Pre-dreadnought secondary batteries helped sink armoured opponents once the range closed. A dreadnought carried enough heavy guns for that work, so its secondary battery could focus on torpedo craft approaching while the main guns engaged capital ships. HMS Dreadnought began with 3-inch or 12-pounder weapons, four-inch guns became the British battlecruiser norm, the Admiral class moved to 5.5-inch guns, and the planned G3s adopted six-inch weapons. The four-inch choice matched small, fast, lightly protected targets. This sequence shows how the dreadnought revolution changed the division of labour among a warship's guns.
Chapter · 5:18-8:45
Why dreadnought secondaries targeted torpedo craft
Once dreadnought main guns could handle armoured opponents, secondary batteries focused on destroyers and torpedo boats.