Ballast and the Shape of a Hull

A ship carrying nothing is in a worse position than a ship fully loaded. [A] An empty hull sits high, so the propeller turns partly in air and the rudder has less water to push against. [B] Worse, the weight that kept the vessel upright has gone with the cargo, and a tall empty ship rolls much further in the same sea. [C] The remedy is older than steam: take on something heavy and low down, in a place where it costs nothing. [D]

For centuries that something was gravel, stone or sand, shoveled in at one port and out at the next. Iron ships changed the practice by making water usable. Pumped into tanks built against the bottom and sides, water can be taken on in minutes, moved from tank to tank to correct a list, and released on arrival. It is also free.

The consequence was not foreseen. A tank filled in one harbor and emptied in another carries whatever was living in the first harbor, and some of it arrives alive. The cheapest answer to an old problem of balance turned out to be an efficient way of moving small animals and plants between continents, and it is now regulated for that reason rather than for anything to do with stability.