Submerged
About four hundred years ago a sailing ship loaded with treasure sailed into a storm. The winds tore at the sails and the waves crashed over the sides. In a few hours the ship had taken on so much water that it sank. Today, imagine that you are to rescue the treasure. How would you go about it? First you would examine maps and written accounts about where the ship was lost, then you would sail on a ship to the area where the sailing ship went down. Finally you could search for the sunken ship and its treasure using a small submarine called a submersible.

When you arrive in the search area your ship stops and the submersible that has been stowed on the back is lifted into the air by a crane. It is carefully lowered into the water. Part of it sinks below the gently undulating surface of the water but part remains above. The reason the submersible does not sink is that it contains tanks which are full of air. When the submersible is roped to the side of the ship, you climb aboard.

Inside the submersible, you find that you are surrounded by many dials and switches. They are there to help you control the vessel. At the front you find a transparent dome which you can look through to view the water around you. When you flick some switches, lights on the front of the submersible come on and you can see into the distance under the waves. Eventually when you have familiarised yourself with all the instruments, you lock yourself in. The door is sealed fast so that no water may enter your living space. You flick another switch and the ropes holding the submersible to the ship fall away. You flick more switches and propellers at the back of the vessel begin to turn. They take the submersible away from the ship and then it is time for it to dive. When more controls are operated, holes in the air tanks are opened to let in water. The water pushes out the air and it forms bubbles as it rushes away to the surface of the sea. A dial close by tells you how fast you are sinking and you work the controls in order to sink slowly.

In time you can see the floor of the sea lit up in the submersible's headlights. Fish flash by and crabs crawl slowly over the rocks. You turn another control to stop more water entering the tanks. This makes the submersible seem to hover a few metres above the sea floor. You turn on more lights and look through the dome at the view around you. To the left you can see a sandy area with a metal pipe sticking out of it. You turn on the motors again and the propellers push you closer. You let more water into the tanks and you sink close to the pipe and discover that it is a cannon.

By carefully controlling the water in the submersible you can move slowly just above the surface of the sea floor. There is a shovel attached to the front of the submersible and you lower it into the sand and start to dig. After some time a shiny metal object glints in the headlights. It is a gold coin. You dig a little further and discover more coins. You use the shovel to scoop up the coins and store them in a tank on the front of the vessel, then prepare to rise to the surface.

On board the submersible are tanks of compressed air. When you open them the air flows into the tanks filled with water and you open the holes in the tanks so the air can push the water out. As the water is replaced by air in the tanks the submersible starts to rise and soon you reach the surface with your treasure. The arrangement of the tanks, pipes and other equipment on the submersible allowed you to control its weight and use the natural upward push of the water to make your journey safely to the ocean floor and back again.

What is compressed air?
It is a large amount of air, which has been squashed into a small space. You compress air when you blow up a bicycle tyre with a pump. The pump has a piston, which you use to push air into the tyre. When you have pushed the piston in you pull it out again but the air does not escape because there is a valve in the tyre, which stops the air moving out. When you push the piston in again the valve opens to let in more air but closes as soon as the piston is pulled out again. When you pump up a tyre you push the piston in many times and each time you add more air to the tyre. The air inside the tyre is compressed air. In a similar way compressed air is made for a submersible but the air is stored in tanks.

Is water let into the compressed air tanks to make the submersible sink?
No. When the submersible is put in the water, it has some tanks full of air which is not compressed, and some tanks full of compressed air. When the submersible is ready to dive, holes are opened in the tanks which hold the air that is not compressed. The water flows in through some of these and pushes the air out of others. The air escapes from the tanks into the sea. Water never enters the compressed air tanks.

How can the compressed air push out seawater?
Air is made from particles of gas, which move about rapidly and push on everything around them. When air is compressed huge numbers of air particles are squashed together. They have little space to move in but still create a push on any surface they touch. The compressed air tanks have to have strong walls to prevent the push of the air tearing them open. When holes in the tanks containing water are opened and the compressed air is allowed to flow into the tanks it pushes with such force on the water that the water is pushed out and the tanks fill with air again.

Why does a submersible float when it is placed in the water at first?
The weight of the vessel pushing down is less that the upward push on the water so the vessel floats.

Why does the water push upwards?
When an object is immersed in water it pushes the water out of the way to find a space for itself. The amount of water pushed out of the way pushes back on the object. It is this force that pushes upwards on the object. This upward pushing force is called the upthrust.

Does the upthrust always make an object float?
No. If the weight of the object is greater than the upthrust the object sinks.

How can the weight of an object be greater than the upthrust?
All substances are made from particles. The size of the particles may vary from one substance to another and the way the particles pack together may vary too. If a substance is made from large particles, which are tightly packed together, it is said to have a high density and will have a heavy weight. If a substance is made from small particles, which are not tightly packed together, it is said to have a low density and has a light weight. The upthrust is made by an amount of water pushing on an object. It is created by the number of particles in that amount pushing on the object. If the number and size of the particles in the object pushing down on the water (the weight of the object) is greater than the number and size of the particles of water pushing back (the upthrust) the weight will be greater than the upthrust and the object will sink.

Why can the submersible hold its position when it is under the water?
This is done by allowing just enough water into the tanks so the density of the submersible is similar to the density of the water. When this happens, the weight and the upthrust balance and the submersible neither rises nor sinks.

Why is it useful to have the submersible the same density of water?
It allows the submersible to move through the water above the sea floor. If the submersible could only sink, it would then have to stay on the sea floor and perhaps have caterpillar tanks to help it move over the surface. This would make the submersible move much more slowly and make searching the sea bed much more difficult.

How does a fish manage to stay underwater yet not sink to the ocean floor?
It has a space inside it called a swim bladder. The fish uses the swim bladder to control the density of its body. Many fish can add gas from their blood to the swim bladder to make the body less dense then water. When a fish does this it rises in the water. They can also take gas from the bladder back into their blood. When they do this the density of the body increases and the fish sinks.

Do all fish control their swim bladders in this way?
No. Some fish gulp in air at the water surface to fill their swim bladder. They then release some air from their mouth to reduce the amount of air in their swim bladder. When they do this they make the density of their body the same as the water around them and can swim about without sinking down too much.

Do all fish have swim bladders?
No. Sharks do not have swim bladders. When they move forwards, they make the water move over their fins so that an upward force is created. This force, called lift, acts against the shark's weight and makes the shark rise in the water. This is why sharks circle around their prey on the surface. They have to keep moving round to gain height in the water so they can reach the surface and attack.