Naval Mines have been used for hundreds of years, so how exactly do these passive weapons work?

While you might think of naval mine technology as one of war's past, these underwater weapons are still crucial aspects of modern naval warfare.

In order to understand this often forgotten aspect of modern warfare, let's take a look back at the history of naval mines, how they work, and how they are used in the modern era.

History of Naval Mines

David Bushnell is known as the inventor of sea mines. During the American revolutionary war in 1777, he set adrift what were known as floating explosive torpedoes in the Delaware River, hoping to take a British ship as a casualty. It took out a small boat by the HMS Cerberus, a British frigate, killing four sailors. These would be the first casualty of sea mines in naval history. In the years after, mines became seen as "devilish" and "unchivalrous" in combat.

For their early lives, sea mines were used sparingly and it wasn't until the American Civil War that the world saw the first large scale use of these explosive torpedoes.

The Confederate Navy was known to be inferior to the Federal Navy during the war, so to compensate they turned to massive mining efforts. They covered their coastline with various types of mines and inflicted heavy losses on the federal navy. During the Battle of Mobile Bay, mines sank 27 Federal vessels.

During World War I, naval mines became a primary weapon against the deadly and feared German U-boats. The Allies laid what is known as the "North Sea Barrage", a minefield that extended 250 miles from Scotland to Norway in 1918. In five months, American and British forces were able to plant 72,000 mines. The war actually ended before the minefield was completed, but during its short time in service, it sank six submarines and damaged even more enemy vessels.

Following the First World War and the subsequent period of peace, mines were forgotten about as effective weapons and not much development was performed in the industry. That is, until World War II.

With the advancement of submarine and airplane technology, both of these crafts were developed to effectively lay sea mines. These mines had now evolved past the early stages of mine design that functioned only on contact, to be what are called influence mines. These mines were now able to actuate based on magnetic, acoustic, or pressure changes in the water due to enemy ships. Technology even existed to program the mines to only detonate on certain ships, i.e. they could program a mine to wait for the Carrier in the middle of a strike group based on various inputs.

Mines became a highly effective and strategic weapon in World War II. One example of such was Operation Starvation. The United States carried out a massive minelaying operation near the end of the war in the Pacific that involved laying 12,000 mines blocking Japanese shipping routes. In total, the U.S. sank 650 Japanese ships and practically halted their shipping activities. Due to the mines' effectiveness as not only an active weapon but a psychological deterrent, nearly all Japanese ships were forced to stay in port or heavily diverted into enemy waters.

After WWII, mines fell again into the background of warfare as the world tried to scale back their militaries. Superpowers assumed that naval mines wouldn't be useful with the state of advanced warfare, but they were soon proven wrong.

During the Korean War, the Korean Navy only had 45 vessels, a tiny navy compared to the U.S.'s 250 invading fleet. Yet, nearly the entire U.S. fleet was held up for a week due to 3,000 mines laid in the North Korean waters. This got the attention of the U.S.'s Chief of Naval Operations, Admiral Forrest Sherman, and the U.S. started heavily investing into mine countermeasures and advanced mining technology.

Another important note is that this re-interest in mines wasn't just due to the week-long holdup, but also that 70% of U.S. naval losses were directly the result of enemy mine warfare in the Korean War. Only 2% of the US naval service was minesweeping personnel, demonstrating a clear flaw in U.S. naval strategy.

The U.S. developed the Destructor class of mines in 1967. These mines contained highly sophisticated, solid-state firing mechanisms that were inserted into the fuze wells of simple, general-purpose bombs.

This class of mines stayed in operation until the Vietnam War era, where a new family of Quickstrike mines was developed. These mines were heavily sophisticated and developed for specific strategic uses. They were also incredibly cheap compared to other weapons, making them perfect for defensive action.

The U.S. continued to build up their mine warfare forces in all types, involving sea and air units, ordnance disposal detachments, reconnaissance units, and minesweeping ships.

To this day, naval mine operations and the skills of the community therein remain the unsung backbone of U.S. naval defensive operations around the world.

Anatomy of a Mine

Modern Naval mines are both fairly simple but also incredibly complex devices. Their simplicity is due to their overall lack of components and their complexity is a result of the high intelligence programmed into them. Modern mines also have a variety of safety features that old mines never had, like the ability to be easily stored or stockpiled and the ability to self-destruct or self-sterilize to keep wildlife and water safe after the mines are no longer needed.

Naval mines usually contain flight gear or handling gear, an explosive case, an arming device with an explosive train, a target detecting device, and a battery.

The flight or handling gear is used to transport and deploy the mine from any one of the deployment techniques in the military's arsenal. The explosive case contains the raw explosive material contained in the mine, usually identical to the types of explosives used in bombs. The arming device and explosive train provide the initial arming of the mine after deployment. You can think of this assembly as the key that allows a mine to be detonated. When the arming device and train are properly aligned, the target detecting device can do its job and actuate the mine.

The target detecting device is the most important part of a mine. It's the one component that makes something a naval mine and not just a bomb. This device senses any number of triggers from the surrounding water, whether that be seismic, acoustic, pressure, or magnetic, and actuates the mine on a given set of constraints.

Different ships will have different signatures in any one of these categories which allows the mines internal technology to make informed decisions on when to activate.

When all of these components are assembled together in proper functionality, you get a naval mine that can be a highly effective intelligent weapon in the naval battlespace.

Mine Types

Among the naval mine weapon type, there are various different types of mines in how they are deployed and used. These usually relate to their final positions, or in other words, where they rest in the water awaiting detonation.

Mines will fall into three main categories: bottom, moored, or drifting.

Bottom Mines

Bottom mines are those that rest on the seafloor, usually in comparatively shallow waters. These mines tend to have large negative buoyancies (they sink), which means after they are deployed they fall and rest on the ground. Certain bottom mines do work in deeper waters that allow them to only go after submarines.

Moored Mines

Moored mines are used against submarines and ships and float a set distance from the seafloor thanks to a weight and mooring, or a tether. The mine itself is held in a case that floats which is attached to an anchoring device that sits on the seafloor. This mine type allows for deeper water deployment as well as other specific use cases.

Mines that are found on beaches that usually make the news are typically old moored mines that have broken loose of their mooring and floated to the surface.

Drifting Mines

Drifting mines are those that float on the surface of the water and explode usually on contact with a ship. Due to their uncontrollable nature, these types of mines were actually outlawed at the Hague Convention of 1907. They have been sparingly used since then.

Mine Delivery

We've discussed the different functional types of mines, but each type also can be deployed in 1 of three ways. Either by Aircraft, submarine, or surface-laying.

Aircraft Laid Mines

Aircraft-laid mines are typically designed to be offensive in nature and are suited for rapid deployment, similar to bombs. Aircraft mines can be used to rapidly provide minefield replenishment without having to send vulnerable ships to the front lines and put them in danger from previously laid mines.

Today, aircraft are seen as an ideal mine delivery technique as the process requires little risk and can be done quickly. Essentially any aircraft that can carry and deploy bombs can also carry and deploy mines.

Submarine-Laid Mines

Submarine-laid mines, like aircraft-laid mines, are typically used in offensive covert operations. These types of mines will fit in submarine torpedo tubes and can be deployed at depth. Typically submarines can only hold a small number of mines, which makes this technique not the best suited for laying large minefields.

During WWII, submarines planted a total of 576 mines, that we know about, which directly resulted in the sinking of 27 ships and the damage of another 27.

Surface-laid Mines

Surface laid mines are the most economical due to the sheer volume of mines that can be transported on a ship. However, surface-laying mines require that the laying country has control of the waters and there are no potential imminent threats. This is not the case in many circumstances, which often leads to submarines and aircraft to be more preferred mine delivery techniques.

Surface-laying is typically used in defensive situations, like defending your harbors. In WWII, this was a very common practice. While there are no documented cases of these defensive minefields taking enemy casualties, there are also no documented cases of enemy vessels getting through the minefields. This underscores one of mines' most essential qualities, their ability to scare off enemies without active engagement – a psychological weapon.

Actuation

Understanding how mines are actuated is crucial to understanding how to protect against them and how to use them. Each type of actuation relies on sensors that are placed inside of the target detecting device, which ultimately triggers the detonation of the mine.

There are four main techniques hinted at earlier: Magnetic, Pressure, Acoustic, and Seismic.

Magnetic Actuation

In cases of magnetic actuation, the target detecting device will contain small magnetometers or sensors that detect magnetic fields. Magnetometers are typically 3-axis detectors, which essentially means they can pick up on magnetic fields in any direction.

Every ship has a magnetic signature of some sort which is specific to size, metal type, and even where the ship was built on earth. As a ship passes, the magnetometer picks up on the signal and converts what it senses into tiny electrical pulses. These pulses are then processed by the internal circuitry and the target detecting device determines whether it wants to actuate or not. When the device determines it has enough input to actuate, the mine is triggered.

Pressure Actuation

Pressure actuation sensors detect negative pressures and convert them into electrical signals. This works similarly to magnetometers, except with pressure. Like ships' magnetic signatures, each ship also has a pressure signature, which is dependent on things like size, shape, and weight. As ships move through the water, they displace a given volume of water. The changes in pressure in water from a ships hull are dictated by Bernoulli's Principle.

After the TDD analyzes the pressure signature and determines that it wants to detonate on the target, it sends signals through the explosive train to finalize the actuation process.

Acoustic Sensors

Acoustic sensors convert soundwaves that travel underwater into electrical signals that the TDD can process. This sound comes from hull noise as the ship moves through the water, propeller noise like water cavitation, and even crew noise inside of a ship. These all create sound vibrations that travel through the water and can meet the requirements of an acoustically trained TDD to detonate.

Seismic Sensors

Seismic sensors are quite similar to acoustic sensors, and they utilize similar acoustic inputs to determine detonation.  The seismic sensors sense case movements that are a result of acoustic stimuli. These sensors are typically slightly more sensitive than acoustic sensors and can be programmed in conjunction to further refine when mines detonate.

How Mines Sink Ships

Now that we've thoroughly grasped what mines are, how they are used, and how they work, the last thing to cover is what happens when they go boom.

You might think that mines just explode and punch a large hole in the side of the ship. This might be the case for the surface mines of old days, but modern mines almost always rest below the surface of the water.

When a mine explodes underwater, the energy is dissipated in three ways.

53% goes to a shockwave

46% is converted to heat

1% is expended as light

The primary ways that mines damage ships are through the initial shockwave and the resultant gas bubble pulse. The shockwave propagates through the water and can damage or crack the hulls of ships if powerful enough. After detonation or rapid expansion underwater of the mine, a gas bubble is created underwater. This gas bubble expands until the pressure of the surrounding water causes it to collapse at incredible speed. After collapsing, it then expands again, and again, and again. Each time losing a little bit of energy. This rapid pulsing is essentially cavitation on a massive scale and it causes ships to break apart, snap in half, and eventually sink.

This combination of effects, shockwave and bubble pulse, causes immense damage to ships and makes mines formattable opponents to even the most advanced naval warfare.

While you might not think of mines as crucial to 21st-century warfare, they make up an essential part of countries' naval strategies around the world. Behind all of the mine technology, there's a complete other science of how to detect, hunt, and neutralize mines that we won't get into here. Naval mines are formidable weapons in a modern battlespace.

(Source: Trevor English, Interesting Engineering- 5/10/2019)

https://interestingengineering.com/how-do-naval-mines-work