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Full Paper Proc. of Int. Conf. on Advances in Mechanical Engineering 2012

Critical Analysis of Design of Bulbous Bow and it’s Optimization Chaitra V. Pawgi1, Kunal N. Parikh2, and Meghana R. Rajan2 1

K. J. Somaiya College of Engineering, Department of Mechanical Engineering, Mumbai, India Email: pawgi.chaitra@gmail.com 2 K. J. Somaiya College of Engineering, Department of Mechanical Engineering, Mumbai, India Email: kunalparikh91@gmail.com, meghanarajan111@gmail.com 2. Transverse waves The interference between these waves gives rise to characteristic bumps and hollows thus giving rise to increased resistance to motion and instability. The purpose of a bulbous bow is to change the nature of this bow wave. It utilizes the principle of destructive interference. Bulb creates its own wave that is farther forward and slightly out of phase with the natural bow wave created by the hull.

The wave system generated by the bulb interferes with the wave system of the ship. Introducing the bulb modifies the entrance angles of the waterlines and changes the volume distribution around the hull. The length of the bulb defines the phase of interference and its volume determines the width of its wave system. The overall effect of the bulb is to effectively subtract the bulb wave from the normal bow wave thus reducing the wave resistance.

Abstract—Bulbous bow is one of the prudent applications of principles of fluid dynamics. It is a fixed domed pipe like structure attached or made integrated with the ship hull. Its function is to reduce run time drag on the ship hull. Studies on such bulbs have been carried out since 19 th century. In recent times the advent of computational fluid dynamics has facilitated a significant amount of innovation when it comes to the profile of the bulb. The paper presents an innovative idea dealing with the possibilities of modifying the form or the inclination of the bulb based on runtime requirements.

Aiming to explain and justify the design of the bulb, focus will be on discussing critically the methods by which the bulbous bow can be modified during motion so as to combat any changes in the surrounding conditions. Use of mechatronic systems is proposed as a preliminary idea. This work intends to predict the use of such systems and ramifications of incorporating such systems in the existing ones. Lastly, conclusions and results are presented as a means of supporting the predictions.

II. DESIGN Index Terms—Bulbous, Bow, Ship, Hull, Froude, Hydraulic

A. Literature Review Design of the bulbous bow is difficult simply because of lack of design data. Many people have studied the complex fluid motion to understand how the bow wave affects the resistance of the ship. Baba et al made the initial observation that there is a significant component of wave resistance associated with the breaking of the bow wave [9]. Patel et al also proved theoretically that incident flow can separate upstream of bow and the location of separation point is a function of Reynolds and Weber numbers [11]. They used a double body approximation to develop the location of the separation point for a circular cylinder. Miyata et al mentioned the possibility of bow wave oscillation [13]. Mori observed that the appearance of bow wave is steady except for random fluctuations because of turbulence [10]. Maruo et al theoretically demonstrated that a hull form having minimum wave resistance has a cylindrical bow form with finite bluntness of the stem edge when the draft is deep and with smaller bluntness when draft is shallow [12].

I. INTRODUCTION The bulbous bow of a ship is an innovation made in the early 19th century. It is a rigid, protruding, capsular structure which is integrated with the ship bow just below the waterline.

Its functions include reduction of drag, modification of flow about the ship and increase in speed limit. Consequently, there is reduction in power required, increase in economy as well as greater stability of motion. The fuel efficiency is increased by a staggering three to fifteen percent. The bulbous bow functions by changing the nature of the bow wave to reduce the resistance to motion produced on the hull. It is best suited for ships operating near their top speeds.

Generally, the bulb is used only for large vessels like war ships, cargo ships, tankers and super tankers since they have been found to be less effective, rather, detrimental to the performance of smaller ships [8]. This is mainly because in case of smaller ships, the drag experienced by the ship hull because of water is much lesser and hence the additional cost of the bulbous bow is not economical. In recent times the advent of computational fluid dynamics has facilitated a significant amount of innovation which has led to major improvements to the profile of the bulb.

Motion of the ship near its top speed creates two major types of waves: [4] 1. Divergent waves © 2012 AMAE DOI: 02.AETAME.2012.3.18

B. Design Procedure Thus based on this literature as well as the Kracht design methodology, three hull parameters and six bulb parameters are taken into account [1]. There are three linear and three non-linear geometric bulb quantities that are put in the form of bulb parameters.

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Full Paper Proc. of Int. Conf. on Advances in Mechanical Engineering 2012 The linear bulb parameters are as follows: The breadth parameter (maximum breadth) BB of bulb area ABT at the front plane divided by the beam BMS of the ship CBB = BB/ BMS The length parameter (protruding length LPR) normalized by the LPP of the ship CLPR=LPR/LPP The depth parameter (height ZB of the foremost point of the bulb over the base) divided by the draft TFP at the front plane CZB=ZB/TFP The three non-linear bulb parameters are as follows: The cross-section parameter: Area of cross-section ABT of bulb at FP divided by area at mid-section of ship CABT = ABT/AMS The lateral parameter: Area of ram bow ABL in longitudinal plane divided by AMS CABL = ABL/AMS The volumetric parameter: Volume PR of the bulb divided by the volume of displacement of the ship WL C PR = PR / WL The most important effect of a bulbous bow is its influence on the different resistance components and hence on the required power. The total resistance is sub-divided as follows: Total resistance (RT) = Viscous resistance (RV) + Wavemaking resistance (RWF) + Wave-breaking resistance (RWB) = Frictional resistance (RF) + Viscous residual resistance (RVR) + Wave-making resistance (RWF) + Wave-breaking resistance (RWB) According to the linearized theory of wave resistance, the free wave systems of the ship and the bulb undergo interference. Thus, depending on phase difference and amplitudes, a mutual cancellation of both interfering wave systems may occur.

At constant Froude number FN the bulb effect is based on the following six parameters (as defined earlier): ΔR = F(C PR, CABT, CABL, CLPR, CBB, CZB) Now power reduction factor is defined as follows: ΔP* = 1.0 – Pw/Po In this form the bulb effect is the power difference of the ship without Po and with bulb Pw related to the power of the bulb less ship. Thus, positive bulb effect corresponds to power reduction.

Now, for block co-efficient of ship as 0.7, the co-relations between bulb parameters and power gain can be referred from “Design of Bulbous Bows” by Alfred M. Kracht [1]. Thus, the modification of these parameters during runtime may lead to constant power gain, eliminating any causes of loss due to the uncertainty of waves.

of the ship.The conditions around the bulb are constantly varying. But, during runtime there is only one fixed bulbous bow. This means that ocean waves, unpredictable as they are, varying from region to region, may act to reverse the benefits of the bulb. These changes are very irregular and hence make the effectiveness of use of the bulb uncertain.

This unpredictability may even counteract the benefits of the bulb and in fact make it a liability.

2. When a bulb is manufactured, the optimal conditions for its use are a major factor in design. Thus, as seen before, this limits the speed range of the vessel. Also, the bulb is designed keeping in mind a specific water body which has a set variation of conditions. Thus, the bulbous bow loses its benefits beyond the set speed range as well as outside the set water body.

3. Let us draw upon an interesting analogy with regard to Formula 1. As any avid fan of Formula 1 would know, the engineers in any team modify the design of the nose slightly for different circuits all over the world. This is taking into account the different drag produced in different conditions around the world. So why not make the same consideration in case of ships and their bulbous bows?

B. Main Idea 1.The present structure of the bulb is rigid and immovable.

This structure is not likely to satisfy the criteria of efficiency and stability at all environmental conditions. This means that if the speed or turbulence of waves in the water body of the bulb changes, the bulb will not work as efficiently as it would have, in the conditions for which it is designed.

2.Thus, to overcome this, the idea is to explore the area of mechatronics and computational fluid dynamics [6] [7] to find out ways to change the parameters of the bulb during run time. For instance, if the turbulence is high, using this system, we can a. move the bulb in desired inclination and change the angle of its nose or b.Change its protruding length to suit the present time requirements for efficiency and stability.

To explain this concept, let’s consider a space diagram as follows: i.Let’s consider efficiency as the measure of performance.

To its simplest form, let the efficiency be a function of three parameters: two bulb parameters, say x and y and third will be the parameter relating to the surrounding conditions, say z. ii.Thus we can say s (p) =f (x, y, z). This expression will establish a relationship between the bulb parameters, environmental parameters and the bulb efficiency. Z can be any parameter describing the scenario around the bulb, for example, it can be the turbulence or wave pattern or wave intensity or wave breaking resistance [3]. iii.A graph can be plotted which locates the point p for different values of x, y and z. let’s define two points p1 and p2 in space. Let point p1 correspond to the design conditions and let point p2 correspond to the required conditions. s (p1) = f(x1, y1, z1) ands (p2) = f (x2, y2, z2). Thus, for any point p, to maintain the efficiency, the attempt is to be made to maintain

III. INNOVATIONS PROPOSED A. Why the Innovations?

1. The bulbous bow is subjected to waves varying in intensity as well as frequency (especially considering that bigger ships travel for longer distances). The bulb has to withstand the vagaries of the weather and changing speeds © 2012 AMAE DOI: 02.AETAME.2012.3.18

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Full Paper Proc. of Int. Conf. on Advances in Mechanical Engineering 2012 its position at p1, for all dynamic changes in the surrounding .This, calls for changes in x, y and z parameters for point p. these changes are to be done with the help of the system that this paper proposes. iv.Approaching the generalization, any other performance parameter, say stability, can be expressed as the dependent variable. The corresponding relationship between f, bulb parameters, and environmental conditions vital for this parameter f. v.Also, instead of two bulb parameters, many more can be considered. Increase in number of parameters will increase the practicality of the analysis.

Procedure: Following procedure is explained with respect to above example.

1.Standard values of all the variables, independent and dependent (say x, y, z and s as mentioned above) that need to be maintained will be provided and stored in the system.

Now, considering above example, as s (p) =f(x, y, z), and varying z is not possible, x and y are to be varied to meet required valued of s (p). This is done with the help of following system.

2.A set of sensors (set1) will sense one or more particular property values of the waves, say pattern in which waves hit the hull or intensity of waves or their amplitude or wave breaking resistance. This will provide approximate predictions about the present scenario. Because the conditions are unpredictable and highly disordered, approximate values of wave properties will be used.

Sensors: These sensors can measure materialistic properties of the wave by: i.Actually staying in contact with the water or ii. They can use sonar and radio technology or remote sensing devices with particular coverage area. iii. Manual design of the sensors to be used in this system can be made. For instance, it can include a floating body which is attached to the ship hull by means of very strong threads.

This body will experience force due to waves and will thus undergo changes in displacement or acceleration. This can be measured which the help of an accelerometer and can be then converted into required form of quantity. This represents a basic idea of how the instrument will work. Experimental data, indeed, is needed to support this claim.

3. A comparator will compare these run time value with given set of standard values to find out the differences in values.

4. This comparison will provide decision information, that is, information on which the decision of changing bulb parameters will be made. Depending on the result from the comparator, signals to actuators will be sent. For example, if the inclination of the bulb is not appropriate for conditions of turbulence, decision will be made to change it in order to match the demands of outside conditions. Referring back to the above example, as we have to maintain the efficiency, based on these values provided by the sensors (set1), calculations can be made to predict the bulb parameters x, y, using relation s(p)=f(x, y ,z).

5. Actuators will activate powerful hydraulically operated cylinders. Motion from these cylinders will be converted into desired motion through linkages to achieve required changes in bulb parameters to change the values of bulb parameters (x and y from the above example).

6. These changes will improve, in turn, the efficiency of bulb s (p).

7. Sensors (set2) will record new changed values of all parameters (x, y z, and s (p)) as feedback to the comparator.

8. Comparator will check the compatibility of new values with the constantly changing parameter (like parameter z, which

Fig. 1. Graph showing points p1 and p2

C. Simulation The changes in inclinations are shown in figure [2].

Fig. 2. Simulation showing X-shift, Y-shift and Z-shift TABLE I. INFORMATION ABOUT VARIOUS SHIFTS

D. Design It demands for the bulbous bow to have an adjustable construction. Š 2012 AMAE DOI: 02.AETAME.2012.3.18

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Full Paper Proc. of Int. Conf. on Advances in Mechanical Engineering 2012 is dependent on surrounding environment) and the procedure will be repeated continuously to keep a check on desired function (that is s (p) from the example).

9. Also predictions can be made based of frequency of variation of wave parameters (z as in above case), ultimately calculating the frequency of changes in bulb parameters required (x and y), in turn, calculating the response time of the system for its design.Flow chart for the example mentioned above is as follows: a. Standard values of s (p) and corresponding values of (x, y, z) are stored in the system. b. Sensors (set1) will sense value of z. c. Comparators will compare it with given standard value. d. Due to the differences in the values of z, signals will be sent to the actuators. e. Actuators will activate hydraulically operated cylinders and achieve required changes in x and y, to maintain value of s (p). f. Sensors (set2) will record new changed value of x and y and provide it as a feedback to the comparator. g. Procedure will repeat.

1. Size and weight of the bulb are too huge to be changed during run time. This might ask for a very powerful system, cost of which might offset the benefits of using it. Calculations for long term and short term advantages and disadvantages need to be made in order to affirmatively state the utility of this system.

2. It is difficult to record correct values of constantly changing wave parameters. Thus very sensitive sensors will have to be used.

3. Response time of this type of system needs to be very small that is, the system needs to be very fast in its actions.

This might ask for development of newer and faster devices.

4. Also the size and weight of such system should be such that it can be accommodated inside the ship. It should not exceed certain conditions like stability of the system and vibrations inside the ship, which are of immense importance.After dealing with these issues if the system can be implemented practically, it would give high long term benefits.

CONCLUSION The concept of the bulbous bow was widely accepted after the 1950’s.Surplus research has been done and modifications have been introduced in the design of the bulb so far. However, all of these modifications have been adopted in the design stage. In order to improve the efficiency and optimize the use of the bulb, a new idea is proposed in this paper for changing the profile of the bulb during runtime.

This proposition exploits the concept of mechatronics in order to achieve modifications during runtime. This paper focuses on the runtime adaptability of the bulb to meet the actual requirement at a particular instant. If this idea is adopted successfully and the problems associated are eliminated the bulb might prove beneficial in terms of fuel economy and overall efficiency of the vessel. The idea proposed in the paper is subject to experimentation. Detailed analysis and research for the execution of this idea is imperative and left for further scrutiny.

1. Further simulation on modelling software with respect to different design and dynamic parameters is to be carried out.

2. Appropriate scaling of the physical models with and without modifications is to be tested in various experimental conditions. Based on this comparison, decisive conclusions have to be drawn with regard to the critical design parameters.

3. Mechatronic and control systems have to be minutely studied in order to find out sensitive and advanced sensors for carrying out the foresaid modifications.

4. Experimental analyses have to be carried out on the bulb after adopting the mechatronic equipment.

5. Power requirement for both the modified and the nonmodified bulb has to be deduced.

6. Detailed cost report is to be tabulated and economic feasibility of the modified bulb is to be studied.

Fig. 3. Proposed system configuration for changing bulb parameters during run time

E. Hindrances It can be predicted that implementation of this system will induce extra costs for actuators [5], hydraulically operated cylinders, sensors and other components. But the hidden advantages include long run benefits. That is, this change in inclination will allow the ships designed for one set of environmental and drag conditions to be used in another set of conditions. For example, a ship hull designed for low draglow turbulence ocean conditions might be used for a high drag-high turbulence conditions with modifications in inclination and length of the bulb. Also, speed range of the ship can be increased.

But at this stage of the project, it would be wise to consider the major hindrances in the working of this system. Above system is just proposed as an idea and needs to be worked upon thoroughly. Future work will include the design of this system to check its validity and feasibility.

There are few major issues related with this idea. They can be as follows:

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Full Paper Proc. of Int. Conf. on Advances in Mechanical Engineering 2012 REFERENCES

a Bulbous Bow in a Ro-Ro Ship”.

[8] Working of bulbous bows:http://en.wikipedia.org/wiki/ Bulbous_bow [9] Baba, E. 1969, A new component of viscous resistance of ships, J. Soc. Naval Arch. Japan, no.125, pp. 23-34 [10] Mori, K. 1984, Necklace vortex and bow wave around blunt bodies, Proc. 15 th Symposium on Naval Hydrodynamics, Hamburg, W.Germany, pp.9-20.

[11] Patel, V.C., Landweber, L., and Tang, C.J. 1984, Free-surface boundary layer and the origin of bow vertices, Iowa Inst. Of Hydraulic Res. Report, No.284 [12] Maruo, H. and Ikehata, M. 1986, Some discussions on the free-surface flow around the bow, Proc. 16th Symposium on Naval Hydrodynamics, Berkeley, Calif. pp. 65-77.

[13] Miyata, H., Kajitani, H., Matsukawa,C., Suzuki, N., Kanai, M., and Kuzumi, S. 1984, Numerical and experimental analysis of non-linear bow and stern waves of two-dimensional body (second report), J. Soc. Naval Arch. Japan, no.155, pp.15-33.

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[2] KarstenHochkirch, Volker Bertram, “Slow Steaming Bulbous Bow Optimization for a Large Containership”.

[3] Hoyte C. Raven, “A Solution Method for Non Linear Ship Wave Resistance Problem”, 1996.

[4] Manuel Ventura, “Bulbous Bow Design and Construction”.

[5] Ryan N. Smith, Dario Cazzaro, Luca Invernizzi ,GiacomoMarani, Song K. Choi , Monique Chyba, “A Geometric Approach to Trajectory Design for an Autonomous Underwater Vehicle: Surveying the Bulbous Bow of a Ship”.

[6] Hydrodynamic services, Force Technology, “CFD Analysis for Improved Hull Performance”.

[7] Prof. Luis Perez Rojas, Prof. Juan M. Sanchez, Prof. Antonio Souto, “A Practical Application of CFD: The Optimization of

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