Because of the pressure of the water, forces act on the boat. The resulting force is the buoyancy force, which pushes the boat up. Have you ever wondered why heavy steel ships float and don't sink? Plain steel would sink in water immediately, as would a wrecked steel ship like the Titanic or the more recently sunken Costa Concordia in Italy. But an intact ship floats.

 

The buoyancy force

The reason that a ship floats is that it displaces a lot of water. The displaced water wants to return to it's original location, where the ship is now, and this pushes the ship upwards. The force which pushes the ship up is called the buoyancy force. It appears due to the Archimedes' principle: “Any object, wholly or partially immersed in a fluid, is buoyed up by a force equal to the weight of the fluid displaced by the object.” That means that when an object displaces one liter of water, the object gets pushed up with the buoyancy force equal to the weight of one liter of water. The more water that is displaced, the stronger the buoyancy force is which pushes the object up.

Floating – equilibrium of forces

Any object which dips into water experiences two forces: the gravity force which pulls it down due to its weight, and the buoyancy force which pushes it up. If these forces are equal, then the object floats. A stone in water also experiences these two forces: a buoyancy force due to the displaced water and the gravity force due to the stone's weight. But a stone's buoyancy force is much smaller than it's gravity force. That's why the stone sinks. However, the buoyancy force makes the stone sink slower than it would fall in the air.
A ship also experiences these two forces. But because of the body design of the ship, it displaces enough water so that the buoyancy force is as strong as the gravity force. These forces neutralize each other and the ship floats. A ship always dips deep enough to displace an amount of water that creates a buoyancy force as strong as the gravity force. If a ship carries heavy cargo, it dips deeper into the water. This creates a stronger buoyancy force which is exactly as strong as the increased gravity force.

 
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