What makes a boat float? And what materials can you use to make the best boat at home?
It's time to set sail! Even if you don’t live near a lake or ocean, you will get to do some sailing in this science activity as you build your own toy sailboat. But first you have to make sure your boat doesn’t capsize! Are you up for the challenge?
Do you remember playing with toy boats in the bathtub—or have you ever been on a real boat? Boats can float because of buoyancy. At the same time, they are pulled down by the force of their own weight (caused by gravity) they are pushed up by the buoyant force, which is equal to the weight of the volume of water they displace. (You can find a more detailed explanation of buoyant force in the “More to explore” section.) Some boats are made of materials that are less dense than water, meaning they have less mass per unit volume. These materials will always float. Other boats, however, are made of metals such as steel, which are much denser than water. So how do they float? They can because they’re hollow, so there is a lot of empty air space inside the boat’s hull. The average density of the boat (including both the metal and the air) is lower than the density of water.
Boats don’t just need to float—they also need to stay upright and avoid capsizing, or flipping over. To do this they need a low center of mass, meaning their weight is concentrated toward the bottom of the boat, not the top. That might seem like it would be a problem for sailboats, which have very tall sails that stick way up into the air. How do they stay balanced with so much mass concentrated way up high? They do so with another part called the keel, which is on the bottom of the boat. (If you’ve only ever seen a sailboat from above the water, you might not even know the keel existed!) The keel is a big part under the boat, shaped like a fin, which serves two purposes. It holds the ballast, or heavy weight, that helps lower the boat's center of mass. It also helps prevent the boat from being blown sideways by the wind. In this project you’ll see how a keel can help keep a sailboat from flipping over and help it go straight.
Your first sailboat was probably pretty stable because it was very wide (made from three corks). When you removed two corks to make it skinnier, however, your sailboat probably became unstable and tipped over. It’s similar to standing with your feet tightly together instead of spread out slightly—it’s harder to balance. When you added nails/screws to the bottom of your sailboat you lowered its center of mass and made it more stable. Individual vertical nails, however, don’t do a very good job pushing against the water—the water can flow right around them. That means the keel doesn’t do a good job making the boat go straight. If you blew on the sail, your boat might have curved off to one side or spun in circles. When you wrapped the nails in aluminum foil you made the keel more like a fin. It can cut through the water very easily in one direction, but it provides a lot of resistance against the water in the other direction. That makes it easier for your boat to move forward, and harder for it to move sideways. This is why real sailboats can be long, skinny and have tall sails—the keel prevents them from tipping over and helps them go straight!
This experiment was selected for Science at Home because it teaches NGSS Disciplinary Core Ideas, which have broad importance within or across multiple science or engineering disciplines.
Learn more about how this experiment is based in NGSS Disciplinary Core Ideas.