Submersion and trimming
An illustration showing submarine controls
USS Seawolf (SSN-21)
Ship Control Panel, with yokes for control surfaces (planes and rudder), and Ballast Control Panel (background), to control the water in tanks and ship's trim
All surface ships, as well as surfaced submarines, are in a positively buoyant condition, weighing less than the volume of water they would displace if fully submerged. To submerge hydrostatically, a ship must have negative buoyancy, either by increasing its own weight or decreasing its displacement of water. To control their displacement, submarines have ballast tanks, which can hold varying amounts of water and air.
For general submersion or surfacing, submarines use the forward and aft tanks, called Main Ballast Tanks (MBT), which are filled with water to submerge or with air to surface. Submerged, MBTs generally remain flooded, which simplifies their design, and on many submarines these tanks are a section of interhull space. For more precise and quick control of depth, submarines use smaller Depth Control Tanks (DCT) – also called hard tanks (due to their ability to withstand higher pressure), or trim tanks. The amount of water in depth control tanks can be controlled to change depth or to maintain a constant depth as outside conditions (chiefly water density) change. Depth control tanks may be located either near the submarine's center of gravity, or separated along the submarine body to prevent affecting trim.
When submerged, the water pressure on a submarine's hull can reach 4 MPa (580 psi) for steel submarines and up to 10 MPa (1,500 psi) for titanium submarines like K-278 Komsomolets, while interior pressure remains relatively unchanged. This difference results in hull compression, which decreases displacement. Water density also marginally increases with depth, as the salinity and pressure are higher. This change in density incompletely compensates for hull compression, so buoyancy decreases as depth increases. A submerged submarine is in an unstable equilibrium, having a tendency to either sink or float to the surface. Keeping a constant depth requires continual operation of either the depth control tanks or control surfaces.
Submarines in a neutral buoyancy condition are not intrinsically trim-stable. To maintain desired trim, submarines use forward and aft trim tanks. Pumps can move water between the tanks, changing weight distribution and pointing the sub up or down. A similar system is sometimes used to maintain stability.
The hydrostatic effect of variable ballast tanks is not the only way to control the submarine underwater. Hydrodynamic maneuvering is done by several surfaces, which can be moved to create hydrodynamic forces when a submarine moves at sufficient speed. The stern planes, located near the propeller and normally horizontal, serve the same purpose as the trim tanks, controlling the trim, and are commonly used, while other control surfaces may not be present on all submarines. The fairwater planes on the sail and/or bow planes on the main body, both also horizontal, are closer to the center of gravity, and are used to control depth with less effect on the trim.
When a submarine performs an emergency surfacing, all depth and trim methods are used simultaneously, together with propelling the boat upwards. Such surfacing is very quick, so the sub may even partially jump out of the water, potentially damaging submarine systems.
Modern submarines are cigar-shaped. This design, visible in early submarines, is sometimes called a "teardrop hull". It reduces the hydrodynamic drag when submerged, but decreases the sea-keeping capabilities and increases drag while surfaced. Since the limitations of the propulsion systems of early submarines forced them to operate surfaced most of the time, their hull designs were a compromise. Because of the slow submerged speeds of those subs, usually well below 10 kt (18 km/h), the increased drag for underwater travel was acceptable. Late in World War II, when technology allowed faster and longer submerged operation and increased aircraft surveillance forced submarines to stay submerged, hull designs became teardrop shaped again to reduce drag and noise. USS Albacore (AGSS-569) was a unique research submarine that pioneered the American version of the teardrop hull form (sometimes referred to as an "Albacore hull") of modern submarines. On modern military submarines the outer hull is covered with a layer of sound-absorbing rubber, or anechoic plating, to reduce detection.
The occupied pressure hulls of deep diving submarines such as DSV Alvin are spherical instead of cylindrical. This allows a more even distribution of stress at the great depth. A titanium frame is usually affixed to the pressure hull, providing attachment for ballast and trim systems, scientific instrumentation, battery packs, syntactic flotation foam, and lighting.
A raised tower on top of a submarine accommodates the periscope and electronics masts, which can include radio, radar, electronic warfare, and other systems including the snorkel mast. In many early classes of submarines (see history), the control room, or "conn", was located inside this tower, which was known as the "conning tower". Since then, the conn has been located within the hull of the submarine, and the tower is now called the "sail". The conn is distinct from the "bridge", a small open platform in the top of the sail, used for observation during surface operation.
"Bathtubs" are related to conning towers but are used on smaller submarines. The bathtub is a metal cylinder surrounding the hatch that prevents waves from breaking directly into the cabin. It is needed because surfaced submarines have limited freeboard, that is, they lie low in the water. Bathtubs help prevent swamping the vessel.
Single and double hulls
, Type VIIC/41 U-Boat of WWII, showing the typical combination of ship-like non-watertight outer hull with bulky strong hull below
Modern submarines and submersibles, as well as the oldest ones, usually have a single hull. Large submarines generally have an additional hull or hull sections outside. This external hull, which actually forms the shape of submarine, is called the outer hull (casing in the Royal Navy) or light hull, as it does not have to withstand a pressure difference. Inside the outer hull there is a strong hull, or pressure hull, which withstands sea pressure and has normal atmospheric pressure inside.
As early as World War I, it was realized that the optimal shape for withstanding pressure conflicted with the optimal shape for seakeeping and minimal drag, and construction difficulties further complicated the problem. This was solved either by a compromise shape, or by using two hulls; internal for holding pressure, and external for optimal shape. Until the end of World War II, most submarines had an additional partial cover on the top, bow and stern, built of thinner metal, which was flooded when submerged. Germany went further with the Type XXI, a general predecessor of modern submarines, in which the pressure hull was fully enclosed inside the light hull, but optimized for submerged navigation, unlike earlier designs that were optimized for surface operation.
U-Boat, late WWII, with pressure hull almost fully enclosed inside the light hull
After World War II, approaches split. The Soviet Union changed its designs, basing them on German developments. All post–World War II heavy Soviet and Russian submarines are built with a double hull structure. American and most other Western submarines switched to a primarily single-hull approach. They still have light hull sections in the bow and stern, which house main ballast tanks and provide a hydrodynamically optimized shape, but the main cylindrical hull section has only a single plating layer. Double hulls are being considered for future submarines in the United States to improve payload capacity, stealth and range.
The pressure hull is generally constructed of thick high-strength steel with a complex structure and high strength reserve, and is separated with watertight bulkheads into several compartments. There are also examples of more than two hulls in a submarine, like the Typhoon class, which has two main pressure hulls and three smaller ones for control room, torpedoes and steering gear, with the missile launch system between the main hulls.
The dive depth cannot be increased easily. Simply making the hull thicker increases the weight and requires reduction of onboard equipment weight, ultimately resulting in a bathyscaphe. This is acceptable for civilian research submersibles, but not military submarines.
WWI submarines had hulls of carbon steel, with a 100-metre (330 ft) maximum depth. During WWII, high-strength alloyed steel was introduced, allowing 200-metre (660 ft) depths. High-strength alloy steel remains the primary material for submarines today, with 250–400-metre (820–1,310 ft) depths, which cannot be exceeded on a military submarine without design compromises. To exceed that limit, a few submarines were built with titanium hulls. Titanium can be stronger than steel, lighter, and is not ferromagnetic, important for stealth. Titanium submarines were built by the Soviet Union, which developed specialized high-strength alloys. It has produced several types of titanium submarines. Titanium alloys allow a major increase in depth, but other systems must be redesigned to cope, so test depth was limited to 1,000 metres (3,300 ft) for the Soviet submarine K-278 Komsomolets, the deepest-diving combat submarine. An Alfa-class submarine may have successfully operated at 1,300 metres (4,300 ft), though continuous operation at such depths would produce excessive stress on many submarine systems. Titanium does not flex as readily as steel, and may become brittle after many dive cycles. Despite its benefits, the high cost of titanium construction led to the abandonment of titanium submarine construction as the Cold War ended. Deep–diving civilian submarines have used thick acrylic pressure hulls.
The deepest deep-submergence vehicle (DSV) to date is Trieste. On 5 October 1959, Trieste departed San Diego for Guam aboard the freighter Santa Maria to participate in Project Nekton, a series of very deep dives in the Mariana Trench. On 23 January 1960, Trieste reached the ocean floor in the Challenger Deep (the deepest southern part of the Mariana Trench), carrying Jacques Piccard (son of Auguste) and Lieutenant Don Walsh, USN. This was the first time a vessel, manned or unmanned, had reached the deepest point in the Earth's oceans. The onboard systems indicated a depth of 11,521 metres (37,799 ft), although this was later revised to 10,916 metres (35,814 ft) and more accurate measurements made in 1995 have found the Challenger Deep slightly shallower, at 10,911 metres (35,797 ft).
Building a pressure hull is difficult, as it must withstand pressures at its required diving depth. When the hull is perfectly round in cross-section, the pressure is evenly distributed, and causes only hull compression. If the shape is not perfect, the hull is bent, with several points heavily strained. Inevitable minor deviations are resisted by stiffener rings, but even a one-inch (25 mm) deviation from roundness results in over 30 percent decrease of maximal hydrostatic load and consequently dive depth. The hull must therefore be constructed with high precision. All hull parts must be welded without defects, and all joints are checked multiple times with different methods, contributing to the high cost of modern submarines. (For example, each Virginia-class attack submarine costs US$2.6 billion, over US$200,000 per ton of displacement.)
The first submarines were propelled by humans. The first mechanically driven submarine was the 1863 French Plongeur, which used compressed air for propulsion. Anaerobic propulsion was first employed by the Spanish Ictineo II in 1864, which used a solution of zinc, manganese dioxide, and potassium chlorate to generate sufficient heat to power a steam engine, while also providing oxygen for the crew. A similar system was not employed again until 1940 when the German Navy tested a hydrogen peroxide-based system, the Walter turbine, on the experimental V-80 submarine and later on the naval
and type XVII submarines.
Until the advent of nuclear marine propulsion, most 20th-century submarines used batteries for running underwater and gasoline (petrol) or diesel engines on the surface, and for battery recharging. Early submarines used gasoline, but this quickly gave way to kerosene (paraffin), then diesel, because of reduced flammability. Diesel-electric became the standard means of propulsion. The diesel or gasoline engine and the electric motor, separated by clutches, were initially on the same shaft driving the propeller. This allowed the engine to drive the electric motor as a generator to recharge the batteries and also propel the submarine. The clutch between the motor and the engine would be disengaged when the submarine dived, so that the motor could drive the propeller. The motor could have multiple armatures on the shaft, which could be electrically coupled in series for slow speed and in parallel for high speed (these connections were called "group down" and "group up", respectively).
recharging battery (JMSDF
Early submarines used a direct mechanical connection between the engine and propeller, switching between diesel engines for surface running, and battery-driven electric motors for submerged propulsion.
In 1928, the United States Navy's Bureau of Engineering proposed a diesel-electric transmission. Instead of driving the propeller directly while running on the surface, the submarine's diesel drove a generator that could either charge the submarine's batteries or drive the electric motor. This made electric motor speed independent of diesel engine speed, so the diesel could run at an optimum and non-critical speed. One or more diesel engines could be shut down for maintenance while the submarine continued to run on the remaining engine or battery power. The US pioneered this concept in 1929, in the S-class submarines S-3, S-6, and S-7. The first production submarines with this system were the Porpoise-class of the 1930s, and it was used on most subsequent US diesel submarines through the 1960s. No other navy adopted the system before 1945, apart from the Royal Navy's U-class submarines, though some submarines of the Imperial Japanese Navy used separate diesel generators for low speed running.
Other advantages of such an arrangement were that a submarine could travel slowly with the engines at full power to recharge the batteries quickly, reducing time on the surface or on snorkel. It was then possible to isolate the noisy diesel engines from the pressure hull, making the submarine quieter. Additionally, diesel-electric transmissions were more compact.
During World War II the Germans experimented with the idea of the schnorchel (snorkel) from captured Dutch submarines, but didn't see the need for them until rather late in the war. The schnorchel was a retractable pipe that supplied air to the diesel engines while submerged at periscope depth, allowing the boats to cruise and recharge their batteries while maintaining a degree of stealth. It was far from a perfect solution, however. There were problems with the device's valve sticking shut or closing as it dunked in rough weather; since the system used the entire pressure hull as a buffer, the diesels would instantaneously suck huge volumes of air from the boat's compartments, and the crew often suffered painful ear injuries. Speed was limited to 8 knots (15 km/h), lest the device snap from stress. The schnorchel also created noise that made the boat easier to detect with sonar, yet more difficult for the on-board sonar to detect signals from other vessels. Finally, Allied radar eventually became sufficiently advanced that the schnorchel mast could be detected beyond visual range.
While the snorkel renders a submarine far less detectable, it is not perfect. In clear weather, diesel exhaust can be seen on the surface to a distance of about three miles, while "periscope feather" (the wave created by the snorkel or periscope moving through the water) is visible from far off in calm sea conditions. Modern radar is also capable of detecting a snorkel in calm sea conditions.
The problem of the diesels causing a vacuum in the submarine when the head valve is submerged still exists in later model diesel submarines, but is mitigated by high-vacuum cut-off sensors that shut down the engines when the vacuum in the ship reaches a pre-set point. Modern snorkel induction masts use a fail-safe design using compressed air, controlled by a simple electrical circuit, to hold the "head valve" open against the pull of a powerful spring. Seawater washing over the mast shorts out exposed electrodes on top, breaking the control, and shutting the "head valve" while it is submerged.
During World War II, German Type XXI submarines (also known as "Elektroboote") were the first submarines designed to operate submerged for extended periods. Initially they were to carry hydrogen peroxide for long-term, fast air-independent propulsion, but were ultimately built with very large batteries instead. At the end of the War, the British and Soviets experimented with hydrogen peroxide/kerosene (paraffin) engines that could run surfaced and submerged. The results were not encouraging. Though the Soviet Union deployed a class of submarines with this engine type (codenamed Quebec by NATO), they were considered unsuccessful.
American X-1 Midget Submarine
The United States also used hydrogen peroxide in an experimental midget submarine, X-1. It was originally powered by a hydrogen peroxide/diesel engine and battery system until an explosion of her hydrogen peroxide supply on 20 May 1957. X-1 was later converted to use diesel-electric drive.
Today several navies use air-independent propulsion. Notably Sweden uses Stirling technology on the Gotland-class and Södermanland-class submarines. The Stirling engine is heated by burning diesel fuel with liquid oxygen from cryogenic tanks. A newer development in air-independent propulsion is hydrogen fuel cells, first used on the German Type 212 submarine, with nine 34 kW or two 120 kW cells and soon to be used in the new Spanish S-80-class submarines.
Battery well containing 126 cells on USS Nautilus
, the first nuclear-powered submarine
Steam power was resurrected in the 1950s with a nuclear-powered steam turbine driving a generator. By eliminating the need for atmospheric oxygen, the time that a submarine could remain submerged was limited only by its food stores, as breathing air was recycled and fresh water distilled from seawater. More importantly, a nuclear submarine has unlimited range at top speed. This allows it to travel from its operating base to the combat zone in a much shorter time and makes it a far more difficult target for most anti-submarine weapons. Nuclear-powered submarines have a relatively small battery and diesel engine/generator powerplant for emergency use if the reactors must be shut down.
Nuclear power is now used in all large submarines, but due to the high cost and large size of nuclear reactors, smaller submarines still use diesel-electric propulsion. The ratio of larger to smaller submarines depends on strategic needs. The US Navy, French Navy, and the British Royal Navy operate only nuclear submarines, which is explained by the need for distant operations. Other major operators rely on a mix of nuclear submarines for strategic purposes and diesel-electric submarines for defense. Most fleets have no nuclear submarines, due to the limited availability of nuclear power and submarine technology.
Diesel-electric submarines have a stealth advantage over their nuclear counterparts. Nuclear submarines generate noise from coolant pumps and turbo-machinery needed to operate the reactor, even at low power levels. Some nuclear submarines such as the American Ohio class can operate with their reactor coolant pumps secured, making them quieter than electric subs. A conventional submarine operating on batteries is almost completely silent, the only noise coming from the shaft bearings, propeller, and flow noise around the hull, all of which stops when the sub hovers in mid-water to listen, leaving only the noise from crew activity. Commercial submarines usually rely only on batteries, since they operate in conjunction with a mother ship.
Several serious nuclear and radiation accidents have involved nuclear submarine mishaps. The Soviet submarine K-19 reactor accident in 1961 resulted in 8 deaths and more than 30 other people were over-exposed to radiation. The Soviet submarine K-27 reactor accident in 1968 resulted in 9 fatalities and 83 other injuries. The Soviet submarine K-431 accident in 1985 resulted in 10 fatalities and 49 other radiation injuries.
Oil-fired steam turbines powered the British K-class submarines, built during World War I and later, to give them the surface speed to keep up with the battle fleet. The K-class subs were not very successful, however.
Toward the end of the 20th century, some submarines—such as the British Vanguard class—began to be fitted with pump-jet propulsors instead of propellers. Though these are heavier, more expensive, and less efficient than a propeller, they are significantly quieter, providing an important tactical advantage.
Magnetohydrodynamic drive (MHD) was portrayed as the operating principle behind the titular submarine's nearly silent propulsion system in the film adaptation of The Hunt for Red October. However, in the novel the Red October did not use MHD, but rather something more similar to the above-mentioned pump-jet.
The success of the submarine is inextricably linked to the development of the torpedo, invented by Robert Whitehead in 1866. His invention is essentially the same now as it was 140 years ago. Only with self-propelled torpedoes could the submarine make the leap from novelty to a weapon of war. Until the perfection of the guided torpedo, multiple "straight-running" torpedoes were required to attack a target. With at most 20 to 25 torpedoes stored on board, the number of attacks was limited. To increase combat endurance most World War I submarines functioned as submersible gunboats, using their deck guns against unarmed targets, and diving to escape and engage enemy warships. The importance of guns encouraged the development of the unsuccessful Submarine Cruiser such as the French Surcouf and the Royal Navy's X1 and M-class submarines. With the arrival of Anti-submarine warfare (ASW) aircraft, guns became more for defense than attack. A more practical method of increasing combat endurance was the external torpedo tube, loaded only in port.
The forward torpedo tubes in HMS Ocelot
The ability of submarines to approach enemy harbours covertly led to their use as minelayers. Minelaying submarines of World War I and World War II were specially built for that purpose. Modern submarine-laid mines, such as the British Mark 5 Stonefish and Mark 6 Sea Urchin, can be deployed from a submarine's torpedo tubes.
After World War II, both the US and the USSR experimented with submarine-launched cruise missiles such as the SSM-N-8 Regulus and P-5 Pyatyorka. Such missiles required the submarine to surface to fire its missiles. They were the forerunners of modern submarine-launched cruise missiles, which can be fired from the torpedo tubes of submerged submarines, for example the US BGM-109 Tomahawk and Russian RPK-2 Viyuga and versions of surface–to–surface anti-ship missiles such as the Exocet and Harpoon, encapsulated for submarine launch. Ballistic missiles can also be fired from a submarine's torpedo tubes, for example missiles such as the anti-submarine SUBROC. With internal volume as limited as ever and the desire to carry heavier warloads, the idea of the external launch tube was revived, usually for encapsulated missiles, with such tubes being placed between the internal pressure and outer streamlined hulls.
The strategic mission of the SSM-N-8 and the P-5 was taken up by submarine-launched ballistic missile beginning with the US Navy's Polaris missile, and subsequently the Poseidon and Trident missiles.
Germany is working on the torpedo tube-launched short-range IDAS missile, which can be used against ASW helicopters, as well as surface ships and coastal targets.
A submarine can have a variety of sensors, depending on its missions. Modern military submarines rely almost entirely on a suite of passive and active sonars to locate targets. Active sonar relies on an audible "ping" to generate echoes to reveal objects around the submarine. Active systems are rarely used, as doing so reveals the sub's presence. Passive sonar is a set of sensitive hydrophones set into the hull or trailed in a towed array, normally trailing several hundred feet behind the sub. The towed array is the mainstay of NATO submarine detection systems, as it reduces the flow noise heard by operators. Hull mounted sonar is employed in addition to the towed array, as the towed array can't work in shallow depth and during maneuvering. In addition, sonar has a blind spot "through" the submarine, so a system on both the front and back works to eliminate that problem. As the towed array trails behind and below the submarine, it also allows the submarine to have a system both above and below the thermocline at the proper depth; sound passing through the thermocline is distorted resulting in a lower detection range.
Submarines also carry radar equipment to detect surface ships and aircraft. Submarine captains are more likely to use radar detection gear than active radar to detect targets, as radar can be detected far beyond its own return range, revealing the submarine. Periscopes are rarely used, except for position fixes and to verify a contact's identity.
Civilian submarines, such as the DSV Alvin or the Russian Mir submersibles, rely on small active sonar sets and viewing ports to navigate. The human eye cannot detect sunlight below about 300 feet (91 m) underwater, so high intensity lights are used to illuminate the viewing area.
The larger search periscope
, and the smaller, less detectable attack periscope on HMS Ocelot
Early submarines had few navigation aids, but modern subs have a variety of navigation systems. Modern military submarines use an inertial guidance system for navigation while submerged, but drift error unavoidably builds over time. To counter this, the crew occasionally uses the Global Positioning System to obtain an accurate position. The periscope—a retractable tube with a prism system that provides a view of the surface—is only used occasionally in modern submarines, since the visibility range is short. The Virginia-class and Astute-class submarines use photonics masts rather than hull-penetrating optical periscopes. These masts must still be deployed above the surface, and use electronic sensors for visible light, infrared, laser range-finding, and electromagnetic surveillance. One benefit to hoisting the mast above the surface is that while the mast is above the water the entire sub is still below the water and is much harder to detect visually or by radar.
Military submarines use several systems to communicate with distant command centers or other ships. One is VLF (Very Low Frequency) radio, which can reach a submarine either on the surface or submerged to a fairly shallow depth, usually less than 250 feet (76 m). ELF (Extremely Low Frequency) can reach a submarine at greater depths, but has a very low bandwidth and is generally used to call a submerged sub to a shallower depth where VLF signals can reach. A submarine also has the option of floating a long, buoyant wire antenna to a shallower depth, allowing VLF transmissions by a deeply submerged boat.
By extending a radio mast, a submarine can also use a "burst transmission" technique. A burst transmission takes only a fraction of a second, minimizing a submarine's risk of detection.
To communicate with other submarines, a system known as Gertrude is used. Gertrude is basically a sonar telephone. Voice communication from one submarine is transmitted by low power speakers into the water, where it is detected by passive sonars on the receiving submarine. The range of this system is probably very short, and using it radiates sound into the water, which can be heard by the enemy.
Civilian submarines can use similar, albeit less powerful systems to communicate with support ships or other submersibles in the area.
Life support systems
With nuclear power or air-independent propulsion, submarines can remain submerged for months at a time. Conventional diesel submarines must periodically resurface or run on snorkel to recharge their batteries. Most modern military submarines generate breathing oxygen by electrolysis of water (using a device called an "Elektrolytic Oxygen Generator"). Atmosphere control equipment includes a CO2 scrubber, which uses an amine absorbent to remove the gas from air and diffuse it into waste pumped overboard. A machine that uses a catalyst to convert carbon monoxide into carbon dioxide (removed by the CO2 scrubber) and bonds hydrogen produced from the ship's storage battery with oxygen in the atmosphere to produce water, is also used. An atmosphere monitoring system samples the air from different areas of the ship for nitrogen, oxygen, hydrogen, R-12 and R-114 refrigerants, carbon dioxide, carbon monoxide, and other gases. Poisonous gases are removed, and oxygen is replenished by use of an oxygen bank located in a main ballast tank. Some heavier submarines have two oxygen bleed stations (forward and aft). The oxygen in the air is sometimes kept a few percent less than atmospheric concentration to reduce fire danger.
Fresh water is produced by either an evaporator or a reverse osmosis unit. The primary use for fresh water is to provide feedwater for the reactor and steam propulsion plants. It is also available for showers, sinks, cooking and cleaning once propulsion plant needs have been met. Seawater is used to flush toilets, and the resulting "black water" is stored in a sanitary tank until it is blown overboard using pressurized air or pumped overboard by using a special sanitary pump. The blackwater–discharge system is difficult to operate, and the German Type VIIC boat U-1206 was lost with casualties because of human error while using this system. Water from showers and sinks is stored separately in "grey water" tanks and discharged overboard using drain pumps.
Trash on modern large submarines is usually disposed of using a tube called a Trash Disposal Unit (TDU), where it is compacted into a galvanized steel can. At the bottom of the TDU is a large ball valve. An ice plug is set on top of the ball valve to protect it, the cans atop the ice plug. The top breech door is shut, and the TDU is flooded and equalized with sea pressure, the ball valve is opened and the cans fall out assisted by scrap iron weights in the cans. The TDU is also flushed with seawater to ensure it is completely empty and the ball valve is clear before closing the valve.