Penny Boat Challenge with LEGOs

Your future engineer will love tackling this exciting LEGO penny boat challenge. It’s a twist on the classic foil boat challenge but using LEGO blocks instead of foil!


You may have done the aluminum foil penny boat challenge before. You simply fold up a piece of aluminum foil so it resembles a boat, and put pennies in it until it sinks. 

Since we are on a LEGO kick lately, we decided to do the penny boat challenge with LEGOs!

Even though LEGO bricks are a lot sturdier than a piece of aluminum foil, building a LEGO boat that floats on water is a challenge of itself. There are a lot of factors to think about, but that’s why this is a great STEM challenge for the little ones.

How to Set Up the Penny Boat Challenge


  • LEGO bricks
  • Pennies
  • Empty container (I recommend a clear one so you can see how far the boat is in the water)


  1. Have your child design a LEGO boat that floats.
  2. Fill the container about ¾ way up with water.
  3. Place the boat in the water.
  4. Slowly place pennies in the boat one by one. Count aloud as you do this.
  5. Continue to put pennies in the boat until the boat sinks to the bottom of the container.
  6. Repeat steps #3-5 with another boat.
  7. The boat that holds the most pennies and stays afloat win!
Penny LEGO Boat Challenge

Unlike aluminum foil, a boat made out of LEGO bricks will have tiny gaps in between the pieces. And unless you use glue to close the gaps, most likely a little water will start entering your boat as soon as you place the boat in the water.

My son learned this the hard way when he didn’t push all his bricks in tight enough. His LEGO boat started leaking water right away and it didn’t take long before the boat sank.

There are so many things to try when it comes to designing the LEGO boats. Here are some design options you can try out to see what factors affect how well the boats can stay afloat with pennies:

  • A wide boat vs. a narrow boat
  • A tall boat vs. a shallow boat
  • A long boat vs. a short boat
  • A heavy boat vs. a light boat

You can also experiment with the pennies! Try placing the pennies all at one end of your boat. Did the boat tilt? Now place them so that you have equal pennies at the bow and stern of the boat. Could you put more pennies in the boat this way before the boat sinks?

The Science Behind the Sink the LEGO Boat with Pennies Challenge

There are two opposing forces in this experiment.

The first force is gravity. Gravity is pulling the LEGO boat and the pennies downward. 

The opposing force is buoyancy. When an object is placed in the water, the object pushes the water aside. This is called displacement. Buoyancy is the power of a fluid(water) to exert an upward force (displacement) on an object placed on it (the boat).

The LEGO boat will continue to flat as long as the force of buoyancy is greater than the force of gravity. However, as you put more and more pennies in the boat, the boat becomes too heavy and gravity wins! The boat will sink far enough to leak water and eventually sink to the bottom of the container.


Final Thoughts on Penny LEGO Boat Challenge

To be honest, I really thought my kids were not into LEGOs. Then I found out that what they needed was a challenge that would give purpose to what they were building. They loved experimenting with different boat designs and if it wasn’t for bedtime, they could have played with LEGO boats for who knows how long.

One thing that amazed me was when my son kept putting pennies at one end of the boat. Then he realized the boat started to tilt and take in water, so he realized that he needed to balance the boat in order to keep it afloat. 

It’s self-learning moments like this that make all the effort to set up and clean up the experiment worth it.

If your kids loved building LEGO boats, they will love powering them with a balloon and watching them move across the water. Combining this Penny LEGO boat challenge with the balloon-powered LEGO boat activity will allow your kids to discover what structural differences are necessary for a stationary boat holding weight versus a moving boat.


Want this activity in an easy-to-print file? I have compiled 5 super fun LEGO STEM activities for kids in a printable PDF.

Each activity includes a list of supplies, step-by-step directions, an explanation of the science behind the activity, and different extensions based on the science concepts.

The ebook includes:

  • Balloon-Powered LEGO Cars
  • LEGO Zip Line
  • Penny LEGO Boat Challenge
  • LEGO Volcano
  • LEGO Plinko Board


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