Submersion and trimming
Control surfaces
All surface ships, as well as surfaced submarines, are in a positively buoyant condition, weighing less than the volume of water they would displace if fully submerged. To submerge hydrostatically, a ship must have negative buoyancy, either by increasing its own weight or decreasing displacement of the water. To control their weight, submarines have ballast tanks, which can be filled with outside water or pressurized air.
For general submersion or surfacing, submarines use the forward and aft tanks, called Main Ballast Tanks or MBTs, which are filled with water to submerge, or filled with air to surface. Under submerged conditions, MBTs generally remain flooded, which simplifies their design, and on many submarines these tanks are a section of interhull space. For more precise and quick control of depth, submarines use smaller Depth Control Tanks or DCTs, also called hard tanks due to their ability to withstand higher pressure. The amount of water in depth control tanks can be controlled either to reflect changes in outside conditions or change depth. Depth control tanks can be located either near the submarine's center of gravity, or separated along the submarine body to prevent affecting trim.
HMS Astute is amongst the most advanced nuclear submarines in the world.[1]
When submerged, the water pressure on submarine's hull can reach 4 MPa for steel submarines and up to 10 MPa for titanium submarines like Komsomolets, while interior pressure remains unchanged. This difference results in hull compression, which decreases
displacement. Water density also increases, as the salinity and pressure are higher, but this does not compensate for hull compression, so buoyancy decreases as depth increases. A submerged submarine is in an unstable equilibrium, having a tendency to either fall or float to the surface. Keeping a constant depth requires continual operation of either the depth control tanks or control surfaces.[2][verification needed][dubious – discuss]
Submarines in a neutral buoyancy condition are not intrinsically trim-stable. To maintain desired trim, submarines use forward and aft trim tanks. Pumps can move water between these, changing weight distribution, creating a moment pointing the sub up or down. A similar system is sometimes used to maintain stability.
Sail of the French nuclear submarine Casabianca; note the diving planes, camouflaged masts, periscope, electronic warfare masts, door and windows.
The hydrostatic effect of variable ballast tanks is not the only way to control the submarine underwater. Hydrodynamic maneuvering is done by several surfaces, which can be moved to create hydrodynamic forces when a submarine moves at sufficient speed. The stern planes, located near the propeller and normally horizontal, serve the same purpose as the trim tanks, controlling the trim, and are commonly used, while other control surfaces may not be present on many submarines. The fairwater planes on the sail and/or bow planes on the main body, both also horizontal, are closer to the centre of gravity, and are used to control depth with less effect on the trim.
When a submarine performs an emergency surfacing, all depth and trim methods are used simultaneously, together with propelling the boat upwards. Such surfacing is very quick, so the sub may even partially jump out of the water, potentially damaging submarine systems.
Submarine hull
Main article: Submarine hull
Overview
The Los Angeles class attack submarine USS Greeneville in dry dock, showing typical cigar-shaped hull.
Modern submarines are cigar-shaped. This design, visible in early submarines (see below) is sometimes called a "teardrop hull". It reduces the hydrodynamic drag when submerged, but decreases the sea-keeping capabilities and increases drag while surfaced. Since the limitations of the propulsion systems of early submarines forced them to operate surfaced most of the time, their hull designs were a compromise. Because of the slow submerged speeds of those subs, usually well below 10 kt (18 km·h−1), 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. On modern military submarines the outer hull is covered with a layer of sound-absorbing rubber [disambiguation needed], or anechoic plating, to reduce detection.
The occupied pressure hulls of deep diving submarines such as DSV Alvin are spherical instead of cylindrical. This allows a more even distribution of stress at the great depth. A titanium frame is usually affixed to the pressure hull, providing attachment for ballast and trim systems, scientific instrumentation, battery packs, syntactic flotation foam, and lighting.
A raised tower on top of a submarine accommodates the periscope and electronics masts, which can include radio, radar, electronic warfare, and other systems including the snorkel mast. In many early classes of submarines (see history), the Control Room, or "Conn", was located inside this tower, which was known as the "conning tower". Since then, the Conn has been located within the hull of the submarine, and the tower is now called the "sail". The Conn is distinct from the "bridge", a small open platform in the top of the sail, used for observation during surface operation.
"Bathtubs" are related to conning towers but are used on smaller submarines. The bathtub is a metal cylinder surrounding the hatch that prevents waves from breaking directly into the cabin. It is needed because surfaced submarines have limited freeboard, that is, they lie low in the water. Bathtubs help prevent swamping the vessel.
Single / double hull
U-995, Type VIIC/41 U-Boat of WWII, showing the typical combination of ship-like non-watertight outer hull with bulky strong hull below
Modern submarines and submersibles, as well as the oldest ones, often have a single hull. Large submarines generally have an additional hull or hull sections outside. This external hull, which actually forms the shape of submarine, is called the outer hull (casing in the Royal Navy) or light hull, as it does not have to withstand a pressure difference. Inside the outer hull there is a strong hull, or pressure hull, which withstands sea pressure and has normal atmospheric pressure inside.
As early as World War I, it was realized that the optimal shape for withstanding pressure conflicted with the optimal shape for seakeeping and minimal drag, and construction difficulties further complicated the problem. This was solved either by a compromise shape, or by using two hulls; internal for holding pressure, and external for optimal shape. Until the end of World War II, most submarines had an additional partial cover on the top, bow and stern, built of thinner metal, which was flooded when submerged. Germany went further with the Type XXI, the general predecessor of modern submarines, in which the pressure hull was fully enclosed inside the light hull, but optimised for submerged navigation, unlike earlier designs that were optimised for surface operation.
Type XXI U-Boat, late WWII, with pressure hull almost fully enclosed inside the light hull
After World War II, approaches split. The Soviet Union changed its designs, basing them on German developments. All post-WWII 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. The double hulls are being considered for future submarines in the United States to improve payload capacity, stealth and range.[3]
Pressure hull
The pressure hull is generally constructed of thick high strength steel with a complex structure and high strength reserve, and is separated with watertight bulkheads into several compartments. There are also examples of more than two hulls in a submarine, like the Typhoon class, which has two main pressure hulls and three smaller ones for control room, torpedoes and steering gear, with the missile launch system between the main hulls.
The dive depth cannot be increased easily. Simply making the hull thicker increases the weight and requires reduction of onboard equipment weight, ultimately resulting in a bathyscaph. This is acceptable for civilian research submersibles, but not military submarines.
WWI submarines had hulls of carbon steel, with a 100 meter maximum depth. During WW II, high-strength alloyed steel was introduced, allowing 200 meter depths. High-strength alloy steel remains the primary material for submarines today, with 250-400 meter 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 is almost as strong as steel, lighter, and is not ferromagnetic, important for stealth. Titanium submarines were built by the Soviet Union, which developed specialized high-strength alloys. It has produced several types of titanium submarines. Titanium alloys allow a major increase in depth, but other systems need to be redesigned to cope, so test depth was limited to 1,000 meters for K-278 Komsomolets, the deepest-diving combat submarine. An Alfa class submarine may have successfully operated at 1,300 meters,[4] 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 during 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 glass pressure hulls.
The task of building a pressure hull is very difficult, as it must withstand pressures up to that of its required diving depth. When the hull is perfectly round in cross-section, the pressure is evenly distributed, and causes only hull compression. If the shape is not perfect, the hull is bent, with several points heavily strained. Inevitable minor deviations are resisted by stiffener rings, but even a one inch (25 mm) deviation from roundness results in over 30 percent decrease of maximal hydrostatic load and consequently dive depth.[5] The hull must therefore be constructed with high precision. All hull parts must be welded without defects, and all joints are checked multiple times with different methods, contributing to the high cost of modern submarines. (For example, each Virginia-class attack submarine costs 2.6 billion dollars, over $200,000 per ton of displacement.)
Propulsion
HMCS Windsor, a Victoria-class diesel-electric hunter-killer submarine
Originally, submarines were human propelled. The first mechanically driven submarine was the 1863 French Plongeur, which used compressed air for propulsion. Anaerobic propulsion was first employed by the Spanish Ictineo II in 1864. Ictineo's engine used a peroxide compound to generate heat for steam propulsion, while also providing oxygen for the crew. The system was not employed again until 1940 when the German Navy tested a hydrogen peroxide-based system employing the same principles, the Walter turbine, on the experimental V-80 submarine and later on the naval U-791 and type XVII submarines.[6]
Until the advent of nuclear marine propulsion, most 20th century submarines used batteries for running underwater and gasoline (petrol) or diesel engines on the surface, and for battery recharging. Early submarines used gasoline, but this quickly gave way to kerosene (paraffin), then diesel, because of reduced flammability. Diesel-electric became the standard means of propulsion. The diesel or gasoline engine and the electric motor, separated by clutches, were initially on the same shaft driving the propeller. This allowed the engine to drive the electric motor as a generator to recharge the batteries and also propel the submarine. The clutch between the motor and the engine would be disengaged when the submarine dove, so that the motor could drive the propeller. The motor could have multiple armatures on the shaft, which could be electrically coupled in series for slow speed and in parallel for high speed. (These connections were called "group down" and "group up", respectively.)
German Type 212 submarine with AIP propulsion of the German Navy in dock at HDW/Kiel
The principle was modified in some designs in the 1930s, particularly those of the U.S. Navy and the British U class submarines. The engine was not connected to the motor/propeller drive shaft, but drove a separate generator to drive the motors on the surface while recharging the batteries. This diesel-electric propulsion allowed greater flexibility. For example, the submarine could travel slowly with the engines at full power to recharge the batteries quickly, reducing time on the surface, or use its snorkel. It was then possible to insulate the noisy diesel engines from the pressure hull, making the submarine quieter.
German Type XXI submarines, also known as "Elektroboote", were the first submarines designed to operate entirely submerged
Other power sources were tested. Oil-fired steam turbines powered the British "K" class submarines, built during the first World War (and later), to give them the surface speed to keep up with battle fleet. The "K" class subs were not very successful, however. German Type XXI submarines were designed 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 Second World War, the British and Russians experimented with hydrogen peroxide/kerosene (paraffin) engines which could be used surfaced and submerged. The results were not encouraging; although the Russians deployed a class of submarines with this engine type (codenamed Quebec by NATO), they were considered unsuccessful. Today several navies use air-independent propulsion. Notably Sweden uses Stirling technology on the Gotland class and Södermanland class submarines. The Stirling engine is heated by burning diesel fuel with liquid oxygen from cryogenic tanks. A newer development in air-independent propulsion is hydrogen fuel cells, first used on the German Type 212 submarine, with nine 34 kW or two 120 kW cells.
Steam power was resurrected in the 1950s with a nuclear-powered steam turbine driving a generator. By eliminating the need for atmospheric oxygen, the length of time that a modern submarine could remain submerged was limited only by its food stores, as breathing air was recycled and fresh water distilled from seawater. 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 and the Royal Navy operate only nuclear submarines,[7] 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. A conventional submarine operating on batteries is almost completely silent, the only noise coming from the shaft bearings and flow noise around the hull, all of which stops when the sub hovers in mid water to listen. Commercial submarines usually rely only on batteries, since they never operate independently of a mother ship.
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. Although these are heavier, more expensive, and less efficient than a propeller, they are significantly quieter, giving an important tactical advantage.
The magnetohydrodynamic drive, or "caterpillar drive", which has no moving parts, was portrayed as a submarine propulsion system in the movie The Hunt for Red October, which portrayed it as a virtually silent system.
Although experimental surface ships have used this system, speeds have been below expectations. In addition, the drive system can induce bubble formation, compromising stealth, and the low efficiency requires high powered reactors. These factors make it unlikely for military usage.
Control surfaces
All surface ships, as well as surfaced submarines, are in a positively buoyant condition, weighing less than the volume of water they would displace if fully submerged. To submerge hydrostatically, a ship must have negative buoyancy, either by increasing its own weight or decreasing displacement of the water. To control their weight, submarines have ballast tanks, which can be filled with outside water or pressurized air.
For general submersion or surfacing, submarines use the forward and aft tanks, called Main Ballast Tanks or MBTs, which are filled with water to submerge, or filled with air to surface. Under submerged conditions, MBTs generally remain flooded, which simplifies their design, and on many submarines these tanks are a section of interhull space. For more precise and quick control of depth, submarines use smaller Depth Control Tanks or DCTs, also called hard tanks due to their ability to withstand higher pressure. The amount of water in depth control tanks can be controlled either to reflect changes in outside conditions or change depth. Depth control tanks can be located either near the submarine's center of gravity, or separated along the submarine body to prevent affecting trim.
HMS Astute is amongst the most advanced nuclear submarines in the world.[1]
When submerged, the water pressure on submarine's hull can reach 4 MPa for steel submarines and up to 10 MPa for titanium submarines like Komsomolets, while interior pressure remains unchanged. This difference results in hull compression, which decreases
displacement. Water density also increases, as the salinity and pressure are higher, but this does not compensate for hull compression, so buoyancy decreases as depth increases. A submerged submarine is in an unstable equilibrium, having a tendency to either fall or float to the surface. Keeping a constant depth requires continual operation of either the depth control tanks or control surfaces.[2][verification needed][dubious – discuss]Submarines in a neutral buoyancy condition are not intrinsically trim-stable. To maintain desired trim, submarines use forward and aft trim tanks. Pumps can move water between these, changing weight distribution, creating a moment pointing the sub up or down. A similar system is sometimes used to maintain stability.
Sail of the French nuclear submarine Casabianca; note the diving planes, camouflaged masts, periscope, electronic warfare masts, door and windows.
The hydrostatic effect of variable ballast tanks is not the only way to control the submarine underwater. Hydrodynamic maneuvering is done by several surfaces, which can be moved to create hydrodynamic forces when a submarine moves at sufficient speed. The stern planes, located near the propeller and normally horizontal, serve the same purpose as the trim tanks, controlling the trim, and are commonly used, while other control surfaces may not be present on many submarines. The fairwater planes on the sail and/or bow planes on the main body, both also horizontal, are closer to the centre of gravity, and are used to control depth with less effect on the trim.
When a submarine performs an emergency surfacing, all depth and trim methods are used simultaneously, together with propelling the boat upwards. Such surfacing is very quick, so the sub may even partially jump out of the water, potentially damaging submarine systems.
Submarine hull
Main article: Submarine hull
Overview
The Los Angeles class attack submarine USS Greeneville in dry dock, showing typical cigar-shaped hull.
Modern submarines are cigar-shaped. This design, visible in early submarines (see below) is sometimes called a "teardrop hull". It reduces the hydrodynamic drag when submerged, but decreases the sea-keeping capabilities and increases drag while surfaced. Since the limitations of the propulsion systems of early submarines forced them to operate surfaced most of the time, their hull designs were a compromise. Because of the slow submerged speeds of those subs, usually well below 10 kt (18 km·h−1), 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. On modern military submarines the outer hull is covered with a layer of sound-absorbing rubber [disambiguation needed], or anechoic plating, to reduce detection.
The occupied pressure hulls of deep diving submarines such as DSV Alvin are spherical instead of cylindrical. This allows a more even distribution of stress at the great depth. A titanium frame is usually affixed to the pressure hull, providing attachment for ballast and trim systems, scientific instrumentation, battery packs, syntactic flotation foam, and lighting.
A raised tower on top of a submarine accommodates the periscope and electronics masts, which can include radio, radar, electronic warfare, and other systems including the snorkel mast. In many early classes of submarines (see history), the Control Room, or "Conn", was located inside this tower, which was known as the "conning tower". Since then, the Conn has been located within the hull of the submarine, and the tower is now called the "sail". The Conn is distinct from the "bridge", a small open platform in the top of the sail, used for observation during surface operation.
"Bathtubs" are related to conning towers but are used on smaller submarines. The bathtub is a metal cylinder surrounding the hatch that prevents waves from breaking directly into the cabin. It is needed because surfaced submarines have limited freeboard, that is, they lie low in the water. Bathtubs help prevent swamping the vessel.
Single / double hull
U-995, Type VIIC/41 U-Boat of WWII, showing the typical combination of ship-like non-watertight outer hull with bulky strong hull below
Modern submarines and submersibles, as well as the oldest ones, often have a single hull. Large submarines generally have an additional hull or hull sections outside. This external hull, which actually forms the shape of submarine, is called the outer hull (casing in the Royal Navy) or light hull, as it does not have to withstand a pressure difference. Inside the outer hull there is a strong hull, or pressure hull, which withstands sea pressure and has normal atmospheric pressure inside.
As early as World War I, it was realized that the optimal shape for withstanding pressure conflicted with the optimal shape for seakeeping and minimal drag, and construction difficulties further complicated the problem. This was solved either by a compromise shape, or by using two hulls; internal for holding pressure, and external for optimal shape. Until the end of World War II, most submarines had an additional partial cover on the top, bow and stern, built of thinner metal, which was flooded when submerged. Germany went further with the Type XXI, the general predecessor of modern submarines, in which the pressure hull was fully enclosed inside the light hull, but optimised for submerged navigation, unlike earlier designs that were optimised for surface operation.
Type XXI U-Boat, late WWII, with pressure hull almost fully enclosed inside the light hull
After World War II, approaches split. The Soviet Union changed its designs, basing them on German developments. All post-WWII 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. The double hulls are being considered for future submarines in the United States to improve payload capacity, stealth and range.[3]
Pressure hull
The pressure hull is generally constructed of thick high strength steel with a complex structure and high strength reserve, and is separated with watertight bulkheads into several compartments. There are also examples of more than two hulls in a submarine, like the Typhoon class, which has two main pressure hulls and three smaller ones for control room, torpedoes and steering gear, with the missile launch system between the main hulls.
The dive depth cannot be increased easily. Simply making the hull thicker increases the weight and requires reduction of onboard equipment weight, ultimately resulting in a bathyscaph. This is acceptable for civilian research submersibles, but not military submarines.
WWI submarines had hulls of carbon steel, with a 100 meter maximum depth. During WW II, high-strength alloyed steel was introduced, allowing 200 meter depths. High-strength alloy steel remains the primary material for submarines today, with 250-400 meter 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 is almost as strong as steel, lighter, and is not ferromagnetic, important for stealth. Titanium submarines were built by the Soviet Union, which developed specialized high-strength alloys. It has produced several types of titanium submarines. Titanium alloys allow a major increase in depth, but other systems need to be redesigned to cope, so test depth was limited to 1,000 meters for K-278 Komsomolets, the deepest-diving combat submarine. An Alfa class submarine may have successfully operated at 1,300 meters,[4] 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 during 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 glass pressure hulls.
The task of building a pressure hull is very difficult, as it must withstand pressures up to that of its required diving depth. When the hull is perfectly round in cross-section, the pressure is evenly distributed, and causes only hull compression. If the shape is not perfect, the hull is bent, with several points heavily strained. Inevitable minor deviations are resisted by stiffener rings, but even a one inch (25 mm) deviation from roundness results in over 30 percent decrease of maximal hydrostatic load and consequently dive depth.[5] The hull must therefore be constructed with high precision. All hull parts must be welded without defects, and all joints are checked multiple times with different methods, contributing to the high cost of modern submarines. (For example, each Virginia-class attack submarine costs 2.6 billion dollars, over $200,000 per ton of displacement.)
Propulsion
HMCS Windsor, a Victoria-class diesel-electric hunter-killer submarine
Originally, submarines were human propelled. The first mechanically driven submarine was the 1863 French Plongeur, which used compressed air for propulsion. Anaerobic propulsion was first employed by the Spanish Ictineo II in 1864. Ictineo's engine used a peroxide compound to generate heat for steam propulsion, while also providing oxygen for the crew. The system was not employed again until 1940 when the German Navy tested a hydrogen peroxide-based system employing the same principles, the Walter turbine, on the experimental V-80 submarine and later on the naval U-791 and type XVII submarines.[6]
Until the advent of nuclear marine propulsion, most 20th century submarines used batteries for running underwater and gasoline (petrol) or diesel engines on the surface, and for battery recharging. Early submarines used gasoline, but this quickly gave way to kerosene (paraffin), then diesel, because of reduced flammability. Diesel-electric became the standard means of propulsion. The diesel or gasoline engine and the electric motor, separated by clutches, were initially on the same shaft driving the propeller. This allowed the engine to drive the electric motor as a generator to recharge the batteries and also propel the submarine. The clutch between the motor and the engine would be disengaged when the submarine dove, so that the motor could drive the propeller. The motor could have multiple armatures on the shaft, which could be electrically coupled in series for slow speed and in parallel for high speed. (These connections were called "group down" and "group up", respectively.)
German Type 212 submarine with AIP propulsion of the German Navy in dock at HDW/Kiel
The principle was modified in some designs in the 1930s, particularly those of the U.S. Navy and the British U class submarines. The engine was not connected to the motor/propeller drive shaft, but drove a separate generator to drive the motors on the surface while recharging the batteries. This diesel-electric propulsion allowed greater flexibility. For example, the submarine could travel slowly with the engines at full power to recharge the batteries quickly, reducing time on the surface, or use its snorkel. It was then possible to insulate the noisy diesel engines from the pressure hull, making the submarine quieter.
German Type XXI submarines, also known as "Elektroboote", were the first submarines designed to operate entirely submerged
Other power sources were tested. Oil-fired steam turbines powered the British "K" class submarines, built during the first World War (and later), to give them the surface speed to keep up with battle fleet. The "K" class subs were not very successful, however. German Type XXI submarines were designed 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 Second World War, the British and Russians experimented with hydrogen peroxide/kerosene (paraffin) engines which could be used surfaced and submerged. The results were not encouraging; although the Russians deployed a class of submarines with this engine type (codenamed Quebec by NATO), they were considered unsuccessful. Today several navies use air-independent propulsion. Notably Sweden uses Stirling technology on the Gotland class and Södermanland class submarines. The Stirling engine is heated by burning diesel fuel with liquid oxygen from cryogenic tanks. A newer development in air-independent propulsion is hydrogen fuel cells, first used on the German Type 212 submarine, with nine 34 kW or two 120 kW cells.
Steam power was resurrected in the 1950s with a nuclear-powered steam turbine driving a generator. By eliminating the need for atmospheric oxygen, the length of time that a modern submarine could remain submerged was limited only by its food stores, as breathing air was recycled and fresh water distilled from seawater. 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 and the Royal Navy operate only nuclear submarines,[7] 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. A conventional submarine operating on batteries is almost completely silent, the only noise coming from the shaft bearings and flow noise around the hull, all of which stops when the sub hovers in mid water to listen. Commercial submarines usually rely only on batteries, since they never operate independently of a mother ship.
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. Although these are heavier, more expensive, and less efficient than a propeller, they are significantly quieter, giving an important tactical advantage.
The magnetohydrodynamic drive, or "caterpillar drive", which has no moving parts, was portrayed as a submarine propulsion system in the movie The Hunt for Red October, which portrayed it as a virtually silent system.
Although experimental surface ships have used this system, speeds have been below expectations. In addition, the drive system can induce bubble formation, compromising stealth, and the low efficiency requires high powered reactors. These factors make it unlikely for military usage.
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