Queen Mary exchanged some armour and one turret for a rise from 21 knots in the contemporary battleship to about 28 knots. Reaching that speed required 42 Yarrow boilers and 70,000 shaft horsepower, exposing the machinery volume behind early battlecruiser performance. Town-class light cruisers in the 1930s could produce similar power with far fewer boilers and turbine units. As a counterfactual, later small-tube boilers and machinery suited to higher pressures might have driven the same hull to roughly 36 to 38 knots. The Yarrow boilers available in 1911 were still medium-to-large-tube designs, unlike the compact small-tube systems developed later.
Notable Point · 2:58-4:54
Queen Mary and the price of speed
Queen Mary's 42 boilers and 70,000 shaft horsepower bought about 28 knots, revealing the machinery cost of early battlecruiser speed.