Editing Submarine (section)

==Technology==
{{See also|Timeline of underwater technology}}

===Buoyancy and trim===
[[File:Submarine control surfaces2.svg|thumb|upright=2.0|An illustration showing submarine control surfaces and trim tanks]]
[[File:USS Seawolf (SSN 21) Control Room HighRes.jpg|thumb|{{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 [[buoyancy|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 tank]]s, which can hold varying amounts of water and air.<ref name="Navpers 16166" >{{cite web |url=https://maritime.org/doc/fleetsub/trim/index.htm |title=The Fleet Type Submarine Online: Submarine Trim and Drain Systems. Navpers 16166|website=maritime.org |access-date=1 January 2022 |via=San Francisco Maritime National Park Association }}</ref>

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,<ref name="Navpers 16166" /> and on many submarines, these tanks are a section of the space between the light hull and the pressure hull. For more precise control of depth, submarines use smaller depth control tanks (DCTs)—also called hard tanks (due to their ability to withstand higher pressure) or trim tanks. These are [[variable buoyancy pressure vessel]]s, a type of buoyancy control device. The amount of water in depth control tanks can be adjusted to hydrostatically change depth or to maintain a constant depth as outside conditions (mainly water density) change.<ref name="Navpers 16166" /> Depth control tanks may be located either near the submarine's [[center of gravity]], to minimise the effect on trim, or separated along the length of the hull so they can also be used to adjust static trim by transfer of water between them.

When submerged, the water pressure on a submarine's hull can reach {{convert|4|MPa|psi|abbr=on|lk=on}} for steel submarines and up to {{convert|10|MPa|psi|abbr=on}} for [[titanium]] submarines like {{ship|Soviet submarine|K-278 Komsomolets||2}}, 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.<ref name=nave>{{cite web|title=Bulk Elastic Properties|author=Nave, R.|work=HyperPhysics|publisher=[[Georgia State University]]|url=http://hyperphysics.phy-astr.gsu.edu/hbase/hph.html|access-date=26 October 2007}}</ref> 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.<ref name="Physics Of Liquids & Gases">{{cite web|url=http://www.vectorsite.net/tpecp_08.html|archive-url=https://web.archive.org/web/20051120100240/http://www.vectorsite.net/tpecp_08.html|url-status=usurped|archive-date=November 20, 2005|title=Physics Of Liquids & Gases|access-date=7 October 2006|work=Elementary Classical Physics}}</ref><ref>{{cite book|author=Richard O'Kane|title=Wahoo|url=https://archive.org/details/wahoopatrolsofam00okan|url-access=registration|publisher=Presidio Press|year=1987|page=[https://archive.org/details/wahoopatrolsofam00okan/page/12 12]|isbn=9780891413011}}</ref>

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.<ref name="Navpers 16166" />

===Control surfaces===
[[File:Kiosk Casabianca.jpg|thumb|[[Sail (submarine)|Sail]] of the French nuclear submarine {{ship|French submarine|Casabianca|S603|2}}; note the diving planes, [[camouflage]]d masts, periscope, electronic warfare masts, hatch, and [[wikt:deadlight|deadlight]].]]

The hydrostatic effect of variable ballast tanks is not the only way to control the submarine underwater. Hydrodynamic maneuvering is done by several control surfaces, collectively known as [[diving plane]]s or hydroplanes, which can be moved to create hydrodynamic forces when a submarine moves longitudinally at sufficient speed. In the classic cruciform stern configuration, the horizontal stern planes serve the same purpose as the trim tanks, controlling the trim. Most submarines additionally have forward horizontal planes, normally placed on the bow until the 1960s but often on the sail on later designs, where they are closer to the center of gravity and can control depth with less effect on the trim.<ref>{{cite book|title=Concepts In Submarine Design|author1=Roy Burcher |author2=Louis Rydill |publisher=Cambridge University Press|year=1995|page=170}}</ref>

[[File:sor.jpeg|thumb|upright|left|Rear view of a model of Swedish submarine [[HSwMS Sjöormen (Sor)|HMS ''Sjöormen'']], 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.<ref>{{cite web |url= https://www.twz.com/sea/chinas-latest-submarine-features-x-shaped-stern |title= China's Latest Submarine Features X-Shaped Stern |author= Thomas Newdick |date=29 July 2024|website= The War Zone |publisher= |language= }}</ref> 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.<ref>{{cite magazine |last=van de Put |first=F.A. |date=September 1986 |title=2. "X" – Roeren.|url=https://www.klaarvooronderwater.nl/kvo/Kvo-016.pdf|magazine=Klaar Voor Onderwater|location=Den Helder|publisher=Onderzeedienst Reünistenvereniging|issue=16|language=Dutch|pages=3–6}}</ref>{{clarify|how does it improve maneuverability?|date=January 2022}} 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.<ref>{{cite journal |last1=Wang|first1=Wenjin|display-authors=etal|date=2020|title=A Fault-tolerant Steering Prototype for X-rudder Underwater Vehicles|journal=Sensors |volume=20|issue=7|page=1816|doi=10.3390/s20071816|pmid=32218145|pmc=7180876|bibcode=2020Senso..20.1816W|doi-access=free}}</ref>{{clarify|how the counteraction works|date=January 2022}}

[[File:USS Albacore (2018) 13.jpg|thumb|right|[[USS Albacore (AGSS-569)|USS ''Albacore'']], 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 (AGSS-569)|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.<ref>{{cite news |url=http://news.usni.org/news-analysis/news/ohio-class-replacement-details |title=Ohio-class Replacement Details|work=US Naval Institute|date=1 November 2012|access-date=26 May 2020}}</ref> Instead, the first to use an x-stern in standard operations was the Swedish Navy with its [[Sjöormen-class submarine|''Sjöormen'' class]], the lead submarine of which was launched in 1967, before the ''Albacore'' had even finished her test runs.<ref>{{cite book|last=Granholm|first=Fredrik|title=Från Hajen till Södermanland: Svenska ubåtar under 100 år|publisher=Marinlitteraturföreningen|year=2003|page=56|isbn=9185944-40-8}}</ref> Since it turned out to work very well in practice, all subsequent classes of Swedish submarines ([[Näcken-class submarine|''Näcken'']], [[Västergötland-class submarine|''Västergötland'']], [[Gotland-class submarine|''Gotland'']], and [[Blekinge-class submarine|''Blekinge'']] class) have or will come with an x-rudder.

[[File:HMS Neptun (Nep) MM10732.jpg|thumb|right|The x-rudder of [[HSwMS Neptun (Nep)|HMS ''Neptun'']], a [[Näcken-class submarine|''Näcken''-class]] submarine in service with the Swedish Navy 1980–1998, now on display at [[Marinmuseum]] in [[Karlskrona]]]]

The [[Saab Kockums|Kockums shipyard]] responsible for the design of the x-stern on Swedish submarines eventually exported it to Australia with the [[Collins-class submarine|''Collins'' class]] as well as to Japan with the [[Sōryū-class submarine|''Sōryū'' class]]. With the introduction of the [[Type 212 submarine|type 212]], the German and Italian Navies came to feature it as well. The US Navy with its [[Columbia-class submarine|''Columbia'' class]], the British Navy with its [[Dreadnought-class submarine|''Dreadnought'' class]], and the French Navy with its [[Barracuda-class submarine (France)|''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,{{citation needed|date=January 2022}} 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.{{clarify|how systems would be damaged, and which systems are vulnerable|date=January 2022}}

===Hull===
{{main|Submarine hull}}

====Overview====
[[File:USS Greeneville (SSN 772) - dry dock Pearl Harbor (1).jpg|thumb|The [[US Navy]] {{sclass|Los Angeles|submarine|0}} {{USS|Greeneville|SSN-772|6}} 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 (physics)|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&nbsp;[[knot (unit)|kt]] (18&nbsp;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 tile|anechoic plating]], to reduce detection.

The occupied pressure hulls of deep-diving submarines such as {{ship|DSV|Alvin}} 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 foam|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 (submarine)|"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 (nautical)|freeboard]], that is, they lie low in the water. Bathtubs help prevent swamping the vessel.

====Single and double hulls====
[[File:U995 2004 1.jpg|thumb|{{GS|U-995||2}}, Type VIIC/41 U-boat of World War II, showing the ship-like lines of the outer hull for surface travel, blended into the cylindrical pressure hull structure.]]

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 (submarine)|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.

[[File:SRH025-p40.jpg|thumb|left|[[Type XXI]] 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.<ref>[http://www.nationaldefensemagazine.org/issues/2000/May/Virginia-Class.htm]. National Defense magazine. {{webarchive|url=https://web.archive.org/web/20080405194626/http://www.nationaldefensemagazine.org/issues/2000/May/Virginia-Class.htm|date=5 April 2008}}</ref>

====Pressure hull====
[[File:Bathyscaphe Trieste.jpg|thumb|In 1960, [[Jacques Piccard]] and [[Don Walsh]] were the first people to explore the [[Challenger Deep|deepest part]] of the world's [[ocean]], and the deepest location on the surface of the Earth's crust, in the {{ship|Bathyscaphe|Trieste}} designed by [[Auguste Piccard]].]]
{{See also|Pressure hull}}

The pressure hull is generally constructed of thick high-strength steel with a complex structure and high strength reserve, and is separated by watertight [[bulkhead (partition)|bulkheads]] into several [[Compartmentalization (fire protection)|compartments]]. There are also examples of more than two hulls in a submarine, like the {{sclass2|Typhoon|submarine|4}}, 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 [[Submarine depth ratings|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 {{convert|100|m|ft|adj=on}} maximum depth. During WWII, high-strength [[alloy]]ed steel was introduced, allowing {{convert|200|m|ft|adj=on}} depths. High-strength alloy steel remains the primary material for submarines today, with {{convert|250|-|400|m|ft|adj=on}} 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 [[ferromagnetism|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 {{convert|1000|m|ft}} for the {{ship|Soviet submarine|K-278 Komsomolets}}, the deepest-diving combat submarine. An {{sclass2|Alfa|submarine|2}} may have successfully operated at {{convert|1300|m|ft}},<ref>{{cite web|url=https://fas.org/man/dod-101/sys/ship/deep.htm|title=Federation of American Scientists|publisher=Fas.org|access-date=18 April 2010}}</ref> 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 [[Poly(methyl methacrylate)|acrylic]] pressure hulls. Although the specific strength and specific modulus of acrylic are not very high, the density is only 1.18g/cm<sup>3</sup>, 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 [[Bathyscaphe Trieste|''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.<ref>{{cite web|url=http://www.history.navy.mil/danfs/t8/trieste.htm |title=Trieste |publisher=History.navy.mil |access-date=18 April 2010 |url-status=dead |archive-url=https://web.archive.org/web/20100317120249/http://www.history.navy.mil/danfs/t8/trieste.htm |archive-date=17 March 2010 }}</ref> 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 {{convert|11521|m|ft|0}}, although this was later revised to {{convert|10916|m|ft|0}} and more accurate measurements made in 1995 have found the Challenger Deep slightly shallower, at {{convert|10911|m|ft|0}}.

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&nbsp;mm) deviation from roundness results in over 30 percent decrease of maximal hydrostatic load and consequently dive depth.<ref>{{cite web|url=http://www.usna.edu/naoe/courses/en200/ch10.pdf|title=US Naval Academy}}</ref> 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 {{sclass|Virginia|submarine|0}} attack submarine costs US$2.6 [[1000000000 (number)|billion]], over US$200,000 per [[long ton|ton]] of displacement.)

===Propulsion===<!-- "diesel–electric transmission" has a see also linking here -->
{{Further|Marine propulsion|Air-independent propulsion|Nuclear marine propulsion|Nuclear submarine}}
[[File:HMCS Windsor SSK 877.jpg|thumb|{{HMCS|Windsor|SSK 877|6}}, a [[Royal Canadian Navy]] {{sclass|Victoria|submarine|0}} diesel–electric hunter-killer submarine]]

The first submarines were propelled by humans. The first mechanically driven submarine was the 1863 French {{ship|French submarine|Plongeur||2}}, 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 [[Hellmuth Walter|Walter]] [[turbine]], on the experimental [[V-80 submarine]] and later on the naval {{GS|U-791||2}} and [[German Type XVII submarine|type XVII]] submarines;<ref>{{cite web|url=http://www.sharkhunters.com/typeadditional.htm|title=Details on German U-Boat Types|access-date=21 September 2008|work=Sharkhunters International|archive-date=24 February 2010|archive-url=https://web.archive.org/web/20100224042841/http://www.sharkhunters.com/typeadditional.htm|url-status=dead}}</ref> the system was further developed for the British {{sclass|Explorer|submarine|0}}, completed in 1958.<ref>{{cite book |author1=Miller, David |author2=Jordan, John |title=Modern Submarine Warfare |location=London |publisher=Salamander Books |year=1987 |isbn=0-86101-317-4 |page =63 }}</ref>

Until the advent of [[nuclear marine propulsion]], most 20th-century submarines used [[electric motor]]s and batteries for running underwater and [[internal combustion engine|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 fuel|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<span class="anchor" id="Diesel-electric transmission"></span>====
[[File:Submarine recharging (JMSDF).jpg|thumb|Recharging battery ([[Japan Maritime Self-Defense Force|JMSDF]])]]
{{Further|Diesel–electric powertrain}}

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.<ref>{{cite book|last=Granholm|first=Fredrik|title=Från Hajen till Södermanland: Svenska ubåtar under 100 år|publisher=Marinlitteraturföreningen|year=2003|pages=12–13|isbn=9185944-40-8}}</ref> 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]], {{ill|HSwMS Hajen (1904)|sv|HMS Hajen (1904)|lt=HSwMS ''Hajen''}} (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 (SS-1)|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.<ref>{{cite book|last=Granholm|first=Fredrik|title=Från Hajen till Södermanland: Svenska ubåtar under 100 år|publisher=Marinlitteraturföreningen|year=2003|pages=12–15|isbn=9185944-40-8}}</ref> By so doing, it took three significant steps toward what was eventually to become the dominant technology for conventional (i.e., non-nuclear) submarines.

[[File:Submarine Hajen 1.jpg|thumb|One of the first submarines with diesel–electric transmission, HMS ''Hajen'', on display outside [[Marinmuseum]] in [[Karlskrona]]]]
In the following years, the Swedish Navy added another seven submarines in three different classes ({{ill|2nd-class submarine|sv|2:a klass ubåt|lt=''2nd'' class}}, {{ill|Laxen-class submarine|sv|Laxen-klass|lt=''Laxen'' class}}, and {{ill|Braxen-class submarine|sv|Braxen-klass|lt=''Braxen'' class}}) using the same propulsion technology but fitted with true diesel engines rather than semidiesels from the outset.<ref>{{cite book|last=Granholm|first=Fredrik|title=Från Hajen till Södermanland: Svenska ubåtar under 100 år|publisher=Marinlitteraturföreningen|year=2003|pages=18–19, 24–25|isbn=9185944-40-8}}</ref> 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 S.p.A.|Fiat]]-[[Cesare Laurenti (engineer)|Laurenti]]) and later German ([[AG Weser|A.G. Weser]] and [[NV Ingenieurskantoor voor Scheepsbouw|IvS]]).<ref>{{cite book|last=Granholm|first=Fredrik|title=Från Hajen till Södermanland: Svenska ubåtar under 100 år|publisher=Marinlitteraturföreningen|year=2003|pages=16–17, 20–21, 26–29, 34–35, 82|isbn=9185944-40-8}}</ref> 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|air-independent propulsion (AIP)]] as provided by [[Stirling engine]]s beginning with [[HSwMS Näcken (Näk)|HMS ''Näcken'']] in 1988.<ref>{{cite book|last=Granholm|first=Fredrik|title=Från Hajen till Södermanland: Svenska ubåtar under 100 år|publisher=Marinlitteraturföreningen|year=2003|pages=40–43, 48–49, 52–61, 64–67, 70–71|isbn=9185944-40-8}}</ref>

[[File:Hajen & Neptun Marinmuseum Karlskrona 002.jpg|thumb|Two widely different generations of Swedish submarines but both with diesel–electric transmission: {{ill|HSwMS Hajen (1904)|sv|HMS Hajen (1904)|lt=HSwMS ''Hajen''}}, in service 1905–1922, and [[HSwMS Neptun (Nep)|HMS ''Neptun'']], in service 1980–1998]]
Another early adopter of diesel–electric transmission was the [[United States Navy|US Navy]], whose Bureau of Engineering proposed its use in 1928. It was subsequently tried in the [[United States S-class submarine|S-class submarines]] {{USS|S-3|SS-107|2}}, {{USS|S-6|SS-111|2}}, and {{USS|S-7|SS-112|2}} before being put into production with the [[United States Porpoise-class submarine|''Porpoise'' class]] of the 1930s. From that point onwards, it continued to be used on most US conventional submarines.<ref name="Book1">{{cite book|last=Friedman|first=Norman|title=U.S. submarines through 1945: an illustrated design history|publisher=Naval Institute Press|year=1995|pages=259–260|isbn=978-1-55750-263-6}}</ref>

Apart from the British [[British U-class submarine|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.<ref name="Book1" /> After World War II, by contrast, it gradually became the dominant mode of propulsion for conventional submarines. However, its adoption was not always swift. Notably, the Soviet Navy did not introduce diesel–electric transmission on its conventional submarines until 1980 with its [[Kilo-class submarine|''Paltus'' class]].<ref>{{cite web|url=http://www.deepstorm.ru/DeepStorm.files/45-92/dts/877/list.htm|title=Проект "Пaлтyc" (NATO-"Kilo")|last=Никoлaeв|first=A.C.|website=Энциклопедия отeчествeннoгo подводнoгo флотa|access-date=2 June 2020}}</ref>

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:<ref name="electrotechnical-officer.com">{{cite web|url=http://electrotechnical-officer.com/what-is-motivations-for-ship-electric-propulsion/|archive-url=https://web.archive.org/web/20190305075645/http://electrotechnical-officer.com/what-is-motivations-for-ship-electric-propulsion/|url-status=live|archive-date=March 5, 2019|title=What is motivations for ship electric propulsion|website=Electro-technical officer|access-date=2 June 2020}}</ref><ref name="diesel eletric drives guideline">{{cite web|url=https://marine.mandieselturbo.com/docs/librariesprovider6/marine-broschures/diesel-electric-drives-guideline.pdf|archive-url=https://web.archive.org/web/20190809071316/https://marine.mandieselturbo.com/docs/librariesprovider6/marine-broschures/diesel-electric-drives-guideline.pdf|url-status=dead|archive-date=August 9, 2019|title=Diesel–electric Propulsion Plants: A brief guideline how to engineer a diesel–electric propulsion system|website=MAN Energy Solutions|pages=3–4|access-date=2 June 2020}}</ref>
* 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:<ref name="electrotechnical-officer.com"/><ref name="diesel eletric drives guideline" />
* It reduces external noise by severing the direct and rigid mechanical link between the relatively noisy diesel engine(s) on the one hand and the propeller shaft(s) and hull on the other. With [[Stealth ship|stealth]] being of paramount importance to submarines, this is a very significant advantage.
* It increases the [[Crash dive|readiness to dive]], which is of course of vital importance for a submarine. The only thing required from a propulsion point of view is to shut down the diesel(s).
* It makes the speed of the diesel engine(s) temporarily independent of the speed of the submarine. This in turn often makes it possible to run the diesel(s) at close to optimal speed from a fuel-efficiency as well as durability point of view. It also makes it possible to reduce the time spent surfaced or snorkeling by running the diesel(s) at maximum speed without affecting the speed of the submarine itself.
* It eliminates the clutches otherwise required to connect the diesel engine, the electric motor, and the propeller shaft. This in turn saves space, increases reliability and reduces maintenance costs.
* It increases flexibility with regard to how the driveline components are configured, positioned, and maintained. For example, the diesel no longer has to be aligned with the electric motor and propeller shaft, two diesels can be used to power a single propeller (or vice versa), and one diesel can be turned off for maintenance as long as a second is available to provide the required amount of electricity.
* It facilitates the integration of additional primary sources of energy, beside the diesel engine(s), such as various kinds of [[Air-independent propulsion|air-independent power (AIP)]] systems. With one or more electric motors always driving the propeller(s), such systems can easily be introduced as yet another source of electric energy in addition to the diesel engine(s) and the batteries.

====Snorkel====
{{Main|Submarine snorkel}}
[[File:Submarine snorkel, 1942, the first used by the Swedish Navy, used on Neptun and later Nacken - Marinmuseum, Karlskrona, Sweden - DSC08950.JPG|thumb|right|Head of the snorkel mast from German [[type XXI submarine]] [[German submarine U-3503|''U-3503'']], scuttled outside [[Gothenburg]] on 8 May 1945 but raised by the Swedish Navy and carefully studied for the purpose of improving future Swedish submarine designs]]

During World War II the Germans experimented with the idea of the ''schnorchel'' (snorkel) from captured Dutch submarines but did not see the need for them until rather late in the war. The ''schnorchel'' is a retractable pipe that supplies air to the diesel engines while submerged at [[periscope depth]], allowing the boat to cruise and recharge its batteries while maintaining a degree of stealth.

Especially as first implemented however, it turned out to be far from a perfect solution. 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 {{convert|8|kn|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.<ref>{{cite book |last=Ireland |first=Bernard |title=Battle of the Atlantic |publisher=Pen & Sword Books |year=2003 |location=Barnsley, UK |page=187 |isbn=978-1-84415-001-4}}</ref>

While the snorkel renders a submarine far less detectable, it is thus not perfect. In clear weather, diesel exhausts can be seen on the surface to a distance of about three miles,<ref>{{cite book|last1=Schull|first1=Joseph|title=The Far Distant Ships|date=1961|publisher=Queen's Printer, Canada|location=Ottawa|pages=259}}</ref> 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.<ref>{{cite book|last1=Lamb|first1=James B.|title=On the triangle run|date=1987|publisher=Totem Books|location=Toronto|isbn=978-0-00-217909-6|pages=[https://archive.org/details/ontrianglerun0000lamb/page/25 25, 26]|url=https://archive.org/details/ontrianglerun0000lamb/page/25}}</ref>

[[File:U-3008 Turm.jpg|thumb|right|[[German submarine U-3008|USS ''U-3008'']] (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.<ref>{{Cite book|url=https://books.google.com/books?id=GLy8quRc-YYC&q=submarine+snorkel&pg=PA86|title=The Submarine|last=Navy|first=United States|date=September 2008|publisher=United States Printing Office|isbn=978-1-935327-44-8|language=en}}</ref>

====Air-independent propulsion====
{{main|Air-independent propulsion}}
[[File:2004-Bremerhaven U-Boot-Museum-Sicherlich retouched.jpg|thumb|[[German Type XXI submarine]]]]
[[File:SS X-1 Midget Submarine.jpg|thumb|American X-1 Midget Submarine]]

During World War II, [[German Type XXI submarine]]s (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 [[United Kingdom|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 {{sclass2|Quebec|submarine|5}} by NATO), they were considered unsuccessful.

The United States also used hydrogen peroxide in an experimental [[midget submarine]], [[USS X-1|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.<ref>{{cite web|title=SS X-1 |url=http://www.hnsa.org/ships/x1.htm |publisher=Historic Naval Ships Association |access-date=24 February 2014 |url-status=dead |archive-url=https://web.archive.org/web/20130818031654/http://www.hnsa.org/ships/x1.htm |archive-date=18 August 2013 }}</ref>

Today several navies use air-independent propulsion. Notably [[Sweden]] uses [[Stirling engine|Stirling technology]] on the {{sclass|Gotland|submarine|0}} and {{sclass|Södermanland|submarine|2}}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 cell]]s, first used on the [[Germany|German]] [[Type 212 submarine]], with nine 34&nbsp;kW or two 120&nbsp;kW cells. Fuel cells are also used in the new [[Spanish Navy|Spanish]] {{sclass2|S-80|submarine|2}}s although with the fuel stored as ethanol and then converted into hydrogen before use.<ref>{{cite news|publisher=Defense Industry Daily|title=S-80: A Sub, for Spain, to Sail Out on the Main|date=15 December 2008|url=http://www.defenseindustrydaily.com/s80-a-sub-for-spain-to-sail-out-on-the-main-02517/|url-access=registration}}</ref>

One new technology that is being introduced starting with the Japanese Navy's eleventh [[Sōryū-class submarine|''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 [[Air-independent propulsion|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.{{citation needed|date=October 2018}}

====Nuclear power====
{{main|Nuclear submarine|Nuclear marine propulsion}}
[[File:Battery well of USS Nautilus.jpg|thumb|Battery well containing 126 cells on {{USS|Nautilus|SSN-571|6}}, 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 [[Distillation|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 submarine]]s,<ref name="Submarine Warfare">{{cite web|url=http://www.odu.edu/ao/hrnrotc/students/ns_courses/101odu/sumbmarine%20presentation%202005.ppt|archive-url=https://web.archive.org/web/20060908003323/http://www.odu.edu/ao/hrnrotc/students/ns_courses/101odu/sumbmarine%20presentation%202005.ppt|url-status=dead|archive-date=8 September 2006|title=Submarine Warfare|access-date=7 October 2006}}</ref><ref>{{cite web|url=http://www.nti.org/db/submarines/france/index.html|title=France Current Capabilities|publisher=Nti.org|access-date=18 April 2010}}</ref> 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.<ref>{{cite book|last=Thompson|first=Roger|title=Lessons Not Learned|publisher=US Naval Institute Press|year=2007|isbn=978-1-59114-865-4|page=34}}</ref><ref>{{Cite book|last=Lee|first=T. W.|url=https://books.google.com/books?id=-nrZqzQs3jMC&q=Ohio+class+submarine+noise&pg=PA343|title=Military Technologies of the World [2 volumes]|date=30 December 2008|publisher=ABC-CLIO|isbn=978-0-275-99536-2|pages=344|language=en}}</ref> Some nuclear submarines such as the American {{sclass|Ohio|submarine|4}} can operate with their reactor coolant pumps secured, making them quieter than electric subs.{{Citation needed|date=April 2020}} 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 [[nuclear and radiation accidents by death toll|serious nuclear and radiation accidents]] have involved nuclear submarine mishaps.<ref name=johnston2007/><ref name=timenuke/> The {{ship|Soviet submarine|K-19}} reactor accident in 1961 resulted in 8 deaths and more than 30 other people were over-exposed to radiation.<ref name=rad>[http://www.iaea.org/Publications/Magazines/Bulletin/Bull413/article1.pdf Strengthening the Safety of Radiation Sources] {{webarchive|url=https://web.archive.org/web/20090326181428/http://www.iaea.org/Publications/Magazines/Bulletin/Bull413/article1.pdf |date=26 March 2009 }} p. 14</ref> The {{ship|Soviet submarine|K-27}} reactor accident in 1968 resulted in 9 fatalities and 83 other injuries.<ref name=johnston2007>{{cite web|url=http://www.johnstonsarchive.net/nuclear/radevents/radevents1.html|title=Deadliest radiation accidents and other events causing radiation casualties|author=Johnston, Robert|date=23 September 2007|publisher=Database of Radiological Incidents and Related Events}}</ref> The {{ship|Soviet submarine|K-431}} accident in 1985 resulted in 10 fatalities and 49 other radiation injuries.<ref name=timenuke>{{cite magazine|url=http://www.time.com/time/photogallery/0,29307,1887705,00.html|archive-url=https://web.archive.org/web/20090328130544/http://www.time.com/time/photogallery/0,29307,1887705,00.html|url-status=dead|archive-date=28 March 2009|title=The Worst Nuclear Disasters|magazine=[[Time (magazine)|Time]]|access-date=1 April 2015|date=25 March 2009}}</ref>

====Alternative====
Oil-fired steam turbines powered the British [[British K-class submarine|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===
[[File:Ocelot-TorpedoTubes.JPG|thumb|The forward torpedo tubes in HMS ''Ocelot'']]
[[File:Sukellusveneestä.jpg|thumb|The torpedo room of ''[[Finnish submarine Vesikko|Vesikko]]'']]
The success of the submarine is inextricably linked to the development of the [[torpedo]], invented by [[Robert Whitehead (engineer)|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 [[Torpedo Data Computer|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 gun]]s against unarmed targets, and diving to escape and engage enemy warships. The initial importance of these deck guns encouraged the development of the unsuccessful [[cruiser submarine|Submarine Cruiser]] such as the French {{ship|French submarine|Surcouf||2}} and the [[Royal Navy]]'s {{HMS|X1||2}} and [[British M-class submarine|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 [[minelayer]]s. Minelaying submarines of World War I and World War II were specially built for that purpose. Modern submarine-laid [[Naval mine|mines]], such as the British Mark 5 [[Stonefish (mine)|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 missile]]s 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 [[Tomahawk (missile family)|BGM-109 Tomahawk]] and Russian [[RPK-2 Viyuga]] and versions of surface-to-surface [[anti-ship missile]]s such as the [[Exocet]] and [[Boeing Harpoon|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 [[UGM-27 Polaris|Polaris]] missile, and subsequently the [[UGM-73 Poseidon|Poseidon]] and [[Trident (missile)|Trident]] missiles.

Germany is working on the torpedo tube-launched short-range [[IDAS (missile)|IDAS missile]], which can be used against ASW helicopters, as well as surface ships and coastal targets.

===Sensors===
{{main|Sonar}}

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 [[sonar]]s 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.

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 {{ship|DSV|Alvin}} or the [[MIR (submersible)|Russian ''Mir'' submersibles]], rely on small active sonar sets and viewing ports to navigate. The human eye cannot detect sunlight below about {{convert|300|ft|m}} underwater, so high intensity lights are used to illuminate the viewing area.

===Navigation===
{{main|Submarine navigation}}
[[File:Ocelot-Periscopes.JPG|thumb|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 (optics)|prism]] system that provides a view of the surface—is only used occasionally in modern submarines, since the visibility range is short. The {{sclass|Virginia|submarine|0}} and {{sclass|Astute|submarine|2}}s use [[photonics mast]]s 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===
{{main|Communication with submarines}}

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 {{convert|250|ft|m}}. [[Extremely low frequency|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 [[underwater telephone|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 reactor|nuclear power]] or [[air-independent propulsion]], submarines can remain submerged for months at a time. Conventional diesel submarines must periodically resurface or run on [[Submarine snorkel|snorkel]] to recharge their batteries. Most modern military submarines generate breathing [[oxygen]] by [[electrolysis]] of fresh water (using a device called an "[[Elektron (ISS)#Elektron|Electrolytic Oxygen Generator]]"). Emergency oxygen can be produced by burning [[sodium chlorate]] candles.<ref name="Smarter every day 251" >{{cite AV media|url=https://www.youtube.com/watch?v=g3Ud6mHdhlQ |title=How Do Nuclear Submarines Make Oxygen? |publisher= Smarter Every Day 251 |first= |last= |type=video |website=www.youtube.com |date=21 February 2021 |access-date=26 January 2022}}</ref> Atmosphere control equipment includes a [[Carbon dioxide scrubber]], which uses a spray of [[Amine gas treating|monoethanolamine]] (MEA) absorbent to remove the gas from the air, after which the MEA is heated in a boiler to release the CO<sub>2</sub> which is then pumped overboard. Emergency scrubbing can also be done with lithium hydroxide, which is consumable.<ref name="Smarter every day 251" /> 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.{{citation needed|date=January 2022}} An atmosphere monitoring system samples the air from different areas of the ship for [[nitrogen]], oxygen, hydrogen, [[Dichlorodifluoromethane|R-12]] and [[1,2-Dichlorotetrafluoroethane|R-114]] refrigerants, carbon dioxide, [[carbon monoxide]], and other gases.<ref name="Smarter every day 251" /> Poisonous gases are removed, and oxygen is replenished by use of an oxygen bank located in a main ballast tank.{{citation needed|date=January 2022}}{{clarify|Is storage of oxugen banks in main ballast tanks a common arrangement, or a special case?|date=January 2022}} 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 risk.

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 [[Blackwater (waste)|"blackwater"]] 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 requires skill to operate, and isolation valves must be closed before discharge.<ref name="Smarter every day 256" >{{cite AV media |url=https://www.youtube.com/watch?v=SYFuA3xnkUE |title=How to Poop on a Nuclear Submarine (256)|publisher=Smarter Every Day |type=video |website=www.youtube.com |date=9 May 2021 |access-date=27 January 2022 }}</ref> The German [[Type VII submarine|Type VIIC]] boat {{GS|U-1206||2}} was lost with casualties because of [[human error]] while using this system.<ref>{{cite web|url=http://www.uboat.net/boats/u1206.htm|title=U-1206|publisher=Uboat.net|access-date=18 April 2010}}</ref> Water from showers and sinks is stored separately in "[[greywater|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.{{citation needed|reason=Totally plausible, but a reference would be nice|date=January 2022}}