Teardrop hull

Because the term refers not to any exact shape, but rather to the abstract concept of a hull shape optimized for underwater travel, and more specifically the physical manifestation of this idea in actual submarines as limited by material sciences and the design requirements placed upon the vessel, whether or not any specific vessel exhibits a teardrop hull depends on the subjective interpretation of the term itself. Depending on this interpretation, several of the submarines shown below may be considered to have been a "first attempt" or a "milestone" of some description.

• A collection of submarines arranged in roughly chronological order to show the progression of submarine hull design.
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  This replica of the Ictineo II of the mid-to-late 1860s may represent one of the earliest attempts at a hull shape optimized for underwater travel.
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  A model of one of John Phillip Holland's designs from the late 19th or early 20th century, showing a very clean hull and relatively modern rudder arrangement.
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  Less than 10 years after HMS Holland 1 entered service, the british B-class submarine shows the transition in hull form from submarine to diving surface vessel. The bow is slightly raised, as is the stern above the two propellers.
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  Entering service in 1917, the steam-powered British K-class may be seen as the peak of surface-optimized submarines in WWI. They achieved 24 knots and in an emergency could submerge in just shy of five minutes, although a half hour was a more reasonable timeframe in normal operation. 103m long, and with an estimated maximum diving depth of 61m, they were 4m short of being able to reach their crush depth without leaving the surface. Combined with an extreme turning radius, poor submerged depth control, and a habit of shipping water through the boiler funnels, it is no wonder that they gained the reputation of "K for Kalamity", with several boats being lost to flooding of the boiler rooms, collisions, reaching crush depth, and one being recorded as having sunk itself, without human assistance, at its mooring in the middle of the night.
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  The world's first hunter-killers, the British R-class submarine of 1918 was a drastic return to the concept of the high-speed undersea warship. Made to hunt enemy submarines, they featured extensive streamlining, a virtually nonexistent casing, and the ability to make 14 knots for a full hour. Only the knife-like bow shape betrays the expectation of the designer that these boats would not operate continuously submerged, only breaking the surface to charge their batteries. They were instead to transit slowly on the surface using their diesel engine, only submerging once they reached their patrol areas.
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  Unrelated to the later German Elektroboot program, the Japanese Submarine no. 71 was launched in 1937 and achieved just over 21 knots submerged, a feat only exceeded by japan's smaller Ko-hyoteki midget submarines at the time, until the German V-80, powered by hydrogen peroxide, achieved 28 knots during testing 1940-1941.
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  Universally optimized for surface cruising, most submarines used in the second world war featured high, somewhat flared bows, as well as propeller and rudder arrangements similar to that of a contemporary cruiser, albeit with an extra set of rudders for depth and pitch control. Here a US Navy Balao-class Fleet Submarine from the early 1940s converted to a museum.
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  German boats, such as this Type VII, featured an overall appearance similar to the Balao, while being substantially smaller (~750 tonnes as opposed to ~1500). The model clearly shows the submarine's pressure hull, its forward and aft ends being enclosed in the lighter casing which contain the boat's forward and aft main ballast tanks. The long cigar-shaped bulge along the submarine's upper side is an external fuel tank. Entering service in 1936-37, the Type VII and the larger Type IX both followed the same "diving torpedo boat" concept that had also spawned the American and British fleet boats and patrol submarines, and facing increasingly effective allied patrol vessels and aircraft, they were operationally obsolete by mid-1941 to 1942, yet remained the mainstay of the increasingly desperate German submarine force throughout the war.
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  As steady improvements to allied anti-submarine warfare resulted in rapidly increasing German losses throughout the second half of WWII, attempts were made to build submarines with hydrogen peroxide turbine propulsion. While the turbine was far from ready to enter service, the submarines were shoehorned into production as regular diesel-electric boats with vastly increased battery capacity replacing the peroxide tanks. Although heavily streamlined, the hull of the late-war German Type XXI (21) remained influenced by previous designs, clearly being expected to operate on the surface in a limited capacity. Even so, it set a new standard for underwater speed, range, and detectability for submarines in the post-war world.
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  Due to neglect of the merchant navy in favour of increased numbers of large surface combatants, in part fueled by an American tendency to downplay the successes of their submarines and overstate the effectiveness of Japanese ASW efforts, the American "Silent Service" faced a comparatively tame Japanese resistance, resulting in great successes against the Japanese merchant fleet. Because of this, American fleet boats saw no serious improvement in overall design throughout the war, with emphasis instead being focused on technical details such as improvements to pressure hull strength and electronics eventually including quieter electric motors, as well as improvements to crew comfort for long patrols in the pacific. Thus, American boats such as this Tench-class left the war looking almost indistinguishable from the classes which were in service at its start.
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  Entering production in 1950, the Soviet Project 613 patrol submarines were heavily influenced by the Type XXI. The deck gun and anti-aircraft autocannon turret were absent on later versions. Although heavily streamlined, like the Type XXI its design made concessions to surfaced operation such as the sharp knife-like bow and stern.
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  A scale model of the USS Albacore undergoing wind-tunnel testing. Apart from the rudder arrangements at its stern and the sail, its general shape is similar to that of the Ictineo II despite the designs being separated by nearly a century.
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  The world's first nuclear submarine, the USS Nautilus (SSN-571) of 1954, shows a very clean stern section, while the bow is clearly to some extent inspired by the Type XXI. Her test depth of 210m was comparable to the German boat, but at 23 knots submerged she was faster by far, even faster than any previous US fleet boat had been on the surface. Unfortunately, serious design flaws in her bow and sail resulted in extreme vibrations at high speed, and worsened over time. At the end of her life 4 knots was enough to deafen her sonar, and the noise drastically increased the risk of detection.
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  Commissioned on the 17th of January 1959, the Soviet Union's first SSN, of the Project 627-class, reached 30 knots with a test depth of 300m. As different from the Nautilus as was possible while still being both a submarine, nuclear powered, and useful as combat vessels, they featured a far more efficient bow and sail shape while still showing the fish-tail of the Type XXI, and where the Nautilus used a nuclear reactor, the Soviet boats mounted two in tandem. Apart from improved hydrodynamics, the rounded bow shape also provided plenty of room for a very large sonar array above the torpedo tubes.
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  Commissioned less than three months after the first Project 627, the USS Skipjack was the first combat vessel in the US Navy to use a "pure" teardrop hull since it was abandoned almost half a century earlier. Because of the high cost of forging large sections of pressure hull in complex shapes, following boats have incrementally "strayed" from this design.
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  One of several new Soviet submarine classes entering service in 1967, the Project 671 was among the first mainline Soviet submarine classes to replace the fish-tail with the more torpedo-like shape that has become as a sign of emphasis on underwater performance.
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  As the Project 611 (Zulu) and Project 641 (Foxtrot) approached obsolescence, the early 1970s saw the USSR beginning construction of a series of 18 Project 641b -class boats. These inherited the three-screw knife-tail arrangement that had been carried down through the 641 from the 611, and which originated in the Type XXI. In stark contrast to their outdated propulsion layout, they were fitted with anechoic tiles and sonar equipment similar to that used by contemporary Soviet nuclear attack submarines.
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  This German Type 212 submarine uses a single hull along part of its length, but is double-hulled throughout its rear half to provide safe stowage space outside the pressure hull for the hydrogen tanks used by its fuel cell-based air-independent-propulsion.
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  On the way to her launch in the late 2000s or 2010s, this Astute-class vessel clearly shows the compromise made by most modern western designs. Instead of completely optimizing the hull for submerged performance, hydrodynamically efficient bow and stern sections made from thinner and more easily shaped steel are welded to the front and rear ends of a constant-diameter pressure hull. Such designs are almost inherently modular as new hull sections can comparatively cheaply and easily be inserted anywhere along the pressure hull's length to provide additional space and weight capacity without seriously impacting the hydrodynamic performance of the design.

Although the overarching theme of the teardrop hull is that of maximizing submerged performance at the cost of surfaced performance, the exact outcome of this process depends on several factors, in no specific order as the order of importance of these concerns is in itself variable:

• The size of the submarine, being a function of the requirements placed upon by the designer in terms of operational endurance, weapons capacity and fitted sensors.
• The arrangement of external and internal equipment.
• Economic factors, primarily related to the expensive process of designing, shaping, and assembling hull sections with 3D curves, and then fitting machinery inside.

On the first of August 1953, the US launched an experimental vessel, Albacore, which featured a hull shape largely based on the "Lyon Shape" named for Hilda Lyon.[1][2] Following successful maneuvering trials and tests of various rudder and propeller arrangements, the same overall hull shape was used for the Skipjack- and Barbel-classes, and most modern US submarines today use a variation of that shape with a central extended cylinder constituting the main pressure hull. Because of the large amount of attention paid to the Albacore in the time before the dawn of the internet made information on the subject of submarine history more accessible to the general public, it is likely that any design which at all resembles it may be referred to as an "Albacore hull", regardless of whether it was inspired by Albacore, was developed independently at a similar moment in time, or predated Albacore outright.