What is used to deballast tanks below the waterline?

The process of deballasting involves removing water from tanks to allow for better stability and handling of a vessel. Compressed air is the most commonly used method for deballasting tanks located below the waterline. This technique works by introducing compressed air into the tank, which displaces the water. As the air rises to the top of the tank, it forces the water out of the bottom, allowing the vessel to control its buoyancy and trim more efficiently.

Using compressed air for this purpose provides several advantages, such as reducing the need for mechanical pumps, which might not be available or convenient in every situation. Moreover, air can be easily compressed and stored onboard, making it a practical solution for real-time adjustments to the vessel's ballast.

On the contrary, freshwater and saltwater are typically used for ballasting, which means adding water to the tanks rather than removing it. Hydraulic fluid is not generally associated with deballasting operations, as it relates more to machinery and engine systems rather than water displacement in ballast tanks.