Why Being Too Stealthy is Bad for Submarines

The missile that just popped out of the ocean, can  hit any place on earth while potentially carrying a nuclear warhead … and it was launched from a  submarine. This is what makes these underwater boats some of the deadliest pieces of equipment  in any military arsenal … until you realize that submarines are really like giant cannons  … made of glass, because they are only as deadly as they are stealthy. Once they are  detected, the jig is up, and that underwater boat just became a giant underwater target.

This is why militaries around the world have poured billions of dollars into trying to keep  their subs as sneaky as possible, while at the same time spending billions in figuring out  how to spot the other guy’s submarines. But why being too stealthy can actually be a  bad thing for submarines, how a small school of random fish swimming around is some of  the best camouflage a giant sub can ask for, and why these underwater boats have to  operate deeper when in the Arctic compared to the equator, is not what you think! The easiest way to pick up a submarine floating around underwater, is by its acoustic  signature.

Every submarine has a unique set of sounds produced by its internal systems  and engines, and even propeller movements. That’s why submarine propellers are typically  covered when in drydock, to prevent enemies from deriving the acoustic signature based on the  shape and size of the propeller. To minimize their acoustic signature, subs use active  noise canceling mounts for onboard machinery, and specialized geometry on their hull to push  through the water as smoothly as possible.

American Virginia-class submarines for example,  on average produce around 90 to 95 decibels when cruising, which is roughly the same noise level  as the currents in the ocean, and marine wildlife around them. This means that even a small school  of fish looking for dinner, adds enough noise to the background to help submarines hide in plain  sight. But that means that in areas where there is no wildlife to make noise, or the water is  particularly calm, the crew has to ensure they stay extra quiet.

They call it “Silent Running”,  because even having sailors walk around with boots on, or having a normal conversation could  produce enough noise to be detected. Sea-wolf class attack submarines, which are  the quietest submarines on earth, have an even smaller acoustic signature. Sounds great,  but that might not always be a good thing.

One way that submarines are able to achieve  such low signatures, is with anechoic tiles, which are these squared off pieces you see  on submarine hulls. These are synthetic or rubber based panels that are installed in  layers on the outside of the superstructure. They work in the same way that foam  panels work in a recording booth, by absorbing soundwaves from the outside to  prevent reflection from any active sonars.

But also from the inside, so that the noises  inside the submarine don’t “leak” out. Because one of the biggest sources of noise on  modern subs, is their powerplant. Modern nuclear power plants on submarines allow  the boats to stay underwater indefinitely, as long as the crew doesn’t run out of  food and toilet paper.

Ironically though, older diesel electric submarines are actually  quieter. Diesel subs run off of battery power when underway, which means there are no moving  parts needed to keep everything powered. But diesel submarines have the massive downside  of needing to surface periodically, to charge the batteries with the diesel generators, which  can take upwards of 8 hours in some boats.

This combined with the limited time the sub can operate  before running out of fuel, means that most countries decide to just stick with the slightly  louder nuclear option … if they can afford it! What’s surprising is that being too quiet  can also be dangerous for a submarine. A submarine produces the same amount of noise,  even when it’s operating in a particularly loud part of the ocean, with things like  dense wildlife, or strong currents.

But if the anechoic tiles absorb most of the  sound waves, they can create a quiet zone, known as an “acoustic hole”. This can be easily  detected by Passive Sonar, as it comes across an unusually quiet space in the middle of all the  ambient noise. Any well-trained sonar tech will be able to tell that there’s probably a sub floating  around in the acoustic hole.

But guess what happens if the sonar tech wasn’t well trained! In the early morning hours of February 4th, 2009, the HMS Vanguard, armed with Trident II  nuclear missiles, was on a routine patrol in the Eastern Atlantic Ocean. Everything was going  normally until the sub collided with a massive underwater object that they had somehow completely  missed on the sonar.

That object was none other than the French submarine, Le Triomphant, which  was also armed with its own complement of nuclear missiles. Both submarines sustained enough  damage to need emergency port repairs. It was later determined that despite both subs  having the latest and greatest underwater sensors, neither was able to detect the other because they  were both so quiet, which led to them unexpectedly bumping into each other in the ocean.

To prevent such accidents, navies may store the acoustic signatures of their submarines in  a database. That way friendly ships and subs are able to look for those exact frequencies  given off, without the submarines having to expose themselves to any nosey onlookers. But because those acoustic fingerprints are so useful in finding your own submarines, they’ve  been hot tickets for spies to grab.

In 2018, Chinese cyber attacks managed to steal a mountain  of classified data from the United States Navy. Experts believe that some of that data included  the specific acoustic signatures of some American submarines, making it easier for them to find for  China’s underwater detection networks. But if you knew there might be a submarine in a particular  area, there is a way to find them from the sky!

Similar to their acoustic signature, submarines  also have a thermal signature. This is the heat emitted from the hull, which can be detected by  sensitive infrared sensors and thermal imaging systems installed on helicopters and satellites  built specifically for anti-submarine warfare. Thermal signatures are a huge factor  that determines how deep a submarine will be patrolling.

In areas closer to  the equator where the water is warmer, subs can swim a bit higher if they need  to. But in colder waters near the arctic, even the smallest heat signatures are easily  detected, so subs have to stay lower. This is why submarines use the ocean’s  thermoclines to their advantage.

Thermocline is the transition layer  between the warmer mixed water at the surface and the cooler deep water below. If a submarine sits below the thermocline, any heat it generates would effectively  dissipate before reaching the surface, so aerial thermal sensors wouldn’t be able to see  them. The challenge is that thermoclines aren’t at a fixed depth and are constantly changing, so if  submarines aren’t careful, they can accidentally reveal themselves to any kind of thermal  sensors that might be looking for them.

The same aircraft looking for thermal signatures  also could deploying these things, and no that’s not the crew’s trash from lunch. These are  called sonobouys. Think of them like sonar bombs.

Once they hit the water, they deploy a  hydrophone sensor, which picks up all the acoustic activity going on underneath it, and  then transmits that information via a radio signal to the aircraft or ship that deployed it. You might think that an obvious thing to look for if you want to spot a submarine, would just be to  look for any periscopes sticking out of the water. The thing is, submarine crews are very selective  when and where they get close to the surface, and even more careful about sticking a giant  periscope tube out.

Many countries will cover the periscope in radar absorbent materials, similar  to a stealth fighter. That way no one would be able to pick it up with any kind of sensor. To get around this, the United States is looking for ways to detect the small waves  that are produced by a periscope, rather than the periscope itself, meaning future  subs will probably need a way to reduce the wake they produce when they do use it.

But what  happens once a submarine is detected? One of the biggest threats to sub crews  are torpedoes, which have a suite of highly advanced sensors for tracking. These are  able to track every bit of positional data that an enemy submarine produces, from sonar and  magnetic signature, to even the wake produced by the propellers.

A modern torpedo can even  tell the difference between a real submarine, and a decoy designed to look like one. Then just  like any other missile, once it gets close enough, it detonates. Keep in mind that being fast and  sneaky means a submarine can’t get bogged down by a bunch of heavy armor, so while they look fairly  sturdy, they can actually be pretty fragile.

Torpedoes these days are launched by pretty  much anything above, on, or under the water. Anti-submarine helicopters come equipped with  torpedo tubes as standard, many modern warships have some kind of torpedo launching capability,  and of course, submarines have torpedo launch tubes that flood with seawater before firing. Old school depth charges used to be a favorite method of taking out a sub.

These  would sink to a predetermined depth, and then automatically explode, either destroying  the sub or forcing it to come to the surface. Some navies, like Russia’s, use something like the  RBU-6000, which can spread a large number of depth charges around a large area. However, most modern  submarines can now get around depth charges.

Arguably, the best way to catch a submarine by  surprise is with a mine. You might be thinking of something that looks like this but these  days you’re more likely to find something like the CAPTOR mine, which sits anchored on the sea  floor, and fires a MK 46 torpedo at any submarines floating by that shouldn’t be there. The depth of the ocean may provide a lot of cover for submarines, but it also makes communications  with the submarine almost impossible.

The high frequency signals that surface ships use  for communications can transmit large amounts of data while being encrypted and resistant to  jamming. But they cannot penetrate past the surface of the water. So how can submarines  communicate with the outside world?

Submarines could use low frequency  communication to get messages out, but these can be easily detected and tapped, so  they only do it if they absolutely have to. The lowest end of radio comms, is the  Extremely Low Frequency range, or ELF, which is able to easily penetrate deep into  the water. It’s just that it carries so little information on it, that it can only be really used  for one thing: launching nuclear weapons.

Nuclear submarines are part of the TACAMO  program, or Take Charge and Move Out. In the event that the United States decides to  start lobbing thermonuclear warheads around, specialized aircraft like the E-6 Mercury will fly  over the patrol paths of American nuclear subs, transmitting launch data on the ELF  spectrum. American submarines looking for these signals will typically deploy a  long trailing wire, which needs to be large enough to pick up on such long wavelengths.

That said, those messages are one-way only, so once a boat receives the signal to start a launch  sequence, that’s it, and they begin initiating a strike on their pre-designated targets. Luckily that’s a worst case scenario. These days, submarines do a lot more spying, wire cutting,  and deterrence than they do blowing up aircraft carriers and launching nukes.

So more and more  countries are investing into spotting them, like the Russian “Harmony” detection network,  and China’s Underwater Great Wall, which looks for submarines in the South China Sea. That said, the submarine stealth technology continues to evolve, because in the  world of submarines, it’s not ‘speak now or forever hold your peace’—it’s ‘stay  quiet and forever hold your breath.