Sustained missions require more hulls than a surge

The Roberts-class monitors and HMS Erebus remained useful during the Second World War, yet the Royal Navy needed fewer monitors because many amphibious operations were discrete events. Support off Belgium, Gallipoli, and other First World War coasts instead demanded ships continuously on station. Sustained commitments therefore required a larger pool for rotation, repair, and rearming than a force assembled for one surge. Keeping six ships deployed could require nine or more hulls once time in port, battle losses, and long refits were included. The force-planning lesson is to count availability, ammunition expenditure, damage, and replacement time rather than treating every commissioned hull as permanently ready.