7. HOW DOES A SUBMARINE GO UNDER WATER AND STAY THERE?
For a diesel submarine to submerge (go underwater) her ballast tanks have to be filled with seawater to make the submarine heavy enough to go deeper. As the submarine uses up fuel to power onboard machinery and equipments, the fuel tanks are also kept full by adding an equivalent weight of seawater. As the seawater is denser (heavier) than the fuel, the fuel always floats towards the top of the tanks and the seawater remains in the lower part of the tanks. When the submarine fires torpedoes or missiles, it must also add seawater to make up the weight loss and keep the boat balanced. Fine adjustments to the boat's balance are made using smaller capacity 'trim' tanks.
A nuclear submarine operates using the same water ballast diving and surfacing methods but is driven by nuclear power which does not require air. The only limitation to how long the crew remain dived is the amount of food carried and the length of time men can be expected to remain underwater, alert and fit. Current Royal Navy nuclear submarines have gone round the world without surfacing and only need refuelling after about 10 years. The latest nuclear submarines being built at Barrow will never require to be refuelled.
