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Learning Intention:


To understand the theories behind floating and sinking. To create a boat that floats and to be able to solve a problem if it initially does not float.

What makes something float or sink? Thanks to kids want to know for the video

How boats float. Thanks to Fun Kids for the video



How does a sailboat float? 


A boat floats because of how it displaces water.

Floating was first recorded by a scientist named Archimedes. He discovered that when an object is placed in water it takes up space and experiences an upward force equal to the weight of the water pushed aside by the object.  This is called displacement. 

Actually, a boat partly floats and partly sinks.   It depends on how much it weighs and how much weight it carries.  There is only so much weight a boat can carry before it sinks completely. As the weight of a boat pushes downward, the boat moves water out of the way - it displaces water.  A force equal to the weight of the displaced water, called the buoyancy force, pushes upward on the bottom of the boat, keeping it afloat.


If a boat weighs less than the maximum volume of water that it could displace (push aside), it will float.

What do you wonder about life jackets?  Why do they float? 


They are small compared to our size, but they allow us to float.  A life jacket is filled with a very light weight material like foam, that can displace a lot of water compared to its weight, so it floats on top of the water. We humans can sink, so that is why it is essential for us to wear a life jacket for all recreational activities on the water. Your body already has some buoyancy, so a life jacket doesn’t need to support all of your weight. It just needs to displace enough to keep your head above the water.

Activity:  Making clay, plasticine or tin foil boats.


You will make little boat hulls, to investigate how their shape and size affects how much weight they can carry.



  • Bowls of water or water trough

  • Clay (approx. 100g each) or plasticine (4cm cube each) or tinfoil (15cm square each)

  • Paper towels (if using clay)

  • Metal washers or gem stones




  1. Use the materials to make an object that will float.  Remake and test the object several times to try and work out what floats best, using paper towels to dry the clay between each design.

  2. Next, make a boat that will float and carry as much weight as possible.

  3. Test in the bowls of water or a water trough to see if it will float.

  4. Once you have a floating boat, make a prediction of how many washers/gems the boat will carry without sinking.  Add the washers/gems, one at a time, counting how many the boat takes before it sinks.

  5. Try making improvements every time the boats sinks so that it takes more weight.


Which boat carried the most weight?  Why?  How does the boat's shape affect how well it floats?




Sail, hēra.


Buoyancy, puhautanga - The ability of something to float in a liquid.


Displacement - The moving of something from its place.


Life jacket, kahu kautere.


Upwind - Sailing in the opposite direction in which the wind is blowing.

Downwind - Sailing in the direction in which the wind is blowing.


Force - A push or pull on an object.

Windward - The side of a boat that is facing the wind.


Leeward - The side of the boat that is sheltered from the wind.


Luff - The front or the leading edge of a sail.


Leech - The back edge of a sail.

Foot (of a sail) - The bottom edge of the sail.


Woolies - Or tell-tales are the strips of ribbon or wool on the side of a sail.


Waka - A traditional Māori canoe.

Kevlar - A very strong synthetic fibre used to make sails and bullet proof vests.


Carbon fibre - A very strong lightweight synthetic fibre used for high performance products like sails, boats and aeroplanes.

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