Bulbous bow

Nov 06, 2009, 7:00AM EST
Bulbous bow
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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Steve Toby
Oddly enough, while the author has the second paragraph (physics of bulbous bows) down cold, with an admirably short explanation too, he's created a confused and often incorrect account of the history behind the device. In fact, Admiral Taylor discusses bulbous bows in the 1933 edition of "Speed and Power of Ships," attributing the detailed experimentation to Capt. E. F. Eggert, CO of the Experimental Model Basin at the Washington Navy Yard in 1921. In "Speed and Power", Taylor provides design charts (pages 62 and 63 of the 1998 SNAME reprint of the 1933 "Speed & Power"), that can be used by the hull designer to develop a bulbous bow appropriate to the ship's dimensions and design speed.

Easier to use design methods are described in Saunders's "Hydrodynamics in Ship Design" (1957). These methods, if correctly used, will result in the "Taylor bulb" which graced the bow of every US Navy cruiser, battleship, and aircraft carrier built between 1935 and 1950 except for the ERIE class gunboats of 1936. This bulb does not extend forward of the fore perpendicular (FP), but includes the blunt, rounded end that creates the extra bow wave crest whose trough cancels the normal ship's bow wave.

The theory of bulbous bows, while it was described by Taylor, was quantified by Havelock and Wigley between 1928 and 1935, using potential flow calculations supported by the tests of simplified models in a towing tank. (This is described in the 1967 issue of SNAME's PNA, pp. 358-9.)

Then in 1979, the Berlin testing tank issued Dr-Ing. Alfred Kracht's paper, "Versuchsanstalt fur Wasserbau und Shiffbau Berlin Bericht No. 811/78", giving design charts for more "modern" bulbous bows with projection further forward of the fore perpendicular. I've used these charts many times. They provided the basis for the bulbous bow used on AOE 6 (USS SUPPLY) when I did her hull design in the 1980's; the lead ship of the class was commissioned in 1994.

Computational Fluid Dynamics (CFD) has indeed changed the methods we use to design bulbous bows, allowing more shapes to be explored, as Mr. Bryant says in paragraph 1. However, bulbous bows were certainly state of the art before World War II; they were not "considered experimental" even in 1940, certainly not in 1950.

In recent evolution, European designers have created the "gooseneck bulb" that has its tip quite far ahead of the ship and almost at the waterline (both Kracht and Taylor warn the designer not to do this) and recent tank tests suggest this is the geometry that results in the best efficiency. The body of the bulb, joining the tip to the hull, is tilted down as you look aft in this type of bulb. In the US, a more horizontal bulb of elliptical section is more often favored. While this shape does not produce as large a cancellation of wave drag at design draft, it is at least somewhat effective over a broader range of drafts. Both of these shapes were refined using CFD.

I hope I have clarified this element in maritime history. The bulbous bow, like the ship herself, is the product of a long, rich tradition to which many famous hydrodynamicists have contributed. While it was certainly enhanced by the advent of CFD, as Mr. Bryant says, functioning bulbous bows were doing good service on naval and commercial ships for at least 40 years before CFD was introduced.
11/11/2009 10:57:11 AM
Dennis Bryant
Many thanks for your erudite comments. I find the bulbous bow to be an interesting development, even though I do not fully understand its history.
11/15/2009 1:43:03 PM
Daryl Wilkes
Another facinating article & reply! Thankyou both gentlemen.
1/28/2010 9:20:46 AM
Pradeep Kumar
Mr. Steve Toby thanks for the comments. Very interesting. Nowadays different shapes of bulbous bow designs have come up , each giving its superiority. I wonder how to decide which is the best shape to adopt.
2/1/2010 10:06:19 PM
Michel Lavergne
Well ...
bulbous bows were built back in 1911 on the USS delaware (1911), while it was used with success on the design of the SS Normandie liner (1935), based on the consulting work of Vladimir Yourkevich, a former naval designer of the Imperial russian navy, in charge of the hull design of the Borodino super dreadnought class battleships. Later on it was used by the Japanese imperial navy on the design of the Yamato/Musashi class battleships and theorized by Dr. Takao Inui. So go back to the beginning of the past century, take a global view, not only US centric, and you'll get an accurate understanding of the topic and contributors.
8/4/2011 5:44:38 PM