What it shows
A motor-driven propeller pushes water backwards and the boat moves forward, which is Newton’s third law. A well-shaped, light hull floats because the water it pushes aside weighs more than the boat.
The science
The propeller shoves water backwards, and the water pushes the propeller and boat forwards with an equal force. The tray floats because of buoyancy: it pushes aside water equal to its own weight. If the boat is too heavy it sinks lower and the motor may touch water. Putting the weight in the middle keeps the boat level, and a flat tray gives stability.
Materials
Cost: under Rs 200. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- A foam meat tray or a thermocol tray as the hull
- A small 3 V DC toy motor
- A 2 x AA battery holder with two cells and a slide switch
- A plastic bottle cap propeller with two or three blades
- A rubber band and tape (waterproof tape if you have it)
- A large tub or bucket of water
Safety
For this project: Cells only, and test in a tub indoors, never in a pond or a river. Keep every wire and the motor top above the water line and keep the switch dry. An adult must be present near water and must dry everything afterwards.
Never use mains electricity: power models with cells, batteries or a low-voltage adapter. Sharp and hot tools, flames and lab chemicals need an adult or your teacher beside you. Keep magnets, button cells and small parts away from young children, and wash your hands after making.
All the safety rulesStep by step
8 steps. Do them in order, and test each part before you go on to the next. Write what you notice in a notebook as you go.
Weigh down a tray in the tub with the cells on it and mark the safe water line you must not exceed.
Tape the motor to the back of the tray so that the shaft points backwards, hanging a little over the edge.
Fix the propeller to the shaft, so its blades just dip into the water when the boat is floating.
Fix the battery holder and the slide switch at the middle of the tray, and keep them dry by raising them on a card platform.
Connect the battery holder, the switch and the motor in series with short wires.
Put the boat in the water and check that it floats level; move the battery holder to fix any tilt.
Switch on, watch it move, and switch off before it reaches the wall of the tub.
Time it over 1 metre with different propellers and compare the speeds.
What you should see
The boat floats level and travels across the tub when the switch is closed, and the best propeller gives the shortest time.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1Why does the tray float?
It displaces water that weighs the same as the boat, giving an upward buoyant force.
2Which law explains the motion?
Newton’s third law of motion.
3Why keep electrical parts above the water?
Water can short the circuit and corrode the contacts.
4Why does the boat tilt if the battery is not centred?
The weight is unevenly spread so one side sinks lower.
Take it further
Make the boat steer with two motors, one on each side, and a DPDT switch for each.
Change one thing, measure what happens, and add a table or chart of your readings: that is what turns a model into your own investigation.
Free ebooks for this project
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