Editing Submarine (section)

===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.