Development of Energy Saving Bow Shape at Sea

ABSTRACT

In the past two decades much effort has been spent for energy saving of ocean going ships. Such energy saving has been achieved by the improvement of hull shape, propeller, main engine, and the development of energy

saving devices. From these results, necessary engine

output has been dramatically reduced. The

improvement of hull shape and the development of

energy saving devices were conducted from the

viewpoint of the

ship's resistance and propulsive performance in still water.

Recently, however, it has been pointed out by ship's

operators that such energy saving ships with smaller

engine output show poorer performance at sea. It seems to be due to the larger nominal speed loss with smaller

engine output in waves. In such circumstances, the

design of

ships has to be based not only on ship's

performance in still water, but on ship's total performance at sea.

One of the aspects to improve the ship's performance at sea is to reduce the added wave resistance. In order to reduce the added wave resistance, new concepts of bow shape ofships have been developed. Their bow shapes above still waterline level are sharpened to forward in comparison with conventional blunt bows, which are named Beak-Bow and Ax-Bow. Model tests in waves are

conducted for bulk carriers. Their results show that

added wave resistance on the new bow shapes is reduced

by 20-30% in comparison with that for conventional

bows. It corresponds to about 10% reduction

of sea margin.

KEY WORDS: Bow Shape, Full Ship, Energy Saving, Added wave resistance

DeIft University of Technology

Ship Hydromechanics Laboratory

Library

Mekelweg 2

2628 CD Deift

Phone: +31 (0)15 2786873

E-mail: p.w.deheertudelft.nI

Development of Energy Saving Bow Shape at Sea

Koichiro MA TSUMOTO, Kazuyoshi HIROTA and Kenji TA KA GJSHJ Ships & Offshore Structures Planning Department, NKK Corporation

Tsu, JAPAN

INTRODUCTION

After Oil-Shock in 1970's, considerable effort has been spent to reduce the fuel oil consumption of ships and the horsepower necessary to the ship has been reduced by almost half in the last two decades. Such horsepower reduction on ships has been based on improving the hull shape, energy saving devices fitted on the ship and the performance of the main engine.

Recently, it has been pointed out that its sea margin and

speed loss has become larger

in

spite of their better

performance in still water. Development of ship hull shape has been focused on the horsepower reduction in still water, but it is also necessary to take that

in waves into

consideration.

The purpose of the present study, therefore, is to develop new ship shapes to reduce the resistance increase due to waves in order to achieve lower sea margin. Here, the sea margin is defined by the ratio between necessary horsepower increase in waves and that in still water on a

same ship speed.

The added wave resistance on full form ships such as

tankers or bulk

carriers is

mainly generated by the

diffraction of the incident waves at the blunt bow. New concepts of bow shape of ships have been developed to reduce the added resistance due to diffraction waves at the bow. The bow shape above tile still waterline is peaked in comparison with ordinary bow shape. Here, the still waterline is the water surface level when a ship stops afloat. Model tests in waves are conducted for bulk carriers, whose results show that added wave resistance on the new bow shape is reduced. And then, the sea margin in the actualsea is estimated by using these tank test results.

in order to decrease the diffraction of waves at the bow, it is necessary to reduce the bluntness of the bow shape especially above the still waterline. To design the optimum bow shape under various practical restrictions, estimation

method of added resistance due to wave considering hull shape above waterline is needed. Conventional method to estimate the added wave resistance can not consider the effect of bow shape above the still waterline. In order to consider its effect, some modifications are necessary for the conventional method and validation of their adequacy are also necessary by comparing calculation results with tank test results.

CONCEPT OF NEW BOW SHAPE

The added wave resistance can be described as sum of two components, one is due to the diffraction at the bow and the other is due to the radiation wave based on ship motions.

As shown in Fig.l, the radiation component is dominant mainly in the

range of longer wavelength and

the diffraction component is in shorter wavelength range where the ship motion is negligible. The added wave resistance

acting on a large full form ship is mainly due to the

diffraction and breaking of waves at its blunt bow and it becomes larger with larger bluntness of the bow.

o) (-i

o)

0)

Totaladdedresistance

Ordinary bow

Sharp bow

-Wave length / Ship length

Shorter wave Longer wave length range

length range

Diffraction wave _{Incident wave}

Due to ship moon

Due to bow reflecon

Fig.2 schematically shows the waterline shapes of the bow. In Fig.2, incident wave is reflected and broken at the blunt bow. Such wave diffraction or breaking generates the reaction force backward on the ship's bow.

A simple idea to reduce the added wave resistance can, therefore, be to sharpen the still waterline shape of the bow. The sharpened bow can reduce such reaction force because the reflected waves in forward direction are decreased. However, the reduction rate of added wave resistance is only 5% even if the shape of still waterline is optimized within some design restrictions or principal dimensions in order to keep the performance in still water. For example, 5% reduction of added wave resistance causes only 1% reduction of sea margin when the sea margin itself is 20%.

When a ship is sailing in the sea, the water surface is elevated at the bow that is called dynamic swell up and incident wave motion is occurred around this swell upped water level. Therefore, to sharpen the bow shape above the still waterline causes to reduce the added wave resistance. lt means that there is room for further modifications of hull shape above still waterline and it does not influence the hull propulsive performance in still water.

Based on this

consideration, a new concept of bow shape was developed through the joint research project with Osaka University.

Fig.l Components of added wave resistance for hull F.P.

ship in head waves

Fig.3 Comparison of bow shape (Beak-Bow:I7OBC)

Fig.5 Scale model of Ordinary Bow and Beak-Bow

Fig.3 shows a comparison between the ordinary bow and a new concept of bow shape applied on a I 70,000DWT bulk carrier. This new bow is named Beak-Bow, because its shape looks like the beak of a bird. For this ship the design draft is 165m. Comparing the 22m waterlines between both of the ordinary and the new shapes, for example, waterline shape of Beak-Bow is sharper than the ordinary bow.

The total ship length with Beak-Bow becomes longer as shown in Fig.3 because the bow is lengthened forward. From the practical viewpoint, the ship length is limited by some port regulations. In the case of 170,000DWT Type Bulk Carrier, the ship length becomes about 300m by adopting Beak-Bow. (See the dashed line in Fig.4)

The ship can not lengthen her length up to 300m, if she enters a port in Europe, because the ship length under its port regulation is to be under 289m.

Cutting out of the tip of the bow shape as described by the solid line in Fig.4 is necessary for satisfying this port

regulation. But only such cutting out of the bow may

increase the added wave resistance because of its triangle section's remaining at the bow front.

Therefore, the bow shape is

modified to shape the

waterline as sharp as possible under keeping the profile of the bow as the solid line in Fig.4 under the condition of the maximum ship length. The above modified bow shape is named Ax-Bow, because of its profile shape's looking like the ax but no more the beak.

The effect to reduce the added wave resistance by these two bow shapes, Beak-Bow and Ax-Bow, is confirmed by model tests in waves.

EXPERIMENTAL RESULTS

Some model tests for Beak-Bow have been preformed and their results are given in Ref.3 and 4. One of the results for Beak-Bow fitted on 170,000DWT Type Bulk Carrier is shown here. The principal dimensions in real ship scale are,

o Bok-bow o Ordrnaxy bow -o

w.

o Beak-bow O Ordinary bow o

CJ

Beak-bow O Ordinary bow

-Cal

J

0.0 06 10 15 20 21L

Fig.6 Added resistance in regular wave (Full Load, l3knot, Head Wave)

00 05 10 1.6 20

2JL

Fig.7 Heaving motion in regular wave (Full Load, I3knot, Head Wave)

00 05 10 1.5 2,0

7JL

Fig.8 Pitching motion in regular wave (Full Load, l3knot, Head Wave)

ship length (Lpp) = 2790m, breadth (B) = 450m and

design draft (d) = 165m. Four meter length (Lpp4.Om) model is used for the model test in regular head waves and the measurement items are resistance and ship motions. The model was towed at the ship speed of 13.Oknot in ship scale in regular head waves. The wave height was 30m in ship scale and the range of the ratio between wave length and ship length( A /Lpp) was 04-- 1 .6. The model tests were performed at Tsu Ship Model Basin of NKK Engineering

25 20 15 10 05 0.0 1.0 0.8 06 04 0.2 00 1.0 08 0.6 04 0.2 0.0

Speed

Fig.9 Estimate power curves in wave (Full Load, BF6. Head Sea)

Research Center, Japan. Fig.5 shows the comparison of the scale model for the ordinary bow and Beak-Bow.

Fig.6 shows the results of measured added wave resistance

in

_{regular head waves. The vertical axis shows the}

non-dimensional value of added wave resistance (Kaw) and horizontal axis shows the ratio of wave length and ship length (A IL).

O

shows the results for ordinary bow and shows the Beak-Bow results. The dash dot line shows the theoretical calculation result and its description is given in the next chapter.

As shown in Fig.6, Beak-Bow gives the smaller added wave resistance than that for the ordinary bow by the ratio of 20-'--30%. There was no difference on the resistance in

still _{water between two bows. Experimentally obtained}

heave and pitch motions given in Fig.7 and Fig.8 do not have much difference between the two bow models. Fig.9 shows the horsepower curves in long-crested irregular waves predicted by using the experimental results of the added wave resistance in the regular head waves given in

Fig.6.

In Fig.9, the sea condition is Beaufort 6 and the significant wave height (H 1/3) is 37m and the mean wave period (To) is 7.5sec. The incident wave direction is head sea. The solid line shows the power curve in still water, dash line and dash dot line are those for the ordinary bow and the Beak-Bow in waves. At the ship speed corresponding to a certain main engine output in still water, the horsepower increase for the ordinary bow is about 20% and that for Beak-Bow is about 15%, which indicates about 5% reduction of horsepower.

Fig.lO Scale model of Ordinary Bow and Ax-Bow

From the viewpoint of speed loss in the same sea condition, the speed loss for the ordinary bow is about 0.9knots. The speed loss for the Beak-Bow is about 0.7knots. Therefore, the reduction of speed loss by adopting the Beak-Bow is about 0.2knots

In the case of Ax-Bow, the scale model of a 170,000DWT Type Bulk Carrier, shown in Fig. 10, was used for the model test.

The model tests in various wave directions were performed by using a 3.5m length model. The tests were in regular waves with their direction of every 30° from _X

1 80°(head wave) to 0°(follow wave). The wave height was 3m in ship scale. The model was towed at the ship speed of l3.Oknot in ship scale. The measurement items were resistance and motions (6-compornents). This model test was performed in Seakeeping & Maneuvering Basin of SUMITOMO HEAVY INDUSTRIES, LTD., Japan where various wave directions can be realized. Figli and Fig.l2 show the results of added wave resistance and heaving motion in various wave directions.

In each figure, Q shows the ordinary bows results and shows the Ax-Bow's results. In Fig. Ii, Ax-Bow gives the smaller added wave resistance than that for the ordinary bow by the ratio of about 20-30% in head and oblique waves( x l80°-- 120°). There is no difference between two bows in beam and follow wave (x 90°-0°). As for

heaving motions, the difference between two bows is very small in all wave directions as shown in Fig.12. There was also no difference in the other measured motions (Surging, Swaying, Rolling, Pitching, and Yawing).

As mentioned above, Beak-Bow and Ax-Bow can reduce the added wave resistance in head sea by 20%-30% with no difference of ship motion.

2.5 2.0 1.5 1.0 0.5 0.0 -0.5 00 't i.0

O Ordinary Bow S Ax-Bow

05 1.0 15

A/L

Added Wave Resistance (=18Odeg.)

Added Wave Resistance (=l2Odeg.)

t

0.5 0.0 2.0 1,5 't 1.0 ra 2.0 1,5 .0 'n ra 0.5 00 20 00 ?s 1.0 AIL

Heaving Motion (x=!8Odeg.)

S S S S

I

15

Heaving Motion (=9Odeg.)

0.0

20 00 05 1.0

AIL

Heaving Motion (=6Odeg.)

'5 20 20

s

S

s

s

s s S o

I

o s

.5,,

5 s

s..

0.0 -0.5 00 0.5 _e

s

r

05 IO 15 ?./L

Added Wave Resistance (x =6Odeg.)

2.5 2.0 '.5 1.0 0.5

,

,I

5

0.0 -0.5 05 lO IS 00 A/L

Added Wave Resistance (X 3Odeg.)

2.5 2.0 1.5 .0 't d 0.5

jj

I,'

0.0 0.5 05 io IS 00 A/L 2.0 1.5 't 1,0 0 ra _0.5 0.0 00 00 05 IO IS AIL

Heaving Motion (xI2Odeg.)

20 e _e

I s.

_.

₅ O S 0 05 lO '5 A/L 20 2.0 15 l'o ra 0.5 0,0 00 05 lO IS A/L

Heaving Mot on (=3Odeg.)

20 S

s

s S 00

I-.

0,5 IO AIL 20 20 1,5 .0 05 e

55

_{e s} S S e 0.0 S 2.5 2.0 1.5 .0 0.5 0.0 -05 00 2.5 2.0 1.5 1.0 0.5 0.0 -0.5 00 o 0 _û

2. So

00

S û s o_S 0.5 1.0 IS A/L

Added Wave Resistance (=15Odeg.)

20 o û o

05 s

s S

0 I

O

S.

05 0 A/L 15 20 2.5 2.0 1.5 1.0 05 0.0 -0.5 00 2.5 2.0 o o

I

SO

s

.5

S

j

s 05 10 5 A/L

Added Wave Resistance (=9Odeg.)

20

1.5

jO Ordinary Bow S Ax-Bow

2.0

1,5

I,0

Added Wave Resistance (=Odeg.) Heaving Mot on (x=Odeg.)

Fig.1 I Added Wave Resistance in Regular Wave Fig. 12 Heaving Motion in Regular Wave

(Vs= 13 .Oknot) (Vs1 3.Oknot)

00 03 iO IS 20

A/L

Heaving Motion (xl5Odeg.)

s e _s s 2.0 IS

.5

e I.0 _{s S s} 0.5 S 00

.

20 S 20

THEORETICAL INVESTIGATION

In order to optimize the design of Beak-Bow or Ax-Bow, it is necessary to estimate the added wave resistance in consideration ofthe bow shape above the still water line.

Added wave resistance acting on a ship

is normally defined by the sum of the resistance due to ship motions

and that due to wave diffraction

at the bow, In the

conventional method both components are calculated under the assumption of small amplitude of incidentwave, small amplitude of ship motion and wall-sided hull form around the still water line. Therefore, conventional calculation method can not calculate added wave resistance correctly for the change of the hull shape above still waterline by

which Beak-Bow or Ax-Bow

is characterized. The conventional method gives the same result for the ordinary bow and Beak-Bow/Ax-Bow because their hull form at and under the still water line ¡s quite the same.

In the future, development of non-liner theories is

expected, which can consider the effect of the change of the hull shape below the moving water surface to the added wave resistance. But they are still under development and need much computing time so that they are notso far usable for the practical design. In order to estimate the added wave resistance practically, the following modifications are

applied to the conventional method.

When a ship is sailing in still water, the water surface is elevated at the bow, which is called the dynamic swell up.

Incident wave motion is occurred around this swell upped water level. Therefore, to use the bow's waterline shape at this swell upped water level seems to be more reasonable in order to calculate the added wave resistance than to use that at still water surface level. The water line shape at swell upped water level is blunt in the case of the ordinary bow, but the waterline shape of Beak-Bow or Ax-Bow is sharp at the swell upped level. By using hull shape at this swell upped water line level, the difference of an added wave resistance between the ordinary and the new bows can be considered theoretically.

By using the above method, the difference of diffraction component of the added wave resistance due to the change of the hull shape above still waterline can be considered. On the other hand, referring to the model test results (Fig.6 and Fig.l I), Beak-Bow/Ax-Bow gives the smaller added wave resistance not only for diffraction component in shorter wave length range (A /L< I) but also for radiation component in longer wave length range (A IL = I) in spite of no difference in motion. (Fig.7, Fig.8 and Fïg.12) It is estimated that is caused by the difference of diffraction component at the bow in motion. In the above method of calculating the diffraction component the input wave amplitude is assumed to be only due to the incident wave without the effect of ship motions. But, from the above estimation, the relative motion amplitude due to ship motions at the bow was used in the calculation instead of the incident wave amplitude.

Fig.13 Comparison of calculation and experiment for added wave resistance

(Full Load, l3knot, Head Wave)

Fig.13 shows the comparison between the theoretical result of added wave resistance in regular head waves given

by the conventional method and that by the modified

method given above. Fig.13 shows the result of I 70,000DWT Type Bulk Carrier and , _O and solid line are same as Fig.6. The dash line and the dash dot line show the modified calculation results for Beak-Bow and ordinary bow respectively. As shown in Fig.13, the present modified method can describe the

difference of added wave

resistance for different bow shape.

In the range of longer wavelength around AIL = I, the modified method gives bigger added wave resistance than the conventional method and give more closer results to the experiments. lt indicates that the added wave resistance due to the wave diffraction at the blunt bow is large also when the ship motions are large. Therefore, the wave diffraction

at the blunt bow does not seem to be only due to the

incident wave amplitude but due to the relative motion amplitude between the incident wave and the ship motion. This modified calculation method can estimate the added wave resistance considering the difference of bow shape. The accuracy of estimation, however, is not enough. Therefore further investigation on improving the calculation method, developing the new estimation method and study for their practical use are necessary.

CONCLUSIONS

Conclusions of the present study are as follows:

I. In order to reduce the diffraction component of added

wave resistance, new bow shapes, Beak-Bow and

Ax-Bow, are developed. Beak-Bow has the sharpen bow shape above the still water line and Ax-Bow has the same under the restriction of ship length.

Beak.Bow

O Ordinary Bow

- Conventana1 Method

- -

Modified Method (Beak.Bow)

- . - Modified Method (Ordinary Bow)

o

,/

0/

00 05

_lo

1.6 2.0 1f L 25 2.0 1.5 10 05 0.0

Model tests on a 170,000 DWT Type Bulk Carrier in regular head waves show the reduction ratio of added wave resistance by 20 to 30% by adopting Beak-Bow.

Based on the model tests results for a 170.000 DWT Type Bulk Carrier in the case of Beak-Bow, the horse power reduction in Beaufort 6 head sea condition is estimated to be 5%, which corresponds to be 0.2 knots reduction of speed loss.

From the results of model tests, Ax-Bow shows almost the same effect of added wave resistance reduction as Beak-Bow.

For designing bow shapes, a practical procedure of calculating added wave resistance is proposed based on modif'ing the conventional method. This procedure can consider the bow shape above the load water line and diffraction of incident wave at bow with ship motions.

The accuracy of this modified theoretical procedure is not enough for practical design. Improvement of the procedure, development of new method and study for practical use is necessary.

This study has been supported by the Technology Development Fund of the Ship & Ocean Foundation and the fund was instituted by donations from the Nippon Foundation derived from revenues from motorboat racing.

REFERENCES

Fujii, H. and Takahashi, T.: Experimental Study on the resistance Increase of a Ship in Regular Oblique waves: Proc. 14th ITTC, Vol.4, Ottawa: 1975

Matsumoto, K. et al.: BEAK-BOW to Reduce the Wave Added Resistance at Sea.: PRADS '98.: Sept. 1998. Matsumoto, K.: Development of hull shape considering the

performance of motion: Japan Towing Tank Conference Symposium: Dec. 1999

Naito, S. et al.: An Experimental Study on the

Above-Water Bow Shape with a Small Added wave

resistance.: Journal of the Kansai Society of Navel