Keeping a pre-dreadnought useful

How should a 21-knot pre-dreadnought with 16-inch guns be modernized after 1917?

Prioritize radar and rangefinding, then add 6-inch and 3-inch secondary weapons to keep smaller attackers away.

Using Swedish coastal-defence ships as the model, the recommended fit includes radar, improved fire control, 6-inch guns against destroyers, and 3-inch weapons against torpedo boats. The central requirement is accurate long-range ranging so the ship can exploit its heavy guns before faster opponents close. The number of main guns also matters: four are far more credible than two.