How would other navies respond if Britain adopted small-tube boilers with the Iron Duke class?
Short answerThey would probably improve their own machinery and investigate small-tube boilers, with the response shaped by the British performance gain.
NotesThe counterfactual begins with Britain introducing small-tube boilers around the Iron Duke generation. Other fleets would probably respond by increasing their own machinery capability rather than ignoring a visible British advantage. They would investigate and adopt comparable boiler systems. The size and speed of that reaction would depend on how much the new machinery improved British ships in practice.