the mid-to-late 1930s, improved power density made the fast-battleship combination more practical. Beyond roughly 32 or 33 knots, rapidly rising power requirements produced diminishing tactical returns, especially once aircraft could overtake any surface ship.