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. 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 principal 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.
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 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.