BULBOUS BOW HULL DESIGN - 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. These forward protrusions are about twice as long as the width of the base and they would usually not extend forward past the top of the bow. The basic principle is to create a low-pressure zone to eliminate the bow wave and reduce drag. Hulls built with bulbous bow sections are common today. Under certain conditions, this type of design is very efficient at redirecting forces of hydrodynamic resistance and drag. There is a movement against bulbous bows that allows greater flexibility of ships at a time when "slow steaming" is a way to save fuel.

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Bulbous Bow Hull Design

What Is a Bulbous Bow?

by Paul Bruno 

The strongest resistance any vessel faces in standard operations comes from displacement as the hull moves through the 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.

These forward protrusions are about twice as long as the width of the base and they would usually not extend forward past the top of the bow.

The basic principle is to create a low-pressure zone to eliminate the bow wave and reduce drag.

First appearing on the USS Delaware in 1910, the bulbous bow was a controversial design of U.S. Navy Ship Architect David W. Taylor.

Much of the controversy disappeared ten years later when passenger ships began exploiting the design to increase speeds.

Hulls built with bulbous bow sections are common today.

Under certain conditions, this type of design is very efficient at redirecting forces of hydrodynamic resistance and drag.

There is a movement against bulbous bows that allows greater flexibility of ships at a time when "slow steaming" is a way to save fuel.

Good Conditions for Bulbous Bows

The design of a ship with a bulbous bow is discussed in many textbooks and technical articles.

It is often referred to as a theory or an art, which is a short way of saying nobody is 100 percent sure of what they are writing.

There are details to be worked out but modern builders have proprietary ways of analyzing and integrating all hydrodynamic aspects of their hulls and these methods are strict secrets.

A bulbous bow works best under certain conditions and good design gives efficiency gains throughout the range of these factors.

Speed – At low speeds, a bulbous bow will trap water above the bulb without forming a low-pressure zone to cancel the bow wave. This leads to increased drag and loss of efficiency.

Each design has what is known as most efficient hull speed, or often just hull speed. This term refers to the speed where the shape of the hull is acting on the water is such a way to produce the minimum possible drag.

This ideal hull speed might not be the top speed of a ship because at some point the lower pressure zone created by the bow features becomes larger than necessary.

A zone of lower pressure water that is larger than the hull is inefficient and leads to reduced rudder response.

Ideally, the cone of lower pressure water will collapse just before the props. This gives the prop blades something to push against and limits cavitation at the props and rudder.

Cavitation will lead to reduced efficiency of props, sluggish steering, and excessive wear of hull and drive components.

Size – Vessels under 49 feet (15 m) do not have enough wetted area to take advantage of a bulbous bow.

The amount of drag on a hull is related to its wetted area. The structure of the bulb also increases drag and at a certain point, the benefits shrink to zero.

Conversely, larger ships with a high proportion of waterline to frontal area use the bulbous bow most effectively.

Bad Conditions for Bulbous Bows

Rough Seas – While a traditional hull rises with the wave, a hull with a bulbous bow can dig in even if it is designed to lift the bow under normal conditions.

The issue of trim is one of the most deeply dividing aspects of bow design among naval architects.

There is also a huge psychological aspect among crews who perceive this bow design as dangerous in storms.

There is some truth that these bows dig into wave faces but there is little proof that it is more dangerous than traditional designs.

Ice – Some ice breaking ships do have a special shape of a bulbous bow that is heavily reinforced. Most bulbous bows are prone to damage since they are the first point of contact with an obstacle.

In addition to ice, large debris and fixed objects like dock faces can damage these extended underwater bows.

Paul Bruno

United States Coast Guard licensed Ship Master with Passenger Certification.

A marine contractor familiar with all ship systems who has worked in the industry for more than 20 years.

Has a USCG Master's License and a degree in Creative Nonfiction and Technical Writing, from the University of Wisconsin.

Experience

Paul Bruno is a former writer for ThoughtCo who contributed work for over five years on the maritime industry. He has worked with a variety of vessels throughout his maritime career, including various commercial ships as well as the Great Lakes schooner reproduction "Dennis Sullivan." As a marine contractor, Paul is familiar with all ship systems, and his lifelong passion for electronics has led to a focus on marine hardware and software development. When he is not working, Paul's favorite hobby is knot work, keeping busy with the nearly 4,000 documented knots, splices, and eyes.

Education

USCG Master's License, 1993

Creative Nonfiction and Technical Writing, University of Wisconsin, Milwaukee and University of Waikato, Hamilton NZ

Paul Bruno

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https://www.thoughtco.com/what-is-a-bulbous-bow-2293301