Bulbous Bow

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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 when used in vessels that meet the following conditions: the waterline length is longer than about 15 metres (49 ft) and the vessel will operate with most of its time at or near its maximum speed. 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 virtually unknown in recreational craft like powerboats, sailing vessels, tug boats, fishing trawlers and yachts. n 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 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 water flow 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. The addition of a bulb to a ship's hull increases its overall wetted area. The problem is that as wetted area increases, so does drag. At greater speeds and in larger vessels it is the bow wave that is the greatest force impeding the vessels forward motion through the water. Thus the beneficial effects of Bulbous bows have a range at which they are seen, generally the upper end of performance for the vessel. For a vessel that is small or spends a great deal of its time at a slow speed, the increase in drag will not offset the benefit in dampening bow wave generation. As the wave counter effects are only significant at the vessel's higher range of speed, bulbous bows are not energy efficient when the vessel cruises outside of these ranges, specifically at lower speeds. 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 the hydrodynamic effects are only incidental. [READ THE REST OF THIS ARTICLE]







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