Key Concepts
Physics
Mass
Volume
Density
Engineering

Introduction
It's one thing to build a boat that can stay afloat in calm water—but what about one that can survive large waves in a storm? In this activity you'll put a new spin on a classic activity. You'll build aluminum foil boats to carry pennies as cargo, but instead of testing them in flat water you'll create your own storm to test them in waves!

Background
People have used boats for transportation for thousands of years. Boats, however, can be vulnerable to storms, when high winds and large waves cause boats to flip over or fill with water and sink. How can you make a boat safer and less likely to sink in water with big waves?

To understand what makes a boat float or sink in still water, you first need to know a little bit about mass, volume and density (mass per unit volume). An object in water is pulled down by its own weight and pushed up by the buoyant force exerted by the water. If the density of the boat (calculated using the total volume and mass of the boat and everything inside it, including air and cargo) is greater than the density of water, the boat will sink. If the density is less than water, the boat will float. That's why boats can be made of materials (such as metal) that are denser than water: the air space inside the hull helps the boat float. But add too much weight (whether from cargo or water), and it will sink.

Waves can make the situation more complicated. Waves that break over a boat's sides can start filling the boat with water, increasing its density and eventually causing it to sink. So even if a boat could hold a certain amount of cargo in perfectly still water, that doesn't mean it can safely carry that cargo across an ocean filled with waves. And that's not the only challenge. A single, large wave can cause a boat to become unstable and flip over. If you rock a boat a little bit, it should return to its upright position. But rock the boat too much, and it can flip.

Can your boat survive the stormy seas? Try this activity to find out!

Materials

• Aluminum foil
• Pennies or other small, heavy objects to use as weights, such as pebbles, nuts and bolts, and so forth.
• Large container such as a plastic bin or large food storage container. You can also use a sink or bath tub.
• Water (enough to fill your large container)
• Heavy ball (or other object you can drop into the water to create waves next to your boat)
• Location that can tolerate some spilled water (such as bathroom or outdoors)
• Towels for cleanup (optional)

Preparation

• Fill your container with several inches of water, and ensure it is in a location where it's okay to spill some water. Bring towels for cleanup if necessary.

Procedure

• Try folding a piece of aluminum foil into a boat shape. There is no single "correct" way to make a boat. (If your first boat doesn't work, you can always make another one.) You will eventually be adding pennies (or other weights) to the boat, so pick a design that will allow your boat to carry this "cargo."
• Place the boat in the water (without any weights), and make sure that it floats. If your boat doesn't float, try modifying it or changing the design. For example, make sure the aluminum foil is folded/crimped tightly. If there are cracks or gaps, water may seep in.
• Try adding pennies to the boat one at a time until it sinks. How many pennies can the boat hold?
• Remove the pennies, and empty your boat of any water.
• Place the empty boat back in the water.
• Now try dropping your ball into the water from about a foot above the water's surface, so that it makes waves. (Make sure you drop the ball to the side of your boat and not directly on top of it.) How big are the resulting waves? What happens to your boat?
• Try dropping the ball from increasingly larger heights. Do the waves get bigger? Does your boat stay afloat?
• Start over and add a few pennies to your boat, and then try dropping the ball again, starting with the ball back at the lower height. Keep increasing the height of the ball until the boat sinks.
• Keep repeating this process—and adding more pennies each time—until your boat no longer floats at all. How does the number of pennies affect the boat's ability to withstand waves?
• Extra: Try building multiple boats with different shapes. Do some designs work better than others?
• Extra: Do this activity with friends, and have a competition to see who can build the sturdiest boat. What happens if you test all the boats at once—do they crash into one another? That's another thing that can cause real boats to sink!
• Extra: Try using other materials to build your boat. For example, make a raft using corks, rubber bands, and wooden craft sticks. Do the same materials and designs hold the most weight and stay floating in the biggest waves?

Observations and Results
You might have found that there are two different ways your boat can sink in this activity. A single, very large wave can cause a boat to flip over. You probably had to drop the ball from very high to cause this to happen (if it happened at all). Even smaller waves, created by dropping the ball from a lower height, however, can cause a boat to sink. They can go over the sides of the boat, gradually filling it with water (and increasing its density as they replace air in the boat), eventually causing it to become too dense. Just because your boat could hold a certain number of pennies in still water did not mean it could hold the same number of pennies in waves. A boat that is overloaded with pennies—and already riding very low in the water—is much easier to sink like this. As you increased the number of pennies, you probably found that your boat was less able to tolerate waves. The same is true for real boats, which is why they have limits on the amount of people or cargo they can safely carry—whether the seas are calm or choppy!

Cleanup
Use towels to clean up any spilled water. You can recycle your aluminum foil boats.

More to Explore
Buoyant Science: How Metal "Boats" Float, from Scientific American
Shipping Science: Building a Boat That Can Carry Cargo, from Scientific American
Rocking the Boat, from Science Buddies
Make Metal Float: Build a Water Strider, from Scientific American
STEM activities for Kids, from Science Buddies

This activity brought to you in partnership with Science Buddies