Bulbous bow

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 twelve to fifteen percent better fuel efficiency than similar vessels without them. A bulbous bow also increases the buoyancy of the forward part and hence reduces the pitching of the ship to a small degree.

Bulbous bows have been found to be most effective when used on vessels that meet the following conditions:

Thus, large vessels that cross large bodies of water near their best speed will benefit from a bulbous bow. This would include naval vessels, cargo ships, passenger ships, tankers and supertankers. All of these ships tend to be large and usually operate within a small range of speeds close to their top speed. Bulbous bows are less beneficial in smaller craft and may actually be detrimental to their performance and economy. Thus, they are rarely used on recreational craft like powerboats, sailing vessels, tug boats and yachts.

The effect of the bulbous bow can be explained using the concept of destructive interference of waves (see graphic):

A conventionally shaped bow (#2 lower left) causes bow wave forms (#4). A bulb alone (#1 top left) forces the water to flow up and over it forming a trough (#3). Thus, if a bulb is added to a conventional bow at the proper position (#1 right), the bulb trough coincides with the crest of the bow wave, and the two cancel out, reducing 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 water flow over the bulb.

...
Wikipedia