Heavy weight, often sea water, which gives a ship stability and improves handling when she is at sea and not carrying cargo. Such a ship is said to be steaming in ballast or ballasting.
Reasons for a ship having to steam in ballast include:
(a) no further cargo being available at the port where the ship discharged her previous cargo. Typically this happens in the case of tramp ships which are not on fixed schedules and are looking for further employment;
(b) in some trades, a ship may perform a series of voyages between two ports, very often on time charter or contract of affreightment, with a (suitable) cargo available in one direction only. This situation applies particularly in certain bulk trades where the ships are specialised and only suited to one type of cargo;
(c) in other instances, it may be more economical, after discharging a cargo, to steam empty to another port to pick up the next cargo than to take cargo which pays a low freight to that port.