Float or sink?

Have you ever wondered why some huge aircraft carriers and luxury cruisers stay afloat on water while small objects such as a metal screw sinks? The reason for this to happen has nothing to do with the size of the objects or their weight. Whether an object sinks or floats depends entirely on its density and its shape.

Long time ago, the Greek mathematician Archimedes discovered that when he got into the bathtub, the water level in the bathtub rose. He then came to the conclusion that for something to float in the water, the upward push of the water must be the same as the weight of the water displaced, or pushed aside by the object in the water. This theory eventually led to inventions of many things.

Crafty vessels

You can test this theory by using different objects. As you need something to float or sink in the water, why not build your own boat?

You can use some modelling clay and mould it into a solid shape like a 'solid boat' (with no hollows) and drop it into a basin of water or a bathtub. Watch how it sinks to the bottom of the water. Try making other solid shapes such as a ball and check what happens when the object is solid. Does it sink or float in the water?

Once you finish experimenting with solid shapes use your hands, a rolling pin or a bottle and roll the clay till its flat and even. Now mould a boat (not a solid blob shaped like a boat - but a typical boat with the hollow in the middle). Curve the edges of the hollowed area to shape the boat. Make sure there are no holes on it or the boat will automatically sink!

Ready with your boat? Well, let's float it then. Gently place your clay boat onto the surface of the water. If there are no holes, the boat will float easily. If you use a glass or a see-through container to float your little boat, you can observe how low in the water your boat sits.

Mould the clay into a boat.

The clay boat is afloat.

Now, mark a water level line on the side of your boat. Because if you want your boat to carry a load of goods safely without sinking, then the boat must sit high in the water. Here are some tips to make your clay boat 'sea-worthy'. You can mould different shapes of boats and experiment but generally a high-sided shape floats better than a shallow one. Make round-shaped boats and long-shapes ones. Don't forget to mark a safe water level on your craft. Do you know what this 'safe water level line' is called? It is known as the plimosoll line after Samuel Plimosoll.

Let's check how your well moulded or hollowed clay boat stayed afloat without sinking, like the solid clay boat.

It is because by making a boat shape (hollowed shape) you changed the density of the solid blob. Solid clay is more dense than water and so it naturally sinks. But the boat shape holds air which is less dense than water.

So, this makes the overall density of the boat less dense than the water. This allows it to float.

So, now you have evidence that the object's shape controls the amount of water it pushes out of the way. If the amount of water pushed aside, or displaced is the same or more than the weight of the object, the object will easily float.

However, if the amount of water weighs less than the object, then it will automatically sink.

Make many more boats of different shapes and sizes and test this theory. See how the different shapes affects the smooth movements of the vessel.

Try moulding passengers or goods to place in your boat to observe how added weight impacts the floating boat.