Influence of ship size, afterbody shape and propeller speed of rotation on propeller performance

ME D DELANO EN

FRAN

STATENS SKEPPSPROVNINGSANSTALT

(PUBLICATIONS OF THE SWEDISH STATE SHIPBUILDING EXPERIMENTAL TANK)

Nr79

GOTEBORG

1977

INFLUENCE OF SHIP SIZE, AFTERBODY

SHAPE AND PROPELLER SPEED OF

ROTATION ON PROPELLER

PERFORMANCE

GILBERT DYNE

Paper presented at

the Symposium of the Swedish and Soviet Specialists

on theme 6: "Improvement of Methods of Design and

Calculation for Different Types of Propellers"

ABSTRACT

The influence of ship size, afterbody shape and propeller rate of revolution on the propulsion, cavitation and pressure fluctuation properties of a full-formed tanker was studied at SSPA in 1971-72. The shaft power and propeller rpm were first predicted from re-sistance, self propulsion and open water tests with 7 m long models in the towing tank. The same models were then tested in SSPA large cavitation tunnel where the wake was measured, the flow around the stern and the Oavitation patterns were observed and the vibratory pressures at the hull above the propeller were recorded and analysed.

1. INTRODUCTION

The size and power of large tankers have been increased considerably during the last years, from about 70 000 TDW and 20 000 -25 000 SHP in 1960 to about 500 000 TDW and 45 000 SHP in 1975. The service speed, on the other hand, has remained constant, 15 -16 knots, while the block coefficient has been inceased from

CE 0.80 to 0.85 today.

Since experience indicated that the development towards larger size, power and fullness might cause cavitation and vibratory problems an investigation was ta±ted in 1970 to clarify these problems. The study described in this paper is a part of this in-vestigation. Its aim is to answer the following questions:

Is it possible to imprOve the propulsive, cavitation and vi-bratory properties of a 300 000 TDW single screw tanker with

given main dimensions, fullness and speed by changing the stern or by varying the propeller diameter and speed of

rota-tion?

How are these properties changed if

the ship ize is changed, the speed being kept constant or

if

o the delivered horsepower and speed are increased, the ship size being kept constant?

The study was ordered by the Swedish Ship Research Foundation and performed at the Swedish State Shipbuilding Experimental Tank in

1971 - 72 [11.

-2. MODELS AND TEST FACILITIES

The tests were carried out with three models of a 300 000 TDW tanker with the following particulars:

ship length = 342 m

ship wetted area S 29 800 m2

length-beam ratio

L/B

beam-draft ratio

B/T

length-displacement ratio L/Vh/3 _5.0

block coefficient _CB = 0.84

The models had a length of L = 7.0 m. They had the same fore-pp

body lines butdIfferent sterns - V-stern, U-stern and bulbous stern. The body plans are shown in Fig 1.

The ship models were tested with three 5-bladed propellers with different diameters. The propellers belonged to the standard family SSPA 5.60 described in [2]. Propeller data of interest are given in Table 1. Table 1 Prop No _DM P/D

D5 ()

The full scale propeller speeds of rotation are approximate and correspond to the trial speed 16 knots.

/

The conventional tests of resistance., self propulsion and propel-ler characteristics in open water were carried out in SSPA towing tank. The main dimensions of this tank are: length 260 m, breadth

10 m and water depth 5 in.

The cavitation tests, the measurements of vibratory pressures on the hull and the major parts of the flow observations and wake measurements were performed in SSPA large cavitation tunnel. The

test section of this tunnel is rectangular with a breadth of 2.6 m, a height of 1.5 m and a length of abOut 10 in. The ship models were mounted in the tunnel as shown in Fig 2. The

water-line corresponding to the level of free water surface in the tow-ing tank was flush with the top of the test- section. Individually

Fig 1. Body plans

\

Maximum woter speed VS 6.8 rn/s

E U,

2.6m

cut wooden plates were then fitted to simulate the free surface. The gap between these plates and the model was very small. The test seätion and the recess were completely filled with water during the tests.

The tunnel is powered by a 1 000 hp niotor giving a maximum water speed of about 6.8 m/s in the test section. The minimum cavita-tion number at the propeller shaft obtained is about a = 1.0.

3 RESISTANCE TESTS

The resistance RM of the models was measured at various ship speeds in the range 10 < V < 20 knots. The ship resistance and the effective power

E were then calculated from

= Cps P V2 S = Rs V as suming CTM = (l+k)CFM + CR CTS = (l+k)CFS + CR + CF where

o the frictional coefficient CF was calculated according to ITTC-57 (3]:

0.075 CF

{101og RflL_2}2

o the form factor k was determined with the method proposed by Prohaska [4]

o the roughness allowance ACF was àhosen in accordance with the trial test experiences at SSPA:

CF = 0.05

o th form factor k and the residual resistance coefficient C were assumed to be the same for ship and model

The result of the resistance tests at V 16 knots is summarized in Table 2.

Table 2

As seen the U-stern ship had about 7% higher resistance than the V-stern ship, the reason being a higher form resistance. Also the

ship fitted with bulbous stern had a somewhat higher resistance -1.5% - than the V-stern ship.

LL SELF PROPULSION TESTS

By combining results from self propulsion, resistance and àpèn. water tests the propulsive factors

o the effective wake WTM (thrust identity method)

o the relative rotative efficiency nR and

o the thrust deduction t

were determined.

The full scale wake WTS was then calculated from a modified ver-sion of the Sasajima formula [5]:

(l+k)C

_+CF

where t according to SSPA trial test experiences was put t =

0.04. Propulsion factors of interest are summarized in Table 3.

Table 3

The propeller characteristics were corrected for scale effects according to the following procedure, see [6]:

CDM = minimum drag coefficient of the equivalent propeller blade at x = 0.75 determined by Lerbs analysis method [7]

CDS = 2(1+2 }(l.89+l.62 log ._)_2.5

where t and c are the maximum thickness and the chord length of the propeller blade at x = 0.75 and k is the blade

roughness (k is put k = 30 pm)

1CD = CDM - CDS

LKT =

-where P/D is the pitch ratio at x = 0.75 and Z is the nuin-ber of blades

KQ

_CD0.25.

KTK = KT - KT Propulsion

factor

Propellers Shape of stern

Bulbous WTM P1079 0.35 0.46 -0.40 P1167 0.36 0.48 0.44 P1337 0.40 0.52 0.47 t all 0.18 0.20 0.17 all 0.97 0.98 0.99 WTS P1079 0.28 0.34 0.30 P1167 0.28 0.35 0.31 P1337 0.30 0.37 0.33

KQK = K0

-Assuming the thust deduction t and the relative rotative effi-ciency

R to be the same in model and full scale the actual values of

TS S

TS

(ltY(1 -wY2

were calculated and

TS and KQK were determined from the

corréc-ted propeller charàctei-istics. The full scale values of propeller, speed of rotation n and shaft horsepower

DT were finally

ob-tained from

(l_wTs)Vs

S _DJTS

DT 2ir p D

KQK

The results are given in Fig 3 where

DT of. the V-stern ship is

plotted against n for the three propellers considered and in

P0j KW 40000 30000 20000 10000 0 06 0.8

Fig 3.. Relation between shaft hOrsepower, number of revs and speed for different propellers. V-shaped stern

1.0 1.2 1.4 _1.6 n5!Im 1.8 n5r/s HP -50000 '40000 -30000 . --10000 . 40 50 6' 70 80 90 100

Fig 4 where the influence of stern shape and number of revolu-tions can be studied.

As seen the lowest shaft horsepower was obtained for the ship with bulbous bow - about 4% lower than for the V-stern ship, the

main reason being a higher hull efficieny. Also the U-stern ship

had a high hull efficiency, but since its resistance was so much higher the shaft horsepower was about 3% higher than for the

V-stern ship.

The shaft horsepower decreased with decreasing number of revolu-tions, e g with increasing propeller diameter. In the range tës-ted this decrease was about 3% for a decrease in n5 of

s = 10 rpm.

The model test results have also been used to prediät shaft horse-power and propeller speed of rotation for tankers of other sizes than 300 000 TDW. The results for 16 knots tankers with V-shaped stern are summarized in Table 4.

Table 4

Propeller Ship size in TDW

100 000 200 000 300 000 500 000 P1337 D (rn) 6.29 7.92 9.07 10.75 DT (KW) 17 480 22 770 27 850 35 700 n (r/s) 2.48 1.87 1.61 1.33 P1167 D (rn) 6.97 8.78 10.05 11.92 DT (KW) 16 620 21 850 26 780 34 340 (r/s) 2.01 1.52 1.31 1.08 P1079 D Cm) 7.64 9.64 11.03 13.08 DT (1(W) 15 740 20 800 24 950 32 150 (r/s) 1.64 1.25 1.07 0.88. Variable D (m) .7.9 8.8 9.4 10.1 DT (1(W) 15 400 21 800 27 500 36 200 n (r/s) 1.5 1.5 1.5 1.5

.1.0 1.1 0.9

I;

-V

Resistance SeLf propuLsion tests

tests

/

Flq 4. Influence of propeller speed of rotation on shaft horse-power at 16 knots

V-shaped stern

DT'DTV-STERN P1167 U-shaped stern

For the same model propeller the number of revolutions is de-creased as seen with increasing ship size. The figures given in the lower part of Table 4 are obtáinéd by interpolation and are valid for constant n = 1.5 r/s = 90 rpm. In this case the rela-tive propeller size is decreased with increasing ship size.

5, WAKE MEASUREMENTS AND FLOW STUDIES

At the wake measurements both the local velocity VA and the local static pressure p were recorded. The result from measurements at 4 in/s in the cavitation tunnel is given in Fig 5, where VA is ex-pressed in the nominal wake w

VA w = 1

-and p in the pressure coefficient C

=-Po

P

where the reference Po is the static pressure in undisturbed flow.

The wake distribution of the V-stern model had a 2-dimensional character with large circumferential wake variations. The distri-bution of static pressure was quite uniform with a maximum devia-tion in C of AC = 0.10.

p p

The wake distributions of the U-stern and the bulbous stern

models, on the other hand, were móré 3-dimensional and the cireurn-ferential wake variations smaller. The static pressure had larger variations with AC = 0.30 behindthe U-shaped stern and AC =

P p

0.20 behind the bulbous stern.

To get an idea of the flow around the stern air bubbles were ejec-ted from holes in the hull during tests in the cavitation tunnel. To keep down the bubble rise caused by buoyancy forces the water velocity was kept high - 4 rn/s. The tests showed, see Fig 6, that

V-stern

U-stern

Fig 5. Nominal wake and static pressure at propeller plane

I

H

__j:

Fig 6. Result of air ejection test

V-shaped stern

U-shaped stern

strong bilge vortices were formed around the U-shaped stern. Also around the bulbous stern the boundary layer was rolled, up into bilge vortices. For both stern s'hape flow separation tendencies were also noticed close to the water surface above the propeller disc. Neither bilge vortices nor other flow separations were re-corded on the V-shaped stern.

The existence of strong bilge vortices around the U-shaped stern and moderate vortices around the, bulbous stern can explain many of the reaults recorded at the other, tests. Thus tI-e bilge vor-tices seem to be responsible for

o the large resistance

o _{the high wake fraction (the vOrtiôes collect water with ].ow}

kinetic energy from the ship boundary layer and supply it. to the propeller) and hull efficiency

o the uniformity of the wake distribution

o the large variations in C.at the propeller disc (zones with low static pressure inside the vortices)

recorded for the U-stern model and in a less degree also for the

bulbous stern model..

6, CAVITATION STUDIES

To obtain reliable cavitation results the model propeller is gen-erally tested at the same advance ratio

TS and cavitation number

o as the full scale propeller in trial condition, the advance velocity VA in a and

TS being determined by the thrust identity

method at nOn-cavitating conditions. Often cavitation tests 'are also carried out at a-and J-values valid for the ship in service conditions. 'The shaft horsepower at constant speed isthen assumed to be 15% higher and the wake Aw = O;05 higher than at trial. With these assumptions the speed at given shaft horsepower will be 0.5,- 0.7 knots lower and the ntimber of revolutions 2.0 2.5%

lower than 'at trial. The o-J-values of the present irivet-igation are given in Figs 7-8 for different speeds,, ship sizes, propeller models and stern shapes.

30 10 30 20 0.25 0.35 0.40 0.45 r 0.40 0.45 TDW V3AT TRIAL. 5000 300000 200000 15 KNOTS 100000 IS KI'IOlS STNDA LOAD coNorms V-SHAPED STERN SHEET CAVITATION THICKER AA1ION INTERMITTENT CAViTATION

rA'°

CAVITATION TP VONTEX CAVITATION ITERMITTENT U-SHAPED STERN 8ULBOUS STERN. SERVIcE CONDII1ON

-r

a

30

20

10

030

Fig 8. Suction side

01.0 045 TOW V AT TRIAL 12 KNOTS 300 000 6 KNOTS 200000 16 KNOTS 100000

P

____

T/cTIl

cavitation at.c 20°. V-shaped stern 20 lO a 30 I SERVICE cowomoN 'p _{STAJOAND LOAD} cONDmoNS P1337 1EET CAVITATION THICKER FORMATION INTERMITTENT CAViTATiON THRUST 8REAI DOWN C'ITATION

.-

'I

I

ISEWQ10Ec

e.

FOAMING CAVITATION CA/TTATION

TIP VORTEX CAViTATION INTERMITTENT

P1157

I

TI6AL

Instead of carrying out the tests at exactly the values discussed above, the cavitation studies were in this investigation made at seven standard load conditions for each propeller, covering a larger region. The result for a specific case must therefore be obtained by interpolation in the diagramxnes of Figs 7-8.

The cavitation extension had large circumferential variations with a maximum when a propeller blade had passed its upper position. The influence of stern shape on the suction side cavitation at position angle 200 is illustrated in Fig 7 and the corresponding

influence of propeller size and number of revolutions in Fig 8. As seen the cavitation extension was increased with decreasing

values of a and J. This means that the cavitation became .worse if

o the ship speed was increased

o the size of the ship was decreased, the relative propeller diameter D/L being kept constant (propeller rpm increased, see,

Table 4)

o the trial condition was changed to service condition by in-creased hull fouling

In general the cavitation was considerably worse if the stern was V-shaped than if it was ti-shaped or bulbous. This was true also if

the differences in actual a-J-values were considered. The cavita-tion seemed also to be slightly worse for the smaller, high speed propeller P1337 than for the larger, slow-rotating P1079.

7. PRESSURE FLUCTUATIONS

The pressure fluctuations at the stern caused by the propeller were measured with two pressure transducers located ih the upper

part of the stern frame,, see Fig 9. The measuring signals were' fed 'into a frequency analyzer giving the mean pressure amplitude at varying frequenciés. Maximum values were in all cases obtained

at' the blade frequency Hz. The variation of this maximum press-ure p with a and J is shown in Figs 10 and 11, where p is ex-pressed in non-dimensional form as

V

2p

K

= 2 D2

BL

Fig 9. Location of pressure transducers

Pressure trdnsducers

1/2

Bulbous stern

As seen the pressure amplitudes measured at the V-stern model were considerably larger than those recorded at. the two other models, especially at heavy propeller loads and low cavitation nunibers. The differences in amplitude between U- and bulbous

sternwere small.

A comparison between Figs 10-11 and Figs 7-8 indicates that there is a strong correlation between the pressure amplitudes and the cavitation. High pressure amplitudes were generally recorded when thick sheet cavitation1 free bubbles and foaming cavit4tion were observed. It is, hOwever, also evident that the existence of cavitation alone is not a criteriüm of high pressure amplitudes.

0 30 20 10 a 30 20 10 0 30 20 10 03 03 04 TDW 100 300 200 000 100000

Fig 10. Pressure fluctuations at hull; transduöer B. Propeller P1167 TRIAL COND.

V-SHAPED

U-SHAPED

\.

H0

a 30 20 10 a 30 20 10 30 20 10 200000 100000 11. KNOTS 02 0.3 0.4 RIAL CONG : STANDARD LOAD CONDIT IONS P1337 P1167 P1079 V5 AT.TRIAL 12 KNOTS KNOTS KNOTS

11TRIAL ND.

I

Fig 11. Pressure fluctuations at hull; transducer B. V-shaped stern

The variation ofpressur-e amplitude p with trial speed, propeller diameter and ship size is given inFigs 12-14 for service condi-tions (speed and propeller rpm being somewhat lower than at trial, see chapter 6). As seen the pressure amplitude, expressed in N/rn2 was

l. l- - 3 times as large for the V-shaped stern as for the other stern shapes at the conditions in question

2. increased considerably with speed, being 4 - 6 times as large at 18 knots as at 12 knots for the V-shaped stern, see Fig. 12

10 12 11. 1,6 18 TriaL speed

in knots

Fig 12.. Variation of pressüré amplitude with trial 'speed. V-shaped stern. Service condition

3. increased slightly with propeller diameter (decreased number of revs) if the cavitation was insignificant, as for all stern shapes at 12 knots and for the U-shaped and bulbous stern at 16 knots, see Fig 13. The reason was probably the smaller clearance between propeller blade tips and top of aperture

p N/rn2 Trial speed 16 knots

A V- shaped stern

o U-shaped stern

o BuLbous stern

Fig 13. Variation of pressure amplitude with propeller diameter. = 16 knots. Service conditions

3000

2000

1000

increased when the propeller cavitation grew worse due to smaller propeller diameter (increased rpm) and unfavouràble stern shape as for the V-shaped stern, propeller P1337, see Figs 12 and 13

decreased with increased ship size, being considerably higher for 100 000 TDW than for 500 000 TDW, see Fig 14. This was also true if the number of revolutions was kept constant.

p 2 N/rn 6000 5000 4000 3000 2000 1000 0

V-shaped stern

U-sha'ed.stern

Fig 14. Variation, of pressure amplitude with ship size.

The pressure fluctuations around a ñon-cavitating p±opeller

de-pend upon the thickness and lift of the blade sections. At

cavi-tating conditions. in the i-rregular flow behind a ship part of the

blades are surrounded by cavities, whose thickness and extension

vary ci-rcurnferentially. These cavities can be treated as an

in-creased and varying blade thickness. Calculations have shown that

very large pressure amplitudes can occur if

he volume of these

cavities varies considerably with time [8]. It therefore seems

clear hat the difference in pressure fluctuation found between

the three stern shapes

t the present investigation is due to the

fact that the propeller caVitation is worse and more varying

be-hind the V-stern models than bebe-hind the other models. For a given

propeller geometry and load condition (a and J) this cavitation

depends upon the wake distribution. The total circumferential

wake variation at radius 80 mm was 0.8 for the V-stern model, but

only 0.3 and 0.2 for the ti-stern and bulbous stern models

respec-tively.. Thus the large wake, variations behind the V-stern model

seem to be responsible for the high pressure ainplitiides.

8. SUMMARY

This paper describes; by an example, how the propulsion,

cavita-tion and pressure fluctuacavita-tion properties Of a ship can be studied

at SSPA by model tests with 7 - 8 m long ship models in the

tow-ing tank and the large cavitation tunnel. In the example given

the investigation concerned three large full-formed tankers with

different stern shapes and fitted with three conventional

propel-lers of different diameters and pitches (different rpm)

The ti-stern ship was shown to. have about 5% higher resistance than

the bulbous stern ship and 'about 7% higher resistance than the

V-stern ship. Flow visualization tests indicated the reason for the

higher resistance to be a formation of strong bile vortices

around the. ti-stern ship. No similar vortices were found around the

V-shaped steln, while the strength of the bilge vor-tices of the

bulbous stern, seemed to be moderate.

The bilge vortices influenced the, propulsive properties, however,

by collecting water with low kinetic energy from the ship boun-dary layer and supplying it to the propeller. The resulting high hull efficiency more or less completely compensated the high re-sistance. Thus the sha't horsepowers f the U-stern ship and the bulbous stern ship were respectively 3% higher and 4% lower than that of the V-stern ship.

Irrespective of stern shape and propeller diameter tested, the cavitation was slight and the pressure fluctuations at the stern were small for the 300 000 TDW tanker both at trial and service'

conditions.. The margins to severe cavitation and high pressure fluctuations were, however, considerably lower for the V-shaped stern than for the other stern shapes! the reason probably being the larger circumferential wake variations and higher wake peaks. The lower wake peaks and the more uniform wake distribution of the U-shaped and bulbous sterns were caused by the bilge vortices.

Thus, the lowest shaft horsepower and the best cavitation and pressure fluctuation properties were obtained with the bulbous stern, where the ship boundary layer was rolled up into two bilge vortices. These vortices were not so strong that.they increased the resistance significantly, but strong enough to increase the mean wake, reduce the wake peaks and make the wake distribution fairly uniform.

The propulsion tests showed that the shaft bosepower decreased with increasing propeller diameter, e g with decreasing speed of rotation. This decrease was about 3% per 10 rpm for all sterns tested. At low ship speeds, where the cavitation was insignifi-cant the small propeller gave lower pressure amplitudes than the larger ones, probably owing to the greater distance between pro-peller tip and stern. At higher speeds, on the other hand, the small propeller caused higher pressure amplitudes than the others becauseof more extensive and fluctuating cavitation.

-If the size of the ship was increased, the speed, the relative propeller diameter ard pitch ratio being kept constant the pro-peller Speed of rotation decreased while the propulsive efficien-cy remained almost unaltered. If, on the Other hand, both ship

size meant a decreased relative propeller diameter and a lower propulsive efficiency. tn both cases the cavitation properties were improved and the pressure amplitude, expressed in N/rn2, de-creased with size.

This result is in contradiction to the fears expressed prior to the start of the investigation that the high shaft horsepowe

ré-quir.ed for large tankers would give vibration problems. As a matter of fact di-fferènt cavitation characteristics had the ef-fect that much higher pressure fluctuations were obtained if the model test results

ker than to a 500 0

9. REFERENCES

[11 Dyhe, G & Idunget, L: Large Tankers. Final Repo-ton

Propulsioii, Cavitation and Vibration Properties of Large

Single Screw Tankers with Di-f-ferent Stern Shapes.. Report

Kl67-lO of the Swedish State Shipbuilding Experimental Tank, Goteborg, April 1973

[2,] Lindgren, H & Bjärne, E: The SSPA Standard Propeller

Family. Open Water Characteristics. Publication No 60 of the Swedish State Shipbuilding Experimental Tank, Gote-borg, 1968

Proceedings of the Eighth International Towin.g Tank Con-ference, Madrid, 1957

Próhaska, C W: A Simple Method for the Eva-luation of the Form Factor and the Low Speed Wave Resistance. Proceed-ings of the Eleventh International Towing Tank Confer-ence, Tokyo, 1966

Sasajirna, H' & Tanaka, I: On the Estimation of Wake of Ships. Proceedings of the Eleventh International Towi1g

Tank.Confeence, Tokyo, 1966

Lindgren, H: Ship Model Correlation Method Based on The-oretical Considerations. Appendix 2 of Report of Perfor-mance Committee in Proceedings of the Thirteenth

Inter-natiOnal Towi-ng Tank Conference, Berlin, 1972

Lerbs, H: On the Effect of Scale and Roughness on Free Running Propellers. Journal of American Society of Naval Engineers No 1, 1951

Huse, E: Pressure Fluctuations on the Hull Induced by Cavitating Propellers. Norwegian Ship Model Experimental Tank Publication No ill, 1972

/

were converted to apply to a 100 000 TDW tan-00 TDW tanker.

CONTENTS

Abstract 1

Introduction 2

Models and test facilities 2

Resistance tests 5

Self propulsion tests 6

Wake measurements and flow studies 11

Cavitation studies 14

Pressure fluctuations 17

Summary 24