Have you ever wondered how colossal metal vessels weighing tons can effortlessly glide beneath the ocean's surface, defying gravity's pull? The secret lies in a fundamental principle of physics, buoyancy. Buoyancy is a force that affects how objects rise or fall in a gas such as air, or in a liquid such as water. Let’s see how submersibles and submarines harness this force to dive, surface, and explore the underwater realm.

A submersible (or any boat) floats when the mass of the water it displaces is equal to the mass of the vessel. This displaced water generates an upward force known as buoyancy, which counteracts the downward pull of gravity. Unlike regular boats however, submersibles and submarines have the ability to control their buoyancy, and dive beneath the water or rise to the surface as needed.

The Principle of Buoyancy in a Liquid

Before we delve into the mechanics of how submersibles and submarines manipulate their buoyancy, let's explore the concept of buoyancy itself. Buoyancy (or upthrust) is a gravitational force, a net upward force exerted by a fluid that opposes the weight of a partially or fully immersed object. In scientific terms, buoyancy is governed by Archimedes' principle, which states that any object submerged in a fluid experiences an upward force equal to the weight of the fluid displaced by the object.

Imagine placing a cork in a glass of water. It floats effortlessly because the upward force exerted by the water, known as buoyant force, is greater than the cork's weight. This upward force is equal to the weight of the water displaced by the cork. Now, consider a steel ball. It sinks because its weight exceeds the buoyant force acting on it.

The key to buoyancy, therefore, is the density of an object relative to the fluid it's in, with density defined as mass per unit volume. Simply put, it does not matter how much something weighs- if an object is less dense, it floats; if it's denser, it sinks. That is why a large and thin steel bowl can float on water, while a small and dense steel ball - of the same weight - will sink!

The Role of Neutral Buoyancy in Submersibles and Submarines

An object that floats in a fluid is known as being positively buoyant. An object that sinks to the bottom is negatively buoyant, while an object that remains in balance at the same level in the fluid is neutrally buoyant.

To better utilize buoyancy to control their depth in water, many submersibles and submarines are designed to easily achieve a Neutral Buoyancy, where their overall weight is equal to the weight of the water displaced. This allows them to hover at specific depths without constant adjustments. Materials such as low density syntactic foams, and other syntactic systems are used to make vehicles neutrally buoyant on their own. Both types of vessels are also designed with special systems that allow them to adjust their buoyancy, enabling them to dive to specific depths and return to the surface.

Submersibles
Submersibles are typically small and agile vessels designed for specific underwater missions. Uncrewed technology systems are often used in ocean exploration, underwater filming, and scientific research of marine life from coral reefs to the deepest ocean floors. There are a variety of submersible Unmanned Underwater Vehicles (UUV), including AUVs, ROVs and HOVs. Unlike submarines, submersibles are usually deployed from a larger ship and are not capable of independent long-distance travel. They typically rely on a combination of factors to achieve buoyancy:

• Structural Materials: Many submersibles incorporate syntactic foam structural materials attached to a rigid frame with composite outer skins to achieve a built-in Neutral or Positive Buoyancy. A UUV with a Neutral Buoyancy can use propellers and diving planes for depth control, and then operate at a fixed position. While an AUV having a Positive Buoyancy would also use thrusters for depth control but is designed to float up towards the surface when the vehicle’s propeller stops turning or it loses power.
  
  
• Ballast Tanks: Like submarines, some larger submersibles such as XLUUVs or HOVs, may have ballast tanks that can be filled with water or air to adjust overall density. An ROV used to retrieve objects from the seafloor might also need a ballast tank to compensate for the added weight when surfacing.
  
  
• External Buoyancy Aids: Some submersibles use external buoyancy aids, such as drop-weights to descend, or inflatable bladders to provide additional lift. These components can be deployed or retracted / dropped as needed.
  
  

Submarines
Submarines are larger, more complex vessels designed for long-term underwater and surface operations. They are used by navies for military purposes and by scientists for extended research missions. Unlike most submersibles, the hull of a submarine is usually made of metal and not lighter syntactic foam material. However, syntactic foam is still an important part of submarine design to assist with buoyancy. Syntactic foam is very flexible and can add to the stability of the of submarine, as it can be used as a void filler in many specific areas of the outer sub structure that needs extra lift.

There is always a trade-off between a vessel's weight elements and its ability to remain buoyant and fast. Having a metal hull provides subs the strength needed to withstand severe conditions for extended periods, to support a variety of complex equipment, and to host crews of over 100 submariners. However, a metal hull and frame also make a submarine very heavy. For example, a current Virginia-class submarine displaces over 10,200 tons of water.

To allow such a heavy vessel to float, submarines are designed to be the masters of manipulating buoyancy. Archimedes' basic principles remain the same for giant submarines as those for smaller submersibles, with multiple ballast systems adjusting density throughout the boat as needed.

• Main Ballast Tanks (MBT): Like some extra-large submersibles, submarines have ballast tanks that can be filled with water or air. When the submarine is on the surface, its Main Ballast Tanks are filled with air, making it buoyant and allowing it to float. To dive, the submarine opens the Main Air Vents on the top of the ballast tanks, allowing air to escape and water to enter from Flood Ports (floods) at the bottom of the tanks. As water replaces the air, the overall density of the submarine increases. This increased density causes the submarine to sink. MBTs are vitally important to submarines, and most have redundant ballast tanks. For example, a sub may have 3 MBTs in the front and 3 in the back, along with multiple other Variable Ballast Tanks. Submarines designed with double hulls might have even more ballast tanks distributed across the entire length of the hull.
  
  
  
  
  
• Variable Ballast Tanks
  • Depth Control Tanks (DCT): The DCTs are used to alter the buoyancy characteristics of the submarine once it is submerged. Environmental factors such as ocean depth or salinity can cause the submarine to move from a neutrally buoyant condition to being negatively or positively buoyant. Consumption of fuel or provisions affect the variable weight onboard, thus changing density as well. Moving water in or out of the DCTs can compensate for this. 
    
  • Trim Tanks: In addition to fine depth adjustments of the DCT, submarines are equipped with trim tanks that allow for more precise control over the vessel's buoyancy and trim (the angle at which the submarine is oriented). By adjusting the amount of water in the trim tanks, the submarine can maintain a level position or tilt upward or downward as needed. Subs have Forward Trim Tanks (FTT) and Aft Trim Tanks (ATT) in the back.
    
    
    
    

• Depth Control Planes: Submarines also have hydroplanes, or diving planes, which are wing-like structures located on the front of the sub's hull (bow planes) or sail (fairwater planes), as well as the back of the sub (stern planes). These planes can be angled to control the submarine's depth as it moves through the water. Adjusting the angle of the hydroplanes relative to the submarine's center of gravity (G), will cause the sub's movement (M) up or down (lift), or maintain a constant depth. 
  
  
• Reserve of Buoyancy (RoB): As with any ship, a submarine needs some volume of its hull above the waterline when surfaced. This area from the waterline to the upper deck level is called a boat's 'Freeboard'. The amount of a boat's Freeboard is created according to its Reserve of Buoyancy (RoB), which pushes some of a boat's structure above the surface. There are two main reasons submarines are designed to have a certain amount of RoB when the MBTs are empty. Firstly, it creates a Freeboard area that provides easy access to hatches and the top deck of the vessel, as well as boosting surface equipment used in the sail far above the waterline.
  
  Secondly, RoB is needed to help the submarine to surface and remain afloat even if part of the submarine is flooded due to a hull breach, or an MBT is damaged. This is particularly relevant in cases of a submarine grounding, hitting an object, or experiencing an explosion. A robust RoB is also needed to break through a surface structure, such as thick artic surface ice.
  
  Different submarines are designed with a range of RoB depending on their size and configuration. The amount of reserve buoyancy is usually between 10% to 20% but can be much higher for double hull submarines. In general, American naval architects consider a 15% reserve of buoyancy sufficient, while RoB on Russian double-hull subs can be 30% or more. 
  
  

The Physics of Diving and Surfacing using a Ballast System

The process of diving and surfacing a submarine (or a XLUUV using a ballast system), is a delicate balance between the forces of buoyancy, gravity, and the vessel's own propulsion system. As noted, most smaller submersibles do not use a ballast system- they dive using their propulsion and fins, and surface using their positive buoyancy.

Diving:
When a submarine begins its dive, it opens the air vents on the top of the ballast tanks. It is the air escaping through the vents that accounts for the spray sometimes seen when submarines dive. The air that leaves the ballast tanks is replaced by the intake of water, which increases the vessel's weight. As the weight becomes greater than the buoyant force, the vessel begins to sink. However, the descent is not solely dependent on gravity. The vessel's propulsion system plays a critical role in controlling the rate of descent and ensuring that the vessel does not descend too quickly.

The use of hydroplanes also helps to control the angle of descent, allowing the vessel to dive at a controlled rate. The combined effect of increased weight, controlled propulsion, and hydroplane adjustment allows the submersible or submarine to reach its desired depth safely. Note that a submersible or submarine could just use their planes for a shallow dive, even if positively buoyant, so long as the vessel's propulsion keeps it moving forward. 

Surfacing:
To surface, the submarine must reduce its density by expelling water from the ballast tanks and replacing it with air. This reduction in weight makes the buoyant force greater than the gravitational force, causing the vessel to rise. High pressure air is held in multiple air banks, so a sub has plenty of compressed air to surface many times. Once on the surface, to conserve supplies of compressed air, a Low-Pressure Blower system (LPB) is then used to remove the remainder of the water from the tanks.

Surfacing a Submarine
Driving a sub to the surface from the depths is a delicate team maneuver involving the planes, propulsion and the blow systems. During the ascent, the vessel's propulsion system and hydroplanes are used to control the rate and angle of ascent. This careful control is necessary to avoid rapid changes in pressure, which can be dangerous for both the vessel and its occupants. 

Note that as the boat surfaces, its speed increases due to a decrease in the immense atmospheric pressures of the deep sea (pressure at 1,000 feet is about 4,400 psi). Firstly, water is expelled slower at depth from a ballast tank, due to the immense atmospheric resistance from the seawater outside the sub. Secondly, as the sub rises, the air in the ballast tanks expands and pushes more water out faster, making the boat even more buoyant and pushing it to the surface even quicker! 

Alternative Submarine Surfacing Methods
There are several alternatives to the usual MBT electrically operated valve ballast blows.

• In case of a loss of power, a submarine can use its Emergency MBT blow system (EMBT). This 'safety' air bank system is completely mechanical (pneumatic), doesn’t need any electricity to function, and quickly blasts air directly from 'bank to tank'. 
• Depending on its depth, a submarine could also do an airless surface, where it powers above the water using only the propulsion system. However, unless it somehow blows out the ballast tank water, it won't be able to stay at the surface or stop moving. 
• On some older subs equipped with a diesel generator, it is said that exhaust could be diverted to blow the ballast tanks if needed.

Note that if there is no propulsion, and more flooding than the reserve buoyancy is designed to counter, then a submarine cannot surface.

Additional Considerations

While the physical characteristics of submersibles and submarines themselves play the primary role in their Diving and Surfacing, other outside forces also influence their buoyancy:

• Salinity: Seawater has a higher density than freshwater, making it easier to float. Even in the ocean, changes in salinity levels are common, especially when moving into new environments. 
• Temperature: The colder the water, the less space it takes up, and the higher its density and buoyancy. 
• Depth: Hydrostatic pressure increases with depth affects the vessel's structure and buoyancy.  A sub is literally 'squished' at extreme depths, and its volume decreases, which in turn reduces its buoyant force.

By understanding the principles of buoyancy and the intricate systems employed by submersibles and submarines, we gain a deeper appreciation for the engineering marvels that allow humans to explore the hidden depths of our oceans.

Trim Tank Control Screen on Virginia-Class Submarine (CNET)

Syntactic Foam Use for Buoyancy 

Syntactic foam is a class of material created using pre-formed hollow spheres (commonly made of glass, ceramic, polymer or even metal) bound together with a polymer. The “syntactic” portion refers to the ordered structure provided by the hollow spheres. The “foam” term relates to the cellular nature of the material. Thanks to its unique properties of high strength at low density, syntactic foam has become widely used in subsea buoyancy applications.

Syntactic materials are resistant to the combined effect of hydrostatic pressure and long-term exposure which make them ideal for challenging ocean projects such as cable and hardball floats and instrumentation support. They also provide strength and structural integrity at a significantly lower weight per volume than most traditional materials which make them an attractive choice in many defense and civil engineering applications. Microsphere syntactic foam can be formulated to meet depth and buoyancy requirements down to 11,000 meters (Hadal Zone), and has been used in missions to the deepest parts of the ocean in the famous Mariana Trench. 

Microsphere syntactic foam is comprised solely of resin and hollow glass microspheres (ESS)

The Importance of Buoyancy in Ocean Exploration

Buoyancy is a fundamental concept in the operation of submersibles and submarines. By controlling their buoyancy, these vessels can explore the ocean’s depths and return to the surface safely. Understanding the principles of buoyancy, the role of ballast and trim tanks, and the use of hydroplanes is essential for anyone interested in the mechanics of underwater exploration. As technology advances, the control of buoyancy will continue to be a critical factor in the design and operation of submersibles and submarines, allowing us to explore even deeper into the mysteries of the ocean.