What Is The Main Purpose Of The Bulbous Bow?

What Is The Main Purpose Of The Bulbous Bow - Merchant Navy Info - News

One thing that often catches the eye of a common man regarding most ships is the bulbous bow at the front end of the ship. It is often located below the waterline.

There is no doubt that at some point in your life, you have asked yourself the reason behind the existence of this structure. Well, since it is generally similar to the shape of a bulb. It is always placed at the bow of the ship. It is known as a Bulbous Bow.

Why Modern Ships Have Bulbous Bows?

Let us look back to about a hundred years. Remember Titanic? You must have observed it did not have a bulbous bow. But try looking at the bows of modern cruise ships, the container ships. LNG carriers, research vessels, etc.

A bulbous bow characterizes all of them. Not only monohull ships but today, almost even catamarans are equipped with bulbous bows rather than straight bows. Why?

When a ship surges, it creates its own Kelvin waves. The ones you see near a ship when it sails in the open sea.

Now imagine it this way: The waves are travelling forms of energy in the water medium. Where did this energy originate from? Who energized the water particles to create these waves?

It is the moving mass of the boat that does this job. Note the word “moving”. The vessel’s movement is powered by its propulsion system. A part of the energy the engine delivers goes into rotating the propeller. In turn, a trace of that thrust generated by the propeller comes in handy when propelling the ship. Where does the remaining energy go? Were the water particles energized to transmit waves? That is the answer. This is also known as the making Resistance of a ship.

Why Are We Discussing This, and What Does This Have To Do With a Bulbous Bow?

Think of a ship with a straight bow (for instance, Titanic). As the vessel surges forward, the water then moves towards the stern along the entire ship’s length. But what about that water particle that is incident exactly at the centreline of the stem?

Its instantaneous velocity is zero. This velocity is known as a Stagnation Point in scientific terms.

Taming the Waves

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

This Wave is known as the bow wave. It is generated by the movement of the bow through the water. So, with a straight bow, a wave is constantly formed, with its crest at the bow. Thus, it is evident that we are not fully using up a part of the engine power in making this Wave. What if this effect of wave-making can be cut down? If yes, then how?

If we introduce one more discontinuity (any structure in the vessel below the waterline that disturbs the laminar flow is considered as a discontinuity) below the waterline of the bow, in front of the stem of the vessel, the discontinuity will itself cause a rise to another wave at its greatest point. Since the stem is still at the waterline, it will lead to normal bow waves.

What if we are able to design the shape and position of the discontinuity so that the bow wave and the Wave made by the discontinuity result in destructive interference? That is the principle behind the design of a bulbous bow. The destructive interference reduces the wave-making of the ship, further reducing the wave-making drag of the hull form. Bow wave and Wave generated by the bulb, both out of phase

Bulbous Bow’s Job

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

Wave-making is a significant characteristic of finer hull forms. You notice prominent Kelvin waveforms in cruise vessels, liners, yachts, also naval cruisers. If you see a bulk carrier or an oil tanker (fuller hull forms). Thus, it is evident that the hull forms do not show a lot of Kelvin wave patterns.

Why? Because the waterline size at the stem itself is so big (or, in other words, the discontinuity inflow is higher), the pressure rises so that the bow wave height is more than the threshold up to which a wave maintains its properties. In this instance, the Wave breaks right at the bow before traveling along the ship’s length.

So, are fuller hull forms better at energy efficiency in this regard? No. Do fuller hull forms have great wave-making resistance? No. Do more full hull forms have high wave-breaking resistance? Yes. This application also introduced bulkers and tanker bulbs to reduce wave-breaking resistance.

The Multifaceted Benefits of Bulbous Bows

The different types of bulbs, as per their shapes, positions, and orientations.

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

Another benefit of the bulb is that it limits 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 the case of high-pitching, the forepeak tank is often ballasted to limit the effect of pitching.

How? Well, the period of pitching is very proportional to the longitudinal distance of objects from the LCG of the vessel. When the fore-peak is hit, it increases weight at a greater distance from the LCG of the ship.

The gyration’s pitch radius improves. Also, the ship’s pitch period should be increased. An increased period of pitching results in less dynamic effects of pitch motion.

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

Bulbous Bows are Wonderful for Housing Bow Thrusters

Bulbous bows have also been useful in housing bow thrusters. As witnessed in modern ships with bow thruster units. In naval ships that utilize high-frequency underwater acoustics like SONAR. Here, bulbous bows play a role as protective housing, in addition to its great effects of reducing drag.

After many repeated model testing methods of a broad range of hull forms and bulb shapes, bulbs have not been proven efficient at all service speeds. In extremely low Froude numbers, bulbous bows have been seen to increase the drag.

Wonder why? A bulb is only effective when it makes its Wave and bow wave. But at very low Froude numbers. Any wave-making hardly occurs. However, the bulb is still below the waterline. It increases the total wetted surface area of the ship. Therefore, it contributes to increasing skin friction resistance.

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