Unit 19. Bulbous bow

I. Master the active vocabulary:

bulbous – бульбообразный

waterline – ватерлиния

to modify – варьировать, видоизменять, модифицировать

drag – сопротивление

fuel efficiency – кпд топлива, топливная экономичность (двигателя)

to increase – увеличивать

freighter – грузовой корабль

to excite – вызывать

to flow up – подниматься

pressure distribution – распределение давления

trim – отделка

II. Read and translate the following text:

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.

The first bulbous bows appeared in the USA being fitted to the USS Delaware [BB28] which entered service in 1910 and the design is credited to David Watson Taylor, naval architect and Chief Constructor of the Navy.

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.

Fig. 22. The bulbous bow of the U.S. Navy carrier USS Ronald Reagan

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.

III. Answer the following questions:

1. What does the bulb modify?

2. Where do bulbous bows achieve maximum effect?

3. Where do bulbous bows have the greatest effect?

4. Does water flowing over the bulb depresses the ship’s bow and keep it trimmed better?

5. When did the first bulbous bows appear?

6. Who created the first bulbous bow?