Bulbous bow
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A bulbous bow, a feature of many modern ship hulls, is a protruding bulb at the bow (or front) below the waterline. Usually visible only when a ship is in drydock, the bulb modifies how water flows around the hull, reducing drag and increasing speed, range, and fuel efficiency. Ships with bulbous bows generally have 12 to 15 percent better fuel efficiency than similar vessels without them.
Bulbous bows achieve maximum effect at a narrow range of speeds over 6 knots (Bray, website). At other speeds, they can increase drag. They have the greatest effect on large ships such as freighters, navy vessels and various passenger ships. They are rarer on recreational boats designed for wide speed ranges and planing over the water.
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[edit] How they work
The fluid dynamics of bulbous bows can be calculated.
Long waves are faster, so a ship that wants to go fast has to excite long waves and not short ones. In a conventionally shaped bow, a bow wave forms immediately before the bow. When a bulb is placed below the water ahead of this wave, water is forced to flow up over the bulb. If the trough formed by water flowing off of the bulb coincides with the bow wave, the two partially cancel out and reduce the vessel's wake. While inducing another wave stream saps energy from the ship, canceling 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.
Some explanations note that water flowing over the bulb depresses the ship's bow and keeps it trimmed better. Since many of the bulbous bows are symmetrical or even angled upwards which would tend to raise the bow further, the improved trim is likely a by product of the reduced wave action as the vessel approaches hull speed, rather than direct action of waterflow over the bulb.
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. Also, in heavy seas, water flowing around the bulb dampens pitching movements like a squiggle keel. The blunt bulbous bow also produces higher pressure in a large region in front, making the bow wave start earlier.
[edit] Development
The first bulbous bows appeared in the USA being fitted to the USS Delaware which entered service in 1910 and the design is credited to David Watson Taylor, naval architect and Chief Constructor of the Navy [USA]. In the 1920s other nations experimented with bulbous bows with the introduction of the Bremen and Europa, two German North Atlantic ocean liners. Bremen, which appeared in 1929, was able to win the coveted Blue Riband of the Atlantic with a speed of 27.9 knots.
Smaller passenger liners such as the American President Hoover and President Coolidge of 1931 began to appear with bulbous bows although they were still viewed by many ship owners and builders as experimental.
In 1935 the French superliner Normandie coupled a bulbous bow with a radically redesigned hull shape and was able to achieve speeds in excess of 30 knots. At the time Normandie was famous for (among other things) her clean entry into the water and her greatly reduced bow wave. Normandie's great rival, the British liner Queen Mary achieved equivalent speeds with a non-bulbous traditional stem and hull design. However, the crucial difference lay in the fact that Normandie achieved these speeds with approximately thirty percent less engine horsepower than Queen Mary—and with a corresponding reduction in fuel use.
Bulbous bows were further developed and used by the Japanese. Some World War II-era Japanese battleships such as the Yamato were fitted with bulbous bows. However, Japanese research into this area did not spread to the western world, and much of the advances were lost post-war.
It is unclear when bulbous bows were conclusively first examined by western researchers, but scientific papers on the subject were first published in the 1950s. Engineers began experimenting with bulbous bows after discovering that ships fitted with a ram bow were exhibiting substantially lower drag characteristics than predicted, and eventually found that they could reduce drag by about 5%. 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 to increase their performance to a practical level in the 1980s.
[edit] Sonar domes
Some warships specialized for anti-submarine warfare use a specifically shaped bulb as a hydrodynamic housing for a sonar transducer, which resembles a bulbous bow but has only incidental hydrodynamic purpose. The transducer is a large cylinder or sphere composed of a phased array of ultrasonic acoustic transducers. The entire compartment is flooded with water and the acoustic window of the bulb is made of fiber-reinforced plastic or another material (such as rubber) transparent to the transmitted and received underwater sounds.
[edit] References
- Bray, Patrick J. (no date). The Bulbous Bow - What is it?. Retrieved April 1, 2005.
