Why Do Ships Have A Bulbous Bow?

A bulbous bow is a large, round-shaped protrusion fitted at the forward end of the ship’s hull. It projects ahead of the stem and usually stays below the waterline.

One thing that often catches the eye of a common man regarding most of the ships is the bulb-like projection at the forward end of the ship, often below the waterline.

Since it generally resembles the shape of a bulb and is always placed at the bow of the ship, it is known as a ‘bulbous bow’.

Let’s look back to about a hundred years from now. Remember Titanic? You must have observed it didn’t have a bulbous bow. But try having a look at the bows of modern cruise ships, container ships, LNG carriers, research vessels, etc.

All of them are characterised by a bulbous bow. Not only monohull ships; today, almost even catamarans are equipped with a bulbous bow rather than straight bows. Why?

Why Are Bulbous Bows Used?

When a ship surges, it generates its own Kelvin waves (the ones you see around a ship when it sails in the open sea), as shown in the figure.

Now visualise it this way: the waves are basically travelling forms of energy in a water medium. Where did this energy come from? In other words, who energised the water particles to form these waves?

The moving mass of the ship does this job. Note the word “moving”. The ship’s movement is powered by its propulsion system. A part of the energy delivered by the engine goes into rotating the propeller, and in turn, a fraction of that thrust generated by the propeller comes in handy in actually propelling the ship. Where does the rest of the energy go? Remember, water particles were energised to transmit waves? That’s your answer. This phenomenon is also called wave-making resistance of a ship.

Consider a ship with a straight bow. As the ship surges forward, the water particles move towards the stern along the entire length of the ship. But what about the water particle incident right at the centreline of the stem? Its instantaneous velocity is zero, which, in scientific terms, is known as a ‘stagnation point’.

If you recall Bernoulli’s Equation, the pressure at a stagnation point will be higher. So the pressure of the water particles at the bow is higher, thus giving rise to the crest of a wave.

This wave is called the bow wave because it is generated by the movement of the bow through the water. So, with a straight bow, a wave is always continuously formed, with its crest at the bow.

Thus, it is evident that we waste some engine power generating this wave. What if we could reduce this wave-making effect?

If we introduce another discontinuity (any structure in the ship below the waterline that disturbs the laminar flow is regarded as a discontinuity) below the waterline at the bow, in front of the ship’s stem, the discontinuity will itself give rise to another wave at its foremost point. Since the stem is still at the waterline, it will generate normal bow waves.

What if we design the shape and position of the discontinuity so that the bow wave and the wave created by the discontinuity result in destructive interference?

That is essentially much the same principle behind the design of a bulbous bow. The destructive interference results in reduced wave-making by the ship, which further reduces the wave-making drag of the hull form.

In the preliminary stages of bulb development, the primary mission was to reduce wave-making drag. But as we moved on, we couldn’t stop delving into the more interesting aspects discussed below:

Wave-making is a significant characteristic of finer hull forms. That is why you notice prominent Kelvin waveforms in cruise ships, liners, yachts, and naval cruisers. If you look at a bulk carrier or an oil tanker (fuller hull form), you’ll notice these hull forms do not show prominent Kelvin wave patterns.

This is because the waterline width at the stem itself is so large (or, in other words, the discontinuity inflow is higher) that the pressure rises to a level such that the bow wave height exceeds the threshold up to which a wave holds its properties. In this case, the wave breaks right at the bow itself even before it travels along the ship’s length.

So, are fuller hull forms more energy efficient in this respect? No. Do fuller hull forms have high wave-making resistance? No. Do fuller hull forms have high wave-breaking resistance? Yes. With this application, bulbs were also introduced in bulkers and tankers to reduce their wave-making resistance.

Advantage And Disadvantage Of Bulbous Bows

The different types of bulbs, according to their shapes, positions and orientations, are as shown below:

The bulb position significantly affects the phase difference between the bow wave and the bulb wave. The volume of the bulb is a deciding factor in the amplitude of the resultant wave.

The advantage of the bulb is that it reduces the dynamic effects of a ship’s pitch motion. In most ships, the bulb’s interior is used as a fore-peak ballast tank. In high-pitching conditions, the forepeak tank is often ballasted to reduce the effect of pitching.

The time of pitching is directly proportional to the longitudinal distance of weights from the LCG of the ship. When the fore-peak is ballasted, it increases the weight at a greater distance from the ship’s LCG (which, in most ideal cases, is abaft midships).

In other words, the pitch radius of gyration increases, increasing the ship’s pitch period. An increased period of pitching results in less dynamic effects of pitch motion.

In ice navigation, the bulb allows broken ice to glide along the hull with its wet side against the hull. The wet side of the ice having a lower friction coefficient reduces the overall drag on the ship.

Bulbous bows also provide space for bow thrusters, as seen on modern ships with bow thruster units. In naval ships that use high-frequency underwater acoustics like SONAR, bulbous bows act as protective housing, in addition to their positive effects of drag reduction.

After repeated model testing of a wide range of hull forms and bulb shapes, it was found that bulbs are not efficient at all service speeds (relate it to Froude numbers). At very low Froude numbers, bulbous bows have been found to increase drag.

Wonder why? This is because a bulb is only effective when it generates its own wave in addition to the bow wave. At very low Froude numbers, wave-making hardly occurs. But the bulb, still being below the waterline, increases the total wetted surface area of the ship, therefore contributing to an increase in its skin friction resistance.

A Short History Of The Bulbous Bow

David W. Taylor first incorporated the idea of a bulbous bow while working for the United States Navy. Referred to as “bulbous forefoot” initially, it was fitted on USS Delaware, which entered service in 1910. However, the performance achieved by the it couldn’t be properly evaluated then.

Later in the 1920s, German liners Bremen and Europa, both fitted with bulbous bows, broke the Atlantic crossing speed record. After this event, commercial shipping began to take the bulbous bow seriously.

In 1935, SS Normandie, fitted with a bulbous bow, reached over 30 knots with around 30% less engine power than the comparable RMS Queen Mary. The Japanese Navy were also reported to have adopted large bulbs on the Yamato-class battleships and on several carriers during the same period.

Several theories and papers were published on the design and applications of bulbous bows. Since the 1980s, computer modelling has replaced much of the trial and error that was involved in tank testing. CFD can now optimise a bulb across a full range of speeds and draughts.

Why Some Modern Ships Have No Bulbous Bow

A couple of years back, a bulbous bow was a common sight on merchant vessels. However, several modern ships are launched without one. One of the main reasons for this change was the introduction of slow steaming.

Earlier container ships were designed to have an average speed of 20 to 25 knots. However, after 2008, operators began sharply cutting ship speeds because of rising fuel costs and stringent emissions regulations. Most merchant vessels now operate at 15 to 18 knots. Many ships already fitted with a bulbous bow saw no efficiency improvement at 15-18 knots.

Thus, to reduce the wetted area of the large bulbous bow and improve efficiency, most modern ships started cutting the bulb and implementing new shapes based on the speed at which the ship actually runs.

This change also led to new bulb designs based on the ship’s purpose, cargo, and speed.

Video: Purpose Of Bulbous Bow

Frequently Asked Questions:

Why do some ships not have a bulbous bow?

If fitting a bulbous bow would create more friction than the efficiency gained from reduced wave resistance, then there is no reason to fit one. Especially ships that work at low speeds or have a wide range of speeds do not benefit from this design. Tugs, fishing vessels, small workboats and many ice-going ships fall into this group.

Does a bulbous bow work at all speeds?

No. The bulbous bow design depends on the speed at which the ship operates. At low speed, it can increase total resistance rather than reduce it.

What is the difference between a bulbous bow and an X-BOW?

A bulbous bow reduces wave-making resistance through destructive interference at a design speed. Ulstein’s X-BOW has no bulb and instead uses a tapered, backwards-sloping design to pierce waves and reduce slamming and vertical accelerations. The bulbous bow is optimised for fuel efficiency at steady speed, while the X-Bow is designed for motion and comfort in rough water.

Can a bulbous bow be removed or replaced on an existing ship?

Yes. Removing and replacing the bulbous bow on an existing ship is a common practice. This process is usually carried out in a dry dock, where a new bulbous bow is welded on based on the vessel’s speed and cargo requirements.