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Bulbous bow

From Biocrawler, the free encyclopedia.

The bulbous bow of the U.S. Navy carrier USS Ronald Reagan is clearly visible in this photograph.

A bulbous bow is a feature of many modern ship hulls. It is characterized by a protuding bulb at the bow (or front) of the ship below the waterline. Due to this fact, a bulbous bow can only be seen while 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 (http://www.dieselduck.ca/library/articles/bulbous_bows.htm)). 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, cruise ships, and navy vessels. Bulbous bows are rare on recreational vehicles; they have wide speed ranges and many recreational boats are designed to plane over the water at high speed.

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How they work

Though the fluid dynamics of bulbous bows are not completely understood, there are theories as to why they have such a pronounced effect. 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 reduces the pressure distribution along the hull, thereby reducing frictional 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 ships bow and keeps it trimmed better, allowing the engines to be more efficient. However, neither of these explanations accounts for the fact that vessels with bulbous bows handle better in large seas.

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Development

It is unclear when bulbous bows were conclusively first examined, but research into the effects of bulbous bows was 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 research conducted at the University of British Columbia in the 1980s increased their performance to a practical level.

Bulbous bows were also developed and used by the Japanese before World War II. One example of this was on the battleship Yamato. This research does not seem to have been connected with western research in any way.

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Additional benefit

Some warships specialized for anti–submarine warfare use a specifically shaped bulb as a housing for a forward–looking sonar transducer. The transducer is a large cylinder composed of an phased array of ultrasonic acoustic transducers. The entire compartment is flooded with water and the steel bulb is transparent to the transmitted and received underwater sounds.

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