8. HOW DOES A SUBMARINE DIVE AND COME BACK UP AGAIN?
To dive, the submarine fills its ballast tanks with seawater to make it heavier and drives forwards with its fore and aft hydroplanes (like short steel wings) angled downwards, making the submarine go deeper. When surfacing, the hydroplanes are tilted upwards and compressed air held in bottles is released into the ballast tanks to blow out the seawater through valves operated from within the submarine hull. This makes the submarine lighter so she rises to the surface as she is driven forwards. The submarine is steered (flown) like an aircraft and travels faster underwater than on the surface because, underwater, the submarine makes fewer energy-wasting waves. (Next time you go swimming you will see how people can swim faster underwater than on the surface!) Underwater, everything is still, calm and quiet and there is no motion to cause seasickness.
