Investigation on the ducted propeller cavitation and the duct erosion prevention by the air injection system

tV. 992

ARCHIEF

SYMPOSIUM ON

"HYDRODYNAMICS OF SHIP AND OFFSHORE PROPULSION SYSTEMS" HØVIK OUTSIDE OSLO, MARCH 20. - 25., 1977

"INVESTIGATION ON THE DUCTED PROPELLER CAVITATION AND THE DUCT EROSION

PRE-VENTION BY THE AIR INJECTION SYSTEM" By

H. Narita and Y. Kunitake, Mitsui Engineering and Shipbuilding Co., Tokyo K. Holden and B. Mugaas,

Det norske Ventas, Oslo

SPONSOR: DET NORSKE VERITAS

PAPER 11/3 SESSION 3

Lab.

y. Scheepsbouwkun

Technische Hogeschool

CONTENTS:

ibIohk

d Rfde!p P a g e: LN b INTRODUCTION PROPELLER CAVITATION OBSERVATIONS 3

2.1. Type and Extent of Cavitation 5

2.2. Cavitation Thickness

Distribution 7

HULL PRESSURE FLUCTUATIONS,

PROPELLER SHAFT FORCES AND

STRUCTURAL VIBRATIONS 8

3.1 Test Equipment and Instrumentation 8

3.2 Results and Discussions 8

NOISE li

4.1 _{Test Equipment and Instrumentation 11}

4.2 Results and Discussions 12

THEORETICAL INVESTIGATIONS 15 CONCLUDING REMARKS ₁₈ AC KNOWLEDGEMENT ₁₉ REFERENCES ₂₁ N' e 'J DOCUMENTA TE J DATUM, I

1. INTRODUCTION

The increasinc size of tankers has required increased propeller loading, and in the last few years the attention is drawn to the use of ducted propeller and its advantages. The

results of such installations have shown that properly designed ducted propellers can

pro-vide a speed increase as compared with

con-ventional propellers /1/, /2/.

However, voyage experiences have also revealed that the duct surface along the propeller tip was susceptible to cavitation erosion,

especi-ally ir the upper part. This problem is one of the most important technical problems to be

solved for successful pplication of larqe

duct-ed propellers.

-1-"Investigation on the Ducted Propeller Cavitation and the Duct Erosion Pre

-2-In order to establish a method to prevent duct erosion,

both theoretical research and model experiments were

conduc-ted in above approaches on the 280,000 dwt motor tanker M/T

"THORSAGA" fitted with a ducted propeller. Through this

re-search the air injection system, which provides air in water

flowing close to the duct surface suceptible to erosion,

appeared to be most promising among the various hydrodynamic

measures investigated and therefore was, installed on M/T

"THOR-SAGA".

For the purpose of confirming the effect of air injection

system the authors conducted the full-scale experiments in

1974 and 1975. Observations of fullscale ducted propeller

cavitation without air injection had already been conducted

by authors in 1973 /1/ /3/ /4/, and other observations have

also been reported /5/ /6/. However, in the present fuliscale

experiments observations of ducted propeller cavitation were

conducted for various amounts of air injected using

cameras, a high speed movie cartiera as well as conventional

TV-camera in order to understand the phenomena clearly.

Further, in order to understand indirectly the effect of air

on the cavitation, erosion reduction noise measurements

near the stern were conducted, and the effect of air

injec-tion on the hull surface pressure and hull vibrainjec-tion levels

were also investigated.

In this paper the results of these fullscale investigations

are presented including cavity thickness contours on the

propeller blades obtained by analysing the stereo photos,

and the effect of air injection is discussed from various

-3

For future design reference theoretical calculations of

the extent of cavitation on the ducted propeller blades were

carried out. Results are compared with fuliscale observations.

Principal characteristics of M/T "THORSAGA" are:

-Deadweight 279,810 tons

Main engine:

Mitsui B&W 9K98FF diesel engine

Output & RPM _{34,200 HP x 103}

Ducted propeller

Propeller diameter 7.3 m

Number of propeller blades 5 Duct inside diameter 7.4 m

Duct length 3.7 m

2. PROPELLER CAVITATION OBSERVATIONS

Tests were made for 3 different drafts at 85,95 and 105 RPM

with air flow rate varying from O to 15 m3/min. Air was

injected in axial direction along the duct inner plate _from two outlets positioned at 00 and 15°.

Hull dimensions

Length BP

Breadth Depth

Draft fully loaded

1080' 170' 91' 71' 0" 0" 0" 5 - 3/4"

Conditions' dF/dA

Ballast I 36'/44'

4, Full load

Ballast II 42'/50'

71 '/71'

RPM Vs SEP Air volume Sea state J 85 o 5 10 95 105 105 85 105 4 15.9 30.700

-O 5 O O 5

iO

14.2 30.500 J 5 10

5-6

O 3 5

1-2

10 15

1-2

Observations of propeller cavitation were made applying TV-camera,

stereo photo technique and high speed movie camera.

The TV-camera was used to observe the general transition of

pro-peller cavitation. The high speed movie pictures were taken at

12 18.000

14.3 23.200

-5-limited occasions to observe closely the dynamic behavior of

propeller cavitation and motion of cavities being ejected from

propeller and hitting the duct. The stereo photos were taken

with intent of estimating cavity thickness on propeller

blades.

2.1 Type and Extent of Cavitation.

For the ballast condition I a large amount of entrained

air-bubbles partly prevented propeller cavitaion observations.

From the television recordings, however, a few pictures of

reasonable quality were obtained for some blade angular

positions. Fig. 2.1 shows sketches from television recordings

for blade augular positions where the viewing conditions were

satisfactory.

For the same RPM and volume of injected air, time variations

of the extent of propeller cavitation were significant. Due to

lack of TV-recordings from some angular positions and the

large time variations of extent of blade cavities, no special

tendency regarding influence of air on propeller cavitation is

found as it is within the range observed without air

injec-tion.

For the ballast condition II, the sketches from the te'evision

recordings are given in Fig. 2.2. Photos of propeller blade

cavitation for various angular positions are presented in Fig.

-6-The varialions of extent of propeller cavitation with injected

amount of air up to 10 m3/min. are small and within time

variations.

For 10 and 15 m3/min. some increase of extent of blade

cavitation was observed. However, the difference may be due

to a delay of the collapsing process of blade cavities. The

rate of collapse of cavities is very high (withdrawal of sheet

towards tip). This process is clearly illustrated on the high

speed movies and may also be seen from the sketches of the

television recordings as well as from the photos. Between the

blade angular positions (referred to the propeller generator

line) 300 and 500 nearly all the developed blade cavities

collapse in the region on the duct where plate damages occurred.

Fig. 2.8 illustrates tip vortex cavitation. RPM of initiation

of tip vortex cavitation is within the 50 - 55 range.

For load condition the same conclusions with respect to

variations of extent of cavities with amount of injected air

may be drawn - Fig. 2.9 - 2.10.

The injected air was observed to mix partly with the cavities

in the blade tip region. Some mixture of air and cavities may

have been obtained before the main part of cavities collapses.

At least a visible thickening of the tip vortex has occured

indicating that the injected air has been sucked into the

It can also be observed that some of the injected air moves

with the sheet/clouds of cavities in the tip region filling

the gap between the tip and the duct inner plate. Further, the

injected air seems to form a layer protecting some parts of

the most exposed area on the duct inner plate.

However, the sheet of cavities on inner parts of the blade is

not mixed with air.

2.2 Cavitation Thickness Distribution.

Stereophoto observations were carried out to find cavitation

thickness distributions. The stereo analyses have been

per-formed by the Institute for Geodesy and Photograrnmetry,

University of Trondheim.

Coordinates of cavity surface are given in cm. and refered

to blade surface. Behind the trailing edge of the blade

thickness of cavities are found extrapolating the reference

blade surface. For one blade angular position, shape of

cavity surface along various blade radial sections are given

(in cm.). Fig. 2.11 - 2.14.

For conditions where there is a volume of injected air,

distances between iso-curves are quite small around the air

outlets. In those areas large inaccuracies may exist because

coordinates of points on the surface of the injected air

have been difficult to define because of a rough and pulsating

3. HULL PRESSURE FLUCTUATIONS, PROPELLER SHAFT FORCES AND STRUCTURAL VIBRATIONS.

3.1 Test Equipment and Instrumentation.

In order to measure the effect of air injection on hull

surface forces pressure tranceducers were fitted on both

port and starboard of the stern hull. Propeller thrust

varia-tions were measured by strain gauges fitted on the propeller

shaft, and superstructure and local vibrations were measured

by velocity transducers.

In the following, the particulars of test instrumentation are

described. Fig. 3.1 shows coupling diagram and positions of

velocity and pressure tranceducers are shown in Fig. 3.2.

Pressure transducers: ENDEVECO

Max range 100 psi

L-

15 psi

Natural frequency 6 kHz

Diametre of membrane 6 min

Velocity transducer: CEC

Linear range 3 - 700 Hz

3.2 Results and Discussions.

a) Ballast Condition I

RPM = 105, V = 17.4 kts, SHP = 32000, Fig. 3.3.

Pressure irnulses on the hull

Blade frequency amplitude increases for amounts of

Above 5 m3/min. the amplitudes were fairly constant.

A reduction of amplitudes of higher frequencies was found

showing a minimum for an air flow rate equal to 3 m3/min.

Shaft excitation forces - dynamic thrust.

Amplitudes of twice blade frequency showed a large

reduc-tion with increasing air flow rate.

Local and superstructure vibration.

Vertical vibrations of bladefrequency measured by

vel-city transducers No. 3 and 4 increase with the air flow rate. The influence on amplitudes of higher frequencies

is negligible.

Superstructure vibrations measured by transducer No. 2, 6

and i increase for amplitudes of all frequencies up to an

3.

air flow rate equal to 5 m ¡min. then the level is

decreasing. For an injected amount of air equal to 10

m3/rnn. vibration amplitude for transducer No. 2 is

below the value with no air.

The reduction of the superstructure vibration level above

5 m3/min. may be due to a corresponding decrease of the dynamic thrust amplitudes as the hull pressure impulses

3.

10

-Ballast Condition II

RPM = 105, vs = 15.9 kts., SHP = 30700, Fig. 3.4.

Pressure impulses on the hull

For airflow rates between O-3 1n3/min. amplitudes of all

frequencies are fairly constant.

The increase of the amplitudes occurs when injecting an

amount of air equal to 5 m3/min. Above that value a

reduction of amplitudes' of higher frequencies is found.

Amplitudes of lower frequences are not influenced in the

air flow rate region 5-15 m3/min.

Shaft excitation forces - dynamic thrust

The influence of air injection on dynamic thrust

ampli-tudes is small for all air flow rates.

Local and Superstructure vibrations

Both the local and superstructure vibration level of

1 x blade and 2 x blade frequency increase

correspond-ing to the increase of induced pressure impulses on

the hull.

Load Condition

RPM = 105, V = 14.2 kts., SHP = 30500, Fig. 3.5.

Pressure imu1ses on the hull

Amplitudes of frequencies below the 4 x blade frequency

in-fluence on amplitudes of higher frequencies.

Shaft excitation forces - dynamic thrust

From 0-5 m3/min. air injected amplitudes of i x blade and 2x blade frequency increase.

Local and su2erstructure vibrations

Local and superstructure vibrations increase for all

frequencies with air flow rate. Only small differences

on the phase angle relationships are found with respect

to injected amount of air.

4. NOISE

4.1 Test Equipment and Instrumentation.

Airborne noise was measured with the following

instruxnen-for calibration tation:

Precision Sound Level meter Brüel & Kjr type 2203

Octave band filter set " " 1613

Microphone " " 4145

Random Incidence Corredor Brüel & Kjr UA 0055

Microphone AKG 1/2" " 451E

Sound and vibration meter Nortronic A/S tI 815

12

-Noise in tfte water was measured with:

Brüel & Kjar Hydrophone type 8100

Charge amplifier " 2633

Narrow band analyses of the hydrophone signals were performed

with the Nortronic Analyzer. Hydrophone signals were also

recorded on a Tandberg Instrumentation tape recorder, Model

115.

4.2 Results and Discussions

Octave band sound measurements were made in some positions

in aft peak tank and engine room. The positions were chosen

because cavitation noise could easily be distinguished and/or

dominated over other noise. In addition to the octave band

analysis, narrow band analyses (5% bandwidth) were made of

signals from a microphone placed in the lower aft .peak tank.

In addition noise was measured in the water outside the

hull, by means of a hydrophone mounted on the port side

just above the observation tubes, close to a pressure

transducer

-Typical results of the octave band analyses are given for

a specified condition in Fig. 4.1. The levels in each

octave band are presented with the air flow as variable

parameter. These diagrams show the noise reduction as a

function of air flow rates, for given frequency bands.

A similar presentation has been chosen for the narrow-band

13

-Fig. 4.3. presents the difference between noise spectra for

two air flow rates by hydrophone measurements for each

condition.

In ballast condition I at 105 RPM, level reduction of high

frequency components was found in the upper aft peak tank.

The damping is significant at small air flow rates. The

tendency towards high frequency damping was found also in

the engine workshop.

Also hydrophone measurements show that low frequency

components are -quite variable, whereas fairly consistent

decrease in levels in shown by components higher than about

150 Hz, as shown in Fig. 4.3.

By octave band analyses in ballast condition II, some level

reduction at high frequencies was found in all measuring

positions. In addition, some increase of the level in band

1 (31.5 Hz) was found in the engine workshop and in the

upper aft peak tank. Level reduction seems to occur for

fre-quencies higher than about 1000 Hz (band 6).

In load condition at 105 RPM, the most pronounced changes

in noise levels occur just when the air flow is turned on.

When the flow is increased beyond 5 m3/min, the high-frequency

noise levels decrease further, but on the whole, less

notice-ably than when the air flow was just turned on. Low frequency

14

-For the microphone signal, the tendency is towards sonie

high-frequency level reduction and some low-high-frequency increase.

The hydrophone measurements confirm this result, with rather

clear low frequency increase and high-frequency level

re-duction. Small air flow rates seem sufficient to provided

good reduction as shown in Fig. 4.4.

The general conclusion of the investigation is that injection

of air into the duct tends to reduce high-frequency noise

levels and increase the levels of low-frequency components.

The hydrophone measurements of noise in the water confirm

the results found by microphone measurements of airborn noise inside the ship.

The injection of air may cause the cavitation bubbles to be

"cushioned" and produce a lesser implosion impact on the

duct. Also it will cause the tip vortices from the blades to

contain more air and may then dampen the sound when they

strike the structure. Both of these effects will tend to

dampen high-frequency noise components.

The level increase of low frequency components does not

cause a noise problem, except in cases of very high levels,

the ear being relatively insensitive to low-frequency sound.

However, increasd low-frequency noise level indicates an

overall increase of vibrations, which may be noticed in the

15

-5. THEORETICAL INVESTIGATIONS

The hydrodynamic force on the propeller and duct, and the

extent of propeller blade cavitation at various angular

blade positions have been calculated applying the following

procedure: At defined angular intervals, the strength _and

radial distribution of propeller bound circulation _are

calcu-lated. When this is done for one complete propeller _revolution,

the wake in way of the duct is corrected for the effect _of

propeller's bound and free circulation. The duct's bound

circulation is calculated and corrected for downwash due to

the free vorticies of the duct, and the wake in way of the

propeller disc is corrected due to the duct's bound

circula-tion. The procedure is repeated in an iterative manner until satisfactory convergence is obtained on the average propeller

thrust. The forces and moments corresponding to the _resulting circulation are then obtained.

Since the numerical approach consists of calculating

pro-perties at defined circumferential stations around the duct,

these stations may well have different geometrical shapes,

i.e. the method is also suitable for analyses of

non-axis-symmetric duct shapes.

Having revealed the propeller blade sectional loadings at

defined angular stations, the detailed pressure distribu-tion on each blade secdistribu-tion is found by linear theory

super-imposing effects from profile camber, thickness and angle of

16

-From the detailed pressure distribution at each blade sec-tion the extent of cavitasec-tion is found applying the

prin-ciple of no thrust reduction at each section.

The results from the analyses are given in Table 5.1 and

Figs. 5.1 - 5.2.

The analyses are carried dut for 3 conditions:

Ballast Condition:

RPM = 85

RPM = 105 and

Load Condition RPM = 105

The only comparison possible with neasured values are the

comparisons between calculated and measured SHP. Taking

into consideration the tolerances on the measurements and

scale effect on the model wake used in the calculation, the

values are comparing reasonably well.

TABLE 5.1

Duct and proller thrust in tons.

Condition Duct Thrust Prop.Thrust Calculated Measured

Ballast _{SHP SlIP} Cond. 1 38 158 19 500 18 000 = 85 RPM Ballast Cond.l 62 213 33 400 32 000

=105

RPM Load Cond. =105 RPM 53 194 30 800 30 500

17

-Comparisons between observed and calculated extent of

cavi-tation are shown in Fig. 5.1 where the extents are indicated

at different angular positions of the propeller blade.

Three typical pressure distributions - one from each of the

three loading cases - are given in Fig. 5.2.

As observed, the pressure on the suction side is below the

cavitation pressure near the leading edge - which corresponds

with the observations. In addition, for the conditions shown

on Fig. 5.2 the safety against cavitation is small also

further back on the chords, and hence cavitation may well

occur on a larger part of the section than the narrow suction

18

-6. CONCLUDING REMARKS

The fuliscale ducted propeller cavitation observations as

well as measurements of noise levels near the stern, hull

surface pressure and hull vibration levels on M/T "THORSAGA"

successfully provided much useful information on the

pheno-mena of ducted propeller cavitation and duct erosion, and

further has revealed the effect of the air injection system

on the duct erosion prevention and hull vibrations.

The main conclusions of these full-scale experiments as well

as the correlation studies between the theoretical prediction

and fuliscale ducted propeller cavitation observations are

mentioned in the following:

The air injection system is effective for preventing

duct erosion. This was assumed from the reduction of

high frequency noise components measured near the duct

and have later been confirmed by inspection of the

ducted propeller of M/T "THORSAGA" after two year's voyage

with the air injection system.

The noise measurements indicated that the significant

part of effect of air was generally attained by the

air flow of approximately 5 m3/min.

Local and superstructure vibration levels particularly

of low frequençies generally tend to increase with the

increase of amount of air but not to any level causing

vibrational trouble by the amount of air required for

19

-The high speed movies revealed how the development and

collapse of cavitation on the blade in the upper

star-board side proceeded.

The angular positions where the blade cavities rapidly

collapse corresponds to the regions on the duct where

plate damages occured.

The injected air appeared to form a layer protecting some

part of the duct inner plate. The injected air bubbles

partly mixed with the cavities in the blade tip region,

and at least a visible thickening of the tip vortex

occured indicating that the injected air was sucked into

the vortex.

From the stereophotos the thickness distribution of

propeller cavitation was obtained.

The influence of propeller RPM and SHP on maximum

thick-ness of sheet of cavities is large, and thickthick-ness of

travelling/tip vortex cavities is noticed somewhat

larger when air is injected.

Extent of cavitation on the propeller blade was

theoretically predicted by the vortex theory, and

the correlation of this result with the fuliscaic

observations was satisfactory.

Acknowledgement

The authors wish to express sincere thanks for Mr. B. Svenning

20

-crew for their kind understanding and fine cooperation in

conducting the tests, onboard M/T '1TIIORSAGA".

This project was sponsored by Mitsui Engineering &

Ship-building Company and the Norwegian Cavitation Committee and

the authors are grateful for their permission to publish

this paper.

The authors are grateful to NSFI for the valueable advices

and fine cooperation in early model test stage and to the

Institute for Geodesy and Photograinmetry, University of

21

-REF ERENC ES

NARITA, H.,KUNITAKE, Y. and YAGI, H.: "Application and Development of a Large

Ducted Propeller for the 280,000 dwt

TankerM/T'THORSAGA' ," Trarìs.SNAME Vol.82(1974)

ANDERSEN, O., TANI, M.:

"Experience withT/T'GOLAR NICHU'",

Symposium on Ducted. Propellers, RINA (1973)

NARITA, H., KUNITAKE,Y. and YAGI, H:

"Correlation Results of Model and Full Scale Ducted Propeller Cavitation Observations", Symposium on High Powered Propulsions of

Large Ships", NSMB (1974)

4 NARITA, H., KUNITAKE,Y, YAGI, H., OOSTERVELD,M.W.C., and HOEKSTRA,M.:

"Model and Full Scale Ducted Propeller Cavitation Observations on a 280,000 DWT

Tanker", Jour. Soc.Nav.Arch.of Japan, Vol. 136 (1974)

OKAMOTO,H., OKADA, K., SAlTO, Y.

and TAKAHEI,T.:

"Cavitation Study of Ducted Propeller on Large Ships", Trans.SNAME Vol. 83 (1975)

HUSE, E:.:

"Air Injection to avoid Cavitation Erosion in

Propeller Duct", The Ship Research Inst.of Norway, Report No. R-49 Sept. 1975

50 s 20

10

did

36'/44'

F A

RPM

105

3/

Inj.air

O Nm/mm

Fig. 2.1

36/ 44'

85

O Nm3/mjn

dF/dA

RPM

tni. air

60 ç,

e 60 40

d I d

42.'/ 50'

F A

RPM

= 105

3

Inj. air

O

Nm/mirt

$ g

d Id

_{F A}

2/50

RPM

=

105

Ij.I r

10 Nm/mm

(D ('J

u-g

-G u) Q IL

50

s

50

o

40

e 40

30

d/d

= F A

RPM =

lnj.uir

s

30

d /d

71'/71

F A

RPM: 105

3

nj air: 10

Nm,'mnin

Fig. 2.9

Fig. 2.10

3 lnj. air

O Nm/mm

d/dF711/ 71

RPM

B5

Fig. 2.fl

i

: 20°

3 lnj. cur 5

Nm/ min

(1

/d

71 / 71 A F R PM

85

Fig. 2.12

L Z 6 i d OL V1d J J V

1tLI1tL

Pl

P o J!01U1 o

3 -cm - 10 3-o - Io r/R 1.00 r IR 0.98 cm 32 15 nR 0.96 nR 0.95 n/R 0.94

Fg 214

35 cm 20

d/d

71'/ 71' RPM 105 p 20 Inj. air O

Velocity transducer1 -hydrophon ( pressure transducers -4

/\

H

6 chan. amplifier! i n te gr a to r

>

Charge amplifier 2 chan. trans-ducer amplifier

.L-1 J

--4 -I Strain Te1inetry gaugo

tansmiter

Telemetry

receiver/

discrimina-tor

¿

i

FIG. 3.1 Ins rumenta tap recordi-'

®CEC CEC CE_Ct

L

®CEC

-PRESSURE TRANSO. - STARBOARD - PORT ® CE C

I1

STRAIN GAUG E S ç

Positions of Velocity (CEC) - and Pressure Transducers

in the centreline on the top of the wheel house horizontal vibrations

in the ceritreline at the aft end of the main deck

vertical vibrations

near to the centr.line at the upper support of the.duct vertical vibrations

near to the centreline above the stern tube bearing horizontal vibrations on the engine

near to the centreline in the steering gear flat pressure transducer starboard

pressure transducer port

--

CEC

VELOCITY T R A N SOU CE R

10

-/.N

5 10 N m3/ min o I 0 5 0 Nm3 Im i n

15

-mm/s lo 5

0i

,4 m3 / min 2 mm/s

©

kp/rn 7 ool 600J

o

20 15 400.

lo I

300-1 ₂₀₀ 0 1001 o4 5 1

x BLADE FREQUENCY

,

2 x

o

3x

's o I, X o o ₁₀ Nm 1mm 2 k p/rn

200-

180- 120e 80

o'

o o 5 tim3 / min

toni ®

i o 8 8 4 o o

15

-rvn /5

d Id

36'/ 44'

mm/s F A

RPM

1O5 15 _lo 5

xx

xX

3 I n

04

I o 5 10 Nm3/ min

d

d2'/5O'

.

i

x BLADE FREO.UENCY

X

2*

'1

RPM1O5

_3*

ii 0

4*

'1

outt

15 mm/s mrrvs 30

-2 k fm 600

500.

400 300 200 100 0

©

25

-20

-15

10i

o 0 5 6*

o

2 kp/m 200 160 120 o o

- -9

5 ₃ 5x Nm ¡min -wÇ3 10 mm's 10 5 o 0 ₁₀ Q 15 Mm 1mm 15

-o,

20- 16- 12-O _o

O

15 0

55

_{10 16} Nm/mm _Mm/min

Fç. 3.4

2510

20 -15 -o X

.

3 Mm/min o X

.0-o s 10 3 15 NmI min

tøb(®

o 36.1 32 2c

XX XÌ

L _'C 24

-mm / s 20 15 lo o k p/rn2 C2) 4 00

.

s 300

/0

100 15 -rn mis

dF/dA71'/71'

i

x BLADE FREQENCY

RPM=105

X 2 X o 3 x fl 4 x -.

©

0 5 10 15 3 Nm 1mm 15 mm / £ lo

5-.

o 2 k P/rn 160- 120 - 80 404 o

/

X I J 5 10 15 3 Nm /mn 15 2 kp/m mm/s s o a c Air outlet Xx

I

20

©

10 ////10 El p J o 10 15 0 5 10 15 3 3 Nm /mÌn Nm ¡min

-0

500

_400-

300

/

o

-.

200-00

100 o o D I I O 0 5 10 15 5 5x Sxx

3.

Nm /mm 3 Mm /mill

03

5 10 15

3.

Nm /mm o to n

©

o 5 10 o 5

Octave band anaLyses, BALLA ST

H-12/3-75.

MANHOLE TO

LOWER AFT PEAK TANK

UPPER AFT PEAK

TANK i

110 2ì

loo:

1*

90

I

902 3

so7

8 4 5

7081111

7 70

6o'

50

°1I

9 40

,iï

60 40 o ic

30

AI

30 20 2O

AIR FLOW RATES

Draughts

Aft

44'

, Fore

36'

105RPM

,

_{17,5 knots}

, 31,200 S.H.P

ENGINE

WORK SHOP

0 3 5 7,5 10 m7 û 5

7,5 10

3

15Hz 85

95 db go

22Hz 85

28Hz 85

110 db 105

36HZ !°°

100 db 95 38Hz 90 110

i05\t

100 45Hz 95

BALL A ST

¡

Draughts

_:

At t: 44'

, Fore

:

36 '

11-12/3-75

54Hz 100 115

db/\

66Hz 110

88H0/\.

ioo'\

105 HzQ 115 db 110 105 115 Hz loo 110 db 200Hz105

ioo,/

290 Hz95 360 Hz90 db

525 Hzco

300-95 db go 500 Hz85 95 db

500

1200 Hz 85 BO db 1 8 KHz7S 80 db

6 KH275

So db

6-9,5 KHzo

Microphone Signal

from lower aft peak tank.

Nor row band analysis e-. P.gk values k- Average values ii, a band C orn ponen t

fruencis

are approximate.

0-35-75-10

Fig. 4.2

loo

db 95 90

30 dB

20

10

-lo

30 dB 20 10

'-lo

30

i

r

r

20

11-12/75, BALLAST,

85 R PM Draughts : Aft :44',3Fore:36'

Ai r fLow rates :

O and 10 Ymin

Level, reduction recorded in hydrophone signal..

Fig. 4.3

95RPM Draughts:Aft:44',Fore:36'

Air tow

r ates: O and IO

njn

Level. difference recorded in hydrophone signai

105RPM Draughts: Aft :44, Fore; 36

Air flowe rates: O and io

L ev eL

diff e r ence recorded

in hydrophone signal.

50 100 200 500 1K

Higher frequencies: Signal too weak to be recorded

Higher frequencies: Signal too weak

to be recorded

Higher frequencies:

Signal too weak to be recorded. A

2k

5K 10)< 15K Level reductior (improvement

with injection

of air. Level reduction with injection of air. lrnproveme n t with injection of

air.

Fig. 4.3

10 0 10 2 O 10 o 1 O 20 Fig. l.A 13-14/3-75

85RPM

Air flow

Level difference

rates

Oraught:71'

:0 and

recorded

I

m/

even

I

keel.

in hydrophone signal.

13-14/3-75

105RPM

Air flow

LeveL

rates

difference

Draught:71'

O

even

and 1Orì

observed

keel.

I

in hydrophone

signal

.

t

&

15 20 50 loo 200 500 1K 2K 5k 10K 15K k .30

dB

20 30

dB

20

I mprove ment

with ar injection

rnro vernerft

with air injetiorì

40

40 e o 40 o

30

'A

20°

d Id

36'144'

F A

RPM

105

d /d

71'/ 71'

F A

RPM = 105

Catcututed

extent

of

cavitation

Fig. 5.1

30 o 20

dF/dA: 36'/44

RPM

85

o 50

cPcQv. k

r IR

0.9

9'

RPM = 85

d

/ d= 36'/ 44

F _A r / R 0.9

9'

d Id

_F

36'/4/,'

A

RPM

105

nR

0.9 =

00

R PM = 105

d

/ d

_36'/4L F A

Fig. 5.2