Submarine Hunting: Anti-Submarine-Warfare (ASW)

Project 877/Kilo-class submarine. (Photo by Guy Toremans, via author)

 

In general, there are three types of ASW-platforms:

-Airborne, including helicopters and planes

-Surface units, including frigates, destroyers as well as small coastal submarine hunters

-Submerged units, including diesel-electric submarines and Nuclear submarines.

The most important thing about hunting a submarine is detecting it.

It consumes a lot of time and effort. Once the submarine is detected, it is – relatively – easy to hit and sink. The searching and detecting of a submarine is still a risk bearing operation, depending on the asset and weapon you use.

Since anyone wants to take the least risky method of finding and killing a submarine, airborne ASW platforms remain the most popular.

In response, some of modern submarines are now equipped with MANPADS (Man-Portable Air Defense Systems), but these have not proved as effective as the submarine has to surface in order to use them, in turn exposing itself to enemy fire. Some research for a torpedo-tube launched anti-helicopter weapon, named “Triton”, was undertaken by Germans, but no such weapon entered service so far.

 

Target

Every submarine is a highly evasive, problematic to detect, “hard” target. Usually, a submarine will be actually invisible even to the eyes of any observer on the surface, and definitely not visible on any radar – as long as it remains submerged.

 The currently available detection technology is therefore based on magnetic deviations and sound waves. Clearly, in response to such threats submarine-constructors adapted their products, making them ever silent.

By nature, a submerged submarine is therefore “invisible”, and therefore an extremely potent weapon. It can manoeuvre, move swiftly, or stand still, dive through so-called thermic layers in order to hide, shoot noise makers, use the bottom of the sea to hide, but also to find a target for itself. This all does not make any submarine a “miracle” by itself, however, then there are still the ways of detecting submarines – even if these are usually complex and full of problems.

Airborne Platforms

As already mentioned, airborne anti-submarine warfare (ASW) platforms are foremost helicopters and aircraft. As seen from the submarine, they are actually invulnerable and far more manoeuvrable – due to their advantage in speed. This speed permits them to change their position much faster than any submarine and thus not only cover large areas when searching for their targets, but also have a choice of time and point from which they attack, relative to the submarine position.

Helicopters are slower than aircraft and usually have a much shorter endurance, but they have the ability to move in a far more methodical way, and even hover over an area. They are also smaller in size and weight and can be carried by small ships.

ASW-Helicopters are usually equipped with MADs (“Magnetic Anomaly Detectors”), dipping sonar (active and passive) and sonobuoys (also active and passive); ASW aircraft are all equipped with MAD and sonobuoys only, while some also have powerful surface-search radars. MAD-detectors can detect submarines only within a quite limited zone. Dipping sonar and sonobuoys are the most effective means of submarine hunting, then they enable the crew of the helicopter to listen to underwater sounds over extended periods of time.

On this photograph of a USN Lockheed P-3C Orion with a Soviet Navy Project 671RTM/Victor III-class SSN, the MAD-detector of the Orion, mounted in the lenghty extension on the rear fuselage, behind the fin, can be seen to advantage. (Photo: Tom Cooper collection)

Modern ASW-Helicopters are mainly equipped with advanced guided torpedoes, but also with depth charges, and light anti-ship missiles.

ASW-Aircraft are much faster than either submarines or helicopters, and usually have an extended range and excellent endurance. However, their speed precludes them from using a dipping sonar, and when it comes to detection means related to sound they are limited to carriage of sonobuoys, and a MAD-detector. The largest disadvantage of ASW-Aircraft is that they most of them – except for such aircraft like Lockheed S-3 Viking (which, however, is not any more used for ASW-purposes) – are limited to the use from land-bases.
Contrary to the ASW-helicopters, however, ASW-aircraft can carry much larger loads of weapons (including guided torpedoes, mines, and depth charges), as well as many more sonobuoys.

Surface Platforms

Surface platforms – usually called “warships” – clearly have the much longer range and endurance than any airborne assets. They are, however, also much heavier and larger, carrying far more equipment and weapons – but also much slower.

There are several different types of surface ASW-platforms: they can either serve as bases for ASW-helicopters (or, in case of some ASW-carriers, for ASW-aircraft), or – in the case of corvettes, frigates and destroyers – can operate in combination with airborne ASW-assets. There are also small and fast coastal-patrol vessels, which usually do not carry any airborne platforms, but can cooperate with these to search for submarines.

The concept of the large, purpose-built ASW-platform, such like ASW-carrier, appears to have been dropped since the end of the Cold War: while in the 1960s and 1970s the USN operated special “ASW carriers” (like re-conditioned ships of Essex-class, carrying S-1 and E-1 Trackers), the Soviet Navy operated helicopter carriers of the Kiev- and Moskva-class, Italians have built their Giuseppe Garibaldi carrier or the cruiser Vitorio Venetto, while later the Japanese followed with their “helicopter-carrying destroyers” of Haruna- and Shirane-class, today nobody is building such ships any more, and the general preference are “multi-purpose ships”.

Smaller surface ASW-combatants usually weight up to 1.000 tons and can reach speeds of up to 40kts. Because of their speed, however, they need powerful machinery, and the space for equipment and weapons on such hulls is therefore usually rather restricted. Nevertheless, even such platforms are usually equipped with hull-mounted and dipping sonar, and some even have – usually very expensive – towed array sonars. Small surface ASW-combatants are mainly equipped with guided torpedoes, anti-submarine mortars and depth charges.

Large surface ASW-combatants usually weight between 5.000 and 8.000 tons, but some classes are well in excess of 10.000 and more tons. Such warships are very stable platforms, with good seagoing capabilities and a lot of space and weight for heavy equipment available. Hence, they not only carry much more and heavier weaponry, but also excellent detection devices in one hull, including bow mounted sonars, hull-mounted sonars, towed array sonars (active and passive), and even the so-called variable depth sonars. Large surface ASW-combatants usually also have specialised ASW-weapons, including light-weight and heavy-weight torpedoes, torpedo-carrying rockets, anti-submarine mortars, depth charges, mines, and – perhaps most important of all – ASW-helicopters.

Submarines

At earlier times, especially during the WWI and WWII, submarines were foremost built with anti-surface warfare, mainly anti-merchant, warfare in mind. The submarine vs submarine combat occurred rather rare, most usually by pure accident. During the 1960s and 1970s, especially the invention of nuclear-powered submarines, equipped with highly advanced passive sonars, and advanced guided torpedoes, enabled the submarines to be developed into potent ASW-weapons as well.

Such “hunter-killer” submarines are, of course, at a disadvantage when compared to airborne platforms in regards to speed, and when compared to ships they frequently also have not only much slower maximal speeds, but also shorter detection ranges. Nevertheless, specific modern-day submarines are almost perfect ASW-weapons, developed and equipped specially with the purpose of detecting and destroying enemy submarines. They do not face problems resulting from swell and can therefore also have quite an advantage in speed.

There are two distinct types of ASW-submarines: most have diesel-electric powerplants (so-called “SSK”s), while larger navies can also afford nuclear powered submarines (so-called “SSN”s). Regardless of their powerplants, all such platforms have a bow-mounted sonar (passive and active), a flank-array (passive linear sonar mounted on submarine’s flank), and usually also a towed array sonar.

They are usually armed with guided torpedoes, but sometimes also with rocket-torpedo combinations (where the rocket powers the torpedo up and over the surface, bringing it to a specific range and then drops it back into the water), rocket-depth charge combinations, mines, and even missiles (used for anti-ship and land-attack roles).

Nuclear-powered submarines (SSNs) are much larger “boats”, and therefore in possession of not only the advantages in speed and endurance, but also regarding the amount of weapons and sensors they can carry. Some of SSN-submarines can even attack enemy submarines on anchor in their own bases, by the means of cruise missiles. SSKs, on the contrary, are usually much smaller, slower, have a much lower endurance, but are also more manoeuvrable. In fact, due to their “air-breathing” propulsion, most of SSKs are – compared to SSNs – exceptionally limited in their capability to remain submerged, then once under the surface they either depend on the power of their batteries, or have to remain directly bellow the surface in order to use the “snorchel” – a special device that supplies air from above the sea surface to diesel engines.

Basic Principles of Anti-Submarine Warfare

The ASW is actually depending on the means of detection.

There are two different large groups of detectors: MADs and sonars.

MAD is short for “Magnetic Anomaly Detector”. Usually deployed as a probe, MAD is towed either behind a helicopter, or aircraft, and based on the fact that any submarine is basically a large mass of steel and other metallic alloys, concentrated in large amounts in an environment otherwise free of such materials. Consequently, the submarines cause fine deviations in the Earth’s magnetic field.
These deviations can be detected – and even tracked with the help of a MAD. Although this detection method is meanwhile increasingly problematic – to no small degree also because of an increasing number of shipwrecks on the bottom of most seas (since shipwrecks can cause a similar deviation and the Earth’s magnetic field) – meanwhile very precise maps of such deviations are available, and they can be taken into account. The largest disadvantage of the MAD remains therefore its limited range: the aircraft or helicopter deploying its MAD-sensor has to fly very low and slow over the area where the submarine is suspected, in order to use it effectively.

The sonar, on the contrary, is based on sound detection: effectively, the sonar is nothing else but a very advance microphone, consisting of all kinds of emitters and receivers, and – in our days – supported by extremely advanced computers and software. There are very different sonars, most of which were mentioned in this article already: bow mounted sonars, hull mounted sonars, sonobuoys, dipping sonars, towed array sonars, and variable depth sonar.

In general, sound is a very uncertain medium, because it has to travel through different other mediums and weather – i.e. water – conditions. Essentially, the detection of underwater sounds depends on four main factors: salinity (amount of salt in water, which varying from one sea to the other), pollution, temperature and pressure (which increases with the depth). These four factors can bend sound waves, bounce them back or even slow them down.

Normaly, a sound wave will be bounced back by hard smooth surfaces, like submarine, but also rock bottom, or stones in the sand bottom (which can even return the same wave into several directions.
The differences in water temperature at different depths form the so-called “thermal layers” (or “thermocyclines”), the borders of which also bounce sound beams. In specific parts of some seas and oceans, these differences are so massive, that they enable even large submarines to hide in one thermal layer, or – better said: bellow them – then the thermocyclines are so massive they bounce sounds from any kind of active sonars, or completely block the sounds from reaching the passive sonar.
On the contrary, in the oceans there is a certain layer that is perfectly “transporting” sound waves. This is the so-called “deep sea sound channel” (DSSC).

The layer on top of this layer has a too high temperature and bounces back the waves sent within the DSSC. The layer below, has a too high pressure and hence bounces the waves back too. By this way, a wave sent within the DSSC will be bounced back by these layers from one another, this will form a sinusoidal movement of the sound wave and transport it for very long periods over several thousands of miles. This layer is mostly located at depths between 800 and 2.000 meters, but is frequently depending on temperature and pressure. As an example what a DSSC can do: in WWII, some bombers and airplanes used to carry a depth charge that was set to explode within the DSSC and in the USA and UK there were several stations with hydrophones inside this layer. By taking bearings from an explosion of such a depth charge, the position of the crashed aircraft could be determined and a rescue team could be sent out….Today, surface ASW platforms equipped with VDS can lower these into this layer enabling them to detect submarines over immense ranges. Of course, any decent submarine-skipper knows this as well, and will attempt to avoid operating within the DSSC.

The Sound wave is determined by the “gain” strength of the emission and the frequency – which is dependent on the wavelength For frequency a simple rule of thumb can be used: the higher the frequency the less of it will protrude the water (meaning the shorter range), but in turn this is making the frequency easier to concentrate – or “aim the beam” (meaning more accurate position fix). The situation is directly opposite when it comes to low frequencies.

All such details and factors fit into the submarine hunting process and procedures: the submarine is most likely to be detected at a long range by low frequency devices, which will provide a blurry – i.e. approximate – position. Once the hunters come closer higher frequencies will be used to track the submarine down and for attack.

Gain is important too: when the gain is set too high, it will be bounced back too strongly, this might cause double targets, the wave gets bounced back by the surface and goes down again, and afterwards will be received again too. This will give double echoes.

This gain might also give echoes of fish and other insignificant objects (wreck masts etc); on the other side, it also helps in detection from longer ranges.

Detection Devices

Sonars are in general capable of being used in active and passive modes. Active means that the sonar sends out its own beam and then listens for echoes.
Passive means that in only relies on the noise from the target, receiving sound waves of certain frequencies.

The largest disadvantage of active seekers is that they can be heard from a much larger range than they can detect themselves. For example when an active sonar has a range of one mile, it can be heard by the passive sonar of a submarine from as far as three or four miles. This would mean that the submarine can detect the presence of the “submarine hunter” in advance and take evasion measures – or even launch a pre-emptive attack.

Therefore, in modern ASW the use of active sonar is preferrable only if the target is already known and tracked, or in desperate situation when there is an urgent need of finding the target the presence of which is known but position of which is unknown.

 

1) Bow mounted sonars

are today mounted on most frigates and destroyers, since they are rather easy to incorporate and do not require any adaptations that might have adverse effects on the construction of the ship.
In general, all bow mounted sonars can be used in both, active and passive modes. They are usually installed in the bulb of the ship, but have the disadvantage of suffering to flow noise. This means that the faster the ship is moving the faster the water passed down the bow, and the bulb is more likely to cause the flow noise, which covers any external noises, making them harder to detect. High speed movements of the ship also create air bubbles in water surrounding the bow: air is especially bad then it bounces sound waves off.
Another problem connected with ship movement is that of machinery noise. Every ship is getting noisier the faster it moves, causing air bubbles to snap in the water, cavitation of propellers, louder engine noises etc. Therefore, slower speeds are advised for submarine hunting.

2) Hull mounter sonars

are usually mounted just behind the bow, at about one third of the hull down from the bow. This position offers the advantage that there is nothing that creates air bubbles – like the bulb in which the bow mounted sonars are usually positioned. Yet, the disadvantage is that hull mounted sonars detoriate the ship’s hydrodynamic form, suffer from additional flow noise, and have a limited field of “view”. Specifically, under specific conditions, hull mounted sonars cannot detect submarines operating near the bottom of the sea. They can also not be used in both, the passive and active modes at once.

3) Variable Depth Sonars

are usually placed in a hull of their own, and towed behind the ship on a line long between 600 and 1.500m. They can be used in both, passive and active modes, and have a steering mechanism that allows them to alter the operating depth, as well as to measure the pressure, temperature and orientation. The VDS therefore offers the crew of any ASW-platform the best grasp of water conditions; yet, its greatest advantage is that it can alter its depth and therefore dive bellow thermal boundaries, in turn enabling detection of submarines that hide in thermal layers – where the sound waves of both, from where no sound waves would reach the bow mounted sonar and the hull mounted sonar, or from where the sound waves would bounce. The VDS can also be driven into the DSSC, and enable the ship to detect submarines from immense distances.

The use of VDS does not permit the warship to operate at high speeds, then towing cable develops very high breaking strength, while the flow noise at high speeds would also cover all the other sounds. Besides, the VDS require a large adaptation of the ship’s stern (rolls to mount the cable, rails to mount the module, winches to launch and retrieve it etc.), and can be very problematic to handle, requiring an excellently trained and experienced crew to operate it. Thefore, such devices are used only on specialised ASW-ships.

4) Towed Array Sonars

are arrays towed behind the ship (or a submarine). Basically, they consist of a cable of up to 1.800m long, with a large “pod” – full fo hydrophones and other sensors – at the end. The cable needs to be as long to be towed behind far enough behind the ship in order to be held out of the zones interfered by the ship’s noise (engines), vibration and cavitation caused by propellers. Originally, all towed arrays were passive, but nowadays there is an increasing number of arrays that can be used in active mode as well. In general, they give a 360 degree search capability and are especially useful for long-range detection, mainly because they monitor low frequency. They can also be used for detection of surface ships, providing bearings to noise-sources.

Compared to VDS, the towed array sensors are rather light, even if they also require a winch for the cable; like in the case of VDS’ they are also useful only at slower speeds. Nevertheless, towed arrays are much lighter and therefore can be found on most of multi-purpose warships too.

Ships equipped with towed arrays usually operate in short dashes: taking one bearing, speeding up to a next position, slowing down and attempting to take a new bearing. This allows for a long-range estimate of the submarine’s position. The towed arrays are also frequently used in conjunction with helicopter’s dipping sonar, or the towed array of another ship, enabling a “triangulation” of the target, i.e. establishing a very precise target fix, including not only the bearing, but also the range to the target.

(All drawings by Roel Van de Velde)

The drawing above shows the function of a towed array sonar. As first while searching for a possible submarine, with the help of the towed sonar the ship can get a bearing (angle between north and the direction of the target), as can be seen on the little compass. Then the ship stores the sonar and accelerates to Point B (usally chosen by the skipper or ASW-officer), where the sonar is deployed again in attempt to get a new bearing. If a second bearing is established, the cross of the two bearings is the approximate position of the submarine.

Of course, it takes some time to move from point A to point B, and so the submarine will move too. But, as submarines usually move at a much slower speeds than warships in order to remain quiet, the warship is in a better position to execute a manoeuvre of this kind and also find the submarine – the possible position of which is marked by a red circle on this drawing. Intermittent line indicates the true course of the submarine. Once such cross-bearing was established, the ship can move in closer and use the active sonar, or – in the case of larger ships – send a helicopter to find the submarine and execute an attack.

This drawing shows also the advantage of having multiple ships equipped with towed array working together. When one ship is at point A and the other at point B simultaneously, they can do the cross bearing immediately and have a very accurate position of the submarine right away. Of course, additional units – including helicopters – with similar capabilities will do even better: the more units an ASW group counts, the better the target fix will be.

5) Dipping Sonars

are mainly used by helicopters and small, fast patrol vessels (Russian Pauk I-class, and different hydrofoil crafts). Diping sonar is basically a small pod with a microphone (1m high and 20-30cm in diameter at most), hanging on the end of a long line. When dipping sonar is operated from helicopter, the helicopter has to stop in the air and hower, lowering the sonar into the war (“dipping it”). Depending on the size of the heilicopter, some dipping sonars can be lowered down to more than 300m bellow the surface. Most of dipping sonars can only work in active mode but an increasing number can also operate in the passive mode. In essence, the dipping sonar is deployed in a similar manner to the towed array sonar, with the difference that the helicopter can swiftly heave the pod and rapidly move to the next spot, thus being able to cross-check own bearings withing shortest possible periods of time.

6) Sonobuoys

Sonobuoys are basically drifting hydrophones, mainly used by ASW-aircraft and -helicopters. They are all connected to a ship or helicopter-based information system via a complex data-link net, which is listening to what the sonobuoys hear. Originally, all sonobuoys could work only in active mode and a single deploying system could usually only listen to two or three at once. Since the mid-1980s much more powerful sonobuoy-supporting systems and computers are in use, enabling the deployment of passive sonobuoys, and listening to all of them at once, as well as automatic modes, which enable sonobuoys to listen on pre-determined frequencies.

The disadvantage of active sonobuoys is obvious: a submarine can hear them not only as they splash into the water, but especially so when they start to “ping” with their active sonars. For this reason the use of also active sonobuoys with active sonars is something modern navies attempt to avoid.

Sonobuoys are relatively cheap and can be used in immense numbers. They are usually dropped in rows, each at a specific different from the other. In this way a helicopter of ASW-aircraft equipped with them can search or monitor either along a specific line, or even a whole area. The advantage of using sonobuoys is obvious: when more than one has a bearing on a possible submarine, the user can almost immediately establish the exact position of his target.

 

Weapons

Through the history a considerable number of different weapons was developed for fighting submarines.

- Torpedoes:

The torpedo is a cylindrical shaped device with a small sonar in its “bow”. ASW-torpedoes are usually driven by an electric engine. In our days, ASW-torpedoes are mainly used by helicopters, which drop them from low altitude while flying at slow speed. Similar or same models are usually also used by ASW-ships. Upon being jettisoned from helicopters, the torpedo is slowed down by a parachute that slows it. The parachute falls off due to the sudden pressure caused by entery into the water and from that moment onwards the weapon is on its own. Modern ASW-torpedoes are self-homing, and have pre-sellected search-patterns, along which they operate once under the surface.

Guided torpedoes used by submarines are not only larger and heavier than those used on helicopters and ASW-aircraft, but also equipped with a small box mounted behind the propeller, which contains wire (usually a fibre-optic cable); there is a similar box with wire inside the torpedo tube.

Once the torpedo is released from the tube it remains connected to the submarine’s weapons system. While torpedoes dropped from helicopters and aircraft are usually short ranged, the weapons deployed from submarines are constructed to be fired from relatively short ranges, do several turns before – under optimal conditions – colliding with target.

Torpedoes used by submarines are far more flexible: they can, for example, be launched at slow speed in order not to create too much noise, then steered to the vicinity of target and kicked at high speed. The crew of the submarine can also decide when to activate their homing system. This is also a fact of potential tactical advantage, then as long as the torpedo’s sonar remains inactive, there is less chance of target recognizing the attack and attempting to evade. In turn, this also means that the torpedo can be used as a remote sonar.

There are, nevertheless, several problem-zones with guided torpedoes. The wires limit the range because only a limited amount of cable can be stored inside the two boxes. The other problem is that while guiding a torpedo via the wire the submarine can only manoeuvre at minimal speeds, and is thus highly vulnerable in the case of a counterattack. If the boat is detected prematurely – i.e. while guiding one of ist torpedoes – it thus has to cut the wires and run: this is making any torpedo useless unless it was previously put in active sonar mode and well on the way to the target.

Of course, guided torpedoes can also be launched in normal mode, aiming at the target: they will normally run until approaching the target and then activate their homing systems. This form of an attack is not the most promissing, then the fire-control solution pre-programmed into the torpedo before launch can be wrong, and torpedo either miss or activate too early, enabling the target to evade.

- Depth Charges:

A depth charge is basically a barrel filled with explosives and a fuze, which usually functions on the basis of pressure that increases with the depth. Another popular method of igniting a depth charge is contact fuze, causing the charge to detonate when hitting the submarine or the sea ground nearby. Generally, depth charges are used in large numbers: they can be dropped by helicopters or ships, or launched by rockets from ships or submarines.

Depth charges are not only dangerous for submarines when they hit directly or detonate nearby: pressure waves created by their detonations are very powerful under the water, and create waves that can not only cause a considerable shock to the structure of the submarine, or disable the electrical system, but even crack the hull and sink the boat. The effective range of the pressure waves caused by a bomb is called the “blast radius”. The most modern depth charges are much lighter than before, and deployed with help of multi-barrel mortars, some of which have a range of between 4.000 and 6.000m. Such mortars can place a number of depth charges into the same part of sea at different depths within very short periods of time, thus creating blast areas of considerable size.

- Mines:

Generally, mines are used to deny the use of specific parts of sea, or to protect a certain area. These weapons exist in all kinds of forms and calibres, and have an even larger variety of fuzing methods. A better part of any mine consists of explosive, but in general there are three types: anchored, floating, and bottom mines. Anchored mines are buoyant, but connected to the bottom with a chain. This can keep them at different depths. Together with floating mines, they are the easiest to dismantle: the chain keeping the mine anchored can be cut by minesweepers, causing the mine to surface (although, specific modern types of anchored mines have an anti-sweeping cable, which have a certain “slip-through” part, through which the sweeping gear can slip). Once there it can be activated by gun or cannon from safe distance.

Bottom mines are the most problematic to find, first of all, as they are usually positioned in the mud or between the rocks on the ocean floor – from where some types rise only if a ship or submarine is passing nearby. Searching for them is immensely problematic, then minehunters require special remotely controlled detection and destruction devices to dismantle them.

Conclusion

Submarine hunting is a very complex operation and requires a large number of assets with a very quick reaction time. Yet, it is a very necessary task because any submarine is a threat that simply cannot be ignored. Like ever since their invention, the submarines remain a latent threat to merchant shipping, and are in position to block even an entire country – if available in sufficient numbers, of course.

With the advent of nuclear submarines this threat grew in intensity, because such boats are capable of hunting down all of enemy’s merchant traffic directly in front of the coast, and only limited by amount of food and weapons they carry.

All submarine-hunters are confronted with immense problems: usually, the areas in which they can operate are immense; the already described problems with the sea can only be worsened by the weather.

Normal patrol duty is endlessly boring for those who serve aboard ASW-warships, aircraft, and helicopters, yet they have to remain vigillant over extended periods of times and keep on trying regardless the circumstances or problems they are confronting.

In general, their opponents – the submarines – can pick the choice of time and place of the engagement, that is they are not detected too early. Submarine skippers will do their best to – patiently – bring thier boats in proper position.

Commanders of SSNs will be in advantage in deeper waters, where there is a need for endurance and flexibility in manoeuvre, as well as long-range sensors and weapons. The SSKs are less mobile and their actions more predictable, resembling rather a “mobile minefield”, which – once it is brought in position – conducts its operation in silence, waiting for a prey to appear.

They can be very effective in defending shores from enemy submarines and warships, but their offensive capabilities are limited also by their inability to dive for longer periods of times without air supply: this inability makes them prone to early detection.

http://www.acig.org/artman/publish/article_491.shtml

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