The Chatham and Birmingham subclasses are close in size, at about 458 feet long and roughly 6,000 long tons at deep load. Chatham carries eight six-inch Mark XII guns and Birmingham nine, with 200 rounds per gun. Both are assigned twelve Yarrow boilers, four shafts, about 25,000 shaft horsepower, and a 25.5-knot maximum speed. Coal is loaded by stokers and oil is sprayed onto the fire to increase heat. Southampton also carried an aircraft platform. These similar arrangements make it practical to consider a common conversion for four surviving ships in the counterfactual.
Notable Point · 36:36-40:55
The Chatham and Birmingham hulls before conversion
Similar hull sizes, four shafts, and large six-inch magazines give the two subclasses a common starting point for conversion.