Balancing with ballast!
Ever heard the word ballast? It comes from the old Nordic word “barlast” which means without (“bar”) cargo (“last”). And there’s a reason for that…
When a ship is fully loaded (say, a ferry carrying hundreds of cars and trucks), the weight of the load submerges the hull in the water. This is good because, as you learned from episode 4 of our series, it lowers the center of gravity and increases the stability of the ship. But with little or no load – or “barlast” – onboard, the opposite happens.
Back in the old days, sailors loaded heavy materials such as rocks into the hull to offset the negative weight of an empty ship. They called it – you guessed it – ballast.
But as you can imagine, it wasn’t very practical. So today we use ballast water in ballast tanks to achieve the same effect. Take a look at this film:
So, to summarise:
Water is pumped in and out of special ballast tanks located at the bottom of the hull.
The amount of ballast water loaded corresponds to the weight of the “missing” cargo.
As a result, the ship floats at the same depth regardless of payload.
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