In simplistic terms, how can a large cruise ship or aircraft carrier stay afloat?

Question

I understand that it has to do with density of the water to the density of the ship, but explaining it to a child doesn't work. I read somewhere that it also has something to do with displacement of water. I will need to explain this to my children (and my wife too).

Answer 1

That's exactly how it works. Anything that can displace a quantity of water equal to its own weight will float, because if it starts going down too far and displacing more water, that water will be pulled back in under it by gravity. Of course, in order to keep floating, it must not leak or otherwise become full of water. Metal ships stay up because the metal is only a thin sheet holding a lot of air. It's the air that is really doing the displacing, the metal just keeps the water out and the ship in a rigid form.

Keep in mind that the object doesn't actually need to be pushing its own weight in water somewhere else. All it needs to do is BE ABLE to do that, and have something to sit in big enough to hold it. There's an interesting question, 'How much water is required to float an aircraft carrier?'. The answer is, you can do it with only a cup of water, so long as you put it in a tub almost exactly the same shape and size as the aircraft carrier's own hull, so that there is a thin film of water between the two. The ship will still float, because it can be considered to be displacing that much water (there'd be that much water there if you took it out and then filled the tub up to the line the water is at when the boat is in it).

You can read this for another, perhaps clearer, explanation:
http://www.madsci.org/posts/archives/nov2000/974385595.Ot.r.html

Answer 2

The ship displaces water that is equal in mass to the ship itself. A practical demonstration of this is good, here are two things that you can do..........

First get three plastic containers from the kitchen (one big one, one medium one and one small one), some glass marbles and a set of kitchen scales.

Put the medium container inside the large one and fill the medium one with water Right to the top. Float the small container on the water. Add marbles to the small container until it has displaced some water which collects in the large container.

Tip the displaced water into the scale pan and weigh it. Poor the water out and weigh the small container with the marbles in it. The two weights will be the same.

Do it again with more marbles. Same result.

Now get some kitchen foil which is metal, just like a ship. Make a nice little ship shape out of it and show that it floats. Now screw the boat up into a small tight ball. Throw it in the water and show that it sinks.

Answer 3

The standard definition of floating was first recorded by Archimedes: An object in a fluid experiences an upward force equal to the weight of the fluid displaced by the object.

So if a boat weighs 1,000 pounds (or kilograms), it will sink into the water until it has displaced 1,000 pounds (or kilograms) of water.

Provided that the boat displaces 1,000 pounds of water before the whole thing is submerged, the boat floats.

It is easy to shape a boat in such a way that the weight of the boat has been displaced before the boat is completely underwater.

The reason it is so easy is that a good portion of the interior of any boat is air (unlike a cube of steel, which is solid steel throughout). The average density of a boat -- the combination of the steel and the air -- is very light compared to the average density of water.

So very little of the boat actually has to submerge into the water before it has displaced the weight of the boat

Answer 4

When an object is placed in water it will begin to displace (push out of the way) the water that is directly beneath it. When the object has displaced it's own weight in water - that is when it has displaced that amount of water the weighs the same as the object, then the object will weigh the same as the water it is setting in. Therefore it floats. If it takes more water to displace it's weight than the size of the object, it will sink. Large boats, cruise ships, air craft carriers etc displace their own weight in water because they are mostly air inside with a thin metal shell around the air. Therefore they can displace their weight in water very easily and float.

Answer 5

A ship is pushed up by a force equal to the weight of water equal to the volume of the submerged part. So if an object is made hollow, like the belly of a ship, it will displace a very large volume of water, creating a strong buoyant force to push it up. If the same material were made into a tight ball, it would sink because it would be heavier than its own volume in water.

Answer 6

Water displacement. The shape of the hull determines it's buoyancy and weight capacity. Example... if you took the cruise ship and gave it a deep and skinny hull vs. the wide and flat one it has... even with the same volume... the deep hull would sink.

Basically take a cigarette box and put 1 lb inside... if you put it on the skinny edge it will sink. But if you lie it flat the same weight will float.

The more water you can displace - the more weight you can effectively carry

That is the simplest way I can think of explaining it. Hope it helps.

Answer 7

I want to echo what several have said about bouyancy and displacement.

I don't know if it helps to explain to a child, but it's a question of equilibrium. When the boat displaces a certain amount of water it reaches equilibrium.

Lots of things are in equilibrium. Like when we stand on the floor, the floor pushes back. If we jump on a trampoline, the trampoline pushes back. How is a floor different than a trampoline?

If we push on a wall, the wall pushes back. If we push on a wall while we have roller skates on, what happens?

Answer 8

Another way of looking at this is when you're swimming. When you are upright, you displace less water than your weight. Therefore you will begin to sink. However, when you lie on your back, you displace more water than your weight and can float.

Answer 9

The ship must way less than the water that would occupy the space that the ship is in. They are mostly hollow so the weight is small compaired to the size.

Answer 10

Things heaver then water sink. So the total weight of the boat is less then the total amount of the water that is displaced by the boat. So, for it's volume, it is lighter then water.