A science fair project based on something as everyday as a boat may not be as flashy or messy as other fair project ideas, but the scientific concepts related to buoyancy make for an interesting and impressive set of experiments. Demonstrate these concepts by designing and building your own functioning miniature boat using household materials, then use your boat to demonstrate the ways in which physical structure can make the difference between floating and sinking.
Determine how you want to demonstrate the scientific principles of buoyancy with your boat. Decide whether you want to make a floating boat and compare it with other non-floating objects or whether you want to make a boat and demonstrate buoyancy failure by making it sink (say, by taking on water or being overloaded).
Choose a material for your boat. For demonstrating buoyancy, materials such as oil-based clay, Styrofoam or aluminum foil will allow you to create boats that you can easily alter for the sake of experiments, making them float or sink at different times. These materials are also inexpensive and will keep your costs down.
Make the boat's basic hollow shape. With a moldable material such as clay or sculpted material such as wood or Styrofoam, build a canoe-like shape by first forming the material into a long, thin log, then flattening one edge by pressing it flat, sawing or sanding it. Carve or cut the other side into a ridge. Hollow out the inside of the boat shape by carving out the material on the flat side. If using aluminum foil, ball the foil into a loose clump, then shape the clump like clay.
Ensure the boat is water-tight. Place the boat in a tank of water to make sure it can float without taking on water. Check for any areas that might need to be thickened or reinforced with some waterproof glue.
Decide on a way to show the difference between a boat that can float and one that sinks. In order to demonstrate buoyancy, you'll want to demonstrate the different ways in which a boat will sink. You might want to make your boat with a hole that can be plugged and unplugged or make sure you can fill it with heavy objects that will sink it.