CHAPTER V

DESIGN OF IMPROVEMENT MECHANISM

5.1 Introduction

The design mechanism chosen here is by modifying the hull form by adding a

bulbous bow. The benefits of bulbous bow are already well known. Trawlers run at a

high value of Fn (0.3-0.37) and will have large wave making resistance. Therefore it

is favourable to use a bulbous bow. According to Doust (1961), a good conventional

design can reduce 10 to15 percent wave making resistance and increase 4 to 5

percent propulsive efficiency. (Comstock. J.P 1967) Reduction of delivered power

also occur as much as 20 percent in a smooth water.

5.2 Basic of bulbous bow

Generally, the bulb looks like a section of a large diameter pipe with a domed

end sticking out of the bow of the vessel, underwater. Although available in many

shapes and sizes such side as bulbs, bilge bulbs and even stern bulbs but the most

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consistent results have been achieved with bow bulbs. Today, to see a large ship

without a bulbous bow is a rare sight indeed. The results have been proven over

countless thousands of deep ocean miles in all kinds of weather by all kinds of

vessels. With model testing and advanced knowledge of hydrodynamics, the bulbous

bow was formulated typically giving a 5 percent reduction in fuel consumption over

a narrow range of speed and draft.

5.3 Function of a bulbous bow

The functioning of bulbous bow is primarily important to the vessel when

moving forward. Hydrodynamically, the interference of the primary and secondary

wave causes an overall reduction in drag, which is beneficial to the vessels resistance

characteristics. When the water flows over the top of the bulb, it could reduce the

pressure above it, thereby reducing the amplitude of the bow-wave.

Figure 5.1: Bulbous bow induces a flow field in the region bow section

The length of bulb is determined by the stem profile, as further forward the

bulb extends more advantage will it have. Nevertheless, it has been generally kept

shorter than the bow overhang because of anchoring problem. The vertical placement

is calculated so that the bulb is just below the surface where it will create a wave in

front of the ship interfering with the natural wave of the vessel, creating a wave

hollow where the crest should be. In this way, the vessel will run flatter and the

overall wave height will be reduced. The vessel can be moved through the water with

less waves and overall disturbance to the surface and as a result, energy will be

transmitted to the water to create those waves and vessels can reach the designed

speed with less horsepower.

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5.4 Advantages and disadvantages of bulbous bow

The benefit of a modern day bulbous bow is the reduction of fuel

consumption by 12 to 15 percent and will give a better range and higher speed. At a

higher speed wave making resistance accounts for the greater part of the drag, and

the slower you go proportionally, more of the resistance is taken up by wetted

surface drag. At a low speed (around 6 knots and lower), the bulb will cause an

increase in drag because of its greater wetted surface area. Luckily, at that low speed,

the added power consumption is negligible and generally, little time is spent in this

speed range. In addition, there will be increased sea keeping ability due to

dampening of the pitching motion. When charging into head seas there is a chance of

slamming the bulb on the troughs, but this is limited to a very narrow range of wave

train and heading.

5.5 Prediction the Advantageous of Bulbous Bow

It is essential to determine the range of benefit in using bulbous bow when

the vessel travels. Establishment of hydrostatic data is important to obtain an early

thought that bulbous bow can reduce the resistance. The range of Froude number is

used to determine the variety of speed in order to get the minimum requirement of

advantageous of bulb. From Watson & Gilfillan’s chart (APPENDIX F), the range of

0.55 is the lowest value that can be obtained by a vessel to get the advantage of

bulbous bow. The minimum condition of speed of a trawler to travel in order to

reduce the wave making resistance is shown below. From the calculation, a

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minimum speed of 7.31 knots must be achieved by the vessel to reduce the wave-

making resistance.

Where,

5.6 Procedure to choose bulbous bow

Basic procedures for choosing the bulbous bow are given by Sounders, H.E.

1957 as follows:-

1. The most important feature of this pattern was a wave trough just aft of the

sphere, which suggested the possibility of partly canceling the bow wave of

the hull by locating a sphere below the surface in the neighborhood of the

stem.

2. Useful range of speed of bulb is generally from about Fn = 0.24 – 0.57.

(Comstock. J.P 1967)

3. The best position for the bulb is with its center at the bow and its nose

projecting forward of the hull.

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4. The bulb should extend as low as possible, and should be as short as

longitudinal and as wide laterally as possible, consonant with fairness in the

lines of the hull.

5. The top of the bulb should not approach too nearly at the water surface.

6. It is significant that the most substantial improvement we found in the ballast

condition is when the bulb is near the surface. The draft forward appears to be

critical and care should be taken in choosing the ballast operating operation.

7. Cost to build and development of the bulb.

8. The bulb must not be treated nearly as an addition or appendage, but the

whole fore body should be redesigned as fine load waterline being used with

half-angles of entrance of 5 to 10 deg less than those of a normal trawler, and

with the LCB as far aft as possible.

9. The bulb area should not exceed 5 % in order to avoid risk of slamming

damage.

10. Unless the lines (forward) are extremely hollow the best position of the bulb

is with its (longitudinal) center at the bow, that is, with its nose projecting

forward of the hull.

11. The bulb should extend as low as possible consistent with the fairness of the

lines of the hull.

12. The bulb should be as short longitudinally and as wide laterally as possible,

again having regard to the fairness of the line.

13. When considering a bulb bow for a new design it is first necessary to

determine whether the speed range is appropriate to its use.

5.7 Bulbous bow shape and parameter

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A methodology of bulbous bow design used is Kracht bulbous bow design

developed for a low-speed full form ship. There are 3 three types of classification

according to the shape of the bulb’s cross section at the forward perpendicular. These

3 types are Δ, O and . (Appendix A). The chosen bulb of a model trawler is type

because its’ advantageous of sea keeping properties. The determination of bulbous

bow parameter is important.

In this initial design, the height of the bulb must be decided first. This is to

ensure that the bulbs are immersed completely in the water surface. After the height,

the other characteristic such as linear and non-linear parameter is established.

(Appendix B) The value of length, and breadth parameter are obtained by

assuming the reference value. (Kracht, A.M. 1978) The specific value for length,

and breadth parameter are given below:

1. = 0.05

2. = 0.204

From this reference value, the length and the breadth can be found out. Other

parameters can be obtained after the modeling of bulb is undertaken.

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5.8 Bulb Design Methodology

The basic particular of the bulb is determined from the initial calculation, and then

the modeling of the bulb is carried out.

HB BB/2 LPR

ZB

Figure 5.2: Linear Bulb Parameter

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1. Drawing of the upper part of the longitudinal profile of bulb

Decide the Forward Perpendicular (FP) in order to attach the bulb to the bare hull.

This procedure is for the upper part to join the FP point. The curves are stopped upon

reaching ZB (1.69 m) at the height above the baseline.

Figure 5.3: Longitudinal profile of bulb

2. Layout of the lower curve of the longitudinal profile.

Taken the distance y(x) below the curve at a longitudinal distance, x forward

perpendicular can be calculated using to the following formula:

Y(x) = [HB² - x²(HB/LPR) ²]0.5

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Figure 5.4: Layout of the lower curve of the longitudinal profile

3. Layout of half-breadth bulb curve

This layout is according to the breadth of the bulb after the calculation. The

extension of the bulb must fair smoothly to the hull form.

Figure 5.5: Layout of half-breadth bulb curve

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4. Create the surface of bulb

After finishing step 3 above, the curve can be plotted to construct a surface of the

bulb according to the initial measurement.

Figure 5.6: Three dimensional surface of bulb

5. Rebuild surface and final fairing

In the final process, the surface was rebuilt and merged together to obtain one solid

bulb.

Figure 5.7: Final process of modeling

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5.9 Bulbous bow design

The procedures of designing bulbous bow are shown in the flow below:

Figure 5.8: Bulb design methodology

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Select one type of bulb

Determine the hull form particular – Lwl, B, T

Predict the minimum speed for advantageous of bulb

Decide HB, Assumption of CLPR, CBB

Calculation of the LPR, BB

Bulb modeling using software

Calculate other linear and non-linear parameter of bulb

Table 5.1: Bulb particulars (Legend refer to Appendix B)

NO Particular Value

1 HB 1.48 m

2 BB 1.14 m

3 BMS 5.60 m

4 LPR 1.19 m

5 LPP 23.9 m

6 TFP 2.43 m

7 ZB 1.69 m

8 ABT 1.12 m²

9 AMS 8.87 m²

10 ABL 1.17 m²

11 VPR 0.9151 m³

12 VWL 132 m³

Table 5.2: Linear and Non-linear Parameter of bulb (Legend refer to

Appendix B)

No Particular

Parameter Linear Parameter Non-Linear

1 CBB 0.2036 CABT 0.133

2 CLPR 0.0498 CABL 0.1319

3 CZB 0.6966 CVPR 0.0069

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5.10 Offset Data

The offset tables for this bulb are from forward perpendicular of the ship.

Spacing station that is used for this offset is 0.15 m. For height above baseline table,

two data are put forward. First data is for below reference waterline and the second

data is for above reference waterline of six. (Refer to offset and lines plan drawing)

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Table 5.3: Offset data for bulb – Half Breadth

Half-Breadth (m)

Station Waterline

0 1 2 3 4 5 6 7 8 9

AP 0.0789 0.2244 0.3463 0.4638 0.5381 0.5702 0.563 0.4946 0.3453 0.1417

1 0 0.1905 0.3124 0.4363 0.5234 0.5608 0.5544 0.4762 0.2939 0

2 0 0.1413 0.2708 0.3994 0.4994 0.5368 0.5422 0.4509 0.2498 0

3 0 0.0632 0.2174 0.3504 0.4636 0.5132 0.5092 0.4171 0.2124 0

4 0 0 0.1477 0.2859 0.4125 0.4721 0.4701 0.3725 0.1711 0

5 0 0 0 0.2008 0.3407 0.4169 0.4179 0.314 0.1172 0

6 0 0 0 0.0862 0.2383 0.3427 0.3497 0.2371 0 0

7 0 0 0 0 0.079 0.2206 0.2489 0.1338 0 0

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Table 5.4: Offset data for bulb – Height above baseline

Height above baseline (Ref = WL 6)

Station Buttock line

Centerline 1 2 3 4

AP 0.7667/2.2461 0.8472/2.1455 1.0151/2.0474 1.1847/1.9291 1.4777/1.6937

1 0.8102/2.1141 0.8914/2.0799 1.0571/2.0036 1.2240/1.8960 1.5462

2 0.8635/2.0867 0.9446/2.0557 1.1048/1.9846 1.2704/1.8662 0

3 0.9279/2.0715 1.0097/2.0135 1.1617/1.9478 1.3310/1.8401 0

4 1.0050/2.0553 1.0880/2.0135 1.2410/1.9196 1.4101/1.7886 0

5 1.0961/2.0296 1.1777/1.9833 1.3287/1.8800 1.5713 0

6 1.2062/1.9876 1.2970/1.9246 1.4412/1.8054 0 0

7 1.3522/1.9172 1.4550/1.8330 0 0 0

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Figure 5.9: Lines plan drawing of bulb

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