Surface Tension and Cargo Boats

Boat Building for Cargo

Foil cargo boat testing surface tension.

Foil cargo boat testing surface tension.

This is an idea that I have seen in many places. Most recently, a second grade teacher at the school in which I substitute did a similar lesson with her students. The purpose of the activity is to explore different variables as they relate to buoyancy and surface tension. Even though the children were learning, they all had a blast. A big part of this type of activity is understanding the variables and the role they play, and then making the connection to real day uses. If you do this activity, take the time to explore changing the variables, and understanding the impact.


With 15 glass rocks, the "boat" began to take  on water.

With 14 glass rocks, the “boat” began to take on water.

Aluminum foil
6 ounce cup
6 ounce flatter container
Flat lid
Tub to hold water deeper than the height of the cup
Glass “rocks” or something of similar size that can be used as “cargo”

Optional: newspaper, salt, popsicle sticks, tape…


  1. Let the children know that eventually they will build their own barge, so they need to pay attention to what factors make the best barge.

    Does it make a difference which side is up for the lid? Try both ways!

    Does it make a difference which side is up for the lid? Try both ways!

  2. Find three objects around the house that will have different buoyancy. I used a lid, a 6 ounce plastic solo cup, and a 6 ounce “to go” container.  Spend some time comparing the attributes of the different containers.
  3. Measure how much water each container can hold.
  4. Have the children predict which container will hold the most glass rocks, and how many.
  5. Provide a bucket of water into which the children can place their “boat”. Start first with the lid. How many glass rocks did the lid actually hold? Does it matter how the glass rocks are placed on the lid? How many rocks if they are stacked in the middle, stacked to one side, stacked along the edges…?
  6. Dry the glass rocks off and repeat the same process with the Solo cup.

    Sunken 6 ounce to go container with 16 glass rocks.

    Sunken 6 ounce to go container with 16 glass rocks.

  7. Repeat the process with the “to go” or flatter 6 ounce container.
  8. Give to each child a piece of aluminum foil. Each piece should be the same size – the width of the foil and about 12 inches long is big enough to build a barge. Let the child create his or her own boat. If you have a bunch of children they can work in teams and collaborate. (This might be a fun activity for a pirate themed birthday party.)
  9. Have them estimate how many rocks their barge can hold, challenge them to see if it can!


  1. Were there any surprises in the results?
  2. Did it matter how the rocks were placed?
  3. Why did the “to go” container hold more rocks than the other containers?
  4. When you build your own boat, what attributes should it have?


  1. What other design improvements could be made that would allow the barge to hold more rocks?
    • Cardboard to reinforce the bottom of the boat?
    • Popsicle stick to reinforce the sides of the boat?
    • Tape to stop the leaks?
    • Higher sides on the boat?
    • ?
  2. Allow students to make the design improvements. How many more rocks does the boat hold now
  3. Challenge students to think about when is a flat bottomed boat not a good choice for a boat. Add some waves to the tub and see how that impacts the boat’s ability to stay afloat, especially when it is carrying a load.
  4. Take a piece of foil. Crumple it up into a tight wad. Ask the children to speculate about what will happen to the ball when it is placed o the water. Put it on the water. What does it do? Take another piece of foil, and have the students predict what will happen to the sheet of foil is placed on the water. Place the foil on the water so that it lays flat. Challenge the students to figure out why they float. Push the foil down so that it sinks. Discuss the definition of the words below and how they impacted the results of the boat building activity.
  5. Would the results of the boat building change if the water were salted versus unsalted? Cold or hot water?  (You may not get appreciable results using the glass stones, and may want to switch to a smaller cargo, such as a penny. I used a smaller container (a 2 ounce plastic container) and with salt water got consistent results of 24 pennies in unsalted water and 26 pennies in highly salted water before they sank.)


  1. 1.      buoyancy

noun \ˈbȯi-ən(t)-sē, ˈbü-yən(t)-\

: the ability of an object to float in water or air

: the power of a liquid to make someone or something float

  1. 2.      Surface tension

Surface tension is a contractive tendency of the surface of a liquid that allows it to resist an external force.  Water has a relatively high surface tension, causing it to act like it has a microscopic membrane over it, allowing some things that are denser than the water to not sink.

A Note on the Glass Rocks: The rocks clearly are not uniform in size or weight, and that may impact the number each boat can hold, especially if the child is smart enough to choose just the smallest of the rocks. The rocks, though, are bright and colorful and are fun to play with. That adds to the interest of the activity. Your choice to use the glass rocks or something more uniform in weight and size.


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