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Concept
Bulbous bow
Summary
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.
Vessels with high kinetic energy, which is proportional to mass and the square of the velocity, benefit from having a bulbous bow that is designed for their operating speed; this includes vessels with high mass (e.g. supertankers) or a high service speed (e.g. passenger ships, and cargo ships). Vessels of lower mass (less than 4,000 dwt) and those that operate at slower speeds (less than 12 kts) have a reduced benefit from bulbous bows, because of the eddies that occur in those cases; examples include tugboats, powerboats, sailing vessels, and small yachts.
Bulbous bows have been found to be most effective when used on vessels that meet the following conditions:
The waterline length is longer than about .
The bulb design is optimised for the vessel's operating speed.
The effect of the bulbous bow can be explained using the concept of destructive interference of waves:
A conventionally shaped bow causes a bow wave. A bulb alone forces the water to flow up and over it forming a trough. Thus, if a bulb is added to a conventional bow at the proper position, 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, cancelling 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.
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.
Official source
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