Why protected cruisers converted well

The Mersey class had been strongly built around a substantial protected deck. That deck provided much of the hull's structural strength and could carry the weight of new machinery and support equipment. At the same time, the rest of the ship lacked the extensive armor plate found in an armored cruiser. Because thick plate is difficult to remove or rearrange, this combination reduced the constraints of rebuilding. The original protective scheme therefore helped Thames and Forth remain useful when submarine support replaced cruising as the more valuable mission.