Engine density as a naval technology turning point

What modest technological advance, sent back ten years, would most rewrite naval history?

Higher-density engines and small-tube boilers: they would make 1910-era warships far faster and reshape later fleet development.

The answer puts machinery density ahead of more conspicuous inventions such as radar or aircraft. Sending early-1920s engines and small-tube boilers back to 1910 would give designers substantially more power within the same machinery weight and volume. Queen Elizabeth-type ships could emerge faster from the outset, pushing the fast-battleship idea into service earlier. Higher speed would change scouting, concentration, withdrawal, and the relationship between battleships and battlecruisers. Because those ships would remain influential into the next war, a seemingly modest propulsion advance could alter both world conflicts. The counterfactual demonstrates how an enabling technology can reshape an entire fleet without changing its guns or armour first.