How Do Boats Float? Understanding the Science Behind It

Boats are such a part of our lives that we may not think twice about all the things that make them tick. We're more than happy to enjoy a ride over the water, but that's one of the more puzzling facts about boats. How exactly do boats float whenever other items that weigh far less than them can sink right to the bottom?

While it may seem illogical once you start to think about it, plenty of factors help floating boats make sense, and they all come down to specific principles of science. Read on to see the forces at work in keeping types of boats on the water instead underneath it.

Displacement, Buoyancy, and the Archimedes Principle

The first two principles that play a part in boats (and anything that floats or sinks) are displacement and buoyancy. Let's start with displacement. This principle goes back to the ancient Greek mathematician Archimedes. When tasked with determining the purity of gold in a crown, he was attempting to figure out the volume of the object to compare to its weight to see if it had the correct density.

When figuring out the volume of objects, such as cubes, prisms, or spheres, there are clear-cut calculations we can use, but something like a crown is much more complicated. In Archimedes's case, he ultimately realized that when he got into the bathtub, the water level rose depending on his mass—and that the crown would do the same. How liquids move about based on the volume of objects inside them is known as displacement.

You've likely experienced this sort of thing when taking a bath yourself, or possibly when adding something to a container of liquid. Displacement is now in use to refer to the water density a boat or other object moves out of the way whenever it rests on or in it. It's also the first part of understanding how boats float.

The other major component is buoyancy, which Archimedes also discovered. The upward force that objects feel whenever partially or fully submerged into water is buoyancy. Whenever objects become fully immersed, they experience enough buoyancy to reduce its weight. Think of how it's easier to move when swimming or how it takes much less effort to lift something the water than the same object on dry land.

Buoyancy and displacement correlate together: the amount of displacement an object equates is equivalent to how much buoyancy it receives. This concept is known as the Archimedes Principle, and it's part of why some objects float and others do not.

Weight and Density

If displacement and buoyancy were the only factors at play in keeping boats afloat, then we wouldn't have any trouble with ships sinking. We also wouldn't have any issues with walking on water ourselves! However, weight and density play a crucial role in determining what floats and what does not.

As we mentioned, two objects can weigh the same but behave very differently in the water. One may float, another may partway submerge in the surface, and one more may sink. The same applies to objects that are the same in volume. Everything comes down to density.

If you don't remember how to calculate density, it involves two things we've already discussed: mass (the weight) divided by the volume. If you have two objects with the same volume, but one weighs more, that heavier one will be denser. And while specific parts of a cargo ship or power boat may be incredibly dense and others are not, the average density will impact if an object or ship floats.

Consider a piece of steel. If you drop a bar of it into the water, it's more than likely to sink. However, we use steel and other dense metals in the construction of boats all the time, and they still float. This result is because the other materials on the ship, along with the air inside, help balance out the average density to a level that it can stay afloat.

Think about a bottle, a cup, or a bowl (or any other similar dish). If you fill up your sink and place an empty container on the surface, it will float. However, as you add water to it, the heavier the object becomes, creating more displacement. Ultimately, there will be too much weight in the cup, and it will sink below the surface. That's because the air is less dense than water, and it balances out the average density. The same applies to boats.

Does Salt or Fresh Water Make a Difference?

Since density is such a critical factor in whether a ship floats or not, salt and freshwater can make a difference in how a boat sits. Saltwater (as it contains salt) is denser than freshwater, and the exact levels will depend on the concentration. Because of this difference, boats will often rest higher on the ocean than they would on a freshwater lake.

Typically, other than a slightly different resting level in the water, you won't see much difference with the same boat in different environments when it comes to floating. On a scientific level, though, there is a distinction between the two.

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How Do Boat Manufacturers Know How to Make Boats That Float?

We've covered the basic principles that have come into play with understanding why boats float, but how can boat manufacturers tell if a design will stay above the water or sink below. Much like how you can calculate the density of an object, it's also possible to calculate how much upward buoyancy an object will create when placed in water.

The calculations can get a little bit complex when you've unfamiliar with all the values. They involve comparing the overall volume and density of an object to the amount of water it displaces, as well as how much buoyancy pressure that displacement will produce at a certain depth in the water.

By calculating these values, it's possible to determine if a boat will displace enough water to provide enough buoyancy to keep its maximum density afloat. Manufacturers use these types of calculations to learn not just if a vessel will work, no matter if it's an outboard motor vessel or a sailboat, but also its maximum carrying capacity—crucial for large cargo vessels that transport massive amounts of weight along with them.

Why Do Boats Sink?

As we've covered all the reasons that boats stay afloat, why do they sink? We've already covered one of the ideas in our bottle/cup/bowl example above: density. Any air inside an object will help raise its average density. If a vessel springs a leak, water will push the air out of place, making the ship weigh heavier and pushing it down further to the point of sinking. But there's a little more science at play.

Remember how we mentioned that the buoyancy is equal to the level of water displacement that an object creates? While you can impact the density of a boat by adding weight, the amount of water a vessel can displace is wholly dependent on its volume.

As a boat sinks further into the water, it displaces more water. This effect is why you can add more weight to a vessel (such as passengers or cargo), and it will sink deeper into the water, but still stay afloat. Ultimately, however, when an object becomes fully submerged, it cannot displace any more water than it already has. As such, there's not enough buoyancy to counteract the downward force of gravity anymore.

When that point happens, the downward force wins, and the boat sinks further beneath the water—especially if there isn't any air left inside to balance out the average density.

While the science behind boats floating is rather involved, the results are the same for us: an enjoyable way to spend some time on the water.