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Bulbous BowDesign 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 designwould 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 thefollowing article we will be going through an overview of the major componentsreuired for the design, following which we shall go through the advantages anddisadvantages of using the bulbous bow.

!ostly 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 beenidentified.

TYPES OF BULB SECTIONS:

The various types of bulbous bow are as follows "•  #" type$ The figure below shows the drop"shaped sectional area A%T of the

delta"type with the centre of area in the lower"half part. This shape indicates aconcentration of the bulb volume near the base. The Taylor bulb and the pear"shaped bulbs belong to this type.

• &" type$ The figure below shows an oval sectional area A%T 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 A%T , but its

centre of area is situated in the upper"half part, indicating a volume concentration

near the free surface, because of its favourable sea"keeping properties, this type isthe common type of bulb.

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INTEGRATION OF THE HULL:

Depending on the shape of the bulb in comparison with the hull shape, bulbs can bedivide into two types"

•  Addition %ulbs$ 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.• 'mplicit %ulbs$ (ectional area curve is changed, part of the volume of the bulb

is distributed aft of the )*D perpendicular. No knuckles are reuired.

'n the bulbous bow by changing the entrance angle of the waterlines and the volumedistribution, represents an effective mean of reducing the wave resistance. (omeauthors limit the usefulness of the bulb to the interval$ +.-/)n/+.01-.

%ulb shape must be adjusted to the design conditions. 2enerally, at low speeds theeffect of the bulb is negative. *hen the )n increases, its effect becomes positive andincreases up to a ma3imum value. )rom this point upwards, when the )n tends to

the infinity, the effect of the bulb tends to 4ero.

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INFLUENCE OF THE BULB ON THE PROPERTIES OF THE SHIP:

%efore discussing the influence of the bulbous bow on the ship resistance andreuired power, respectively, we should mention other important hydrodynamic

ualities which play a role in the decision whether a bulb should be used or not. Thechange of resistance influences the thrust loading of the propeller and conseuently,other propulsive characteristics of the ship like the wake and the thrust deductionfactor.

The figure shown below5left6 gives us the indirect influence of the bulbous bow onthrust deduction fraction. %oth are increased by an additive as well as by implicitbulb, if the bulb ship has a lower total resistance than the bulbless form.

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Advantages of bulbous bow:

• 7educes the bow wave, due to the wave generated by the bulb itself, making

the ship more efficient in terms of energy.

• 'ncreases the ships waterline length, slightly increasing the ship speed,

reducing the installed power reuirements and so the fuel oil consumption.

• *orks 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.

• 7educes pitch movement.

(trictly speaking there is hardly any disadvantage of using a bulbous bow, however afew factors must be always kept in mind before installing one, that is a bulb is only

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useful for long voyage ships, especially, !erchant 8essels. 't operates only for agiven range of speed, hence ships like 9oastal 7esearch 8essels, or for coastalfishing vessels it may be useless. 2enerally displacement ships whose froudenumber  is less than +.-1 may find it advantageous.

Aspets of In!t!al Bulb "es!gn:

The bulb shall never emerge completely. The point at the forward e3tremity shall beat the level of the waterline.

Distribution of the bulb volume

• Too much immersion does not produce any effect.

•  8olume concentrated longitudinally near the free surface increases the effect

of interference in waves.

The waterlines at the bulb e3tremity should have a thin shape but not circular, toavoid the flow separation.The bulb is advantageous in ice navigation : the ice blocksslide along the bulb with their ;wet< side, which has a lower friction coefficient.

9omputation )luid Dynamics of the bulbous bow is very much necessary if you haveto know the distribution of the hydrostatic pressure and the flow lines of the wavesaround the hull due to the bulbous bow.

%ased on the e3perience, the designer must evaluate if$• The length of the bulb is adeuate to the considered speed.

•The over pressure generated at the sides of the bulb is adeuate.

• The bow flare does not increase too much the height of the wave at the bulb.

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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 reuired operational profile shall be the one selected.

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

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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 reuired operational profile shall be the one selected.

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

I#ages of d!ffe$ent t%pes of bulbs !n &e$'ant S'!ps:

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LSD

 Article By: Tanu#o% S!n'a

 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 therespective 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