![Passenger ship Zaandam in drydock](https://upload.wikimedia.org/wikipedia/commons/9/92/Passenger_ship_Zaandam_in_drydock.jpg)
A bulbous bow is a protruding bulb at the
bow (or front) of a ship just below the
waterline
The waterline is the line where the hull of a ship meets the surface of the water. Specifically, it is also the name of a special marking, also known as an international load line, Plimsoll line and water line (positioned amidships), that indi ...
. The bulb modifies the way the water flows around the
hull, reducing
drag and thus increasing speed, range,
fuel efficiency
Fuel efficiency is a form of thermal efficiency, meaning the ratio of effort to result of a process that converts chemical potential energy contained in a carrier (fuel) into kinetic energy or work. Overall fuel efficiency may vary per device, wh ...
, and stability. Large ships with bulbous bows generally have twelve to fifteen percent better fuel efficiency than similar vessels without them. A bulbous bow also increases the buoyancy of the forward part and hence reduces the pitching of the ship to a small degree.
Vessels with high
kinetic energy
In physics, the kinetic energy of an object is the energy that it possesses due to its motion.
It is defined as the work needed to accelerate a body of a given mass from rest to its stated velocity. Having gained this energy during its accele ...
, which is proportional to mass and the square of the velocity, benefit from having a bulbous bow that is designed for their operating speed; this includes vessels with high mass (e.g.
supertanker
An oil tanker, also known as a petroleum tanker, is a ship designed for the bulk cargo, bulk transport of petroleum, oil or its products. There are two basic types of oil tankers: crude tankers and product tankers. Crude tankers move large quant ...
s) or a high service speed (e.g.
passenger ship
A passenger ship is a merchant ship whose primary function is to carry passengers on the sea. The category does not include cargo vessels which have accommodations for limited numbers of passengers, such as the ubiquitous twelve-passenger freig ...
s, and
cargo ship
A cargo ship or freighter is a merchant ship that carries cargo, goods, and materials from one port to another. Thousands of cargo carriers ply the world's seas and oceans each year, handling the bulk of international trade. Cargo ships are usu ...
s).
Vessels of lower mass (less than 4,000
dwt) and those that operate at slower speeds (less than 12
kts) have a reduced benefit from bulbous bows, because of the eddies that occur in those cases;
examples include tugboats, powerboats, sailing vessels, and small yachts.
Bulbous bows have been found to be most effective when used on vessels that meet the following conditions:
*The waterline length is longer than about .
*The bulb design is optimised for the vessel's operating speed.
Underlying principle
![Bulbous Bow Combined Wave Effects](https://upload.wikimedia.org/wikipedia/commons/d/df/Bulbous_Bow_Combined_Wave_Effects.png)
The effect of the bulbous bow can be explained using the concept of
destructive interference
In physics, interference is a phenomenon in which two waves combine by adding their displacement together at every single point in space and time, to form a resultant wave of greater, lower, or the same amplitude. Constructive and destructive ...
of waves:
A conventionally shaped bow causes a
bow wave
A bow wave is the wave that forms at the bow of a ship when it moves through the water. As the bow wave spreads out, it defines the outer limits of a ship's wake. A large bow wave slows the ship down, is a risk to smaller boats, and in a harbor ...
. A bulb alone forces the water to flow up and over it forming a trough. Thus, if a bulb is added to a conventional bow at the proper position, the bulb trough coincides with the crest of the bow wave, and the two cancel out, reducing the vessel's
wake. While inducing another wave stream saps energy from the ship, cancelling out the second wave stream at the bow changes the pressure distribution along the hull, thereby reducing wave resistance. The effect that pressure distribution has on a surface is known as the
form effect.
[
A sharp bow on a conventional hull form would produce waves and low drag like a bulbous bow, but waves coming from the side would strike it harder. The blunt bulbous bow also produces higher pressure in a large region in front, making the bow wave start earlier.][
The addition of a bulb to a ship's hull increases its overall wetted area. As wetted area increases, so does drag. At greater speeds and in larger vessels it is the bow wave that is the greatest force impeding the vessel's forward motion through the water. For a vessel that is small or spends a great deal of its time at a slow speed, the increase in drag will not be offset by the benefit in damping bow wave generation. As the wave counter effects are only significant at the vessel's higher range of speed, bulbous bows are not energy efficient when the vessel cruises outside of these ranges, specifically at lower speeds.][
Bulbous bows may be configured differently, according to the designed interaction between the bow wave and the countering wave from the bulb. Design parameters include a) upward curvature (a "ram" bulb) versus straight forward (a "faired-in" bulb), b) bulb position with respect to the waterline, and c) bulb volume.] Bulbous bows also decrease a ship's pitching motion, when they are ballasted, by increasing the mass at a distance removed from the ship's longitudinal centre of gravity.
Development
Towing tests of warships had demonstrated that a below-water ram shape reduced resistance through the water before 1900. The bulbous bow concept is credited to David W. Taylor
David Watson Taylor (March 4, 1864 – July 28, 1940) was a U.S. naval architect and an engineer of the United States Navy. He served during World War I as Chief Constructor of the Navy, and Chief of the Bureau of Construction and Repair. Ta ...
, a naval architect who served as Chief Constructor of the United States Navy
The United States Navy (USN) is the maritime service branch of the United States Armed Forces and one of the eight uniformed services of the United States. It is the largest and most powerful navy in the world, with the estimated tonnage ...
during the First World War
World War I (28 July 1914 11 November 1918), often abbreviated as WWI, was one of the deadliest global conflicts in history. Belligerents included much of Europe, the Russian Empire, the United States, and the Ottoman Empire, with fightin ...
and who used the concept (known as a bulbous forefoot) in his design of the , which entered service in 1910. The bow design did not initially enjoy wide acceptance, although it was used in the to great success after the two ships of that class which survived the Washington Naval Treaty
The Washington Naval Treaty, also known as the Five-Power Treaty, was a treaty signed during 1922 among the major Allies of World War I, which agreed to prevent an arms race by limiting naval construction. It was negotiated at the Washington Nav ...
were converted to aircraft carrier
An aircraft carrier is a warship that serves as a seagoing airbase, equipped with a full-length flight deck and facilities for carrying, arming, deploying, and recovering aircraft. Typically, it is the capital ship of a fleet, as it allows a ...
s. This lack of acceptance changed in the 1920s, with Germany's launching of and . They were referred to as Germany's North Atlantic greyhounds, two large commercial ocean liner
An ocean liner is a passenger ship primarily used as a form of transportation across seas or oceans. Ocean liners may also carry cargo or mail, and may sometimes be used for other purposes (such as for pleasure cruises or as hospital ships).
Ca ...
s that competed for the trans-Atlantic passenger trade. Both ships won the coveted Blue Riband
The Blue Riband () is an unofficial accolade given to the passenger liner crossing the Atlantic Ocean in regular service with the record highest average speed. The term was borrowed from horse racing and was not widely used until after 1910. T ...
, ''Bremen'' in 1929 with a crossing speed of , and ''Europa'' surpassing her in 1930 with a crossing speed of 27.91 knots.
The design began to be incorporated elsewhere, as seen in the U.S. built ''SS Malolo
SS ''Malolo'' (later known as ''Matsonia'', ''Atlantic'', and ''Queen Frederica'') was a passenger liner, later cruise ship, built by William Cramp & Sons, Philadelphia, in 1926 for the Matson Line. She was the first of a number of ships designed ...
'', ''SS President Hoover
SS ''President Hoover'' was an ocean liner built for the Dollar Steamship Lines. She was completed in 1930 and provided a trans-Pacific service between the US and the Far East. In 1937 she ran aground on an island off Formosa (now known as Taiw ...
'' and ''SS President Coolidge
SS ''President Coolidge'' was an American luxury ocean liner that was completed in 1931. She was operated by Dollar Steamship Lines until 1938, and then by American President Lines until 1941. She served as a troopship from December 1941 until ...
'' passenger liners launched in the late 1920s and early 1930s. Still, the idea was viewed as experimental by many shipbuilders and owners.
In 1935 the French superliner was designed by Vladimir Yurkevich combining a bulbous forefoot with massive size and a redesigned hull shape. She was able to achieve speeds in excess of 30 knots (56 km/h). ''Normandie'' was famous for many things, including her clean entry into the water and markedly reduced bow wave. ''Normandie''s great rival, the British liner , achieved equivalent speeds using traditional stem and hull design. However, a crucial difference was that ''Normandie'' achieved these speeds with approximately thirty per cent less engine power than ''Queen Mary'' and a corresponding reduction in fuel use.
Bulbous bow designs were also developed and used by the Imperial Japanese Navy
The Imperial Japanese Navy (IJN; Kyūjitai: Shinjitai: ' 'Navy of the Greater Japanese Empire', or ''Nippon Kaigun'', 'Japanese Navy') was the navy of the Empire of Japan from 1868 to 1945, when it was dissolved following Japan's surrender ...
. A modest bulbous bow was used in a number of their ship designs, including the light cruiser and the carriers and . A far more radical bulbous bow design solution was incorporated into their massively large , including , and the aircraft carrier .
The modern bulbous bow was developed by Dr. Takao Inui at the University of Tokyo
, abbreviated as or UTokyo, is a public research university located in Bunkyō, Tokyo, Japan. Established in 1877, the university was the first Imperial University and is currently a Top Type university of the Top Global University Project by ...
during the 1950s and 1960s, independently of Japanese naval research. Inui based his research on earlier findings by scientists made after Taylor discovered that ships fitted with a bulbous forefoot exhibited substantially lower drag characteristics than predicted. The bulbous bow concept was first definitively studied by Thomas Havelock, Cyril Wigley and Georg Weinblum, including Wigley's 1936 work "The Theory of the Bulbous Bow and its Practical Application" which examined the issues of wave production and damping. Inui's initial scientific papers on the effect of bulbous bow on wave-making resistance were collected into a report published by the University of Michigan
, mottoeng = "Arts, Knowledge, Truth"
, former_names = Catholepistemiad, or University of Michigania (1817–1821)
, budget = $10.3 billion (2021)
, endowment = $17 billion (2021)As o ...
in 1960. His work came to widespread attention with his paper "Wavemaking Resistance of Ships" published by the Society of Naval Architects and Marine Engineers
The Society of Naval Architects and Marine Engineers (SNAME) is a global professional society that provides a forum for the advancement of the engineering profession as applied to the marine field. Although it particularly names the naval archit ...
in 1962. It was eventually found that drag could be reduced by about five per cent. Experimentation and refinement slowly improved the geometry of bulbous bows, but they were not widely exploited until computer modelling techniques enabled researchers at the University of British Columbia
The University of British Columbia (UBC) is a public university, public research university with campuses near Vancouver and in Kelowna, British Columbia. Established in 1908, it is British Columbia's oldest university. The university ranks a ...
to increase their performance to a practical level in the 1980s.
Design considerations
Bulbous bows embody the following defining characteristics:
* Length-wise shape
* Cross-section
* Length of forward projection
* Position of the shape's axis (e.g. forward or upwards)
While the primary purpose of such bulbs is to reduce the power required to drive a vessel at its operating speed, their sea-keeping characteristics are also important. A ship's wave-making characteristics at its operating speed are reflected in its Froude number
In continuum mechanics, the Froude number (, after William Froude, ) is a dimensionless number defined as the ratio of the flow inertia to the external field (the latter in many applications simply due to gravity). The Froude number is based on t ...
.[In marine hydrodynamic applications, the Froude number is usually referenced with the notation and is defined as:
:
where is the relative flow velocity between the sea and ship, is in particular the acceleration due to gravity, and is the length of the ship at the water line level, or in some notations.
]
It is an important parameter with respect to the ship's drag, or resistance, especially in terms of wave making resistance. A ship designer can compare the length at the water line for a design with and without a bulb necessary to power the vessel at its operating speed. The higher the speed, the bigger the benefit of the bulbous bow in diminishing the necessity for a longer water line to achieve the same power requirement. Bulbs typically are v-shaped on the bottom to minimise slamming in rough seas.
Sonar domes
Some warships
A warship or combatant ship is a naval ship that is built and primarily intended for naval warfare. Usually they belong to the armed forces of a state. As well as being armed, warships are designed to withstand damage and are usually faster and ...
specialised for anti-submarine warfare
Anti-submarine warfare (ASW, or in older form A/S) is a branch of underwater warfare that uses surface warships, aircraft, submarines, or other platforms, to find, track, and deter, damage, or destroy enemy submarines. Such operations are t ...
use a specifically shaped bulb as a hydrodynamic housing for a sonar
Sonar (sound navigation and ranging or sonic navigation and ranging) is a technique that uses sound propagation (usually underwater, as in submarine navigation) to navigation, navigate, measure distances (ranging), communicate with or detect o ...
transducer
A transducer is a device that converts energy from one form to another. Usually a transducer converts a signal in one form of energy to a signal in another.
Transducers are often employed at the boundaries of automation, measurement, and contr ...
, which resembles a bulbous bow but the hydrodynamic effects are only incidental. The transducer is a large cylinder or sphere composed of a phased array
In antenna theory, a phased array usually means an electronically scanned array, a computer-controlled array of antennas which creates a beam of radio waves that can be electronically steered to point in different directions without moving th ...
of acoustic transducers. The entire compartment is flooded with water and the acoustic window of the bulb is made of fiber-reinforced plastic
Fibre-reinforced plastic (FRP; also called fibre-reinforced polymer, or in American English ''fiber'') is a composite material made of a polymer matrix reinforced with fibres. The fibres are usually glass fibre, glass (in fibreglass), Carbon fib ...
or another material (such as rubber
Rubber, also called India rubber, latex, Amazonian rubber, ''caucho'', or ''caoutchouc'', as initially produced, consists of polymers of the organic compound isoprene, with minor impurities of other organic compounds. Thailand, Malaysia, and ...
) transparent to underwater sounds as they are transmitted and received. The transducer bulb places the sonar equipment at the greatest possible distance from the ship's own noise-generating propulsion system.
Notes
References
{{DEFAULTSORT:Bulbous Bow
Watercraft components
Fluid dynamics
Nautical terminology
Interference
de:Bug (Schiffbau)#Wulstbug