The Interesting Engineering Behind Submarines

As far as recent records indicate, humans have only been exploring deep below the surface in the last few hundred years. Although it is heavily disputed as to when the first submarine was made, many owe the title out to Cornelis Drebbel, a Dutch inventor acclaimed to have invented the world’s first navigable submarine in 1620. Since, submarines have improved in mobility, strength, and stealth – some of which are capable of remaining submerged for up to three months underwater. Here is the interesting engineering of some of the world’s most advanced submarines.

[Image Source: United States Navy/Wikimedia Commons]

The Interesting Engineering of Submarines

Only a select few have descended into the deepest part of the world’s oceans called the Mariana Trench. In fact, more people have explored the moon compared to those who have conquered the absolute deepest spot on Earth deemed the Challenger Deep. The first people to ever descend to the bottom of the Challenger Deep are hydronauts Don Walsh and Jacques Piccard. The team sunk their submersible, the Bathyscaphe Trieste, to the bottom of the ocean in 1960.

Bathyscaphe Trieste [Image Source: Wikipedia]

The Bathyscaphe Trieste

The bathyscaphe vessel dives below the surface and is supported by a surface float, instead of a crew. Previously, propelled deep sea submersibles were supported by a float that remains at the surface of the water. Its cabin is narrow and small with just enough room for a two person crew. The rest of the hull is taken up by scientific equipment used to investigate one of the world’s most hostile regions on the planet, the bottom of the ocean.

Since the two hydronauts explored the bottom of the trench in 1960, only a handful of humans have gone back.

The Challenger Deep speaks for one of the most difficult challenges engineers face – pressure and life support.

High pressures threaten to crumple the hull of any vessel. For every 10 meters of water descended the pressure goes up by an entire atmosphere. That is the same amount of pressure experienced on the surface of an object from the entire weight of the atmosphere from sea level, all the way up until the reaches of space. At the bottom of the Challenger Deep, the pressure exceeds 1000 atmospheres (over 1500 PSI).

Only small spherical vessels can withstand the intense pressures at the bottom of the ocean. Manned submarines tend not to sink farther than one kilometer bellow the surface of the water. Modern submarines can operate independently and support a crew for weeks on end. The most advance nuclear submarines can operate independently and supply the crew with life-support systems for months on end.

There are many types of submarines, some of which are electric, diesel, or nuclear powered. There is a large misconception that diesel engines directly power the propellers of a sub. Many people question how a diesel engine could possibly run underwater with enough oxygen for it and the crew, while somehow hiding the exhaust as well. Solving the problem is a much more trivial solution, not using the engine underwater at all.

How Crews and Engines Breath Underwater

The solution to keeping a diesel supplied with enough oxygen is simple. Diesel powered submarines are typically hybrids. The diesel engine typically powers a generator which powers a battery pack. But the engine is only used while the submarine is surfaced. When the batteries run low, the submarine resurfaces and runs its engines to power its battery.

Carrying oxygen on board is the simple solution. Massive canisters are pumped full of air every time the submarine resurfaces. The system is fine for missions no longer than a month. However, governments are always pushing the bounds of naval capabilities. Most of the time, stealth is the number one consideration.

While submerged in water, the vehicle is hidden, on the surface of the water, however, a submarine becomes a sitting duck.

Of course, the problem remains – surfacing submarines and making them vulnerable.

BrainStuff explains the process of how engines breath in diesel submarines.

For electric and diesel powered subs, the problem of oxygen still persists.

The solution does exist – electrolysis – the process of splitting water into hydrogen and oxygen using electricity. Unfortunately for fully electric and diesel subs, splitting water is too power intensive for their reserved power supply. Electric and diesel powered subs simply do not carry enough energy on board to provide enough oxygen for the crew for extended periods of time. The process drains the batteries that drive the motor, forcing the vessel to surface, defeating the purpose entirely.

Electrolysis

Electrolysis is typically reserved for the largest and most advanced subs – nuclear submarines. The process works by passing an electric current through water. The electricity provides the energy necessary to split the bonds of water, or H2O.

Electrolysis of water. [Image Source: Nevit Dilmen/Wikimedia Commons]

In a typical day, one person will consume about a kilogram of oxygen, along with a concoction of mostly inert gasses. Commercial electrolysis systems typically require approximately 50 kilowatt-hours of power to produce 1 kg of H2 and 8 kg of O2 from 9 kg of water. Instead of constantly draining batteries, nuclear submarines provide a constant source of power – enough to power a small city.

The system supplies more than enough oxygen, incidentally, hydrogen is made in the process. In normal operation, nuclear subs disperse the hydrogen directly into the water. However, during covert operations, secret methods are used to scrub the hydrogen from the exhaust.

Since some navies can detect the hydrogen releases in water, most scrubbing methods are kept secretive. Although, it is likely the subs use catalytic converters to add hydrogen to another compound. However, it is not the only stealth consideration submarine engineers must consider.

The Stealth Technologies of Submarines

Submarines make a ton of noise through their engines, propellers, and general operation. The noise causes vibrations which easily carry through the water for kilometers on end. Every part on a military submarine is built with the consideration of stealth. Every vibrating part is held in place with rubber mounts that actively dampen most of the vibrations. The technique works well on the outside, although it is not the only source of noise on the vessel.

Nearly all submarines are propeller driven. As the propellers cut through the water, they produce noise for a couple of reasons. One, the blades send out vibrations as they slice through the air, and two, the blades create cavitation bubbles which collapse, sending more vibrations throughout the water – as well as damaging the blade itself.

Cavitation bubbles form on the suction side of a propeller when the pressure of the water drops below the vapor pressure of water. As quick as the bubbles are produced, they almost just as quickly disappear. The bubble collapses on the propeller blade, causing them to wear down and release noise.

The noise is easily detected, therefore, it is imperative for submarine engineers to design sound suppressive blades that reduce cavitation.

Submarine engineers modify the shape of the propeller to reduce turbulence and reduce the overall noise of the blades as they slice through the water. Often times, there are four or more blades which force more water out of the way while spinning at slower speeds. The result is a propeller that does not need to spin as fast to produce the same speeds as propellers with fewer blades. Since the blades spin slower, there is less turbulence and less formation of cavitation bubbles. Sometimes, special groves are carved into the blades which further guide the water along with as little disturbance as possible, once again reducing the formation of cavitations.

The submarine is also streamlined to direct water through the blades, reducing as much turbulence as possible. If the design is not sufficient enough, a ring or cage is sometimes installed around the propeller which guides the water through more linearly.

[Image Source: AndrejS.K/Wikimedia Commons]

Other Sound Suppression Technologies

Suppressing the noise from the submarine itself is one challenge. But developing a sub capable of evading enemy sonar presents a different predicament.

Sonar is a device used to detect objects underwater. Sound pulses are emitted and a detector records any pulses that return. By determining the time it takes for a pulse to return, the distance between the detector and the object can easily be determined.

The massive hull of large submarines is hardly stealthy. As a metallic shell, the surface of a submarine readily reflects sounds, making it highly visible to enemy radar. The solution for hiding from radar comes from an unlikely source: bubbles.

Under water, air bubbles easily absorb noise. As sound waves propagate through water they slowly disperse over time. Introducing bubbles disturbs the sound, forcing it to travel through one medium to another, absorbing a little energy with each bubble.

Incoming sound vibrations collide with the bubbles, forcing the bubble to vibrate or contract and expand. The fluctuations cause the air inside to compress, creating heat. Essentially, the sound energy is converted from noise into heat.

Counter-intuitively, producing bubbles could actually disguise the sound signature of a sub. Although, it is nearly impossible to absorb all of the sound, hence why anechoic tiles were introduced.

Sound Suppressing Tiles

Lining the outskirts of modern submarines are layers of anechoic tile, a rubber material with thousand of tiny voids. The material continues the effect of bubbles in water, without the need to dispense bubbles and reveal a position.

[Image Source: NZSnowman/Wikimedia Commons]

The material actively absorbs sonar while speakers may play predictable opposite tones to cancel some noise. Together, the two along with other technologies keep submarines hidden at all times, plotting on the floors of the ocean.

The extreme capabilities of modern engineers are the result of hundreds, if not thousands of years of trial and error. The interesting engineering of submarines keeps the crews alive, and well hidden in the most challenging of conditions. Space may be difficult, but it would seem the bottom of the ocean possesses the greatest challenge.

Sources: MIT, BrainStuff, MarineInsight, Geology, BBC

SEE ALSO: Understanding the Incredible Engineering of Cruise Ships