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Thursday, January 19, 2017

Bulbous bow

Posted to Maritime Musings (by on November 6, 2009

Sometimes efficiency takes a long time to catch on

A bulbous bow is a protruding bulb at the bow of a ship located just below the normal waterline. As the ship moves through the water, the bulbous bow modifies the flow of water around the hull, thereby reducing drag and increasing speed, range, and stability. Large ships (those with lengths of about 15 meters or more) with bulbous bows generally experience a 12% or greater fuel efficiency compared to similar vessels without bulbous bows. Fuel efficiency tends to increase with speed. Modeled somewhat on the ramming bows of ancient Greek triremes, the first modern bulbous bow was on the USS Delaware launched in 1910. A few other ships followed suit, but it was clearly viewed as experimental. Engineering research on the design was conducted in the 1950s, but the concept was not widely accepted until computer modeling in the 1980s proved the value of the additional work and steelweight. Now, only the smallest of commercial vessels are constructed without a bulbous bow. 
As a ship with a conventionally-shaped bow moves through the water, a bow wave forms immediately in front of the bow and a stern wave is generated at the stern of the vessel. The ship consumes energy in creation of these waves. If the ship has a bulbous bow and is moving at speed, the trough formed by the bulb coincides with and partially cancels out the usual bow wave. Cancellation of the wave stream changes the pressure distribution along the hull and reduces wave resistance. Evidence indicates that a bulbous bow can also improve the trim and stability of the ship at speed. Thus, there are few downsides to including a bulbous bow on a moderately large commercial vessel. Maybe those ancient Greeks were even smarter than we assumed.


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