How do Boats Float?

Mary McMahon
Mary McMahon

If someone throws a crowbar in the ocean, it sinks, but a ship, which has a weight many times that of a crowbar, manages to float in the water. This interesting phenomenon known as buoyancy is the reason that boats float. The properties of buoyancy were first described by the Greek mathematician Archimedes, in what we now call Archimedes' Principle. This principle states that any object, wholly or partly immersed in a fluid, is buoyed up by a force equal to the weight of the fluid displaced by the object.

A boat's hull, which can be seen in its entirety when laid up on dry land, displaces an equal area of water when it is floating.
A boat's hull, which can be seen in its entirety when laid up on dry land, displaces an equal area of water when it is floating.

Archimedes is said to have discovered the principle in the bathtub, which makes sense, because a bathtub is a great place to research buoyancy and displacement. He discovered that when an object (such as a scientist) is placed into a fluid, the level of the fluid rises, because the object has displaced some of the liquid. Archimedes realized that any object placed in water will displace its own weight or volume in water, which ever comes first. This is called the weight-to-surface area ratio.

The deck of a modern lifeboat can be sealed off to prevent swamping in rough weather.
The deck of a modern lifeboat can be sealed off to prevent swamping in rough weather.

He realized that different materials which have different densities but equal volume will have a different buoyancy. A bowling ball will sink, while a balloon filled with air will float, even though they occupy the same volume in space. This is due to the greater density of the bowling ball. Unlike a balloon, a bowling ball weighs more than the weight of the water it displaces, so the ball has a low surface area-to-weight ratio. A boat is essentially a hollow shell filled with air, meaning that it has a large surface area-to-weight ratio. Therefore, the boat will still be well above the surface when it has displaced its equivalent weight in water.

Boats float because of their design. When a boat is heavily laden, it will settle lower in the water, because its surface area-to-weight ratio is different than when the boat is light. A boat will remain floating and stable provided it is not overloaded.

Once someone knows how boats float, he or she can imagine how it is that other objects float. From pieces of wood to ducks, an object's weight-to-surface area ratio dictates its buoyancy. This also explains why boats sink: if the hull of a boat is breached, it begins to take on water, which makes it denser, causing it to displace more water. If the boat takes on enough water, it will become too heavy to remain buoyant.

In the case of the traditional Irish currach, an exterior of stretches leather is used to seal the hull off from water.
In the case of the traditional Irish currach, an exterior of stretches leather is used to seal the hull off from water.
Mary McMahon
Mary McMahon

Ever since she began contributing to the site several years ago, Mary has embraced the exciting challenge of being a wiseGEEK researcher and writer. Mary has a liberal arts degree from Goddard College and spends her free time reading, cooking, and exploring the great outdoors.

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Discussion Comments


What is the relationship between surface area to weight ratio and density? Isn't it simply that an object with a density lower than that of the fluid will float, while an object with a density higher than that of the fluid will sink?

Why does surface area come into it?


I find it amazing that boats and humans can float so easily. Both weight a good amount, but because of this, both displace a large amount of water, so it all balances out in the end.


@kylee07drg – I know what you mean. It seemed strange to me before that big branches or even entire trees would float on water. They are so heavy that I often can't lift them on land, but the water just carries them as if they are almost weightless.

I never understood how boats made of wood could float, but it's basically the same principle. Wood floats because it isn't very dense, so boats made with wood have no trouble floating along.

I suppose the ideal boat would be one made of light wood, like driftwood. That stuff is so light that I can toss around a big piece of it with no problem.


I always thought that boats floated because of their shape. I had never considered the fact that they are mostly open air inside.

It also makes sense that a boat sinks when it takes on water. I have tried to pick up a five gallon bucket after pouring water up to the brim before, and it was nearly impossible for me. Water is extremely heavy.

It's strange that something so heavy can be easily pushed aside by your arms when you're swimming. Lifting it and pushing it over are two different things, though.


Just last week, my daughter asked me, “Why do boats float?” I was ashamed that I had no idea how to answer her. Thank you for helping me with this.


anon28472, to figure this out, figure out if the weight and space that the ice cubes will take up when melted compared to before(when they were still ice) with the change. since water is the only liquid that expands when turning into its solid form(ice), the glass' density will rise when the ice melts. temperature is a key feature in density. the weight never is changed when temperature interferes. instead, the space taken decreases when the ice melts, meaning the weight/space ratio(which you probably know as density) increases, meaning that the glass COULD sink. the fact is that it is impossible for the glass to not sink if everyone's interpretations were different.

I'm not naturally knowing of this. i actually have a science fair in a couple of weeks that i really need to catch up on and am working hard on. peace


my family argued this answer. --connor


It will stay the same because ice is the same amount of the water it started as, so therefore when it melts the glass of water will have the same water level.


When water freezes it increases in volume about 9%. The ice cubes are about 9% bigger than its comparative water volume, but the weight displacement is the same. Therefore the water level will be the same when the ice cubes melt.


Here's a related question my family has argued about for hours. If you have a glass of water with ice-cubes floating in the water. Will the water level rise or sink when the ice-cubes have melted?

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