Why are all navy boilers laid-up when idle for more than 24 hours?

The correct answer is centered on the importance of preventing oxygen corrosion in the watersides of the boiler. When a boiler is idle for extended periods, especially beyond 24 hours, the water within can become stagnant and exposed to oxygen. This combination can lead to corrosion, which significantly increases the likelihood of damage to the boiler interior.

By laying up the boiler, which involves removing or treating the water and properly preparing the boiler system, actions are taken to minimize or eliminate the presence of oxygen. This helps in maintaining the integrity of the boiler materials and prevents long-term deterioration, which can be costly to repair and can impact the operational readiness of the vessel.

Other options, while they address important boiler considerations, do not focus specifically on the corrosive effects of oxygen on the water surfaces of the boiler, which is the primary reason for the lay-up procedure.