This design, visible in early submarines, 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. 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, or anechoic plating , to reduce detection. 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. The bathtub is a metal cylinder surrounding the hatch that prevents waves from breaking directly into the cabin.
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It is needed because surfaced submarines have limited freeboard , that is, they lie low in the water. Bathtubs help prevent swamping the vessel. Modern submarines and submersibles, as well as the oldest ones, usually 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 , a general predecessor of modern submarines, in which the pressure hull was fully enclosed inside the light hull, but optimized for submerged navigation, unlike earlier designs that were optimized for surface operation.
After World War II, approaches split. The Soviet Union changed its designs, basing them on German developments. American and most other Western submarines switched to a primarily single-hull approach. They still have light hull sections in the bow and stern, which house main ballast tanks and provide a hydrodynamically optimized shape, but the main cylindrical hull section has only a single plating layer. Double hulls are being considered for future submarines in the United States to improve payload capacity, stealth and range. 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.
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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 bathyscaphe.
This is acceptable for civilian research submersibles, but not military submarines. To exceed that limit, a few submarines were built with titanium hulls. Titanium can be stronger than 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 does not flex as readily as steel, and may become brittle after many dive cycles.
Despite its benefits, the high cost of titanium construction led to the abandonment of titanium submarine construction as the Cold War ended.
Deep-diving civilian submarines have used thick acrylic pressure hulls. The deepest deep-submergence vehicle DSV to date is Trieste.
Building a pressure hull is difficult, as it must withstand pressures at its required diving depth. When the hull is perfectly round in cross-section, the pressure is evenly distributed, and causes only hull compression. If the shape is not perfect, the hull is bent, with several points heavily strained. 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.
The first submarines were propelled by humans. The first mechanically driven submarine was the French Plongeur , which used compressed air for propulsion. Anaerobic propulsion was first employed by the Spanish Ictineo II in , 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 when the German Navy tested a hydrogen peroxide -based system, the Walter turbine , on the experimental V submarine and later on the naval U and type XVII submarines.
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 dived, 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.
Early submarines used a direct mechanical connection between the engine and propeller, switching between diesel engines for surface running, and battery-driven electric motors for submerged propulsion. Instead of driving the propeller directly while running on the surface, the submarine's diesel drove a generator that could either charge the submarine's batteries or drive the electric motor.
This made electric motor speed independent of diesel engine speed, so the diesel could run at an optimum and non-critical speed. One or more diesel engines could be shut down for maintenance while the submarine continued to run on the remaining engine or battery power. The first production submarines with this system were the Porpoise class of the s, and it was used on most subsequent US diesel submarines through the s.
No other navy adopted the system before , apart from the Royal Navy's U-class submarines , though some submarines of the Imperial Japanese Navy used separate diesel generators for low speed running. Other advantages of such an arrangement were that a submarine could travel slowly with the engines at full power to recharge the batteries quickly, reducing time on the surface or on snorkel. It was then possible to isolate the noisy diesel engines from the pressure hull, making the submarine quieter. Additionally, diesel-electric transmissions were more compact.
During World War II the Germans experimented with the idea of the schnorchel snorkel from captured Dutch submarines, but didn't see the need for them until rather late in the war. The schnorchel was a retractable pipe that supplied air to the diesel engines while submerged at periscope depth , allowing the boats to cruise and recharge their batteries while maintaining a degree of stealth.
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It was far from a perfect solution, however. 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. 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.
While the snorkel renders a submarine far less detectable, it is not perfect. In clear weather, diesel exhaust can be seen on the surface to a distance of about three miles,  while "periscope feather" the wave created by the snorkel or periscope moving through the water is visible from far off in calm sea conditions.
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Modern radar is also capable of detecting a snorkel in calm sea conditions. 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 use 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.
These batteries have about double the electric storage of traditional batteries, and by changing out the lead-acid batteries in their normal storage areas plus filling up the large hull space normally devoted to AIP engine and fuel tanks with many tons of lithium-ion batteries, modern submarines can actually return to a "pure" diesel-electric configuration yet have the added underwater range and power normally associated with AIP equipped submarines.
Initially they were to carry hydrogen peroxide for long-term, fast air-independent propulsion, but were ultimately built with very large batteries instead.