Buoyancy and 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 and weight, submarines have
ballast tanks, which can hold varying amounts of water and air. For general submersion or surfacing, submarines use the main ballast tanks (MBTs), which are ambient pressure tanks, filled with water to submerge or with air to surface. While submerged, MBTs generally remain flooded, which simplifies their design, 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 longitudinal trim, submarines use forward and aft trim tanks. Pumps move water between the tanks, changing weight distribution and pitching the sub up or down. A similar system may be used to maintain transverse trim. , the first production submarine to feature an x-stern An obvious way to configure the control surfaces at the stern of a submarine is to use vertical planes to control yaw and horizontal planes to control pitch, which gives them the shape of a cross when seen from astern of the vessel. In this configuration, which long remained the dominant one, the horizontal planes are used to control the trim and depth and the vertical planes to control sideways maneuvers, like the rudder of a surface ship. Alternatively, the rear control surfaces can be combined into what has become known as an X-stern or an X-form rudder. Although less intuitive, such a configuration has turned out to have several advantages over the traditional cruciform arrangement. First, it improves maneuverability, horizontally as well as vertically. Second, the control surfaces are less likely to get damaged when landing on, or departing from, the seabed as well as when mooring and unmooring alongside. Finally, it is safer in that one of the two diagonal lines can counteract the other with respect to vertical as well as horizontal motion if one of them accidentally gets stuck. , the first submarine to use an x-rudder in practice, now on display in
Portsmouth, New Hampshire The x-stern was first tried in practice in the early 1960s on the
USS Albacore, an experimental submarine of the US Navy. While the arrangement was found to be advantageous, it was nevertheless not used on US production submarines that followed due to the fact that it requires the use of a computer to manipulate the control surfaces to the desired effect. Instead, the first to use an x-stern in standard operations was the Swedish Navy with its
Sjöormen class, the lead submarine of which was launched in 1967, before the
Albacore had even finished her test runs. Since it turned out to work very well in practice, all subsequent classes of Swedish submarines (
Näcken,
Västergötland,
Gotland, and
Blekinge class) have or will come with an x-rudder. , a
Näcken-class submarine in service with the Swedish Navy 1980–1998, now on display at
Marinmuseum in
Karlskrona The
Kockums shipyard responsible for the design of the x-stern on Swedish submarines eventually exported it to Australia with the
Collins class as well as to Japan with the
Sōryū class. With the introduction of the
type 212, the German and Italian Navies came to feature it as well. The US Navy with its
Columbia class, the British Navy with its
Dreadnought class, and the French Navy with its
Barracuda class are all about to join the x-stern family. Hence, as judged by the situation in the early 2020s, the x-stern is about to become the dominant technology. When a submarine performs an emergency surfacing, all depth and trim control methods are used simultaneously, together with propelling the boat upwards. Such surfacing is very quick, so the vessel may even partially jump out of the water, potentially damaging submarine systems.
Hull Overview in dry dock, showing cigar-shaped hull Modern submarines are cigar-shaped. This design, also used in very early submarines, is sometimes called a "
teardrop hull". It reduces hydrodynamic
drag when the sub is 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. 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 are spherical instead of cylindrical. This allows a more even distribution of stress and efficient use of materials to withstand external pressure as it gives the most internal volume for structural weight and is the most efficient shape to avoid buckling instability in compression. A frame is usually affixed to the outside of 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 standard submarine accommodates the
periscope and electronics masts, which can include radio,
radar,
electronic warfare, and other systems. It might also include a 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" or "fin". 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 Modern submarines and submersibles usually have, as did the earliest models, 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 at the surface, and construction difficulties further complicated the problem. This was solved either by a compromise shape, or by using two layered hulls: the internal strength hull for withstanding pressure, and an external fairing for hydrodynamic shape. Until the end of World War II, most submarines had an additional partial casing 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 World War II, 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.
Pressure hull and
Don Walsh were the first people to explore the
deepest part of the world's
ocean, and the deepest location on the surface of the Earth's crust, in the designed by
Auguste Piccard. The pressure hull is generally constructed of thick high-strength steel with a complex structure and high strength reserve, and is separated by watertight
bulkheads into several
compartments. There are also examples of more than two hulls in a submarine, like the , 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, all surrounded and supported by the outer light hydrodynamic hull. When submerged the pressure hull provides most of the buoyancy for the whole vessel. The
dive depth cannot be increased easily. Simply making the hull thicker increases the structural weight and requires reduction of onboard equipment weight, and increasing the diameter requires a proportional increase in thickness for the same material and architecture, ultimately resulting in a pressure hull that does not have sufficient buoyancy to support its own weight, as in a
bathyscaphe. This is acceptable for civilian research submersibles, but not military submarines, which need to carry a large equipment, crew, and weapons load to fulfill their function. Construction materials with greater
specific strength and
specific modulus are needed. WWI submarines had hulls of
carbon steel, with a maximum depth. During WWII, high-strength
alloyed steel was introduced, allowing depths. High-strength alloy steel remains the primary material for submarines today, with 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 alloys can be stronger than steel, lighter, and most importantly, have higher immersed
specific strength and
specific modulus. Titanium is also 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 for the , the deepest-diving combat submarine, 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. Although the specific strength and specific modulus of acrylic are not very high, the density is only 1.18g/cm3, so it is only very slightly denser than water, and the buoyancy penalty of increased thickness is correspondingly low. 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, crewed or uncrewed, had reached the deepest point in the Earth's oceans. The onboard systems indicated a depth of , although this was later revised to and more accurate measurements made in 1995 have found the Challenger Deep slightly shallower, at . 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 deflects more in some places and
buckling instability is the usual
failure mode. 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 attack submarine costs US$2.6
billion, over US$200,000 per
ton of displacement.)
Propulsion diesel–electric hunter-killer submarine The first submarines were propelled by humans. The first mechanically driven submarine was the 1863 French , 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; the system was further developed for the British , completed in 1958. Until the advent of
nuclear marine propulsion, most 20th-century submarines used
electric motors and batteries for running underwater and
combustion engines on the surface, and for battery recharging. Early submarines used
gasoline (petrol) engines but this quickly gave way to
kerosene (paraffin) and then
diesel engines because of reduced flammability and, with diesel, improved fuel-efficiency and thus also greater range. A combination of diesel and electric propulsion became the norm. Initially, the combustion engine and the electric motor were in most cases connected to the same shaft so that both could directly drive the propeller. The combustion engine was placed at the front end of the stern section with the electric motor behind it followed by the propeller shaft. The engine was connected to the motor by a clutch and the motor in turn connected to the propeller shaft by another clutch. With only the rear clutch engaged, the electric motor could drive the propeller, as required for fully submerged operation. With both clutches engaged, the combustion engine could drive the propeller, as was possible when operating on the surface or, at a later stage, when snorkeling. The electric motor would in this case serve as a generator to charge the batteries or, if no charging was needed, be allowed to rotate freely. With only the front clutch engaged, the combustion engine could drive the electric motor as a generator for charging the batteries without simultaneously forcing the propeller to move. 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).
Diesel–electric transmission ) While most early submarines used a direct mechanical connection between the combustion engine and the propeller, an alternative solution was considered as well as implemented at a very early stage. That solution consists in first converting the work of the combustion engine into electric energy via a dedicated generator. This energy is then used to drive the propeller via the electric motor and, to the extent required, for charging the batteries. In this configuration, the electric motor is thus responsible for driving the propeller at all times, regardless of whether air is available so that the combustion engine can also be used or not. Among the pioneers of this alternative solution was the very first submarine of the
Swedish Navy, (later renamed
Ub no 1), launched in 1904. While its design was generally inspired by the first submarine commissioned by the US Navy,
USS Holland, it deviated from the latter in at least three significant ways: by adding a periscope, by replacing the gasoline engine by a semidiesel engine (a
hot-bulb engine primarily meant to be fueled by kerosene, later replaced by a true diesel engine) and by severing the mechanical link between the combustion engine and the propeller by instead letting the former drive a dedicated generator. By so doing, it took three significant steps toward what was eventually to become the dominant technology for conventional (i.e., non-nuclear) submarines. in
Karlskrona In the following years, the Swedish Navy added another seven submarines in three different classes (
Undervattensbåten No 2,
Laxen, and
Abborren class) using the same propulsion technology but fitted with true diesel engines rather than semidiesels from the outset. Since by that time, the technology was usually based on the diesel engine rather than some other type of combustion engine, it eventually came to be known as
diesel–electric transmission. Like many other early submarines, those initially designed in Sweden were quite small (less than 200 tonnes) and thus confined to littoral operation. When the Swedish Navy wanted to add larger vessels, capable of operating further from the shore, their designs were purchased from companies abroad that already had the required experience: first Italian (
Fiat-
Laurenti) and later German (
A.G. Weser and
IvS). As a side-effect, the diesel–electric transmission was temporarily abandoned. However, diesel–electric transmission was immediately reintroduced when Sweden began designing its own submarines again in the mid-1930s. From that point onwards, it has been consistently used for all new classes of Swedish submarines, albeit supplemented by
air-independent propulsion (AIP) as provided by
Stirling engines beginning with
HMS Näcken in 1988. , in service 1980–1998 Another early adopter of diesel–electric transmission was the
US Navy, whose Bureau of Engineering proposed its use in 1928. It was subsequently tried in the
S-class submarines , , and before being put into production with the
Porpoise class of the 1930s. From that point onwards, it continued to be used on most US conventional submarines. Apart from the British
U-class and some submarines of the Imperial Japanese Navy that used separate diesel generators for low speed running, few navies other than those of Sweden and the US made much use of diesel–electric transmission before 1945. If diesel–electric transmission had only brought advantages and no disadvantages in comparison with a system that mechanically connects the diesel engine to the propeller, it would undoubtedly have become dominant much earlier. The disadvantages include the following: • It entails a loss of fuel-efficiency as well as power by converting the output of the diesel engine into electricity. While both generators and electric motors are known to be very efficient, their efficiency nevertheless falls short of 100 percent. • It requires an additional component in the form of a dedicated generator. Since the electric motor is always used to drive the propeller it can no longer step in to take on generator service as well. • It does not allow the diesel engine and the electrical motor to join forces by simultaneously driving the propeller mechanically for maximum speed when the submarine is surfaced or snorkeling. This may, however, be of little practical importance inasmuch as the option it prevents is one that would leave the submarine at a risk of having to dive with its batteries at least partly depleted. The reason why diesel–electric transmission has become the dominant alternative in spite of these disadvantages is of course that it also comes with many advantages and that, on balance, these have eventually been found to be more important. The advantages include the following: In clear weather, diesel exhausts 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. (former German submarine
U-3008) with her snorkel masts raised at Portsmouth Naval Shipyard, Kittery, Maine 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 have 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. US submarines did not adopt the use of snorkels until after WWII.
Air-independent propulsion 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 by NATO), they were considered unsuccessful. 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 and s. 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. Fuel cells are also used in the new
Spanish s although with the fuel stored as ethanol and then converted into hydrogen before use. One new technology that is being introduced starting with the Japanese Navy's eleventh
Sōryū-class submarine (JS
Ōryū) is a more modern battery, the
lithium-ion battery. These batteries have about double the electric storage of traditional batteries, and by changing out the lead-acid batteries in their normal storage areas plus filling up the large hull space normally devoted to
AIP engine and fuel tanks with many tons of lithium-ion batteries, modern submarines can actually return to a "pure" diesel–electric configuration yet have the added underwater range and power normally associated with AIP equipped submarines.
Nuclear power 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 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 reactor accident in 1968 resulted in 9 fatalities and 83 other injuries. The accident in 1985 resulted in 10 fatalities and 49 other radiation injuries.
Alternative 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.
Armament '' The success of the submarine is inextricably linked to the development of the
torpedo, invented by
Robert Whitehead in 1866. His invention (essentially the same now as it was 140 years ago), allowed the submarine make the leap from novelty to a weapon of war. Prior to the development and miniaturization of sonar sensitive enough to track a submerged submarine, attacks were exclusively restricted to ships and submarines operating near or at the surface. Targeting of unguided torpedoes was initially done by eye, but by World War II
analog targeting computers began to proliferate, being able to calculate basic firing solutions. Nonetheless, multiple "straight-running" torpedoes could be required to ensure a target was hit. With at most 20 to 25 torpedoes stored on board, the number of attacks a submarine could make was limited. To increase
combat endurance starting in World War I submarines also functioned as submersible gunboats, using their
deck guns against unarmed targets, and diving to escape and engage enemy warships. The initial importance of these deck guns encouraged the development of the unsuccessful
Submarine Cruiser such as the French and the
Royal Navy's 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 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. Guided torpedoes also proliferated extensively during and after World War II, even further increasing the combat endurance and lethality of submarines and allowing them to engage other submarines at depth (with the latter now being one of the primary missions of the modern
attack submarine). 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.
Sensors 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 not 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. Global
climate change and warmer oceans may complicate detecting submarines at depth in most places in the world. 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 or the
Russian Mir submersibles, rely on small active sonar sets and viewing ports to navigate. The human eye cannot detect sunlight below about underwater, so high intensity lights are used to illuminate the viewing area.
Navigation , 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 and s 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.
Communication 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 .
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 fresh water (using a device called an "
Electrolytic Oxygen Generator"). Emergency oxygen can be produced by burning
sodium chlorate candles. Atmosphere control equipment includes a
Carbon dioxide scrubber, which uses a spray of
monoethanolamine (MEA) absorbent to remove the gas from the air, after which the MEA is heated in a boiler to release the CO2 which is then pumped overboard. Emergency scrubbing can also be done with lithium hydroxide, which is consumable. The German
Type VIIC boat 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. ==Crew==