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Thursday, June 5, 2014

bulb ~ Onion ~ bulbous bow - something on a Marine term...

In the intersection of Triplicane High road and Bharathiar Salai (Pycrofts Road) is ‘Ratna Café’ – people thronging for ‘sambar idly’ – a delicacy of idlies with buckets of sambar made of onion.

The onion (Allium cepa) (Latin 'cepa' = onion), also known as the bulb onion or common onion, is used as a vegetable and is the most widely cultivated species of the genus Allium. The onion is most frequently a biennial or a perennial plant, but is usually treated as an annual and harvested in its first growing season. In botany, a bulb is a short stem with fleshy leaves or leaf bases that function as food storage organs during dormancy. A bulb's leaf bases, also known as scales, generally do not support leaves, but contain food reserves to enable the plant to survive adverse conditions.  Actually, it is not this bulb onion, which is used to make tasteful sambar…. ..it is the smaller ones – ‘the shallot’ (Allium cepa var. aggregatum).

I had recently posted about electric bulbs – the old gundu bulb (the 40W/60W – zero W and more) – CFL – and LED…… an incandescent light bulb, is an electric light which produces light with a wire filament heated to a high temperature by an electric current passing through it, until it glows.  The hot filament is protected from oxidation with a glass or quartz bulb that is filled with inert gas or evacuated.

Bulb, abbreviated B, is a shutter speed setting on an adjustable camera that allows for long exposure times under the direct control of the photographer. With this setting, the shutter simply stays open as long as the shutter release button remains depressed.

~ Away from all these, the subject-matter of this post is ‘bulbous bow’ – something connected to Marine Hull – that of a Ship.  

The strongest resistance any vessel faces in standard operations comes from displacement as the hull moves through water. Waves that climb the bow are water being pushed aside faster than it can move away. It takes a lot of power to overcome the viscosity and mass of water and that means burning fuel which adds to costs. A bulbous bow is an extension of the hull just below the waterline. It has many subtle shape variations but it’s basically a rounded front portion that flares out slightly as it blends into the traditional displacement hull construction.  In principle, it is  to create a low pressure zone to eliminate the bow wave and reduce drag.

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 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.  It is stated that 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, fishing trawlers and yachts.

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 the bulb coincides with the bow wave, the two partially cancel out and reduce the vessel's wake.

With regards – S. Sampathkumar
29th May 2014.

PS: the description of bulbous bow is taken from various sources – primarily : http://en.wikipedia.org/wiki/Bulbous_bow &  http://maritime.about.com/od/shipbuilding/a/What-Is-A-Bulbous-Bow.htm



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