Ballast

Ballast solves a basic problem of ship stability. A large cargo ship is designed to float safely when loaded, but with little or no cargo it would sit too high in the water: its propeller and rudder would be partly exposed, its hull would be vulnerable to slamming in a seaway, and its stability and handling would suffer. To correct this the ship floods dedicated ballast tanks (in the double bottom, wing tanks and forepeak and afterpeak) with seawater, lowering itself to a safe operating draught. As cargo is loaded the ballast is pumped out, and the process is reversed when the cargo is discharged, so ballast and cargo trade off against one another throughout a voyage.

Beyond keeping the ship deep enough, ballast is used actively to control trim, list and the stresses on the hull. By moving water between tanks the crew can adjust the fore-and-aft trim for good steering and fuel economy, correct a list, and keep bending and shear forces within safe limits during loading and discharging, which is especially important on bulk carriers where cargo is worked quickly in large amounts.

Ballast water carries an environmental problem, however. Water taken up in one port contains living organisms (larvae, plankton and small marine life) that can be carried across the world and released alive in another ecosystem, where they may become invasive species that damage native habitats and fisheries. To control this the IMO's Ballast Water Management Convention now requires ships either to exchange ballast in mid-ocean or, increasingly, to treat it through an approved ballast water treatment system that filters and disinfects it before discharge. Ships carry a ballast water management plan and record book, and compliance is checked by Port State Control.