How do boats and huge ships float?

1. Here are two pieces of steel that weigh exactly the same. The ball sinks to the bottom, but the bowl-shaped boat floats. The difference is their shape.
2. Water presses on every part of the boat that is underwater. The push on the bottom is biggest, so the water pushes the boat up. This push is called buoyancy.
3. A floating boat pushes water out of its way. If we catch that water and weigh it, it weighs exactly the same as the boat. Look, the scale balances!
4. Each block makes the boat heavier, so it sinks a little deeper until it pushes aside enough extra water. Ships have a load line painted on the side to show how deep is safe.
5. With the same cargo, a wide boat sits high, but a narrow boat sits so low that water nearly comes in. A tall, thin boat is wobbly and can tip over.
6. A giant cargo ship is made of steel, but inside it is mostly air and cargo. Its whole shape weighs less than the water it pushes aside, so it floats. A big propeller pushes water backward to move it forward.

Tap each part of the ship. Many of them move when you find them!

• Hull. The hull is the watertight steel shell. Its wide, hollow shape pushes aside a huge amount of water, which pushes the ship up.
• Bulbous bow. The round bulb under the front makes its own little wave that cancels some of the ship's bow wave, so the ship uses less fuel.
• Ballast tanks. When the ship is empty, it takes seawater into ballast tanks low down. The extra weight keeps it steady and deep enough for the propeller to work.
• Watertight bulkheads. Steel walls divide the hull into separate rooms. If one room leaks, the walls stop the water spreading, and the ship can still float.
• Cargo and containers. Containers are giant metal boxes that stack like blocks. Cranes lift them on and off, and heavy ones go lower down to keep the ship steady.
• Engine. Deep in the ship, a huge engine as tall as a house turns a long shaft that spins the propeller.
• Propeller. The propeller's twisted blades push water backward. The water pushes back on the ship, moving it forward.
• Rudder. The rudder is a big flat fin behind the propeller. Turning it sends water sideways, which swings the back of the ship around.
• Bridge. From the bridge, high up, the crew can see far ahead. They use radar, maps and radios to steer safely.
• Load line. The load line is painted on the side. If the water comes above it, the ship is carrying too much.

Shape the same lump of clay into different boats. Add blocks one at a time. Which shape carries the most before the water comes in?

Experiment: Using the same clay, which boat shape can carry the most blocks?

1. Try the ball first: does it float?
2. Choose a boat and add blocks until water nearly comes in.
3. Try the other shapes and compare.

What we found: The wide boat carries the most. It can push aside lots of water before water reaches its sides, and it is steady. The narrow boat fills up sooner, and the tall cup tips over.

Turn on JavaScript to try it yourself.

Carry at least 12 blocks across the pond. Small waves splash in if the boat sits too low!

When you get into a bathtub, the water rises. Your body pushes water out of the way. The water pushes back by pressing up on you. This upward push is called buoyancy, and it is what holds boats up.

How big is the push? It is equal to the weight of the water that gets pushed aside. A boat sinks into the water until it has pushed aside exactly its own weight of water. Then the push up equals its weight, and it floats.

A lump of steel or clay is small, so it can only push aside a little water, and it sinks. Spread the same material into a hollow boat shape, and it can push aside much more water before the water reaches the top of its sides. That is why a giant steel ship floats.

When you add cargo, the boat sinks deeper so it can push aside more water. If the water reaches the top of the sides, it pours in and the boat sinks. Wide, steady boats can carry the most, and ships have a load line to show the safe limit.

Key words

Buoyancy

The upward push that water gives to things in it.

Displacement

The water pushed out of the way by a boat.

Density

How much mass is packed into a certain volume.

Draft

How deep the bottom of a boat sits below the waterline.

Freeboard

The height of the boat's side above the waterline.

Hull

The main watertight body of a boat or ship.

Check your thinking

1. What is the upward push of water on a boat called?

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   Buoyancy. Buoyancy is the water's upward push.

2. Why does a clay boat float when a clay ball of the same clay sinks?

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   The boat shape pushes aside much more water. Same weight, but the hollow boat can push aside more water, so it gets a bigger push up.

3. What happens to a boat when you add more cargo?

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   It sinks deeper until it pushes aside more water. To balance the extra weight, it sinks until it pushes aside enough extra water.

4. Why do ships float a little higher in the sea than in a river?

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   Salt water is denser, so it pushes up harder. Each bucket of salt water weighs more, so the ship needs to push aside less of it.

5. A clay boat has a mass of 50 g. The most water it can displace before it floods is 270 cm³. About how much cargo can it carry in fresh water?

   Show the answer

   About 220 g. 270 cm³ of fresh water weighs about 270 g; subtract the 50 g boat to leave about 220 g for cargo.

6. Why is a tall, thin boat more likely to tip over?

   Show the answer

   Its weight is high up and it is narrow. A narrow boat with weight up high wobbles and rolls over easily. Wide and low is steady.