A submarine is a type of watercraft designed to float on the surface of the water, and also to operate submerged under the water. Submarines range in size from small single-man submersibles, to giant navy boats carrying hundreds of men. All work basically the same way. The principle of buoyancy is what allows a submarine to do what it does.
In general, a submarine is a cylinder with a double hull. The space between the two hulls is a ballast tank filled with air. Vents are positioned at the top to expel air, and at the bottom to admit water. The sub is also fitted with auxiliary trim tanks that can admit or expel water, and large air flasks of compressed air inside the inner hull that vent to both.
When floating on the surface, the ballast tanks between the two hulls is filled with air. This makes it lighter in weight than the water it displaces, which allows the sub to float – the same principle that serves any water vessel. To dive, all vents are opened. Water enters from below and fills the ballast tanks, making the boat heavier and allowing it to sink. Moveable fins, or hydroplanes, on the stern (rear) direct water under the stern, raising it so the boat is angled downwards. Descent is controlled by the amount of water admitted and the angle of the hydroplanes.
When the desired depth is reached, the ballast vents are closed and the boat levels out. Water is forced through fore and aft trim tanks that keep a controlled balance between air and water, thus holding the boat at the desired depth.
To return to the surface, interior vents from the air flasks admit compressed air to the ballast tanks, expelling water from the lower vents. As water level decreases, the boat rises. Hydroplanes control the angle of the boat and up it comes. The link above includes an action diagram of a submerging submarine, courtesy of How Stuff Works (a Discovery Company; www.HowStuffWorks.com).
A crude model can be built of a submarine that demonstrates how buoyancy works. You will need:Plastic bottle with lid, like a soda bottle. Flexible straw. 24 pennies Wide tape
Lay the bottle on its side and punch three holes along an imaginary line stretching from bottom to lid. The holes should be about the diameter of a pencil. Arrange the pennies in three piles; one with 12 pennies, one with 8, and one with 4. Tape the piles so the pennies are secure. Take the largest pile and position it just behind the hole nearest the base of the bottle, and tape it there. The medium penny pile is taped just behind the middle hole, and the smallest pile is taped just behind the hole nearest the bottle’s lid.
Punch a hole in the lid large enough for a straw to fit through. Put the lid on and fasten tightly. Insert the short end of your flexible straw and tape in place.
Fill a tub or bucket with water and position your boat with the pennies down. Adjust the straw so that it sticks up, so water will not enter it. Now place your boat in the water.
The boat stays on top of the water at this point. Now push the boat down so water begins to fill it. The more water in the jug, the lower the boat will ride until it is completely under water. To raise the boat back to the surface, start blowing into the straw. As you blow air in, water is pushed out of the holes and the boat rises. This is the basic principle of buoyancy.
Submarines are closed environment, so complete life support systems must be maintained. Along with air quality, personnel need a fresh water supply and controlled temperature.
Fresh air is supplied either from pressurized tanks, releasing air periodically, or through an oxygen generator. This generator draws oxygen from sea water and releases it continuously, via a computerized system that monitors the air and adjusts accordingly. Scrubbers remove carbon dioxide, and dehumidifiers remove excess moisture. Special burners are also installed to remove gasses like hydrogen generated by equipment and cigarette smoke. Filters remove dirt and dust. Some subs also have a snorkeling system to draw in outside fresh air when near the surface.
Fresh water is supplied on larger boats with a distillation plant that converts seawater to fresh water. Temperature is carefully monitored and controlled. Onboard machinery creates heat, but seawater averages about 39 degrees and the metal hull conducts heat out.
Diesel submarines use diesel fuel generators to charge a large bank of batteries, and sometimes also to directly run propellers. The boat then runs off the electricity stored in the batteries. When power gets low, the sub must surface in order for the diesel engines to repower the batteries.
Nuclear submarines have their own small nuclear power plant onboard, which generates both direct power and battery-stored electricity. The batteries are used to run most on-board systems and also for back-up power in case of emergency. These subs can stay submerged for months at a time without resurfacing.
Along with the hydroplanes which direct the boat up or down, the rudder at the stern direct the boat right or left. Propellers move the boat forward and backward. Navigators use navigational charts based on topography, latitude, and longitude to plot their course.
Since they are virtually blind when travelling, they use a sophisticated GPS system when on or near the surface. Underwater an inertial guidance system, both electrical and mechanical, tracks the ship’s motion from a fixed starting point using gyroscopes. They are quite accurate for a set time, about 150 hours, after which they must be realigned to a new fixed reference point using GPS, radio, radar, or satellite.
Sonar, both active and passive, helps by locating objects. Active sonar emits sound waves in pulses that travel through the water, reflect off an object, and return to the boat. The sonar operator uses a computer to calculate the distance between them using the speed of sound in water, much the same as whales and bats use echoes to navigate. By sending a bank of sound waves, the computer can even generate a rough picture of the object. Passive sonar is listening to the sounds generating from a nearby target or object.
Another navigation tool is the periscope, which can be used close to the surface. It is a long tube that can be raised to see what is above the water. Mirrors are inset all around, and the periscope has a radius of 360 degrees. It is used to see what is going on; look for threats, check the weather, or make other general assessments.