A bulbous bow is a protruding bulb at the bow (or front) of a ship just below the waterline. The bulb modifies the way the water flows around the hull, reducing drag and thus increasing speed, range, fuel efficiency, and stability. Large ships with bulbous bows generally have a 12 to 15 percent better fuel efficiency than similar vessels without them.

Bulbous bows have been found to be most effective under the following conditions:

• when used on hulls with waterline lengths of more than about 15 m (50 ft)

• when used on long, narrow hulls

• when used at speeds close to the vessel's maximum speed

These points make them a standard feature for cargo ships, naval vessels and passenger ships, all of which are large, narrow and usually operate within a small range of speeds close to their top speed.
On the other hand, they are virtually unknown in recreational craft like yachts, especially sailing vessels, which operate in a wide range of speeds and even heel over.

How they work

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.

Development

The first bulbous bows appeared in the USA being fitted to the which entered service in 1910 and the design is credited to David W. Taylor, naval architect and Chief Constructor of the Navy [USA]. In the 1920s other nations experimented with bulbous bows with the introduction of the and , 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 (51.7 km/h).

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 coupled a bulbous bow with a radically redesigned hull shape and was able to achieve speeds in excess of 30 knots (56 km/h). 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 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 ships such as the battleship Yamato and light cruiser Ōyodo 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.

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 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.