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Why steel giants are able to float

Fully laden and fuelled, one of Hapag-Lloyd’s biggest ships from the “Hamburg Express” class weighs up to 180,000 tonnes – and still these giants float on the world’s oceans. How do they do it?

The explanation was provided by the Greek scientist Archimedes more than 2,000 years ago. This is why we speak of “Archimedes’ Principle” in physics today when we talk about the buoyancy or upward force of a solid body immersed in a fluid. The principle describes a very simple phenomenon, according to which the buoyancy of a ship is equal to the weight of the water displaced by the ship.

The buoyancy acts against the weight and pushes the ship upwards out of the water. As long as this buoyancy is greater than the weight of the ship that forces it down into the water, the ship will float. Anyone who has tried to push an empty bucket down (base first) into water knows how strong the upward force can be. As long as the density of a body is lower than that of the fluid, the body will float in the fluid.

With a container ship, the air pockets in the ship’s belly ensure that the density of the freighter, in spite of its impressive total weight of several thousand tonnes, is always less than the density of the water. However, if the ship is loaded with cargo, this cargo gradually replaces the volume of air in the belly of the ship. The density of the ship as a body then increases in mathematical terms and it sinks deeper into the water – meaning that the ship’s draught increases.

However, water does not always have the same density either. Fresh water is less dense than salt water, so the latter supports a ship better. For example, if a freighter sails from the salty North Sea into the fresh water of the River Elbe, towards the Port of Hamburg, its draught increases as the salt content of the water decreases. For the “Hamburg Express”, this can mean up to 30 centimetres more draught.
Source: Hapag-Lloyd AG