Build a cargo boat

by Amy Collins June 8, 2018

Ahoy there shipmates! Your STEM challenge is to use a Gratnells Mini Tray to make a boat that can carry as heavy a cargo as possible. This activity can be carried out by individuals or small teams and the majority of the equipment is reusable.

You will need…

• One shallow Gratnells tray with appropriate insert to hold and neatly organise the below components
• One Gratnells Mini Tray or food tub e.g. cream cheese or margarine
• Drinking straws x 4
• Insulating tape small roll x 1
• Sharp scissors x 1
• Tin foil x 1 roll
• Cocktail sticks x 4
• Blue or white tac half a pack x 1
• Elastic bands x 4
• Disposable plastic shot glasses/cups x 4
• Lollypop sticks or craft sticks x 4
• Wine bottle corks x 4
• One penny pieces x 200
• Gratnells A3 or deep tray half filled with water x 1

What to do…

• Use only the equipment in the shallow tray to make a boat that can carry as heavy a cargo (1 penny pieces) as possible. You must use the Mini Tray or tub, all other equipment use is optional. It is up to you to work out how to make your boat as good as possible at carrying a heavy cargo.
• If you need some inspiration, watch our short YouTube video. Younger participants may also benefit from seeing one you made earlier.
• Once your boat is finished, float it on the surface of the water in the A3 or deep tray.
• Make a prediction, how many one penny pieces will your boat hold before it sinks?
• Top Tip: It helps to distribute the pennies evenly across the boat.
• Record the number of pennies your boat was able to hold before it sunk.

What is happening?

Key words: Buoyancy, Water Displacement, Gravity, Force, Archimedes’ Principle.

Water pushes upwards with a force called ‘upthrust’. You can feel upthurst if you try to push a light object such as a balloon under water. If the weight of an object placed on the water is equal to or less than the force of the upthrust then it floats. When something floats we say it is buoyant. The shape, design and construction of a boat determines its buoyancy and therefore the weight of cargo it can hold. If the weight of the boat is greater than the force of the upthrust then it sinks.

If the force of gravity on the boat is stronger than the force of the upthrust then the boat will sink.

Archimedes’ Principle tells us if the mass of the water displaced is equal or more than the mass of the boat, the boat will float. Therefore, the more water that the boat displaces the better it will float and the more weight it can carry. Once the boat’s mass is heavier than that of the displaced water then it will sink.

Some of the objects in the equipment tray are more buoyant than others and displace more water than others. Adding these items to your boat design will increase the weight of cargo it can carry.

Other things to try:

• Modify your boat using the equipment available. Make a new prediction and then re-test your modified boat. Have your modifications improved the weight bearing capability of your boat? Record the type of modifications you made and the number of pennies your boat was able to hold before it sunk.
• Try again, with further modifications, predication and re-testing. Be sure to record your findings.
• Which elements make for the most effective cargo boat?
• Challenge yourself to make the most effective boat with the least amount of equipment/materials.

Health and Safety

As with all Gratnells What’s In My Tray activities, you should carry out your own risk assessment prior to undertaking any activities with children.