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. There is no doubt in the fact that at some point of your life, you have questioned yourself regarding the reason behind the presence of this structure. Well, since it generally resembles the shape of the bulb, and 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 characterized with a bulbous bow. Not only monohull ships, today almost even catamarans are equipped with a bulbous bows rather than straight bows. Why?
When a ship surges, it generates its own Kelvin waves (the ones you see around a ship when it sails in open sea) as shown in Figure 1.
Now visualize it this way- the waves are basically travelling forms of energy in water medium. Where did this energy come from? In other words, who energized the water particles to form these waves?
It is the moving mass of the ship that 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 handy in actually propelling the ship. Where does the rest of the energy go? Remember, water particles were energized to transmit waves? That’s your answer. This is also called Wave Making Resistance of a ship.
Now, why are we discussing this, and what does this have to do with a bulbous bow? Read on.
Consider a ship with a straight bow (for example, Titanic). As the ship surges forward, the water particles move towards the stern along the entire length of the ship. But what about that water particle which is 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 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, since it is generated due to the movement of the bow through the water, as shown in Figure 2. So with a straight bow, there is always a wave continuously formed, with its crest at the bow. Thus, it is evident, that we are wasting a part of the engine power in generating this wave. What if this effect of wave making can be reduced? If yes, then how?
If we introduce another discontinuity (any structure in the ship below the waterline which disturbs the laminar flow is regarded as a discontinuity) below the waterline at the bow, in front of the stem of the ship, 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 can design the shape and position of the discontinuity in such a way so that the bow wave and the wave created by the discontinuity result in a destructive interference? (Refer to Figure 3) Well, that is pretty much the principle behind the design of a bulbous bow. The destructive interference results in reduced wave making of the ship, and which further reduces the wave making drag of the hull form.
In the preliminary stages of development of the bulb, the primary mission of the design was to reduce the wave making drag. But as we moved on, we couldn’t stop delving into more interesting aspects as discussed below:
Wave making is a significant characteristic of finer hullforms. That is why, you notice prominent Kelvin waveforms in cruise ships, liners, yachts, and naval cruisers. If you notice a bulk carrier or an oil tanker (fuller hullforms), it is evident that these hullforms do not show prominent Kelvin wave patterns. Why? Because the waterline width at the stem itself is so large (or in other words, the discontinuity in flow is higher) that the pressure rises to a level such that the bow wave height exceeds the threshold upto which a wave holds its properties. In this case, the wave breaks right at the bow itself even before it travels along the ship length.
So, are fuller hullforms more energy efficient in this respect? No. Do fuller hullforms have high wave making resistance? No. Do fuller hullforms have high wave breaking resistance? Yes. With this application, bulbs were also introduced in bulkers and tankers to reduce their wave breaking resistance.
The different types of bulbs according to their shapes, positions and orientations are as shown below :
The position of the bulb significantly affects the phase difference between the bow wave and the bulb wave. The volume of the bulb is a deciding factor of the amplitude of the resultant wave.
Another advantage of the bulb is that it reduces the dynamic effects of pitch motion of a ship. In most ships, the interior of the bulb is used as fore-peak ballast tank. In case of high pitching, the forepeak tank is often ballasted to reduce the effect of pitching.
How? Well, the time period 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 weight at a larger distance from the LCG of the ship (which in most ideal cases is abaft the midship). In other words, the pitch radius of gyration increases, therefore increasing the pitch period of the ship. Increased period of pitching results in less dynamic effects of pitch motion.
In case of 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 less friction coefficient, reduces the overall drag on the ship.
Bulbous bows have also been advantageous in housing bow thrusters, as can be seen in modern ships with bow thruster units. In naval ships that use high frequency underwater acoustics like SONAR, bulbous bows act as a protective housing, in addition to its positive effects of drag reduction.
After repeated model testing procedures of wide range of hullforms and bulb shapes, it has been found that bulbs are not efficient at all service speeds (relate it to Froude numbers). In very low Froude numbers, bulbous bows have found to increase the drag. Wonder why? Because a bulb is only effective when it makes its own wave, along with the bow wave. But 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 increase in its skin friction resistance.
Over to you?
Do you know more about the importance of bulbous bow of ships?
Let’s know in the comments below.