Why is the main engine lube oil temperature lowered to 100 deg F upon reaching port?

Lowering the main engine lube oil temperature to 100 degrees Fahrenheit upon reaching port is primarily aimed at minimizing moisture buildup and rust formation on the reduction gears. When a vessel is in port, the main engine is not in operation, and the lube oil can cool down significantly. At higher temperatures, lube oil can facilitate condensation that leads to water accumulation, promoting corrosion and rust in the engine's components, such as the reduction gears. By maintaining a warmer temperature of 100 degrees Fahrenheit, the oil helps to prevent moisture from condensing inside the system. This practice is crucial for prolonging the life of mechanical components and ensuring they remain in good working condition when the engine is not in use.

The other options, while relevant to engine maintenance and operation, do not directly address the primary reason for lowering the lube oil temperature in this context. Therefore, they do not align with the main objective of preventing moisture and rust within the engine's lubrication system.