Model tests on contra-rotating propellers

MODEL TESTS ON CONTRA-ROTATING

PROPELLERS

by

J.D. van MAN EN and M.W.C. °OSTER VEL D

MINISTERIE VAN DEFENSIE (MARINE)

HOOFDAFDELING MATERI EEL

BUREAU SCHEEPSBOUW

TORENSTRAAT 172

's-GRAVENHAGE

NEDERLAND

PREFACE

The Royal Netherlands Navy is by its nature interested in the

hy-drodynamics of ship propulsion. This attention is primarily

focussed on propellers having the highest degree of performance with respect to efficiency in conjunction with superior cavitation and propeller induced vibration characteristics.

Because of the physical limits of conventional screw propellers, the Netherlands Navy decided to start a survey of unconventional propulsors and to study the benefits that could be expected from, application of such devices on ships.

In, his inaugural lecture at the Technical University of Delft, Professor Van Manen mentioned already the importance for the Netherlands Navy to tackle the problems of application of uncon-ventional thrust devices for ships. In addition, he stated that the results of such investigations will be of use for merchant ships,.

Subsequently the interest of the Netherlands shipbuilding industry

was aroused. As a result a project concerning investigations on

unconventional propulsion devices came into existence. This

project was sponsored jointly by Government and industry (viz., the shipyards that make up the Netherlands United Shipbuilding Bureaux, Ltd. and LIPS N.V., Propeller Works).

One of the subjects of this project was the investigation of the hydrodynamic characteristics of contra-rotating propellers. This

report gives a summary of the test results.

Jr. MEIJER

The Director of Naval Construction J. F.

CONTENTS

page

Abstract 5

Introduction 5

Test results with systematic series of contra-rotating

propellers 5

Comparative tests with ship models equipped with

con-ventional and contra-rotating propellers 6

Description of hull forms and propellers 6

Model resistance and self-propulsion tests 11

Cavitation tests 14

Measurements of propeller induced vibratory forces 14

Determination of stopping abilities 19

Conclusions 20 Acknowledgement 21 References 21 Nomenclature 22 ,.. ., ..

Introduction.

During the past years the trend of most ship designs has been toward higher speeds (cargo liners) and/or larger displacement (tankers, or carriers) and, therefore, high-powered ships. As a result the problems of propeller cavitation

and propeller induced vibration became matters of great concern.

In an attempt to provide merchant ships with

propulsion devices with superior cavitation and

propeller induced vibration characteristics in addition to a high propulsive efficiency, the application of contra-rotating propellers have been the subject of several investigations. [1],

[2], [3]. This paper presents the results of

in-vestigations on contra-rotating propellers which has been performed at the NSMB during the past five years.

These investigations covered the following details. A systematic series of contra-rotating

propeller systems was designed and

manu-factured. These systems, consisting of a four

bladed forward screw and a five bladed aft screw,

were designed for equal power absorption by the forward and the aft screw. Tests were carried out

in the towing tank to determine the open-water characteristics of this series of contra-rotating propellers.

Based on

these open-water test

results. contra-rotating propellers were designed for a

tanker and a cargo liner. Comparative tests have

.) Netherlands Ship Model Basin, Wageningen.

MODEL TESTS ON CONTRA-ROTATING PROPELLERS

by 1.D. van MANEN and M.W.C. OOSTERVEL D

Abstract.

This paper presents the results of open-water tests with a systematic series of contra-rotating

propellers, consisting of a four bladed forward screw and a five bladed aft screw.

Based on the open-water test results, contra -rotating propeller systems were designed for a tanker and a cargo liner. Comparative tests have been carried out with the tanker and the cargo liner both equipped with contra-rotating propellers and with a conventional screw. The propulsive efficiencies,

the cavitation characteristics, the propeller induced vibratory forces and the stopping abilities are

dealt with.

been carried out with both ships equipped with a contra-rotating propeller system and with a con-ventional single screw arrangement. In Fig. 1 the

contra-rotating propeller arrangement on the

stern of the cargo liner is shown. The

pro-pulsive efficiencies, the cavitation

character-istics, the propeller induced vibratory forces and the stopping abilities are dealt with.

The investigations on contra-rotating propel-lers were given in detail in [4], [5], [6]. [7] and

[8]; a recapitulation of the results is given here.

Test results with systematic series of conIra-rotating propellers.

An important method of screw design is that

which is based on the results of open-water tests

with systematically varied series of screw models. [9] and [101.

According to the lifting line theory, as

described in Ref. [11], a systematic series of

contra-rotating propellers, consisting of a four bladed forward propeller and a five bladed aft

propeller, was designed. A problem which may

occur on contra-rotating propellers is that the

cavitating tip vortices generated by the blades of the forward propeller may impinge on the blades of the aft propeller and cause damage there. This problem was avoided by reducing the diameter of the aft propeller. This reduction was based on the expected slipstream contraction at design condi-tion. In addicondi-tion. this reduction is attractive with regard to efficiency as for equal screw loadings,

a five-bladed propeller has a smaller optimum

6

Figure 1. Arrangement of contra-rotating propellers for a cargo liner model.

diameter than a four-bladed propeller with equal

blade area ratio. The sets of contra-rotating

propellers were designed in such a way that one

set is representative for tanker application and another set for cargo liner application. Three

additional sets complete the systematic series.

The particulars of the propeller models are given in Table 1 and Fig. 2.

Tests were carried out in the towing tank to determine the open-water characteristics of the series. The open-water test results were faired and plotted in the conventional way using the

_ T n2D4, K = Q/pn2D5 _and

coefficients K_T

_"

o = as functions of the advance

coefficient J - VA/nD. The diagram is given in

Fig. 3. In this diagram each set of contra -rotating propellers is considered as one propulsion unit,

the thrust T and the torque Q are based on the

sum of the thrusts and torques respectively of the forward and aft screw. The diameter D denotes

the tip diameter of the forward propeller.

In

addition, the aft propeller thrust-total

thrust ratio Taft/T and the aft propeller

torque-total torque ratio Qaft/Q are presented in Fig. 3. For design purposes, various practical results can be derived from Fig. 3. In the case where the

power P and VA and n are given, thedetermination

of the optimum diameter from a point of view of efficiency of the contra-rotating propeller system

can be solved by plottingn 0 and 5 (5=101. 27/s) as functions of the coefficient

Bp, (Bp = 33.08 KQ1/2/J5/2)

As a comparison, the optimum curves for

efficiency, n and diameter coefficient, 8 , of

the contra-rotating propeller series and

the

B 4-70 screw series are given in Fig. 4. Screws

of he B 4-70 screw series are usually applied

behind single screw ships [10], and [12].

At the top of Fig.4 the ranges of Bp-values,

typical for different ship types are indicated. The lightly loaded screws of fast ships are at the left

hand side and the heavily loaded propellers for

towing vessels are at the right. This diagram

gives quick information which type of propeller

will be the best with regard to efficiency for a certain ship type. For fast ships, (cargo liners)

contra-rotating propellers appear to give a higher

efficiency than conventional screws. It must be

noted, however, that by application of a

conven-tional screw behind a ship, the rudder partly

removes the rotational velocity from the

propel-ler jet and hence improves the efficiency of the

propulsion device. It is obvious that this

improve-ment in efficiency will not be found by application of contra-rotating propellers behind a ship.

It can be seen from Fig. 4 that the optimum diameter of the contra-rotating propeller series is considerably smaller than the optimum

dia-meter of the conventional screw series.

Comparative tests with ship models equipped with consentional and eontra-rotating propellers. Description of hull forms and propellers.

Comparative tests have been carried out with a 32500 DWT tanker model and a .cargo liner

model both equipped with successively a conven-tional screw propeller and contra-rotating

propel-lers. The principal dimensions of these ships

are given in Table 2; the hull forms and the stern

Table 1

Principal characteristics of screw models of contra-rotating propeller series.

SET 1 2 3 4

5

Forward Aft Forward Aft Forward Aft Forward Aft Forward Aft

Diameter D (mm) 210. - 179.34 208. - 182.72 210.- 191.01 217.50 203.33 210.- 198.90

Number of blades 4 5 4 5 4 5 4 5 4 5

Pitch ratio at 0.7 R 0.627 0.957 0.779 1.034 0.931 1.110 1.083 1.196 1.235 1.306

Expanded blade area ratio 0.432 0.507 0.432 0.515 0.432 0.523 0.432 0.531 0.432 0.539

Daft/Dforward 0.854 0.878 0.910 0.935 0.947

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Figure 5. Body plan and stern arrangement of tanker.

0 0 20

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Figure 6. Body plan and stern arrangement of cargo liner.

0 50 7o opt 400 300 Cs 200 100 BP

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arrangements are given in Figs. 5 and 6.

The propeller designs for both ships were based on 16,000 metric DHP at 120 RPM, and with ship speeds of 16.5 knots for the tanker and 20.5 knots

for the cargo liner. The conventional screws

were designed according to the circulation theory

for wake adapted propellers: The principal full

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Table 2

Principal dimensions of tanker and cargo liner.

scale characteristics of the propellers are given in Table 3; further details of the conventional

screw propellers for the tanker and the cargo

liner are presented in Figs. 7 and 8. The

particu-lars of the contra-rotating propellers for the

tanker (set 2) and the cargo liner (set 4) were

already presented in Fig. 2.

Figure 8. Particulars of propeller model for cargo 'incl.

Ship type 32,500 DWT tanker Cargo liner

Loading condition Loaded Ballast Loaded

Length between perpendiculars (m) 195.07 195.07 158.50

Breadth moulded (m) 25.91 25.91 22.40

on F. P. (m) 10.331 ' 6.401 5.839

Draft moulded on A. P. (m) 10.331 6.706 8.839

mean (m) 10.331 6.554 8.839

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Model resistance and self-propulsion tests.

Model tests have been carried out to obtain a comparison of the propulsive quality of the tanker

and the cargo liner both equipped with

success-ively contra-rotating propellers and a conven-tional screw propeller.

Resistance and self-propulsion tests were con-ducted in the deep-water basin of the NSMB, in

accordance with established procedures. All

model data were extrapolated to full-scale ship values using Schoenherr's friction coefficients

with an

addition

of 0.00035 for correlation

allowance. For turbulence stimulation,

a trip

wire of lmm diameter was fitted to girth each

model at a section 5 percent LBP at aft FP. The

tanker model was tested at the loaded and the ballast condition; the cargo liner model only at

the loaded condition. The results of the resistance and self-propulsion tests are presented in

Tables 4 through 6. Figs. 9 and 10 show the per-formance predictions for the tanker and the cargo

liner.

Table 7 compares the results of

the propulsion tests with the ship models equipped

with conventional screws and the contra-rotating

propellers. Table 7 shows that application of contra-rotating propellers on both ships gives a significant reduction in DHP. The DHP of the tanker model with contra-rotating propellers is

about 4.5 percent less in the loaded condition and 8 percent less in the ballast condition, when com-pared to the model with the conventional screw propeller. The contra-rotating propellers behind

the cargo liner require about 6.5 percent less

Table 3

Principal full scale characteristics of tanker and cargo liner propellers.

DHP than the conventional screw propeller in the

loaded condition of the ship. The gain in trial speeds, due to application of contra-rotating propellers, is at maximum power absorbtion

(16000 DHP):

tanker in loaded condition 0.12 knots

tanker in ballast condition 0.30 knots

cargo liner in loaded condition 0.21 knots

The tanker with the

conventional screw

arrangement suffered from air sucking into the

propeller plane in the ballast condition, whereas this phenomenon did not occur when the contra-rotating propellers were fitted to the model. This must be attributed to the smaller diameter of the

contra-rotating propellers.

An analysis of the various propulsion factors shows that the wake fraction was larger for the contra-rotating propellers than for the conven-tional screws. This is due to the smaller

diame-ters of the contra-rotating propellers. In the case of the tanker, the thrust deduction factor did not

differ very much. This factor was somewhat larger for the cargo liner with contra-rotating

propellers than with the conventional screw.

For the tanker,

the increase in propulsive

efficiency due to contra-rotating propeller ap-plication was principally obtained by a better hull efficiency whereas for the cargo liner this in-crease was obtained by both a better hull

efficien-cy and a higher open-water efficienefficien-cy of the

contra-rotating propellers. More detailed data

must be available, however, to give a complete

explanation of the obtained reduction in DH P.

II

Ship 32,500 DWT Tanker Car rip liner

Propeller type Single

screw

Contra -rotating Single screw

Contra -rotating

forward aft forward aft

Diameter (D) (mm) 6,400 5,720 5,025 6,000 5,220 4,880

Number of blades 4 4 5 4 4 5

Pitch at blade root (mm) 3,875 3,175 4,655 5,284 5,383 5,513

Pitch at blade tip (mm) 4,825 4,240 4,850 5,799 5,251 5,488

Pitch at 0.7 R (mm) 4,696 4,455 5,195 5,796 5,652 5,838

Blade area ratio 0.624 0.432 0.515 0.621 0.432 0.531

Direction of turning Right Left Right Right Left Right

aft forward - 0.878 0.935

D /D

-12

Table 4

Results of resistance and self-propulsion tests with tanker model in loaded condition.

Table 5

Results of resistance and self-propulsion test,s with tanker model in ballast condition.

Table 6

Results of resistance and self-propulsion tests with cargo liner in loaded condition.

E

Resistance tests Self-propulsion tests Coeffic lents Propeller Speed EI.IP Resistance DHP RPM Thrust

arrangement knots metric tons metric tons 113 t a 90

'. 4

I

'7 5/6- V). N -0 Conventional 14 5273 54.91 7574 92.8 74.50 0.696 0.263 0.396 1.221 0.536 1.063 screw 14.5 5986 60.18 8689 97.1 81.64 0.689 0.263 0.393 1.215 0.53-1 1.063 I.-)-1- ,-. 1 -. 15 6813 66.22 9972 101.4 89.64 0.683 0.261 0.392 1.215 0.531 1.060 15.. 5 7839 73.73 11473 105.8 98.91 0.683 0.255 0.393 1.227 0.527 1.057 16 9125 83.15 13206 110.5 109.46 0.691 0.240 0.396 1.258 0.521 1.056 DH P DHP Thrust Thrust 16-5_ 10685 94.41 15343 115.7 122.15 0.696 0.227 0.395 1.278 0.516 1.056

17 12474 106.97 18138 121.9 137.81 0.688 0.224 0.392 1.277 0.509 1.059 forw. pr. aft. pr. forw. pr aft. pr.

Contra-rotating propellers 14 7193 90.5 75.57 0.733 0.273 0.465 1.358 0.512 1.055 3667 3526 40.40 35.1-14.5 8246 94.5 82.82 0.726 0.273 0.483 1.359 0.508 1.051 4224 4022 44.24 38.58 /5 9445 98.6 90.98 0.721 0.272 0.466 1.364 0.504 1.050 4844 4601 50.78 30.20 15.5 10914 103.2 100.40 0.718 0.266 0.464 1.371 0.500 1.046 5634 5280 53.92 46.47 16 19647 108.1 111.38 0.722 0.253 0.462 1.387 0.496 1.048 6558 6089 6.0.11 51.27 16.5 14805 113.6 124.28 0.722 0.240 0.459 1.405 0.492 1.045 7700 7105 67.36 56.92 17 15371 115.6 139.84 0.718 0.235 0. 455 1..104 0.486 1.051 9062 83119 76.10 63.74

Resistance tests Self -propulsion tests Coefficients

Propeller Speed EliP Resistance DHP RPM Thrust

arrangement knots metric tons metric tons 1D t w 1 II ' 11 Conventional 14.5 4941 49.68 6508 90.1 66.30 0.759 0.251 0.399 1.247 0.557 1.093 screw 15 5578 54.21 7560 94.4 73.33 0.738 0.261 0.397 1.226 Oi 544 1.087 15.5 6346 59.69 8775 98.8 81.32 0.723 0.266 0.399 1.221 0.549 1.080 16 7253 66.08 10176 103.4 90.17 0.713 0.267 0.396 1.213 0.546 1.077 -16.5 8333 73.63 11890 108.5 100.40 0.701 0.267 0.393 1.208 0.541 1.072. 17 9610 82.41 14009 11.1.6 112.02 0.686 0.264 0.378 1.184 0.541 1.072 DHP DHP Thrust Thrust

17.5 11160 92.97 16773 122.2 125.77 0.665 0.261 0.3-17 1.132 0.546 1.077 forw. pr. aft. pr forw. pr. aft. pr. Contra-rotating propellers 14.5 6083 85.1 66.51 0.812 0.253 0.518 1.550 0.508 1.031 3010 3073 34.32 N. 32.19 15 6940 88.8 72.80 0.804 0.255 0.518 1.547 0.505 1.030 3445 3495 37.52 35.28 15.5 8040 93.0 80.76 0.789 0.261 0.516 1.528 0.502 1.028 4005 4035 41.64 39.12 16 9352 97.5 89.64 0.776 0.263 0.515 1.518 0.497 1.028 4476 4676 46.15 43.49 16.5 10967 102.5 99.98 0.760 0.264 9.512 1.508 0.492 1.024 5488 5479 51.59 48.39 17 12858 107.9 111.81 0.747 0.263 0.507 1.498 0.186 1.029 6464 6394 57.88 53.93 17.5 15151 113.8 125.45 0.737 0.259 0.500 1.481 0 . 182 1.031 7669 7482 65.34 60.11

Resistance tests Self-propulsion tests Coefficients

Propeller Speed EliP Resistance DHP RPM Thrust

/

p/

arrangement knots metric tons metric tons t , 10 0 R _{0 s 11-} 0 (xl 2 o, --,

Conventional 18 6889 55.79 9437 99.7 66.24 0.730 0.158 0.246 1.120 0.632 1.032 screw 18.5 7537 59.40 10366 103.0 70.85 0.727 0.162 0.246 1.111 0.632 1.035

19 8156 62.58 11324 106.1 75.38 0.720 0.170 0.243 1.097 0.632 1.038 19.5 8848 66.15 12435 109.3 80.70 0.712 0.180 0.242 1.083 0.631 1.041

20 9766 71.19 13821 112.9 87.14 0.707 0.183 0.243 1.079 0.629 1.091

.1)....5__ 11109 79.00 15705 117.2 95.72 0.707 0.175 0.243 1.090 0.625 1.038 DIIII DHP Thrust Thrust

21 12805 88.89 18072 122.0 106.48 0.709 0.165 0,243 1.104 0.619 1.037 form. pr. aft. pr. "forw. pr. aft. pr. Contra-rotating propellers 18 14 - 8997 99.2 72.03 0.766 0.226 0.303 1.110 0.650 1.060 4654 4343 37.48 34.57 18.5 - -' c9788 102.1 76.09 0.770 0.219 0.298 1.112 0.632 1.063 5059 4729 39.60 36.49 19 .3 10600 105.0 80.27 0.769 0.220 0.297 1.110 0.652 1.063 5477 1123 41.80 38.47 19.5 6, 5 11584 107.9 85.44 0.764 0.226 0.300 1.106 0.650 1.063 5979 5605 44.49 40.95 20 6 , 51 12927 111.6 92.46 0.755 0.230 0.298 1.097 0.648 1.063 6654 6273 48.18 44.28 20.5 5,1 14743 116.1 101.95 0.754 0.225 0.296 1.101 0.643 1.065 7581 7162 53.21 48.74 21

S 0

17163 121.5 113.92 0.746 0.220 0.289 1.097 0.638 1.066 8833 8330 59.72 54.20 4 I' 11 I ,

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20.000 15.000 z ct 10 0 00 90 145 15 155 16 165 Speed in knots 18 130 120 90 I opeller RPM onventiona screw ntra-rotating prope Co Ocr 20 000 15000 10.000 Speed in knots

Figure 10. Power and RPM curves for cargo liner.

Table 7

Contra-rotating propellers better (+) or worse (-)

than conventional screw propellers.

13 Propeller RPM Conventional Contra- rota tmg screw propellers

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Tanker Cargo Liner loaded condition Speed knots Loaded condition Ballast condition 14 14.5 + 5.1% + 6.5% 15 + 5. 3% + 8. 2% 15.5 + 4. 8% + 8. 4% 16 + 4. 2% + 8. 2% 16.5 + 3. 4% + 7. 8% 17 + 4. 2% + 8. 2% 17.5 + 9. 6% 18 i 4.6% 18.5 + 5. 4% 19 + 6. 4% 19.5 + 6. 8% 20 + 6. 6% 20.5 + 6. 2% 21 + 5. 1% 17 175 18 18 18.5 19 19.5 20 205 21 21.5 Speed in knots 18 18.5 19 195 20 205 21 215 17 17.5 14.5 15 155 16 16,5 Speed in knots

Figure 9. Power and RPM curves for tanker.

130 120 110 3. 100 110 100 D P + 5. 1%

-1

14

Cavitai ion tests.

Cavitation tests were conducted in the 40 cm

diameter slotted wall cavitation tunnel with flow

regulator of the NSMB, 1[131 and [141, simulating

the full load operating conditions. The axial wake distributions behind the two models, as measured in the deep-water basin by means of a pitot-tube were simulated in the tunnel_ The results of the

velocity surveys in the way of the propeller are

described in Fig. 11 for the tanker and the cargo

liner. Tanker 0A0 30 'Wake 't ;action, 9, o.es 0.80 .S pee d,=16.5 knots Cargo Lire, -0.20 03 Wake, Vractioni y 0.50.060 01.0 'Basel in Speed-205 knots 070 075 0801

Figure 11. Wake distributions in way af the propeller of tanker and of cargo liner in loaded condition.

The results of the cavitation tests are presented

in Figs. 12 and 13. From an examination of the

various test results, it can be concluded that the conventional screw and the forward propeller of the contra-rotating propellers are quite compar-able as far as blade cavitation is concerned. This

holds as well for both the tanker and the cargo'

liner. The extent of sheet cavitation on the back of the aft propeller of the contra-rotating propel-lers was for both ship types considerably smaller

than that on the forward propeller. Apparently the forward propeller smoothes the peripheral irregularities' in the flow behind the ship and

consequently in the inflow to the aft propeller.

From the test results it can be seen that with

regard to the strength of the tip-vortex cavitation,

the contra-rotating propellers

were slightly

better than the conventional screws. This was to

be expected since the 9 blades of the

contra-rotating propellers were less loaded than the 4

blades of the individually screws..

In the upper and lower part of the aperture,

the tip-vortex of the forward propeller of the

contra-rotating propellers interfered with the

blades of the aft propeller, especially for the

cargo liner propellers, which led periodically to unfavourable cavitation phenomena. The chance for the appearance of these phenomena depends

on the angular conjunction of the blades of forward

and aft propeller,, In order to avoid these plieno-mena it may be useful to reduce the di.ametjer of

the aft propeller slightly more.

Measurements of propeller induced vilbral ry forces

42o

Comparative tests on propeller induced v

bra-'tory forces have been carried out on the cargo

liner model equipped with successively the con-ventional screw propeller and the contra-rot ting

screw propellers.

To measure these forces, a special arr

ment had to be made in the case of the co ira-rotating propellers in order to use the existing

measuring equipment [15]. The forward propeller was driven by the normal dynamometer, installed

in the ship model. The aft propeller was driven by a dummy dynamometer. This dummy dynamo-meter was installed in an open-water test boat,

mounted behind the ship model. A stiff coupling

shaft synchronized the combination. By I ex-changing the real dynamometer and the dummy dynamometer, the vibratory outputs of both

pro-pellers were determined. II

The results of the measurements of the propel-ler induced vibratory forces are given in Figs. 14 through 17. Samples of the instantaneous torque

and thrust of the conventional screw and of the forward and aft propeller of the contra-rotating propellers are shown in Figs.14 and 15. The in-stantaneous thrust eccentricity for the different propellers is given in Fig. 16, whereas the in-stantaneous transverse forces are presentel, in. Fig. 17.

Figs. 14 and 15 show clearly that the variationS in torque and thrust of the forward screw of the contra-rotating propellers are about the same in magnitude as those of the conventional screw,, which implies that these variations, expressed

in percentages of the mean values are about twice

as larger for the forward screw of the

'Conventional screw

KT-0.174 o-0.1238

contra - rotating ,propellers

KT=0169 C0-12.41

forward 'propeller aft ,propeller

Figure 12. Cavitation patterns of conventional and contra-rotating propellers behind tanker.

Conventional screw K1=0.186 Co=4.94 Contra-rotating propellers Kr=0.186 C0=480 forward propeller

ir

ii

AN/isdkve

111111171

aft !propeller

Figure 13. Cavitation patterns of conventional and contra-rotating propellers behind cargo liner.

TO 9 II ,c 0 8 7 6 5 40 0 11_i, 8 7 6 5 0 360° ting

WINIEFAIIII

lim m

1111"

111.1"

1W

Conventiona4 screw

111/11

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.a:4 OUNME&I WAMMEIV

Contra-ro - / % . -

--

Z .2 :,_,__ ..71_: \ //

.N___

propellers

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i ,

\

\

/

1 'r v

1111111111TAL

MEM

11111111111NIMMILIIMINIAMII1

V/

1111/

in

Alk

Kw

Ell

IN

C.onvenional screii

NV

NI/

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A

Aft

wA____ A

MINNp

ProPe1ter__

ow / -1 mrelix.

_{- ,}

1 hi -

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FEE

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45,090°

1350 180 225° 270° 315° 360° 431

Figure 14., Instantaneous torque of cargo liner propellers..

.45° 90° 1350 180 225° _ 270 315

0

Figure 15. Instantaneous thrust of cargo liner propellers.

17

atingl

10

90

18 -06 -0,4 -02 06 01. a, w 0.2

Forward propeller Aft propeller

as

0.2

E Hor in meter

Figure 16. Instantaneous thrust eccentricity of cargo liner propellers.

screw. The variations of the aft screw are lower

than those of the forward screw of the

contra-rotating propellers. Apparently the forward

pro-peller smoothes the peripheral irregularities of the flow in the way of the aft propeller as was also

evident from the cavitation tests. It appears from Fig. 16 that the thrust eccentricity of the forward

screw of

the contra-rotating propellers is

considerably larger than that of the conventional screw. However, the thrust of this forward screw is about half as large as that of the conventional

screw, so that the maximum bending moments due to the eccentricity of the thrust will not change

very much. This implies that the stresses due to these bending moments must be almost equal if the diameter of the outer shaft is the same as that of the shaft of the conventional screw.

Since the inner shaft diameter is smaller,

possibly the loading of this shaft increases, since

its stiffness against bending is only about a fourth

Conventional screw 0-90°

era

AIWIIM

6° s° 02 04 0.6 E _{Hoc, in meter} 0.6 -6 -4 -2 0 2 F Hor. in tons 1.

Figure 17. Instantaneous transverse forces of cargo liner

part of that of the conventional propeller shaft,

whereas the moments are in the order of half of those of the conventional propeller.

Fig. 16 shows that in many cases the

eccentri-cities for the contra-rotating propellers are in opposite direction, so that the bending moment

on the forward screw may be reduced by that on the aft screw. The variations in thrust and torque

of the forward and aft screw, however, may

re-inforce each other.

It can be seen from Fig. 17 that the transverse

force variations of the conventional screw are large, whereas those of the contrarotating

pro-pellers are negligible both in quantity and in

direction.

Additional measurements were conducted to measure the effect of a change of the angular positions of the mutual blade encounter of the contra-rotating propellers. From these tests it was concluded that no significant differences

occur for different angular positions of the mutual

blade encounter. ID 8=0%90° 'IL 12 10 o ,

i....

Conventional screw .. >`" u_ Contra-rotating propellers

r/

11

Aft °propeller 1 Forward propeller

z45e

0 450 . -8 propeller:"

15

0

2

Determination of stopping ahilities.

Investigations have been carried out to compare the contra-rotating propellers and the conven-tional screw with respect to their ability to stop the 32500DWT tanker. The propelling machinery was supposed to be a Steam Turbine orDiesel Engine, each capable of developing 16000 DHP at

120 RPM.

The comparison of the stopping abilities of the

contra-rotating propellers and the conventional

screw is based on a stopping manoeuvre as illus -trated in Fig. 18. This manoeuvre is divided into four phases:

phase 1. Steam or fuel supply to the engine is shut, the propeller is running slack and the

ship speed decreased due to the hull

resistance until the RPM are sufficiently low to enable reversing of the engine

rotation.

phase 2. The ship is further slowed down by the propeller system running full astern, till

a forward speed of about six knots is achieved. At this speed the ship will

loose steerability and tugs will have to

render assistance.

Head reach in kin

Figure 18. Schematic representation of stopping manoeuvre of a large tanker.

7 8

50

0 t

-50

phase 3. The propeller is stopped and tugs make

fast

phase 4. With the propeller slowly turning astern

the stopping manoeuvre is completed. In

this phase the steering of the ship is

accomplished by tugs.

For a comparison of the conventional screw and the contra -rotating propellers, the

character-istics for phases 1,

2, 3 and 4 of the stopping

manoeuvre were derived from the results of

model tests. During these tests the total braking

force (hull resistance and propeller force) was

measured at different speeds of the model and at different propeller RPM. The speed of the model was kept constant during a test. For the

calcula-tion of the head reach, it was assumed that the ship's speed changes so slowly during the stopping manoeuvre that the values of the total braking force, as measured during the stationary tests, were correct. Thus a quasi-steady approach (as described in [12] and [16] has been used for analysing the stopping manoeuvres. This approach is correct for large ships having

relatively low powers installed, so that long stopping times occur [16]. To determine the added

,-nass of the ship during the stopping manoeuvre

19

Steam

screwsupply shutturning slack

1

down Full power astern

I

Screw stopped tugs make

I

Tugs made fast. fast ....Lturning slowly

screw astern

Propeller RPM

Speed of ship

PhaseI Phasell Phase DI

---,._-...

Phase 12---._

20 200 100 a. 0. ce o -100 15 250 500 750 /000 Time in seconds 1250 1500 750 6000 5000 4000 000

Figure 19. Comparison of stopping abilities of conven-tional and contra-rotating propeller for a steam turbine tanker.

000

4400

000:

000

Figure 20. Comparison of stopping abilities of conven-tional and contra-rotating propeller for a tanker equipped with Diesel engine which can be reversed at 20 RPM.

additional dynamic stopping tests were perform-ed.

A comparison between the stopping abilities of

the conventional screw and the contra-rotating

propellers can be made from the results present-ed in Figs. 19 through 21. Fig. 19 shows the head reaches of the turbine tanker to be almost equal

for the conventional screw and the contra-rotating

1 0 750 1000 Time in seconds 1250 1500 1750 5000 000 4000 3000

Figure 21. Comparison of stopping abilities of conven-tional and contra-rotating propeller for a tanker equipped with Diesel engine which can be reversed at 15 RPM.

propellers. For the Diesel engine tanker (Figs.

20 and 21) the contra-rotating propeller reduced

the head reach in comparison with the conven-tional screw. The RPM at which the Diesel engine is reversed affects the head reach of the tanker

considerably.

Conclusions.

As a result of these investigations the following

conclusions can be made:

contra -rotating propellers have an open -water

efficiency which is slightly higher (about 2 percent) than that of conventional screw propel

-lers ; the optimum diameter of contra-rotating

propellers is less (about 15 percent) than that

of conventional screws.

contra-rotating propellers offer a means of

improving the propulsive efficiency of ships. The reduction in DHP due to application of the

contra-rotating propellers for a tanker was

about 4.5 percent in loaded condition and 8

percent in ballast condition of the ship if com-pared with the ship with conventional screw

The contra-rotating propellers behind the cargo liner compared with the conventional

screw requires about 6.5 percent less DH P.

Diesel engine

screw

Conventional

---C'thalgstI

IIIISIIIIIIIIM

EEO=

pir7:___

Pro. tIer RPM

EIM

..., Head reac

111M111

VA_

Steam turbine Conventional Screw propellers --Contra-rotating }Propeller RPM I

1

Head reach 'e-A6Mma.--_

-glimumul

Diesel engine Conventionalscrew Mg propellers ---Contra-rota I

I

PrrellerRPM

MI EN"

III

r

IlLpeed

in knots

Il

Eft.-20 100 -10 1 St 250 SOO 750 1000 Tirne in seconds 1250 1500 1750 3000 2000 -15. the. Speed

from the point of view of cavitation it was found

that conventional screws and the forward

pro-peller of the contra-rotating propro-pellers are

quite comparable as far as blade cavitation is concerned. The extent of sheet cavitation on the

back of the aft propeller of the contra-rotating

propellers is relatively small. With regard to

the strength of tip-vortex cavitation, the

contra-rotating propellers were slightly better than the conventional screws.

with regard to the propeller induced vibratory

forces, it was concluded that the thrust and

torque variations,

as well as

the thrust

eccentricity of conventional screws and

contra-rotating propellers did not differ very much,

although the average thrust and torque of each of the contra-rotating propellers are about half

of that of a comparable conventional screw. The application of cc,ritra-rotating propellers

causes a considerable reduction in transverse

forces compared to a conventional screw.

These forces are practically constant both in

quantity and in direction for the contra-rotating

propellers.

with regard to the stopping abilities of contra-rotating propellers it was concluded that for a

ship with steam turbine machinery the head

reaches corresponding to the conventional and the contra-rotating propeller are nearly equal.

For a ship with Diesel machinery, the

appli-cation of a contra-rotating propeller leads to a

decrease in head reach.

Acknowledge m en!.

We wish to express our appreciation to Mr.

J. aufm Keller for his valuable assistance in

designing the contra-rotating propeller series.

Reference..

1. Hadler, J. B. , Morgan, W. B. , and Meyers, K. A. , "Advanced propeller propulsion for high -powered

single-screw ships". Trans. SNAME, 1964. Lindgren, H. , Jung, I. , and Hillander, H. ,

"Contra-rotating propellers". "Analysis and overall arrangement". Stal-Laval. Technical Information

Letter 5/57, 1967.

Glover, E. J. , "Contra-rotating propellers for high speed cargo vessels:A theoretical design study". Trans. NEC!, 1966-1967.

Aurm Keller, W. H. , "Comparative tests with various

propellers for a tanker and for a cargo liner".

NSMB Towing tank test report No. 217 (June 1995).

Esveldt, J. , "Comparative testzwith various propel-lers for a tanker and for a cargo liner". NSMB Cavitation tunnel test report No. 615 (June 1965). Wereldsma, J. , "Investigation into the propeller

excited vibratory forces for various propulsion

devices ". NSMB report No. 67-210-AS (July 1968).

Hooft, J. P. , "Onderzoek naar de stopeigenschappen

van drie soorten voortstuwers". NSMB report

No. 66-021-BT, April 1966.

Hooft, J. P. , and Manen, J. D. van. "The effect of propeller type on 'the stopping abilities of large

ships". RINA 1967; De Ingenieur, 1967;

International Shipbuilding Progress, 1967. Manen, J. D. van, "Fundamentals of ship resistance

and propulsion. Part B, Propulsion". International Shipbuilding Progress, 1967.

"Principles of Naval Architecture", Published by the Society of Naval Architects and Marine Engineers,

New York (1967).

Manen, J. D. van, and Sentia, A., "Contra-rotating propellers". Trans. I. N. A. ,1956; International Shipbuilding Progress, 1956.

Manen, J. D. van, "The choice of the propeller"

Marine Technology, 1966.

Lammeren, W. P. A. van, "Testing screw propellers in a cavitation tunnel with controllable velocity distribution over the screw disc". Trans. SNAME, 1955; International Shipbuilding Progress, 1955. Manen, J. D. van, "On the usefulness of a test with a propeller model in a cavitation tunnel with a simulated non-uniform flow". Symposium on

Testing Techniques in Ship Cavitation Research,

Trondheim 1967; International Shipbuilding

Progress, 1967.

Wereldsma, J. , "Some aspects of the research on propeller-induced vibrations ".:International Ship-building Progress, 1967.

Jaeger, H. E. , "Le freinage de grands navires (V) correlation entre navire et modele en ce qui con-cerne rarret par le propulseur". ATMA, 1967.

21 -, , , 2., , , 3. 4, , '12. 13. 141. 15. , ,

22

Nomenclature.

B Taylor's propeller coefficient based on

5,

delivered horsepower, 33.08 KQ2 AJ

D propeller diameter

J advance coefficient, VA/nD

KT thrust coefficient, T/pr? D'

KQ torque coefficient, Q/pn2 D5

n

number of revolutions per second of the

propeller

Q torque

R propeller radius

T thrust

thrust deduction fraction VA speed of advance of propeller

Taylor wake fraction

8 Taylor's advance coefficient, 101.27/3 nD propulsive efficiency

nH hull efficiency

no propeller efficiency in open water

IR relative rotative efficiency

go cavitation number based on speed of advance of propeller