How Do Boats Float?

An amateur comedian once said that in Las Vegas you could pretty much do whatever “floats your boat.” His comment was a play on words, of course. There is a very specific factor that makes a boat float. It goes by the name “displacement.”

As the great scientist of the ancient world, Archimedes, stated, there is an upward force exerted on an object placed in a fluid. This force is equal to the weight of the fluid displaced by the object. In our case, the object is a boat. Use a boat of any weight and it will sink into water until it displaces its weight. If this happens before the boat goes under, it will continue to float.

Shape, Size

The above explanation is very basic, by necessity. The shape of a boat is also very important. If the craft is constructed so that there is a lot of air in the interior it is going to float rather lightly. This is sometimes called buoyancy. This is why a ship made of some of the heaviest materials known to man will still float.

But we have to go beyond the basic idea of weight to really understand why boats float. It is necessary to grasp the idea of pressure when we are discussing water displacement. A designated amount of water will exert a specific amount of pressure (measure in pounds per square inch). This is the force that Archimedes referred to so many centuries ago.

As one academic text states it, each square inch of the boat’s bottom has a specific amount of water pressing upward. The combined pressure over the entire surface of the boat serves to “float your boat.”

Opposite is True

Sometimes it is necessary to see the opposite side of the issue to really understand how things work. We have seen that a ship or boat with high sides, a lot of surface on the hull and plenty of air or empty space inside is ready to float on most bodies of water. But if the boat is too big and the body of water is too small, there won’t be enough water to be displaced or to put sufficient pressure on the hull. The boat will sink.

In a similar, negative view: a small piece of steel that is very dense and heavy doesn’t have enough surface or interior air space to compensate for its weight. It will displace a small amount of water quickly and sink. Think of it as the heavy steel “looking for” enough water to equal its weight. If the same amount of weight was distributed over a larger area the water could exert enough upward pressure to keep the steel afloat.

One of the factors that makes shipping on the ocean a bit easier than on fresh water is the buoyancy of salt water compared to fresh water. It is easier to float a large ship or boat on salt water than on a fresh-water lake, for example. The science involved requires an understanding of how salts are dissolved in water.