What's with the Bulb? - Part Three

If you are a beginner, read the first and second part of this series.

Design of a bulb is one of the most challenging works for a Naval Architect. A major portion of the economy is dependent on the design of the bow. The better design would lead to better efficiency and lesser fuel consumption. Hence, it is very important to know and understand every concept of the bulb design. However, in the following article we will be going through an overview of the major components required for the design, following which we shall go through the advantages and disadvantages of using the bulbous bow.

Mostly the design and shape of the bulbous bow is determined by model testing. However, on the basis of the design used till date, three major shapes have been identified.

TYPES OF BULB SECTIONS:

The various types of bulbous bow are as follows - 

• Δ- type: The figure below shows the drop-shaped sectional area ABT of the delta-type with the center of area in the lower-half part. This shape indicates a concentration of the bulb volume near the base. The Taylor bulb and the pear-shaped bulbs belong to this type.
• O- type: The figure below shows an oval sectional area ABT and a center of area in the middle with a central volumetric concentration. All the circular, elliptical, and lens- shaped bulbs as well as the cylindrical bulbs belong to this type.
• ∇- type: The nabla-type also has a drop-shaped sectional area  ABT , but its center of area is situated in the upper-half part, indicating a volume concentration near the free surface, because of its favorable sea-keeping properties, this type is the common type of bulb. 

INTEGRATION OF THE HULL:

Depending on the shape of the bulb in comparison with the hull shape, bulbs can be divide into two types-

• Addition Bulbs: The bulb shape is completely independent from the hull shape. Typically there is a knuckle resulting from the intersection of the bulb and the hull.
• Implicit Bulbs: Sectional area curve is changed, part of the volume of the bulb is distributed aft of the FWD perpendicular. No knuckles are required.

In the bulbous bow by changing the entrance angle of the waterlines and the volume distribution, represents an effective mean of reducing the wave resistance. Some authors limit the usefulness of the bulb to the interval: 0.238<Fn<0.563. 

Bulb shape must be adjusted to the design conditions. Generally, at low speeds the effect of the bulb is negative. When the Fn increases, its effect becomes positive and increases up to a maximum value. From this point upwards, when the Fn tends to the infinity, the effect of the bulb tends to zero.

INFLUENCE OF THE BULB ON THE PROPERTIES OF THE SHIP:

Before discussing the influence of the bulbous bow on the ship resistance and required power, respectively, we should mention other important hydrodynamic qualities which play a role in the decision whether a bulb should be used or not. The change of resistance influences the thrust loading of the propeller and consequently, other propulsive characteristics of the ship like the wake and the thrust deduction factor.

The figure shown below(left) gives us the indirect influence of the bulbous bow on thrust deduction fraction. Both are increased by an additive as well as by implicit bulb, if the bulb ship has a lower total resistance than the bulbless form.

Advantages of bulbous bow:

• Reduces the bow wave, due to the wave generated by the bulb itself, making the ship more efficient in terms of energy.
• Increases the ships waterline length, slightly increasing the ship speed, reducing the installed power requirements and so the fuel oil consumption.
• Works as a robust bumper in the event of collision.
• Allows the installation of the bow thrusters at a foremost position, making it more efficient.
• Allows a larger reserve of flotation or a larger ballast capacity forward.
• Reduces pitch movement.

Strictly speaking there is hardly any disadvantage of using a bulbous bow, however a few factors must be always kept in mind before installing one, that is a bulb is only useful for long voyage ships, especially, Merchant Vessels. It operates only for a given range of speed, hence ships like Coastal Research Vessels, or for coastal fishing vessels it may be useless. Generally displacement ships whose froude number is less than 0.36 may find it advantageous. 

Aspects of Initial Bulb Design:

The bulb shall never emerge completely. The point at the forward extremity shall be at the level of the waterline.

Distribution of the bulb volume

• Too much immersion does not produce any effect.
•  Volume concentrated longitudinally near the free surface increases the effect of interference in waves.

The waterlines at the bulb extremity should have a thin shape but not circular, to avoid the flow separation.The bulb is advantageous in ice navigation – the ice blocks slide along the bulb with their “wet” side, which has a lower friction coefficient.

Computation Fluid Dynamics of the bulbous bow is very much necessary if you have to know the distribution of the hydrostatic pressure and the flow lines of the waves around the hull due to the bulbous bow.

Based on the experience, the designer must evaluate if:

• The length of the bulb is adequate to the considered speed.
• The over pressure generated at the sides of the bulb is adequate.
• The bow flare does not increase too much the height of the wave at the bulb.

The bulbs shall be designed for the operational profile of the ship and not only to the full load condition. The bulb that reduces the most fuel needed for the required operational profile shall be the one selected.

Note the flow around the bulbous bow in the video below- 

                                    

Images of different types of bulbs in Merchant Ships:

LSD

Article By: Tanumoy Sinha

Author's Note: This is the third and last part of the series on bulbous bows. If you haven't gone through the first and second parts, please click on the respective words on this line. This article is the Third and the final part of the topic 'What's with the Bulb?'. It expounds to us about the various design aspects, CFD requirement and advantages & Disadvantages of the Bulbous Bow. I have written this article based on my experience and technical knowledge on the same as a Naval Architect. The picture and the video used above does not belong to LSD, and full credit for the same goes to their respective owners. If you have any queries or doubts, write to me at learnshipdesign@gmail.com