How Do Boats Float?

You take a boat on the water, floating down the Indian River in the warm sun. But have you ever wondered how that boat stays on top of the water? Especially considering all the weight it’s holding?

Two-thirds of the earth is covered with water. The world would be mighty small if we were held to just land. So how is it that we can use these heavy crafts to float on top the water? Let’s start with the basics.

What is a Boat?

A boat is a vehicle that can float or move on a body of water such as an ocean, river, or lake. It moves either by its own power, usually using a motor, or by using power from the elements like wind, waves, or the sun.

Most boats move part through the water and partly above the water. Some boats are able to lift up and move on top of the water like hovercrafts and hydrofoils. Others move entirely under the water like submarines and submersibles, or small submarines.

So what are the differences that allow them to do this?

Why Boats Float (Or Don’t)

All boats keep from sinking through buoyancy, the act of floating. An objects density is what forces it to either float or sink. The density of an object is how much a certain volume of it weighs. If the object is denser than the water it’s in, it will sink. If the object is less dense than the water it’s in, it will float. The size of the object is irrelevant. For example, a gold ring will sink if you put it in water but a piece of plastic the size of a football field will float. The basic rule of buoyancy is that an object will sink if it weighs more than exactly the same volume of water.

Types of Buoyancy

There are three types of buoyancy, positive, negative, and neutral. Positive buoyancy means that the object will float. Negative buoyancy means that the object will sink. And neutral buoyancy means that the object will float at a middle depth without floating or sinking.

Think of a submarine. A submarine has ballast tanks that take in water or air to make it rise and fall as needed. If the tanks are filled completely with air, it will float toward the surface because the tanks will weigh less than the equal volume of water. If the tanks are partly filled with air, it will float at a middle depth without floating or sinking. If the tanks are filled completely with water, the submarine will sink to the bottom.

While a submarine is either on top of the water or under the water, most boats partly float and partly sink according to their own weight and how much weight it carries. The greater the total of these two weights are, the lower it sits in the water. However, boats can only withstand so much weight before sinking.

Archimedes’ Principle

Greek mathematician Archimedes is the father of buoyancy. He first came up with the answer to floatation some time in the third century BCE. Legend has it that he had been given the job of finding out whether a crown made for the king was solid gold or a cheap fake partly made from a mixture of gold and silver. The most popular version of the legend says that Archimedes was taking a bath and noticed that the water level was rising as he immersed himself in the tub. He then realized that if he dropped the gold crown in the bathtub, it would push out or “displace” its own volume of water of the side effectively giving him an easy way to measure the volume of a complex object.

He figured out that by weighing the crown, he could easily work out its density and compare that with the gold. An objects density is its mass divided by its volume. If the density of the crown happened to be lower than the gold, the crown would be fake.

Later Archimedes came up his famous law of physics, which today is known as Archimedes’ Principle. This law states that when something rests in water, it feels an upward, or buoyant, force equal that the weight of the water that it pushes aside, or displaces. If the object is completely submerged in water, it effectively loses its weight.

This is why objects seem to weigh less under water than they do when they’re on dry land. An example is if you take a rubber diving brick, a brick used to train with in a pool, it will feel lighter under water than it will when brought to the surface. You’re getting help from the buoyant force under water.

Next time you’re out on a boat from 321 Boat Club and you wonder why you’re able to float, just remember the concept of buoyancy and Archimedes’ Principle.