Why do Ships Float? Why Don't They Sink?

Most recent answer: 10/22/2007

Why do ships float? Why don't they sink?
- Anonymous (age 10)
Lexington, Ky USA
It seems like they ought to sink because we're used to seeing things fall. But for the ship to sink it has to push aside some water, which has nowhere to go but up. So it's a question: does the ship 'want' to sink more than the water 'wants' not to rise? It turns out that just depends on whether the ship weighs more or less than the amount of water that would fill the same space. Real ships have lots of air inside, so they weigh less than the same volume of water, so they float.

Mike W.

Here's a cool experiment you could try. Fill your sink with water, then put a bowl in it. Now see how much weight you can put in the bowl before it sinks. You could even pour in water as the weight. If you use water, look at how high the water comes up on the inside and on the outside of the bowl as you add more water. Is it the same or different?

And by the way, isn't it cool that it doesn't matter how deep or shallow the water is? If something floats in 2 feet of water, it will float in 2000 feet of water!

I hope this makes sense.


(published on 10/22/2007)

Follow-Up #1: how things float

how do they float
- Anonymous
We’ve revised the original answer, so maybe that helps. The water pushing up on the bottom of the boat is what keeps the boat afloat.

Mike W.

(published on 11/08/2007)

Follow-Up #2: Sink or float?

I thought it had to do with the density of the ship, versus the density of the water? Or does it just have to do with the weight of the displaced fluid?
- Alina (age 13)
New York
It depends on both.  Whether or not a ship sinks or floats is determined by both its total weight and the weight of water it displaces.  If the former is bigger it sinks, if the latter is bigger it floats.  Try the experiment suggested in the previous question.


(published on 05/20/2008)

Follow-Up #3: Floating Ships and Buoyancy

what all factors help the ship to float?
- achu (age 12)

There are a few things that determine whether or not a boat will float, such as its shape, its load, and where the boat is sailing.  First, let's talk about density.  Density is a measure of how much mass of something is packed into a certain volume.  For example, iron is denser than wood because a volume of iron (say a cube one centimeter on each side) weighs more than the same cube if it were made of wood; there's more "stuff" inside the cube of iron than in the cube of wood because its atoms are mostly heavier but take up about the same amount of space.  If something is denser than water, in general, it will sink.  If it is less dense than water, it will float.  Cooking oils are less dense than water, which is why they sit on the top of pots of water.

Ships are often made of wood, some kinds of which are less dense than water, and some of which are more dense.  Ships can also be made of metals like steel (denser than water) or tough plastics (usually denser than water).  You would imagine that a boat made of dense stuff would sink, but the boat floats primarily because of its shape.  Boats stay afloat with heavy loads because they're hollow; they aren't solid hunks of wood or metal.  This means that the boat experiences a really strong "buoyant force," upward, against the pull of gravity which is downward.  You can see this for yourself if you put a plastic bowl upright in a pot of water or a bathtub.  If you set the bowl in the water, it will float, but if you fill the bowl with water, it may sink to the bottom (depending on if the plastic is denser than the water or not).

The other factor that determines the buoyancy of a boat is the salinity (the salt content) of the water.  Saltier water is denser than fresher water, so denser things can float in saltier water.

I hope that answers your question!

(published on 03/05/2011)

Follow-up on this answer