Nautical term -
Ballast: Any weight or weights used to keep the ship from becoming top-heavy. Usually a heavy material such as iron, lead, bags of sand or shot, barrels or tanks of water, carried in the bottom of the ship to steady her or prevent her from leaning over too easily when the wind blows against her side or sails. A ship which does not carry any cargo need more ballast than when carrying heavy cargo in the
hold
