Why Royal Navy gun calibers often rose by 1.5 inches

Why did the Royal Navy favor half-inch calibers such as 13.5 inches?

A 1.5-inch step was large enough to justify new logistics and performance while allowing the leading navy to manage escalation more gradually than a two-inch jump.

A 1.5-inch increase is presented as large enough to justify a new calibre in performance and logistical terms. A one-inch rise from 12 inches would offer too little improvement for the Royal Navy's purposes. The US Navy's preference for two-inch steps provides a different approach. Smaller steps also leave more opportunities to escalate in an arms race. As the leading naval power, Britain could influence its rivals' choices and draw on its industrial capacity for another increase when needed.