How do boats float?

Answered by Randy Haws, Environmental Scientist, Pro2Serve

Whether an object floats or not depends on several things. Solid objects, like wood or apples, float because they have lots of air in them and are lighter than the water around them. Objects like pennies or paper clips sink because the atoms in them are close together and they are heavier than water.

Objects like boats float, even if the metal that makes up their hull is heavier than water, because of a force called buoyancy. That means the water pushes up against them. You can feel this force by holding a block of wood under water. How does it feel to your hand? It almost feels like the wood is struggling against you to get to the top of the water. That push is buoyancy.

If the metal in a ship was all squashed into a ball, it would sink. Because a boat is built to spread out the metal over a wide area, it floats because the water pushes up on a lot of the metal. Remember your last swimming adventure. If you wrapped your arms around your legs and curled up into a ball, did you sink? When you stretched out flat, did you float? That’s because more water pushes against you since your body is laid out flatter. When you curl up into a ball, less water is pushing against you.

Another thing that determines if an object will float is its displacement, or the amount of water it pushes away. When you get in the bathtub, the water rises because your body pushed some of the water away. An object will float if it is lighter than the water it pushes away. The heavy metal in a ship is formed into a hull, which is basically a metal shell filled with air. The metal and air together are lighter than the water they push away, and a heavy ship will float.

Place a slice of bread in water. Does it float? Now wad up a slice of bread into a ball and place it in water? What happened? Floating is like a pushing contest between the water and objects in the water. Water can push up harder than the flat slice of bread can push down, so the bread slice floats. When you wadded up the bread you changed the amount of space the bread took up. That changed its density. Bread that’s denser pushes down harder than the force of the water pushing back up, so the ball of bread sinks.

Think of it this way. The weight of the bread stayed the same, but you crammed it into a smaller space. If you put the same stuff (mass) into a smaller space (volume), it becomes more dense.

Testing Density
Materials: Small plastic boat, a container or sink of water,
pennies, a balloon and a rubber ball.

  1. Float a blown-up balloon on the water. It doesn’t sink very far because it’s not very heavy, but do you see a little dent in the water where the balloon is floating? The water is being pushed out of the way.
  2. Float a ball the same size as the balloon. Does it sink deeper into the water? It has to push more water out of the way before it floats because it is denser.
  3. Float a small plastic boat in water and notice how deep the boat sinks when it is empty.
  4. Drop a penny in the boat. Does the boat sink deeper in the water? Add more pennies to the boat and watch what happens.

Float a Boat
Materials: aluminum foil, scissors, about 50 pennies,
tape, large container that will hold water, water

  1. Cut one or more squares of aluminum foil, each approximately 5-6 inches square.
  2. Fold the foil squares into creative boat shapes, using tape if necessary. Try different shapes and side heights.
  3. Carefully float the boat in a container of water. Does it float?
  4. Carefully add one DRY penny at a time, taking care to add pennies to alternate sides of the boat so the load is balanced. Add pennies until the boat sinks and record the number of pennies that were able to float. The penny that sank the ship doesn’t count.
  5. Try the experiment using each of the different foil boats you made. Which one was able to float the most pennies?

Pennies seem too heavy to float, but they actually can— but only if the pennies are spread out over a large enough area. When pennies are added, the boat will float if the combined density of the pennies and the boat is still less than that of the water. When too many pennies are added, the density becomes too large and the boat will sink.

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