Queen Mary's 42 Yarrow boilers produced 75,000 shaft horsepower. In the comparison, the Queen Elizabeth class reaches comparable output with 24 oil-fired boilers. A Queen Mary-derived design using roughly 30 oil-fired boilers might therefore aim at 30 or 31 knots while freeing internal volume. Tiger's 39 mixed-firing water-tube boilers generated 85,000 shaft horsepower. The later Renown class retained 42 oil-fired boilers, delivered 112,000 shaft horsepower, and reached 32 knots. Oil could reduce the machinery count or turn the same boiler volume into much greater speed, so propulsion choices directly shaped the possible armament and hull layout.
Chapter · 24:41-27:08
Oil firing and the Queen Mary boiler case
Queen Mary's 42 boilers become the basis for asking how oil firing might release space or power for a faster, differently armed battlecruiser.