Summary
Introduction
It's time to set sail! Even if you live nowhere 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?
Materials
- Wine corks (3)
- Rubber bands (2)
- Toothpick
- Several screws or nails
- Craft foam, wax paper, or paper milk carton to make a sail
- Aluminum foil
- Sink, bathtub, or a large container you can fill with water. The container should be deeper than the length of your nails/screws.
- Tap water
Prep Work
- Fill your container with water. Make sure you can put your longest nail/screw vertically into the water and completely submerge it.
Instructions
- Line up three corks (side by side, not end-to-end).
- Use two rubber bands to hold the corks together, forming a "raft."
- Poke a toothpick into the center cork, so it sticks straight up. This is your boat's mast (the part that holds the sail).
- Cut a square of thin waterproof material (see materials list - don't use regular paper) to make a sail. It should be about 6 cm x 6 cm.
- Poke the toothpick through opposite ends of the sail (near the edges) to hold it in place. Your completed boat should look like this:
- You've made your first sailboat! Put it in the water. Blow on the sail from behind.
What happens? - Now make a skinnier boat by removing the rubber bands and the two outer corks. Keep the sail in place. Rotate your sail 90 degrees so it matches the next picture.
- Put your new sailboat back in the water.
What happens?
- Uh-oh! Your sailboat probably fell over! That's not good. To fix it, try adding a keel. Stick a nail or screw into the bottom of the boat, directly under the sail.
- Try putting the boat back in the water. If it doesn't stay upright, keep adding nails or screws (in a straight line with the first one) until it can float without tipping over.
- Now try blowing on the sail again.
What happens? Does your boat move in a straight line?
- Right now, your keel is made of one or more nails/screws, but they are not connected to each other. Cut a rectangular piece of aluminum foil and tightly wrap it around the nails/screws to make a "fin" shape.
- Put your boat back in the water and try blowing on the sail again.
- Try making different boats and compare their performance.
What design is the most stable? Which one goes the fastest?
What Happened?
Your first sailboat was probably pretty stable, because it was very wide (made from three corks). However, when you removed two corks to make it skinnier, your sailboat probably became unstable and tipped over. It's similar to standing with your feet tight 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. However, individual vertical nails don't do a very good of job pushing against the water—the water can flow right around them. That means they don't do a good job of 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 shape more like a fin. It can cut through the water very easily in one direction, but 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—they have a part called the keel that prevents them from tipping over and helps them go straight! Read the Digging Deeper section to learn more about the science behind this project.
Digging Deeper
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. While 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 that they displace. Some boats are made of materials that are less dense than water, meaning they have less mass per unit volume. These materials (like cork) will always float. However, other boats are made of metals like steel that are much denser than water—so how do they float? They can float because they're hollow, so there is a bunch 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.
But boats don't just need to float - they also need to stay upright and avoid capsizing (flipping over). To do this, they need a low center of mass, meaning their weight is concentrated towards the bottom of the boat, not the top. That might seem like it's a problem for sailboats, boats with 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 (so if you've only seen a sailboat from above the water, you might not even know the keel exists!). The keel is a big part under the boat, shaped like a fin, that 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 made a keel from nails and aluminum foil, which helped prevent your sailboat from flipping over and helped it go straight.
Ask an Expert
For Further Exploration
- Try making a bigger sail and using part of a wooden skewer for the mast instead of a toothpick. How heavy does your ballast need to be to balance the boat with a bigger sail? Hint: try attaching a horizontal nail/screw to the bottom of your keel to act as a ballast. That way you don't have to keep poking more nails/screws into the cork.
