Bulbous bow
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 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). 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 as these vessels have wide speed ranges are often designed to plane over the water at high speed.
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.
Development
Bulbous bows are thought to have been first 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.
Additional benefits
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.
References
- Bray, Patrick J. (no date). The Bulbous Bow - What is it?. Retrieved April 1, 2005.