By design, cargo weight is considered in determining the ship stability. The weight distribution on a vessel is an important factor in ship stability. For a ship to stabilize on a voyage, sufficient immersion of the hull in water is required.
When a cargo is discharged from the ship, it becomes light in weight thus unstable. Ballasting is required to complement the lost weight of the ship due to cargo discharge for stability of the vessel.
There are segregated tanks on a vessel for the purpose of ballasting called ballast tanks with ballast piping system and high capacity pumps for ballast operations.
What is Ballasting and De-ballasting?
Ballasting is the process of loading the ballast tanks of a ship with sea water while de-ballasting is the process of discharging sea water from the ballast tanks of a ship.
Ballast or ballast water is the water (mostly sea water) carried in ballast tanks of a vessel to aid its stability and maintain structural integrity when a cargo is being discharged.
In medieval age, ballasting and de-ballasting was done with solid ballast for ship stability but the difficulties and time-consuming process of loading and discharging solid ballast paved way for the advent of sea water ballast since it is readily available in good quantity.
Ballasting is done for cargo unloading and De-ballasting is done for Cargo loading. Ballasting and De-ballasting processes can be performed by use of ballast water pump or gravity.
Ballast Water System comprises of :
- Ballast tanks
- Ballast pump
- Ballast water lines
- Ballast Water Treatment.
Purpose of Ballasting and De-ballasting in a ship
- To manage the trim of the ship
- To reduce free board and provide sufficient immersion of propeller (proper immersion of propeller reduces slip and vibration)
- To provide better weight distribution along ship to reduce bending stress.
- To reduce pounding or slamming action in rough weather.
- To provide satisfactory positive metacentric height.
- To provide better rudder action due to greater draft so that water flow around the rudder is stream line water flow.
- Better sea-keeping.
What is a ballast tank?
Ballast Tanks are dedicated tanks for ballast water storage. It can be wing tanks, double bottom, hopper tanks (bulk carriers).
What is a ballast water pump?
Ballast water pump is a centrifugal pump used for transferring or discharging the ballast water. It also serves as the stand-by sea water circulating pump. Valves are used for tanks where pumping is not suitable.