by Barry Parker
Although several crude designs for submarines appeared before the 1700s, one of the first to build an operational model was the American engineer Robert Fulton. Between 1793 and 1797 he built the first working submarine while living in France. It could stay underwater for seventeen minutes, and it was about twenty-four feet long. He called it the Nautilus. Submarines were also used in the American Civil War. The Confederates built four submarines, the most famous of which was the H. L. Hunley. After the war, research on submarines continued, and this research is usually associated with two names: Simon Lake and John Holland. Lake began experimenting with the idea of using buoyancy to submerge and surface a submarine. Holland worked on various methods of propulsion. The US Navy's first commissioned submarine, the USS Holland, was built by Holland in 1898. It was fifty-three feet long, weighed seventy-five tons, and it had an internal combustion engine for running on the surface and an electric motor for use while submerged.
All submarines depend on a principle that was formulated many years earlier by Archimedes of Syracuse, Sicily. We discussed it briefly earlier; let's look at it now in more detail.
ARCHIMEDES’ PRINCIPLE
Archimedes’ principle is related to the pressure on an object in water or another liquid, or more exactly the buoyancy on an object in a fluid.1 To understand it, let's begin with the concept of pressure; it is defined as force per unit area, or algebraically, as P = F/A, where P is pressure, F is force, and A is area. If you're considering the pressure on a given surface under a certain amount of water, it's easy to see that the pressure comes from the weight of the column of water above it acting on the surface. And the weight of this water depends on its density, which is defined as its weight per unit volume. The density of water is sixty-two pounds per cubic foot.
But we're mainly interested in buoyancy, so let's consider a solid cube within a tank of water and determine the buoyant force on it. This is the force pushing it upward. Archimedes’ principle states that the upward force on any object in water or other fluid is equal to the weight of the fluid displaced. Archimedes (278–212 BCE) arrived at his principle when he was asked by the king of Syracuse to find out if a blacksmith had stolen some of the gold he had been given to make a crown by substituting silver for it. And, as it turned out, he had.
Archimedes’ principle is valid if the body is totally submerged, or if it is floating. In fact, it's easy to see that if the weight of the body is less than the weight of the water displaced (when it is totally submerged), the body will float. This means that if its density is less than that of water, it will float.
Let's look now at our solid cube in the tank of water. Assume it is totally submerged. The pressure on all sides will equalize because there is an equal opposing force across from any force acting on it. But the force on the top and the bottom will be different because the upward force on the bottom is greater than that of the force on the top, since the bottom of the cube is deeper. The difference will, in fact, be equal to the weight of the water the cube displaces. This is the buoyant force. And if it is greater than the weight of the cube, the cube will move upward and float. This is, of course, what Archimedes’ principle tells us. It occurs for any object in water when the object's density is less than in that of water, and this is, in fact, why ships, which are made of heavy steel, can float. Steel has a high density, but the ship is made up mostly of air, which has a density much less than that of water, so it has an average density less than that of water.
PHYSICS OF SUBMARINES
A submarine can, of course, float on the surface, and it can also dive under the water. And when it is floating its average density has to be less than that of water, but when it dives its average density has to be greater. So it obviously has to change its density, and it does this using ballast tanks that are on its outer surface. When these tanks are full of air the average density of the submarine is less than that of water, so the submarine floats. To submerge, the submarine releases the air through small vents and allows the tanks to fill with water. When they are full (or partially full), the average density of the submarine is sufficient for it to sink. To surface, air is pumped into the ballast tanks from a compressed air tank. It forces the water out.2
Hydroplanes are also used to assist in the process of diving and resurfacing. They are at the rear of the submarine and look like the wings of an airplane. They help steer the submarine up and down in the same way that rudders on an airplane do.
It is also important to keep the submarine level and steady at various depths when it is underwater. In practice there are several problems. For example, water density increases with depth, so buoyancy increases as depth increases. The temperature of the water also has a small effect. Because of these and other problems, the submarine is in unstable equilibrium when it is submerged, and it therefore has a tendency to rise or sink at one end or the other unless adjustments are made continuously. This is referred to as maintaining trim. To achieve it, submarines use smaller forward and aft tanks. Pumps move water back and forth between them, changing the weight distribution almost continuously. A similar system is also used for stability.3
Ballast tanks on a submarine.
POWER FOR THE PROPELLERS
A submarine needs power to turn the propellers, and over the years the power source has changed. The earliest submarines were powered by human muscle. A number of men actually cranked the propeller by hand. But it wasn't long before engines of various types were introduced to do this. By about 1900 gas-powered engines were used on the surface, and electric motors were used when the submarines submerged. Gasoline engines were, however, soon replaced by diesel engines. In the first submarines of this type, the diesel and electric motors were separated by a clutch, so they were both on the same drive shaft to the propeller. This allowed the engine to drive the electric motor as a generator that could be used to recharge the battery that was used for electric power. One of the main problems with the submarines of World War I and World War II was that they had to resurface to recharge their batteries quite frequently. Eventually a snorkel device was invented so they could recharge while still submerged, but they still had to be quite close to the surface.
SHAPE AND PERISCOPES
One of the major problems associated with submarines is hydrodynamic drag. Drag is also a problem in relation to cars, and they are therefore designed with a shape that minimizes it. In the case of submarines, the medium through which they travel is water, and water creates a much greater drag than air does. A teardrop shape for the front was used to keep drag to a minimum on most submarines deployed during World War I and World War II; more recently, however, a slightly different surface shape has been used, although the teardrop shape is still used to some extent.
Submarine showing periscope, sail, and rudder.
On the top of the submarine is a raised tower, known as the conning tower, which accommodates the periscope, various electronic devices, and the radio. In many early submarines the control room was also located here. The control room is now located within the submarine and the raised tower is now called the sail. The periscope allows an observer in the submarine to see what is happening on the surface when the submarine is submerged. It consists of a system of mirrors and lenses that bend and reflect images down a long tube. In newer submarines photon masts have superseded periscopes. They are high-resolution, color cameras that send images via fiber optics to a large screen (in fiber optics, pulses of light are sent along a long optical fiber).
NAVIGATION
A modern submarine can use GPS (the global-positioning system) to help guide it while it is on the surface, but when it is submerged GPS does not work. Newer submarines therefore have underwater inertial guidance systems that keep track of their position by noting their motion away from a fixed stationary point. These systems are quite complex, and they generally use gyroscopes to track the location of the submarine. American submarines use a system called SINS (ships inertial navigation system); it keeps track of
the location of the submarine by following its course changes using gyroscopes. Numbers are fed to a computer and compared to the starting coordinate. With this system submariners can quickly determine where they are at any time.
Gyroscopes are not only useful for underwater navigation, but, as we will see, they are also used to guide torpedoes to a target. A gyroscope is helpful because it exhibits a fundamental property called gyroscopic inertia, which gives it rigidity in space. As we saw earlier, this is a consequence of Newton's first law of motion, which states that a body tends toward a continuous state of rest or uniform motion unless subjected to an outside force. This means that when a gyroscope is set spinning in a particular direction, it takes a force to change it. Gyroscopes are, in fact, not only used in submarines and torpedoes; they are also used in spacecraft, rockets, guided missiles, and ships, so they obviously play an important role in warfare.
SONAR
Also important in relation to underwater navigation is sonar. When a submarine is submerged it is generally cut off from the region around it because light does not penetrate very far into water. So even if it had video cameras attached to its exterior, they would be of little use. Sonar is similar to radar except that it uses sound waves rather than microwaves. In the last chapter we saw that radar systems send out an electromagnetic pulse then look for an echo, or reflection, of the pulse. By analyzing the echo, radar systems can determine what is around them, even if the objects can't be seen directly. In the same way, sonar allows submariners to see what is in the water around them.4
Two types of sonar are used in submarines: active and passive. Active sonar is an analog to radar in that the travel time for a reflected wave is recorded along with any change in frequency of the initial signal. An active signal transmitter, or signal generator, creates a pulse of sound that is referred to as a ping. This pulse is concentrated into a relatively narrow beam, so that it is going in a particular direction. It is used mainly for detecting other submarines, ships, or other objects around the submarine. Analysis of the echo gives information about the distance to the object and the direction and speed at which it is traveling. Its distance can easily be determined from the time between the release of the signal and the return of the echo. Its speed can be determined from the Doppler Effect.5
One of the problems with active sonar in warfare is that any ship or submarine in the neighborhood can easily pick it up, which could allow the enemy to determine the vessel's position. Because of this, passive sonar is used in many situations. It is simply a very sensitive underwater microphone that is used to listen for noises in the water around the submarine. The problem, of course, is identifying the sound that the microphone picks up. In most cases, however, this is left to a computer. A large database of different sounds, along with the things that cause them, is stored in the computer. When a particular sound is detected, it is fed to the computer for identification. In general, passive sonar has a greater range, and it has the advantage of being undetectable.
In World War II, active sonar was generally kept to a minimum, so most submarines relied heavily on passive sonar. But modern techniques and devices have improved active sonar, so both active and passive sonar are now used extensively. There are, of course, other problems with both types of sonar. The signal is influenced by the depth of the water and by the water's temperature and solubility, and these factors have to be taken into account. In addition, there is what is called a thermocline in the ocean. It's a sharp division between the warm surface water and the cold, still waters below. As sound waves pass through it they tend to be deflected, and this has to be taken into account as well.
Sonar is used not just by submarines. Objects called sonobuoys were used extensively during World War II, and they are still used. Sonobuoys are small systems, about three feet long and five inches across, which can be easily dropped or ejected from an airplane or ship. They float in the water and can be either active or passive. Their signal is picked up by a nearby ship or an airplane. They do have limitations, however. They have a limited lifetime (depending on their batteries) and a limited range, but they have proven to be useful.
TORPEDOES
Robert Fulton is believed to have been the first to equip a submarine with a torpedo. His submarine, the Nautilus, was equipped with a torpedo that was actually little more than a box of dynamite designed to explode beneath enemy ships. He used it in a demonstration in France in 1801 to sink a small ship, and again in a demonstration in England, but he didn't manage to get much interest from either government.6
Torpedoes first came into their own during the American Civil War, and they were used most effectively by the Confederate navy. At that time they were mounted on a boom or spar in the front of the submarine, from which they were attached to an enemy vessel. Sometimes they were detonated by the blow they received when they struck the ship, and sometimes timing devices were used. Free-floating torpedoes, or what we today would call mines, were also used. Twenty-two Union ships were sunk by Confederate torpedoes, while only six Confederate ships were destroyed by Union torpedoes. One of the most famous sinkings was achieved by the Confederate submarine H. L. Hunley. On the night of February 17, 1864, it rammed and sank the Union ship USS Housatonic, but the explosion was so great that it damaged the Hunley, and it also sank with all men aboard. In 2004 the remains of the Hunley were located and raised.
One of the most significant advances in torpedo technology came in 1864. An Englishman, Robert Whitehead, who was working in Austria, became interested in the torpedo and decided to build a model that would run just under the surface of the water. In October 1866, he had his model ready. It was driven by a two-cylinder compressed-air engine, had a top speed of about seven and a half miles per hour, and a range of approximately two hundred yards. Officials in Austria were so impressed that they immediately purchased it. He did, however, also sell the rights to manufacture it to several other countries. Strangely, the US Navy was not interested in it.
HOW TORPEDOES WORK
A modern torpedo is a self-propelled projectile. It is stored in the launching area until it is fired. When fired, it is given an initial velocity, but as it moves out into the water several other forces act on it. Gravity pulls it downward, and the drag of water on it creates a friction that slows it down. The friction created by the drag of water is, indeed, quite large—about a thousand times greater than the drag created by air. Depending on design, there is another force that also comes into play, namely the buoyancy of the torpedo itself. All of these forces have to be taken into consideration.
The earliest torpedoes used compressed air to turn their propellers. Within a few years, however, it was found that compressed oxygen was more efficient, but oxygen systems posed a danger to submarines when they came under attack. Because of this, the Germans used a small electric motor powered by batteries. It had an additional advantage in that bubbles were not released as a torpedo moved toward its target. It was slower and had a more limited range than previous torpedoes, but it was much cheaper to build. The United States also soon introduced an electric motor model called the Mark 18.
Torpedoes can be aimed at a target and fired in the same way that an artillery shell is fired. In this case, there is no control over the torpedo once it leaves the submarine, and no changes can be made if the target sees it and tries to outmaneuver it. Because of this, guided torpedoes are frequently used. In some cases they are guided to the target by its sound or by the use of sonar. They are referred to as acoustic torpedoes. The first acoustic torpedoes were employed late in World War II by the Germans, and they proved to be quite effective against both surface ships and other submarines.
Acoustic torpedoes are equipped with acoustic sensors and transmitters in their nose. They are therefore able both to detect sound coming from the target and to produce sonar reflections. Usually the torpedo starts by using passive sonar. Once the passive sonar has detected the enemy, it switches over to active sonar, which allows it to send out a
sound beam to locate the enemy more exactly. It then attacks.
Another effect that is useful in the case of torpedoes is what is called supercavitation. When an object moves through water at high speed, the pressure behind the object is lowered, and, as a result, a bubble is formed that can encompass the object. This is particularly useful in the case of a torpedo because water creates a large frictional drag on it. If the object is in such a bubble, however, the drag is significantly reduced. Torpedoes are therefore designed to produce supercavitation bubbles.
SUBMARINES IN WORLD WAR II
Submarines were used extensively in World War II by both sides. They were particularly effective for the Germans at the beginning of the war and for the Americans against the Japanese toward the end of the war. Although they were limited in speed, range, and endurance while they were underwater, they could attack with total surprise and inflict devastating damage, so they were highly lethal. Although early submarines were basically underwater crafts, they spent a lot of time on the surface, submerging only when they were engaged with the enemy.7
The Treaty of Versailles that ended World War I did not allow Germany to build either surface ships or submarines. But the Germans soon found that they could build submarines much more quickly and with more secrecy, so they concentrated on them, and by the beginning of World War II they had the largest fleet of submarines in the world. Furthermore, they had also now developed several new technologies and techniques, so their submarines were better than the British and American ones. The early U-boat success of the Germans was mainly due to a World War I U-boat captain Karl Doenitz. He built up the submarine fleet and equipped it with highly trained crews, and he developed what was called the Wolfpack tactic, which was particularly effective. To initiate the tactic, German U-boats would spread out across a large section of the ocean looking for convoys. When one was located, the captain of the U-boat that located it would signal the other U-boats, and they would group themselves around and ahead of the convoy. All convoys were escorted and protected by destroyers and other battleships at that time, so the U-boat captains had to outsmart the escorts. They would therefore attack together at night, creating as much chaos as possible; this would give the German submarines a much better chance of escaping. Allied losses were high when this tactic was first used.