Why new boilers cannot turn the R class into 28-knot ships

How fast could an upgraded R-class battleship become with small-tube boilers?

Probably no more than about 25 knots, because reaching 28 knots would require major changes to machinery space, shafts, hull proportions, and possibly armament.

The R class starts around 21 knots, so a 28-knot target demands roughly a one-third increase rather than the smaller gain contemplated for the Queen Elizabeth class. Its shorter, narrower machinery arrangement leaves less room for the additional power plant. Removing a turret to create boiler space would turn the work into a fundamental reconstruction. The better fleet choice is to retain the ships for second-line duties and spend modernization resources on hulls that can join the fast battle line.