Bulbous bow
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A bulbous bow is a feature of many modern ship hulls. It is characterized by a protruding bulb at the bow (or front) of the ship below the waterline. Due to this, a bulbous bow is usually only visible when a ship is in drydock. The presence of this bulb modifies how water flows around the hull, thereby reducing drag and affording an increase in speed, range, and fuel efficiency. In ships that have had bulbous bows fitted, gains in fuel efficiency of between 12-15% are standard. As these factors are particularly important for almost all applications of maritime vessels, bulbous bows have seen widespread adoption since their development.
Despite their advantages, bulbous bows only achieve maximum effect at a narrow range of speeds, and at speeds exclusively over 6 knots (Bray, website). At other speeds outside the range for which they were designed, bulbous bows can have the opposite effect and actually increase the drag. Finally, bulbous bows have the greatest effect when applied to large ships such as freighters, navy vessels and various passenger ships. Bulbous bows are rare on recreational boats as these vessels have wide speed ranges and are often designed to plane over the water at high speed.
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[edit] How they work
The fluid dynamics of bulbous bows can be calculated numerically and various non-technical explanations are given.
As a prerequisite it is stated that long waves are faster, therefore a ship which wants to go fast has to excite long waves and no short ones.
A common explanation concerns the wake produced by a ship. 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 greatly reduce the vessel's wake. While inducing another wave stream actually 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.
Another explanation focuses on the fact that water flowing over the bulb depresses the ship's bow and keeps it trimmed better, allowing the engines to be more efficient.
A sharp bow would also produce waves and low drag like a bulbous bow, but waves coming from the side would strike it harder. Also, when the bow goes up and down in heavy seas the water has to flow around the bulb, damping the movement 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 1920s 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.
