Carrier tonnage, conversions, and the transition to new builds

Treaty delegates appear unusually restrictive toward carriers even though no existing project approached the permitted maximum. The eight-inch gun ceiling is understandable because Furious, Courageous, and Glorious provided precedents for carrier hulls associated with 15- and 18-inch weapons, while negotiators still regarded gunfire as more practical than aircraft. The alternative allocation applies the 10:10:7:4:4 ratio to carrier tonnage and limits construction in progress to one-fifth of a navy's allowance. It sets a 33,000-ton unit ceiling, requires early carriers to be conversions, permits a mixed program after 1926, and allows wholly new construction after 1930. Replacement rules let pre-treaty conversions retire from 1930 and post-treaty conversions or pre-treaty new builds from 1934, while post-treaty new builds serve at least eighteen years. Because conversions use tonnage less efficiently per hull, the timetable forces a choice between obtaining airpower sooner and waiting for better purpose-built ships.