RESEARCH ON BULBOUS BOW SHIPS Part II.B

REPORT No. 72 S June 1965

(Sgo/74-81)

STUDIECENTRUM T.N.O. VOOR SCHEEPSBOUW EN NAVIGATIE

NETHERLANDS' RESEARCH CENTRE T.N.O. FOR SHIPBUILDING AND NAVIGATION

SHIPBUILDING DEPARTMENT MEKELWEG 2, DELFT

RESEARCH ON BULBOUS BOW SHIPS

Part I I .B

BEHAVIOUR OF A 24,000 DWT BULKCARRIER WITH A LARGE BULBOUS BOW

IN A SEAWAY

(HET GEDRAG IN ZEEGANG VAN EEN 24.000 DWT BULKCARRIER VOORZIEN VAN EEN GROTE BULBSTEVEN)

by

PROF. DR. IR. W. P. A. VAN LAMMEREN

and

IR. F. V. A. PANGALILA

(Netherlands' Ship Model Basin)

Issued by the Council This report is not to be published

This report is the second one in the series on bulb-ous-bow research, a project strongly supported by the shipowners' and the shipbuilders' associations

in the Netherlands, the Koninklijke

Nederland-sche Reedersvereeniging" and the Centrale Bond van Scheepsbouwmeesters in Nederland"

respec-tively.

The investigation reported here is to be consid-ered as a continuation of a former report in which the performance of the same bulkcarrier has been investigated in smooth water.

Again the actual work has been carried out by

the Wageningen towingtank, the Nederlandsch Scheepsbouwkundig Proefstation".

The aid received from Messrs. N.V. Koninklijke

Paketvaart-Maatschappij is gratefully

acknow-ledged.

CONTENTS

page

Summary 5

1 Introduction

2 Particulars of ship and model 6

3 Test programme and analysis of records 6

4 Discussion of the results 8

5 Conclusions 14

References 14

RESEARCH ON BULBOUS BOW SHIPS

BEHAVIOUR OF A 24,000 DWT BULKCARRIER WITH A LARGE BULBOUS BOW

IN A SEAWAY

by

PROF. DR. Jr. W. P. A. VAN LAMMEREN and IR. F. V. A. PANGALILA

Summary

The propulsive performance, the bending moments amidships and the motions of a ship model representing a 24,000 DWT bulkcarrier were measured in smooth water and in irregular long-crested waves, over a speed range corresponding to 10 knots through 16 knots.

The model was tested with a conventional bow and with a 9% bulb added respectively.

In the loaded condition and for the speed range considered with smooth watertests, no saving in power was measured for the model with the bulbous bow in comparison with the model with the conventional bow. In ballast, however, smooth water tests showed that a reduction in power was obtained for speeds above 13 knots when the ship model was fitted with

a bulb [3].

The waves in which the models were tested correspond to long-crested irregular head seas, windforce Beaufort 6 with a significant wave height of 2.90 m and an average period of 7.2 seconds. In the ballast condition for the sea described, the model with the bulbous bow still required less power than that with the conventional bow to attain speeds above 13 knots. Only small differences in bending moments due to rough seas were measured between the two models and hardly any difference in pitch angle. Smaller amplitudes of the relative motion of the bow were measured at the model with the bulbous bow when it was in loaded condition. In the ballast condition the average immersion of the propeller was reduced by the bulb.

1

Introduction

It is known that in smooth water, at the higher speeds, where the wave-making resistance accounts

for an important part of the total resistance

expe-rienced by a ship, a correctly shaped bulb may reduce the resistance considerably. LINDBLAD [1] and others [2], [3] showed that especially by adopting large bulb-areas great reductions can be obtained.

Nevertheless a bulbous bow is not commonly adopted, one of the principal reasons being the feeling of shipowners and shipbuilders that apart from anchoring and docking difficulties

the bulb would produce an unfavourable effect on ship performance in rough water such as:

the shipping of green seas speed loss

slamming pitching

Tests by GERRITSMA [4] on a series-sixty model block coefficient 6 = 0.60 and also by LASKEY [5] on a 45,000 DWT bulkcarrier = 0.83, show that in very rough seas the power increase at a certain

speed will be larger for a ship with a bulbous bow

than for a ship with a conventional bow. But for wind forces up to Beaufort 5 the speed losses of

both ships will be substantially the same.

TSUNODA [6]

reported the design of a bulb

having a radius of 2% of the ship's length for a

high speed cargo-liner. A power reduction of 9%

was realized with this bulb in smooth water at Froude number 0.256 (19 knots). This

saving in power decreased with increasing

rough-ness of the sea and changed into an increase of

power for wind speeds above about 24 knots. GERRITSMA, LASKEY and TSUNODA conducted tests in regular waves. By determining frequency response functions, the performance in irregular

seas could be predicted, by using various wave

spectra for the different sea states.

It was decided to conduct the tests reported here in irregular waves and to measure the various parameters directly. In the seakeeping basin of the Netherlands' Ship Model Basin (N.S.M.B.), long-crested irregular waves can be generated having an energy distribution along the various frequencies corresponding approximately to the

distributions as reported by oceanographers. An

advantage of tests in irregular waves is the

pos-sibility of detecting slamming" because

it is

difficult to base a prediction of this phenomenon on tests in regular waves.

The tests were conducted on a model of a

24,000 DWT bulkcarrier for Froude numbers

ranging from F, = 0.125 through = 0.188. The area of the bulb was 9% of the immersed midship

section area when fully loaded and the radius

equal to 1.45% of the length between perpendic-ulars. For this size of bulb at not too high Froude numbers, considerable reductions in power may not be expected for the ship in fully loaded

condi-5

d.

6

tion. The owners expect, however, that the ship

will often sail at a considerable smaller draft than

the full load draft. For such a loading condition the ratio bulb area/midship section area is rel-atively large ,(14%) and more effect of the bulb

can be expected. Moreover a ship in ballast condi-tion will sail in general about one knot faster than

in the loaded condition, at the same power. This results in a relatively greater wave making part,

which can be 'decreased by the bulb..

Because a sea state higher than' corresponds with Beaufort 6 occur only very sporadically on the route where the ship is to sail, the comparative model tests were conducted in waves corresponding

to sea state Beaufort 6.

A few tests were conducted with an additional

ballast condition (ballast condition II) in order to determine whether for the ship with a bulb a

large trim angle is allowable..

'The tests were carried out for the following speed ranges:

loaded condition: 10 knots through 15 knots ballast condition: 10 knots through 16 knots.

2

Particulars of ship and model

2'.1 The main particulars of the ship are given

table I.

TABLE I. Particulars of the ship fully loaded and in two

ballast conditions

2.2 The model of the ship scale 1: 45, had a sep-arate removable bulb made of wood as shown in

figures 1 and la.

For measuring bending moments and the

detec-tion of slamming the model was cut amidships and the two model-halves were connected by a strain-gauge dynamometer. A thin plastic strip covered the opening between fore and aft parts.

The weight, longitudinal centre of gravity and the longitudinal moment of inertia of each part were

appropriately scaled down from the equivalent,

parts of the ship. The weight distribution used for

table I is valid for. the conventional bow. It was assumed that the

additional weight forward

caused by the presence of the bulb is equal to the extra displacement.

2.783 Z

Fig. 1.. Model with conventional bow..

4

15'ig.

Ia. Model with bulbous bow.

3 The test programme and analysis of

records.

3.1 General

The smooth water tests were conducted in the deep water basin with a 6.90 m wax model. The tests in irregular headseas were conducted in the sea-keeping basin with a smaller model of 3.83 m

Loading condition 1 Full 1 load Ballast I Ballast 1 II Length between perpendiculars rn 172.21 172.21 172.211 Breadth moulded m 92.86 1 29.86 99.861 Draft: foreward m 9.731 51.21 4.13 I mean m 9.73 6.27' 5.14' aft m 9.73 7.33 6.141 1 Displacement: total ma 29230 r 18200 14680 fore body ma 15158 pi 8904 70221 after body ma 14072' 9296 7658 Block coefficient 0.764 1 0.732 0.721'

, Midship section coelTicient 01.991 , 0.986 0.9851 Centre of buoyancy with

respect to section

10:-total m + 1.591 1.40

1.40

fore body m +34.861+36.53

after body m _34.241-37.76

Longitudinal radius of

gyration in per cent of 1

LBP total % fore body % 23.591 25.43 12.351 13.491 24.93, after body % 12.53 L' 13.461 Additional displacement of the bulb Ma 1471 147

Bulb area in per cent of

1

1

immersed midship section

area _% 9 14' 17'

Radius of bulb. m 2.5 2.51 2.3

in

-length, made of wood. In waves the following

quantities were measured simultaneously: propeller thrust

number of revolutions of propeller

C. bending moment amidships

relative motion of the bow at station 19 relative vertical motion of propeller pitch angle

g speed

The relative vertical motion of the propeller was

measured because TAKEZAWA reported in [7] that

the propulsive efficiency of a ship with a bulbous bow in waves will be reduced because of a reduc-tion of the mean propeller immersion.

3.2 Sea state

For the irregular waves a sea state was simulated with the shape of a Neumann spectrum

fhh( co ) = A co-6 . . .. m2- sec.

Iii terms of significant height and average period

it becomes:

fhh, (co)

= 6 A/6 .7 A/7r ( T (0) . e-67rz(

R,132T

in which:

fhh(w) =- spectral density = circular frequency

= significant wave height (average of

1/3 highest waves) T = average period ilia is defined as i-Lia2 = 4A/mo and T is defined as T = 2V:z in which

mo = f jh' h(co)dco and

77/2 fil;h (CO) (02C1.10

0

Only the shape of the applied spectrum shown in

figure 2 is that of a Neumann spectrum. Ji and

T are the average values as measured by weather ships at Beaufort 6 and reported by ROLL in [8]. 3.3 Analysis of the oscillating records

The records were analysed by measuring the

difference between the still water value and the extremes of the record where the tangent is horizon-tal. However, in order to avoid confusion, the negative maxima and the positive minima were left out of consideration. The following sketch

mo nZ2 1.0 hogging still water saggi ng

moments follow the well known Rayleigh

dis-tribution. In a wide spectrum, however, these

negative peaks and positive troughs do occur and therefore the distribution functions follow a

dif-ferent pattern, as can be seen in the figures 8, 9 and 10. A steep slope of the cumulative curve,

combined with a relatively high value of the first block in the histogram, indicates the existence of

a wide spectrum.

3.4 Determination of power in waves

In the seakeeping experiments the thrust increase due to waves was determined by subtracting the thrust measured in still water from that measured

in the irregular sea. The propeller thrust for the

ship in waves was obtained by adding this thrust

increment, extrapolated to full size to the predicted thrust in the trial condition as derived from the

still water tests with the 6.90 m long wax model.

This trial prediction was also used to find the

relation between thrust and power at each ship

speed. Therefore the power and revolutions given in this report are related to the same propeller as

is to be fitted to the ship. This propeller had the

full size characteristics, according to table II.

7

SIGNIFICANT WAVE HEIGHT AVERAGE WAVE PERIOD

Hb.2.90m T .72 sec.

/ \

MEASURED SPECTRUM

/

_\

/ THEORETICAL SPECTRUM

\

\

/

\

/ /

\

0.5 10 15 CO INsec'

Fig. 2. Wave spectrum for wind force Beaufort 6.

gives an example of the manner in which the

distribution functions were obtained for the

bend-ing moment.

In a narrow spectrum, where no negative peaks or positive throughs occur, the hogging and sagging a.

e.

a

TABLE II. Propeller characteristics..

This method was adopted because a stock

propel-ler model with other characteristics was used in

the seakeeping experiments.. The underlying as-sumption is that the thrust increment necessary to

maintain speed in waves is independent of the

properties of the propeller.,

4 Discussion of the results

4.1 The average period of the sea spectrum

As mentioned in section 3.2 the average period of the sea spectrum is defined by:

T =

2nV

mo

n22

Recent publications, see for instance [9] suggest that it is better to relate the observed period to the

first moment of the spectrum than to the second

moment. In that case T should be defined by: T 2.7"±b

7n1

03

where ml =7- f fhh(())r- co.- cloy.

If this definition is used, the spectrum simulated for the tests belongs to an observed period of 7.8 seconds instead of the previously mentioned 7.2

seconds.

4.2 The sea state selected for the tests Beaufort 6 is rather low for measuring serious slamming"

or considerable speed loss. However; this sea state

was selected for practical reasons, since the owners

reported that on the route where the ship is to

sail, sea states above Beaufort 6 occur only sporad-ically.

4..3 Power

In the figures 3 and 4, power is plotted versus

speed for smooth water and Beaufort 6 in the fully

loaded and the ballast I condition respectively. The bulb, having an area of 9% of the immersed midship section area in the loaded condition, did not affect the wave making resistance very much.

llq 9 8 7 6 3 2 V IIN KNOTS

Fig. '3'. Power and propeller revolutions for the full 'load

condition in a Beaufort 6 sea. )000 MOO ' BULBOUS BOW I BOW 1 I 11560 OP I 90 80 70 60 50 _._ --.L. CONVENTIONAL 1 'BEAUFORT 6

A

Ai

Frzl

.BEAUFORT O. 1 1 _ 3000 7000 1 s000 5000 4000 . ''''

V

i

Appip

BEAUFORT 6.

r

I 1

'Er

-1 , -1

A/

BEAUFORT 0:

In

gy

,

AI

I

, 0.1-1.P., I 2000 1000 - - -II 1 a_I in 1116 12 IT IL lB TB N 000 BULBOUS BOW - 11 10

A

,0(

T

CONVENTIONAL BOW ' I )0a 1 1 i , BEAUFORT6.

A /

re

MO

pr.'

I 1 .411BEAUFORT0 91 /4

00CM-add

300

- wpm 11111.

pier,.."

71 I! BEAUFORT

6.DM

004-

61 I

Pr

11

1 000 i

.44

ri p

A BEAUFORT M 000 rp- Aldr pei

/-000

-,

ripp

r

v

coo.A- ,

_r,

i

-

- -I i I Diameter 5.70 m Pitch at 0.7 R. 4%40 m

Developed blade area ratio 0.532

Number of blades 4

IN KNOT'S

Fig. 4. Power and propeller revolutions for the ballast

condition in a Beaufort 6 sea.

10 11 12 14 15 I

/

D.H.P. 13 V 15 300

Fully loaded at 7500 HP the following speeds

were obtained:

'Conventional bow Bulbous bow Beaufort 0 14.95 knots 14.90 knots

Beaufort 6 114.3 knots 14.2 knots

In the loading condition ballast I (draft forward

=-5.21 m, draft aft = 7.33 m) the bulb area was

14% of the immersed midship section area.

For this ballast condition at

7000' HP the following speeds were obtained:

Conventional bow Bulbous bow Beaufort 0. 15.4 knots 16.2 knots Beaufort 6 14.5 knots 15.0 knots

Figure 4 shows that the saving in power in still

water, due to fitting the bulb, is reduced in a,

Beaufort 6 sea, but is still extant.

An estimate has been made for the required

power in sea state corresponding with Beaufort 7,, starting from the assumption that the ratio

propel-ler torque increase / (significant wave height)2 remains constant. Actually this is only correct if with a changing sea state only the significant

wave height changes and the average period

remains constant.

Where the average period for Beaufort 7 (T 7.8 sec), deviates only a little from the average

period for Beaufort .6, (T 7.2 sec) the above assumption can be used,.,

The calculated results are shown in the figures. 5 and 6.

Figure 6 indicates that for the ballast I

condi-tions, as far as power is concerned, in a Beaufort 7

sea at 7000 HP the ship with the bulb will have

approximately the same speed as the ship with the

conventional bow. When for other reasons' the master of the ship is forced to reduce power, the

bulbous bow will be distinctly disadvantageous.

4.4 Bending moments

Figure 7 shows smooth water bending moments for the various loading conditions. Since a bulb

affects the wave pattern caused by a moving ship in smooth water, a difference is expected between the still water moments of the ship with

conven-tional and of that with bulbous bow. The figure

shows an increase in hogging moment for the ship

with the bulb in the higher speed range, which

is logical because. a bulb is expected to reduce the

bow wave, /0000 9000 18000 7000 6000 5000 400 3000 200 1000 10000 9000 7000 6000 5000 4'00 300 200 1000 V IN KNOTS

Fig.. 6. Estimated power and propeller revolutions for ,the ballast I condition in a Beaufont 7 sea.

.70 60, 00 90 60 ce; 70. 60 '50 ! BULBOUS BOW BOW 1 -FORPrr - ----A

_

CONVENTIONAL BEAUFORT ESTIMATED 7.4 Grey ' ,

w

'Ippr

0

Fir.

A

v

A

AirA

7HP

Al

A,BEAUFORT

ar

' 1 I d.. BULBOUS BOW BOW' 1 ESTIMATED BEAUFORT CONVENTIONAL I

\

FOR 7.

.

Aide

AO

A

1

Fil

Fr

FIRM=

m"wmpm-ipr-e' .4

V

AI

4 ' R.P.M,. 0

/41

RUH

ilj

I,

mi

Pr'

All

D.H.P.

1111

r 1

BEAUFORTa.

lirM /

I Pr-

r

1 1 1 i V IN 'KNOTS

Fig., 5., Estimated power and propeller revolutions for the full load condition ill a Beaufort 7 sea.

10 tr 12 43- TO ts 16 IS 12 10

=

8000 100 90 80 9 13 15 16 110

10 3000 100 50 BULBOUS BOW

- -

CONVENTIONAL BOW

The distribution of wave bending moments at service speed in percentages of occurrence in a Beaufort 6 sea state is given in the figures 8, 9 and

10 in histograms as well as in cumulative curves.

The cumulative curve represents the percentage of occurrence of bending moments below a certain value.

Figure 8 shows that in the full load condition

for bulbous and conventional bow the wave

sagging moment was larger than the wave hogging

moment. Of the wave hogging moments only 5%

SPEED OF SHIP 15 KNOTS MEAN DRAFT 9.73m 10000 ///"--CONVENTIONAL ROW HOGGING MOMENT -20000 30000 0 BULBOUS BOW 10000

was higher than 19,000 tm, while about 10% of

the wave sagging moments exceed this value. In

the conditions ballast I and ballast II, however,

the hogging moment was larger than the sagging

moment.

For ballast I as shown in figure 9, 5% of the sagging

moments exceed 16,000 tin and 10% of the hog-ging moments exceed this value.

For ballast II as illustrated in figure 10, 5% of the

sagging moments exceed 15,800 tm and 11.5%

of the hogging moments exceed this value.

SPEED OF SHIP 15 KNOTS

"CONVENTIONAL BOW

SAGGING MOMENT

---=9----,

20000 30000

Fig. 8. Histograms and cumulative curves for wave bending moments. Full load condition, speed 15 knots in a sea state corresponding with Beaufort 6.

BULBOUS Bow BOW 1 - CONVENTIONAL MEAN DRAFT 6.27m 4. _ PP-MEAN DRAFT 5.14m 2000 1000 10 11 12 13 14 15 16 10 12 13 15 16 V IN KNOTS V IN KNOTS

Fig. 7. Smooth water bending moments amidships.

IN trT1 _{IN trfl}

'0

'BULBOUS BOW

100

50

100

50

CONVENTIONAL BOW

Fig. 9. Histograms and cumulative curves for wave bending moments. Ballast I condition, speed 16 knots in a sea state corresponding with Beaufort 6.

Only the bulbous bow model was tested for the

loading condition ballast II (draft forew-ard

=-4.13 m, draft aft = 6.14 m). The reason for testing this condition was to determine whether if the

test results would show that a bulbous bow is

favourable this loading condition will be

allow-SPEED OF SHIP 16 KNOTS

BULBOUS BOW

HOGGING MOMENT

CONVENTIONAL BOW

SPEED OF SHIP 16 KNOTS

BULBOUS BOW SAGGING MOMENT

able or not. In the Beaufort 6 sea the bending

moments amidship proved to be not higher than for

the ballast

I condition (draft forward =

5.21 m, aft = 7.33 m).

The significant (average of 1/3 highest) wave

bending moments are given in figure 11. For

11

Fig. 10. Histograms and cumulative curves for wave bending moments. Ballast II condition, speed 16 knots in a

sea-state corresponding with Beaufort 6.

30000 0 10000 20000 30000

20000 30000

10000

10000 20000 30000 0

M IN t M IN t en

SPEED OF SHIP 16 KNOTS BULBOUS BOW

HOGGING MOMENT

20000

10000

SPEED OF SHIP 16 KNOTS

BULBOUS BOW

SAGGING MOMENT

12 2000 Fg ono 0 "E' ?X' CD 0 1000 2000 10

calculating the bending stresses, the smooth water

moment at zero speed has to be added to the

bending moments of figure 11.

The smooth water bending moments at zero

speed were 4200 tm hogging moment in the fully loaded condition and 44000 tm hogging moment

in the ballast I condition respectively. It appears that in the considered sea state the bulbous bow

does not affect the bending moments adversely. 4.5 Relative motion of the bow

The relative motions of the bow with regard to the

wave surface measured at station 19 (0.05 LBP

aft of foreward perpendicular) are shown in figure 12. Smooth water values and significant (average

of 1/3 highest) values of bow submergence and

emergence in the Beaufort 6 sea are plotted..

The addition of the bulb affected the relative motions considerably. In the loaded condition submergence as well as emergence were larger

12 0 14

V IN KNOTS

Significant values of bow

13' 1_4 15 16 10 11 12 13 th

IN KNOTS _{V pg KNo.Ts}

Fig. 11. Significant wave bending moments ,ainidships.

16 10

for the conventional bow model than for the

bulbous bow model and the figure shows also the

reduction in bow wave caused by the bulb in

smooth water Beaufort 0. In the ballast I condition the model with bulb experienced more emergence

due to its lower wave surface in smooth water

and its larger emergence amplitudes.

From cumulative oscillation amplitude

distribu-tion curves of which figure 12 has been composed, it followed that in the fully loaded condition no

complete emergence of the keel at station

19' occurs. In ballast I about one per cent of the

am-plitudes were so high that the keel at station 19 was completely emerged, which does not mean

that the whole bulb came out of the water. Not so much because the motion amplitudes are different

more forward, but due to the fact that the ship's smooth water bow wave is higher there. In the

ballast II condition five per cent of the emergence

values were larger than the ship's draft at station

19.. III BULBOUS BOW BOW I L-- CONVENTIONAL MEAN DRAFT 9.73m

\

11 IMEAN DRAFT 9.73 m is. BOW BOW

..,

I BULBOUS CONVENTIONAL I I

/

'MEAN DRAFT 6.27 m

\

II I MEAN DRAFT 5.14 m

/

_A MEAN IDR-AFr_S.2 11 I 7 m , I

\

---"7"---,

I MEAN DRAFT 5.14 m--"''" ; B6Lsous CONVENTIONAL eisw SOW

WEAN/ DRAFT 9.73 rn MEANDRAFT

/ 6.27m MEAN 1 DRAFT 10r, I /

Mill

FA

F.

rammo

, BEAUFORT 16. BEAUF0 / 7 6. A BEAUFORT 0. ,

MIMIlliiiIME

Al

BEAUFORT -"MEI

man

O. , BEAUFORT0. 41 _....1111

.

111Limi

12 0 IL IS 16 10 fl 17 V IN KNOTS, V IN KNOTS

submergence and emergence relative to wave surface at station 19:

15 16 .25 20 It Fig. 12.

-11 12

-

--/

16 V

4.6 Slamming

Since one of the most important reasons

for

slowing down in rough seas is the occurrence of

slamming, it is necessary to investigate the effect of a bulbous bow in this respect. The model being divided into two parts and rejoined by a

dynamo-meter, slamming can be detected as a vibration superimposed on the bending moment records.

Serious slamming shows up clearly in a record as in the following sketch.

slamming vibration

Such vibrations did not appear at all in the present test series. Only unimportant vibrations which can

be characterized as shudders" were found. The

following sketch shows the nature of these minor

vibrations.

There was no difference between the model with

bulb and the conventional bow model in this

respect. This is also shown in the figures 8 and 9

because serious slamming will manifest itself in the

occurrence of high bending moments.

Another indication for the probability of slam-ming could be found in the records of cumulative oscillation amplitude distribution for the relative

bow motion of which figure 12 has been composed.

3

Usually no slamming occurs as long as the keel of the

ship remains under water. It is evident that slam-, ming will not occur in the full load condition. In the ballast I condition the probability of complete emergence at station 19 is somewhat larger for the model with the bulbous bow than for the conven-tional shape. As. already mentioned for ballast II

condition the keel will emerge in 5% of the

oscillation-occurrences and so it is to be expected

that slamming can become ,a problem in that

condition.

It is concluded from the tests that in the full load condition there is no danger for slamming

both with and without bulb. In the ballast I condi-tion there does not seem to be a serious problem

although it might be that the bulbous bow ship will slam slightly now and then. The ballast II condition for the bulbous bow configuration is

expected to cause difficulties in an average Beau=

fort 6 sea.

4.7 Relative vertical motion of the propeller

TAKEZAWA reported in [7] that the average propel-ler immersion will be reduced when a bulb is fitted to the ship's bow. Figure 13, also derived from cumulative 'oscillation amplitude curves, shows

that an immersion reduction was also found from

the tests reported here. It appeared that the

rel-ative motions of the propeller are only about half the relative bow motions. In the ballast I condition the motions are larger than in the full load

condi-tion which is

unfortunate since the propeller

shaft is already about 1.19 m closer to the water

surface. From the test results it could be estimated that during 5% of the time more than 20% of the

propeller radius extent above the water surface.

5' IN KNOTS V IN KNOTS V IN KNOTS

Fig. lg. Significant vertical propeller motions, relative, to wave surface..

13

I

i

MEAN DRAFT 913 n I MEAMJAFT 6.27 rn

, 1

DI

MEAN I DRAFT 114 rn I , BULBOUS BOW BEAUFORT 0 A 1 CONVENTIONAL BOW

.111

BEAUFORT 6. 1

--.-

--0--BEAUFORT O. BEAUFORT .6 I BEAUFORT'S I 1 i BEAUFORT 0:

Mill

pra

VIII I -, , 1 1

1

II 11 1 1 II 1

!

in 12 13 TL n 1E IQ IT IS 15 16 10 It 12 13 IC- 115 16

/

-/

14

3

2

4.8 Pitch

Figure 14 shows that the average of1/3 highest

pitch angles are small and almost equal for both

bow configurations in the three loading conditions investigated. It is felt, however, that on merchant

ships with the accomodation amidships, pitch

motions have practically no effect on the opinion of the crew about the sea-kindliness of a ship.

5 Conclusions

In the loaded condition application of a bulb

has hardly any effect on power both in smooth water and in waves.

In the ballast condition a gain in speed was obtained by application of the bulbous-bow for speeds above 13 knots. This gain is still extant in waves of a Beaufort 6 sea state,

al-though the gain is smaller than in smooth

water.

At speeds below speed losses.

Bending moments were

by the bulbous-bow.

13 knots, the bulb caused

not affected adversely

Since the simulated sea state was not very

severe, slamming phenomena which increase the bending moment amidships considerably, could not be measured.

U. Relative motion of the bow was decreased

noticeably by the bulb for the ship in full load

condition.

In the ballast condition the amplitudes of the

bow motion were about the same for the

bulbous- and conventional bow models. g. At sea states above Beaufort 6 the bulbous-bow

is expected to emerge completely if the ship's draft forward is 4 m or less.

References

. LINDBLAD, A., Further experiments with bulbous bows.

Publication of the Swedish State Shipbuilding Tank, Goteborg, 1948.

LAMMEREN, W. P. A. VAN and R. WAHAB, Research On

bulbous-bow ships. Part IA. Still water investigations into bulbous-bow forms for a fast cargo-liner. Report

no. 74S. Netherlands' Research Centre T.N.O. for

Shipbuilding and Navigation (to be published).

LAMIVIEREN, W. P. A. VAN and J. J. MUNTJEWERF,

Re-search on bulbous-bow ships. Part IIA. Still water performance of a 24,000 DWT bulkcarrier with a large bulbous-bow. Report no. 71S. Netherlands'

Research Centre T.N.O. for Shipbuilding and Navi-gation. May 1965.

GERRITSMA, J. and W. BEUKELMAN, The influence of a

bulbous bow on the motions and the propulsion in longitudinal waves. Report no. 50S. Netherlands'

Research Centre T.N.O. for Shipbuilding and Nav-igation. April 1963.

LASKEY, N. V. and G. T. R. CAMPBELL, Large Bulbous Bow, Maritime Reporter/Engineering News, Februa-ry 15, 1964.

TSUNODA, R. KYOSHI, K. and TAKEKUMA, K., A

High-speed Cargo Liner design based upon the with-bulb waveless concept. The University of Michigan,

August 1963.

TAKEZAWA., S., A study on large bulbous bow on a high

speed displacement ship, part II. Journal of Zosen

Kiokai, June 1962.

ROLL, H. U., Die Grosze der Meereswellen in

Abhan-gigkeit von der Windstarke. Deutscher Seewetter-amt, Hamburg 1954.

Proceedings, International Ship Structures Congress

20-24 July 1964.

WALDEN, H. and J. PMST, Vergleichsmessungen des

Seeganges. Deutscher Wetterdienst. Seewetteramt

Hamburg 1961.

KORVIN-KROUKOVSKY, B. V., Theory of Seakeeping. Society of Naval Architects and Marine Engineers,

1961.

/

MEAN DRAFT 9.73m MEAN DRAFT BULBOUS 6.27m',. BOW BOW MEAN t DRAFT 5.14m c BULBOUS BOW CONVENTIONAL ---CONVENTIONAL 10 11 12 13 15 16 10 11 12 14 16 V IN KNOTS V IN KNOTS

Fig. 14. Significant pitch angle.

h. In the ballast condition the average immersion of the propeller was reduced by the bulbous-bow.

Pitch angles were practically the same for the models with and without the bulbous-bow.

C. BO ,a. 1. 9,. 10, . IL 13 15 14

APPENDIX

MEASURING INSTRUMENTS

a Thrust

Thrust was measured by means of a strain-gauge

dynamometer.

b Bending moments

For measuring bending moments the model was

cut amidships into two parts. The two model

halves were connected by a strain-gauge

dynamo-meter.

In order to cover up the opening between the

fore- and afterbody a thin plastic strip was glued

on the hull. The instrument is calibrated in the model by applying known bending moments to

the hull and consequently the effect of the plastic connection is included directly.

c

Pitch

Pitch motion was measured with the aid of an

artificial horizon, that is a gyroscope suspended in

gimbals. Recording was effected by means of a potentiometer.

d Wave height

Wave height was measured by means of a trans-ducer, placed in front of the towing carriage. This transducer consisted of two thin brass wires,

sus-pended in a streamlined frame. The resistance variations in the water caused by the passage of

current are a measure for the wave height.

e

Relative motion

Relative motion of the bow and the propeller with

regard to the waves were measured in the same

manner as the wave height.

f

Irregular waves

The wave generator is controlled by a programmed

regulator which varies the number of revolutions

automatically. An energy distribution according

to a prescribed wave spectrum is obtained by

trial and error" by putting more of certain

frequencies and less of others in the programme until the desired distribution is obtained as close as

possible. The wave height variations are recorded by an analog to digital" converter, and evaluated

by the Xi computer in the usual way [10], [11]

using the autocorrelation method. The maximum number of lags used was m = 50.

PUBLICATIONS OF THE NETHERLANDS' RESEARCH CENTRE T.N.O. FOR SHIPBUILDING AND NAVIGATION

Reports

The determination of the natural frequencies of ship vibrations (Dutch). By prof. ir H. E. Jaeger. May 1950.

Practical possibilities of constructional applications of aluminium alloys to ship construction. By prof. ir H. E. Jaeger. March 1951.

Corrugation of bottom shell plating in ships with all-welded or partially welded bottoms (Dutch). By prof. ir H. E. Jaeger and ir H. A. Verbeek. November 1951.

Standard-recommendations for measured mile and endurance trials of sea-going ships (Dutch). By prof. ir J. W. Bonebakker, dr ir W. J. Muller and ir E. J. Diehl. February 1952.

Some tests on stayed and unstayed masts and a comparison of experimental results and calculated stresses (Dutch). By ir A. Verduin and ir B. Burglzgraef. June 1952.

Cylinder wear in marine diesel engines (Dutch). By ir H. Visser. December 1952.

No. 8 M Analysis and testing of lubricating oils (Dutch).

By ir R. N. M. A. Malotaux and ir y. G. Smit. July 1953.

No. 9 S Stability experiments on models of Dutch and French standardized lifeboats.

By prof. ir H. E. Jaeger, prof ir j. W. Bonebakker and J. Pereboom, in collaboration with A. Audigi. October 1952. No. 10 S On collecting ship service performance data and their analysis.

By prof ir J. W. Bonebakker. January 1953.

No. 11 M The use of three-phase current for auxiliary purposes (Dutch).

By ir 3. C. G. van Wijk. May 1953.

No. 12 M Noise and noise abatement in marine engine rooms (Dutch). By "Technisch-Physische Dienst T.N.0.-T.H." April 1953.

No. 13 M Investigation of cylinder wear in diesel engines by means of laboratory machines (Dutch). By ir H. Visser. December 1954.

No. 14 M The purification of heavy fuel oil for diesel engines (Dutch).

By A. Bremer. August 1953.

No. 15 S Investigation of the stress distribution in corrugated bulkheads with vertical troughs. By prof. ir H. E. Jaeger, ir B. Burghgraef and I. van der Ham. September 1954. No. 16 M Analysis and testing of lubricating oils II (Dutch).

By ir R. N. M. A. Malotaux and drs J. B. Zabel. March 1956.

No. 17 M The application of new physical methods in the examination of lubricating oils. By ir R. N. M. A. Malotaux and dr F. van Zeggeren. March 1957.

No. 18 M Considerations on the application of three phase current on board ships for auxiliary purposes especially with regard to fault protection, with a survey of winch drives recently applied on board of these ships and their in-fluence on the generating capacity (Dutch).

By ir J. C. G. van Wijk. February 1957. No. 19 M Crankcase explosions (Dutch).

By ir J. H. Minkhorst. April 1957.

No. 20 S An analysis of the application of aluminium alloys in ships' structures.

Suggestions about the riveting between steel and aluminium alloy ships' structures. By prof ir H. E. Jaeger. January 1955.

No. 21 S On stress calculations in helicoidal shells and propeller blades. By dr ir y. W. Cohen. July 1955.

No. 22 S Some notes on the calculation of pitching and heaving in longitudinal waves. By ir J. Gerritsina. December 1955.

No. 23 S Second series of stability experiments on models of lifeboats. By ir B. Burgizgraef. September 1956.

No. 24 M Outside corrosion of and slagformation on tubes in oil-fired boilers (Dutch). By dr W. J. Taat. April 1957.

No. 25 S Experimental determination of damping, added mass and added mass moment of inertia of a shipmodel. By ir J. Gerritsma. October 1957.

No. 26 M Noise measurements and noise reduction in ships. By ir G. J. van Os and B. van Steenbrugge. May 1957.

No. 27 S Initial metacentric height of small seagoing ships and the inaccuracy and unreliability of calculated curves of

righting levers.

By Prof ir J. W. Bonebakker. December 1957.

No. 28 M Influence of piston temperature on piston fouling and piston-ring wear in diesel engines using residual fuels. By ir H. Visser. June 1959.

No. 29 M The influence of hysteresis on the value of the modulus of rigidity of steel. By ir A. Hoppe and ir A. M. Hens. December 1959.

No. 30 S An experimental analysis of shipmotions in longitudinal regular waves. By ir J. Gerritsma. December 1958.

No. 31 M Model tests concerning damping coefficients and the increase in the moments of inertia due to entrained water of ship's propellers.

By N. y. Visser. October 1959.

No. 32 S The effect of a keel on the rolling characteristics of a ship. By ir J. Gerritsina. July 1959.

No. 33 M The application of new physical methods in the examination of lubricating oils. (Continuation of report No. 17 M.)

By ir R. N. M. A. Malotaux and dr F. van Zeggeren. November 1959. No. 34 S Acoustical principles in ship design.

By ir J. H. Janssen. October 1959. No. 35 S Shipmotions in longitudinal waves.

By ir J. Gerritsma. February 1960.

No. 36 S Experimental determination of bending moments for three models of different fullness in regular waves. By ir J. Ch. De Does. April 1960.

No. 37 M Propeller excited vibratory forces in the shaft of a single screw tanker. By dr ir J. D. van Manen and ir R. Wereldsma. June 1960.

No. 38 S Beamknees and other bracketed connections.

By prof ir H. E. Jaeger and ir J. J. W. Nibbering. January 1961.

No. 39 M Crankshaft coupled free torsional-axial vibrations of a ship's propulsion system. By ir D. van Dort and N. y. Visser. September 1963.

No. 40 S On the longitudinal reduction factor for the added mass of vibrating ships with rectangular cross-section.

By ir W. P. A. Joosen and dry. A. Sparenberg. April 1961.

No. 41 S Stresses in flat propeller blade models determined by the moire-method.

By ir F. K. Ligtenberg. June 1962. No. 1 S No. 3 S No. 4 S No. 5 S No. 6 S No. 7 M