Tests with interlocking and overlapping propellers

ARCHIEF

Hydrodynamics

Section

Report No. Hy- 12.. January 1969

HYDRO- OG AERODYNAMISK

,LABORATORIUM

HYDRO- AND AERODYNAMICS LABORATORY

Lyngby Denmark

Tests with

Interlocking and

Overlapping Propellers

BY

T. MUNK

and

C. W. PROHASKA

IN COMMISSION:

DANISH TECHNICAL PRESS

SKELBAEKGADE 4 DK-1717 COPENHAGEN DENMARK

Lab.

v.

Scheepsbouwkunde

Technische Hogeschoor

Deift

fi .11

Hydro- and Aerodynamics. According to its by-laws, confirmed by His Majesty the King of Denmark, if is governed by a council of eleven members, six of which are elected by the Danish Government and by research organizations, and five by the shipbuilding industry.

Research reports are published in English in two series: Series Hy (blue) from the Hydrodynamics Section and Series A (green) from the Aerodynamics Section.

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Series Hy:

No.: Author: Title: Price: D. Kr.

Hy-1 PROHASKA, C. W. Analysis of Ship Model Experiments

and Prediction of Ship Performance 5,00 (Second printing)

Hy-2 PROHASKA, C. W. Trial Trip Analysis for Six Sister Ships 6,00

Hy-3 'SILOVIC, V. A Five Hole Spherical Pitot Tube for 6,00

Three Dimensional Wake Measurements

Hy-4 STROM-TEJSEN, J. The HyA ALGOL-Programme for Analysis 6,00

of Open Water Propeller Test

Hy-5 ABKOWITZ, M. A. Lectures on Ship Hydrodynamics 20,00 Steering and Manoeuvrability

Hy-6 CHISLETT, M. S., and Planar Motion Mechanism Tests 12.00

STROM-TEJSEN, J. and Full-Scale Steering and

Manoeuvring Predictions for a

MARINER Class Vessel Hy-7 STROM-TEJSEN, J., and

CHISLETT, M. S.

Hy-8 CHISLETT, M. S., and

BJORHEDEN, 0.

Hy-9 BARDARSON, H. R.,

WAGNER SMITT, L., and CHISLETT, M. S.

Hy-10 WAGNER SMITT, L.

A Model Testing Technique and Method of Analysis for the Prediction

of Steering and Manoeuvring Qualities of Surface Vessels

Influence of Ship Speed

on the Effectiveness of a Lateral-Thrust Unit

The Effect of Rudder Configuration on Turning Ability of Trawler Forms. Model and Full-Scale tests with

special Reference to a Conversion to Purse-Seiners

The Reversed Spiral Test.

A Note on Bech's Spiral Test

and some Unexpected Results

of its Applications to Coasters

12,00

12,00

20,00

10,00

Hy-11 WM. B. MORGAN, Propeller 25,00

VLADIMIR S'ILOVIC, and Lifting-Surface Corrections

STEPHEN B. DENNY

Hy-12 MUNK, T., and Tests with 20,00

PRO HASKA, C. W. Interlocking and

HYDRO- OG AERODYNAMISK

LABOR ATORIUM

Lyngby - Denmark

Tests with Interlocking and Overlapping Propellers

by

T. Munk and C. W. Prohaska

This publication is based on a paper entitled "Unusual Two-Propeller Arrangements" presented at the 7th Symposium on Naval Hydrodynamics, Rome 1968.

Hydrodynamics Department

Abstract

Comparative self-propulsion tests with a tanker

model propelled in turn by a small single screw, a large

single screw, twin screws, interlocking propellers and finally overlapping propellers were carried out together

with stress measurements on one blade of the propellers. On the basis of the results the working conditions for the interlocking and the overlapping propellers were

in-vestigated, and the possibilities of these systems and

their advantages as compared to the conventional propulsion systems are discussed.

Introduction

The large horsepowers installed in the ships of today make heavy demands on the propellers. High thrusts combined with diameters, which are kept too low either due to limitation of draught or to

relatively high revolutions, result in heavy propeller loadings and

consequently in poor propeller efficiencies. The high wake behind a

full stern further increases the loading, but will on the other hand

improve the hull efficiency.

Where the loading on a single screw would be excessive, twin

screw arrangements are often adopted, but the gain in propeller efficiency is counteracted by a loss in hull efficiency.

In the following an endeavour is made to prove that in many cases considerable savings in horsepower may be obtained through the adoption of two propellers so closely spaced that the propeller discs overlap. It should further be possible due to the less ir-regular wake field to reduce the risk of cavitation and also to mini-mize propeller induced vibrations.

The interlocking propeller arrangement

The above-mentioned problems were treated by Pao C. Pien and

J. Strem-Tejsen in Ref. (1). Several stern arrangements were dis-cussed with regard to total efficiency and ability of reducing cavi-tation and vibration. In addition a new stern arrangement was pro-posed.

2

-In this arrangement the two propellers of a normal twin screw

system were moved aft to the longitudinal position of a normal single screw propeller and inwards until the distance between the shafts was less than the diameter of the propellers, which therefore interlocked in the centre line zone. This combined the advantages of the twin screw system, which are high propeller efficiency and reduced

gene-ration of cavitation and vibgene-ration due to the smoothness of the wake field, with those of the ordinary single screw system, which are low appendage resistance and high hull efficiency due to the high viscous

wake behind the ship.

Tests were carried out with the system adapted to a tanker model and the results compared with results from previous tests with

other propulsion systems on the model. It was evident that the

inter-locking propeller arrangement offered great advantages and could be

used in ships without any technical difficulties. The shafts would

have to be connected by a gear, and could be driven by one large engine

or by several smaller units. The cost of the gear and the extra shaft

was judged to be modest in comparison to the gain in total efficiency of the system, and the gear would permit an optimum number of

revolu-tions to be chosen.

Separate drives of the two shafts might even be adopted if the two propellers were placed clear of each other in the longitudinal

direction. This arrangement will in the following be termed the over-lapping propeller system.

Testing of the systems

At the Hydro- and Aerodynamics Laboratory, Lyngby, Denmark, (HyA), the results from Ref. (1) were found of the greatest interest,

and as no further treatment of interlocking or overlapping propeller arrangements was available, it was decided to carry out supplementary

tests with these systems in order to confirm the results of Ref. (1), and to get more knowledge of the interaction between the propellers. It was considered of special interest to know how the

vibration-generating variation of the forces on a propeller blade would compare

with the variation on a normal single screw, and how the wake for._one

propeller is influenced by the induced velocities from the other pro-peller, as this will be of importance in the design of interlocking

propellers. It was further decided to seek information as to the

op-timum distances between the two shafts.

The system should be of particular interest for large tankers and bulk-carriers, which usually have a high and very irregular wake, and where the diameter of the propeller often is kept small on account of the relatively high number of revolutions of diesel marine engines for large ships, and was therefore tested on a model of a tanker. The principal data of model and ship fully loaded and ballasted appear from Table No. 1.

The interlocking propeller arrangement was tested with the pro-pellers turning both inwards and outwards and with three different

distances, ab. 0.7, 0.8 and 0.9 times the diameter, between the pro-peller axes.

Table 1. Data of model and ship ballasted and fully loaded

Scale 1:35

Ballasted Fully loaded Model Ship Model Ship

Length pp Lpp in 7.200 252.000 7.200 252.000 Length wl

Li

in 7.000 245.000 _7.321 256.235 Breadth Bm in 1.114 38.990 1.114 38.990 Draught forw. df in 0.214 7.490 0.382 13.37 Draught aft da in 0.229 8.015 0.382 13.37 Mean draught dm in 0.221 7.752 0.382 13.37 Displacement V m3 1.398 59951.8 2.523 108176.9 Wetted surface S m2 9.146 11203.4 11.582 14188.0 Block coeff. 0.810 0.810 Prism. coeff. 0.820 0.813

Midsh. sec. coeff. 0.988 0.996

Waterline coeff. 0.867 0.879 Lwl/V1/3 6.26 5.375 B/d 5.029 2.916 L/B 6.284 6.57 Long. c. of buoyancy aft of Lpp/2 LCB % -2.405 -1.702

-4

The test with outward turning propellers and the most

ad-vantageous distance between the shafts was repeated with

over-lapping propellers placed with a longitudinal distance of about

0.2 x D. The shafts were still. coupled together, and the propellers therefore rotated at the same number of revolutions.

In order to get an impression of the influence of the induced velocity from one propeller on the wake at the position of the other propeller, nearly all the tests were repeated with only one propeller

working and the other removed and compensated by a tow rope force.

For comparison self-propulsion tests were also carried out with

two normal single screws and with normal twin screws turning outwards.

The model fitted with the different propellers is shown on Fig. 1-4.

The stress at the root of one propeller blade was measured in

a number of the tests. _{In the case of the overlapping propeller system}

stress measurements were carried out on both propellers running at the

same number of revolutions as well as at different numbers of

revolu-tions.

In Table No. 2 a list is given of the total number of tests

carried out.

Fig. 1.

The single screw arrangement, small propeller.

-Ira.--3410

'Fig. 2.

The single screw arrangement, large propeller.

Fig. 3.

The twin screw arrangement.

'NW

Fig. 4.

The interlocking propeller arrangement.

Stress measurements carried out. Stress measurements carried out with one and with both propellers working.

Table 2.

Tests carried out with different propulsion_ systems

Test Type of Propulsion system Prop. No. test No. 1,2 Resist.

Fully loaded and ballasted

3,4

Selfprop. 2 interl.prop.inward rot. Dist.btw.axes

0.9 D

6137

II It II II

5,6

II 1 II II II II It II I I

0.9 D

6137

II I/ II

7,8

It 2 ii u II H It II u

0.8 D

6137

It It II II

9,10

II 1 It II II II It II II

0.8 D

6137

II II It II 11,12 It 2 u tl II It It It II

0.7 D

6137

II It II It

1)

13,14

II 1 II II II It II II II

0.7 D

6137

It II II II

15

II 2 It I/ It It It II II

0.68 D

5915

II It 16,17 II 2 II It outward " It It II

0.77 D

5915

If and ballasted 1)

18,19

11 1 It II Il II II II II

0.77 D

5915

tl tt It tt 1) 20 II 2 II /I II It It II It

0.68 D

5915

I/ II 2) 21 II 2 II II It II II II

0.86 D

5915

II It 22 II 2 overlap. " It It It It II

0.77 D

5915

II II

2)

23,24

II twin screw II It

5915

II and ballasted

25,26

II single screw

6507

It II It II

1)

27

II II II

6535

It II [II

1)

Stock propellers were used for all the tests. The diameter of one of the single screw propellers was chosen as optimum for a number of revolutions in agreement with that of large slowly-running diesels,

while the diameter of the other was taken as large as practically

possible, and the shaft should consequently be driven by a geared power unit. The diameter of the interlocking propellers was taken as 0.9 times that of the small single screw propeller. This diameter gave an optimum number of revolutions somewhat below that of an ungeared

marine diesel, but this was found permissible as a gear in any case

would be required for the connection of the two shafts.

Two sets of stock propellers were used as interlocking pro-pellers. Both had pronounced rake and were therefore not quite suitable for the purpose.

The data of the propellers are given in Table No.

3.

Testing methods

The resistance tests were carried out in the normal way and

the self-propulsion tests with a tow-rope force in accordance with the Hughes friction line for a form factor of

1.36

for fully loaded condition and

1.28

for ballasted condition, and with CA

0.15

10-3.

Torque and thrust were measured by mechanical dynamometers,

and in the two-propeller cases the two dynamometers were connected to

one motor to ensure uniform running of the propellers.

Table 3. Propeller data

Propeller no.

6507

6137

5915

6535

Diameter, model ram

201.9

180

186.4

251

Diameter, ship mm

7050

6300

6520

8800

Number of blades 4 4 4 4

Pitch ratio

0.754

0.963

1.27

0.775

Dev. blade area ratio

0.458

0.524

0.43

0.465

Rake deg. 6 8 15 0

Purpose single interl. interl. single screw prop. prop.

twin screw

8

-The stress at the root of one propeller blade was measured

by means of strain gauges. These were placed on each side of the pro-peller blade to prevent signals caused by temperature expansion. The

strain gauges were through a hollow shaft wired to a unit, placed on

the shaft inside the model and consisting of the remaining resistances of a Wheatstone's bridge, an amplifier for amplification of the signals from the bridge and an accumulator for feeding the amplifier. The

measuring circuit was fed by an accumulator through slip rings, and the amplified signal was led through slip rings to an oscilloscope and a direct-recording ultraviolet oscillograph with a sensibility enabling it to follow the variation of the stress through a revolution. (See

Fig. 5).

Fig. 5.

Shaft equipment for measuring the stress at the propeller blade root.

Analysis of the results

The resistance tests were extrapolated to ship scale by using the Hughes method and a form factor of 1.36 for fully loaded condition

and 1.28 for ballasted condition, and a CA-value of 0.15- 10-3. The

self-propulsion tests were analysed by the method described in Ref. (2), which method normally is used at HyA. The analysis work was done

The wake coefficient corrections used in the different cases are given in Table No. 4.

The results of the stress measurements were given by the re-corder in the form of curves showing the stress caused by the bending moment on the blade as a function of time or of the blade position, which was also recorded. (Fig. 12-16).

The blade stress is proportional to the bending moment, and

after calibration the curves therefore represent the bending moment at the blade root as a function of the blade position. The bending

moment is a function of the resultant force on the blade normal to the

section where the stress is measured, in the following called the

normal force, and of the distance of the centre of pressure from the

root, which may be assumed to be nearly constant. The stress conse-quently in the first approximation is proportional to the normal vector force, which in turn is a function of the inflow velocity, or to the vectorial sum of the ship speed, the circumferential velocity and the wake speed. As only the latter is not constant, the fluctuating curves

illustrate the variation of the wake during one revolution.

Test results

The results of the test series are given graphically in Fig.

6-16.

Fig. 6-9 give the required propeller horsepower for the diffe-rent propulsion systems as a function of the ship speed. The

corre-sponding numbers of revolutions are given in Fig. 10.

The model torque wake coefficients for all the tests are given

graphically as a function of the speed in Fig. 11. The thrust deduc-tion coefficients and the relative rotative efficiencies varied little and not systematically and were of the order of 0.29 and 1.00

respec-tively for fully loaded ship, and 0.27 and 1.00 for ballasted ship. The results of the stress measurements are shown on Fig. 12-16.

For the interlocking propellers only the stress curves for a distance

between the shafts of 0.7 D are given, as the measurements with a distance of 0.8 D gave nearly the same results.

30000 20000 10 000 10 -Ii-' NORMAL NORMAL LARGE INTERL.PROPS.OUTW. SINGLE SCREW TWIN SCREW SINGLE SCREW P No.6507 P No.5915 DIST.0,77xD,P P No. 6533 No.5915

--mrs.--0 0

---FULLY LOADED RED.

20%

0/

RED

15°

III.A

=mum in

-,..

RED. 6 °/

W

Alir

4-4 rAm

/

/

,/

,6

0

e

co-v9k5)

4

+.'

/

0/ /4

,

/,

0

...

,

.../-

/

.

0

a

+ --

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,6

kr

Z./-.

+

7

.

--/

,'`'''

.,

7+

7,,,,

SAL A STE D

....Pscroir

..0 --- ..,--0 44.

---0 -0 SERVICE SPEED KNOTS 12 13 14 15 16 17 18 Fig. 6.

Comparison between required propeller horsepowers

for the different propeller arrangements.

0

ei

30000 20000 10000

-

---

-

.

I " " DIST. 0,8 x D,P DIST. 0,7 xD,P DIST. 0,68x D,P No. 6137 No. 6137 No. 5915

i

i

el

4,4,

li

'

,, /

fil

1

/ 1 / . ,

,

1,, , ,, , ,

h

/i/

' /

I t

,'

//

/

FULLY LOADED

/4

/,

,/

,

,,

,

1 / /

/

///

I/

'BALLASTED

/

BALLASTED P)

/ /

r ei

/7

o,

/ /

1

/,

i

//r/

'

.-.#,r-,.

-,..,

,

--

y

7

z

,.:,

-.?

z,. v

4'-=

//

/,-,

- ....-:,.----,.,-- ..--,---

,-..---..

.-- ,...

--!..----,

.2

--.;

--,---=,---P SERVICE

SPEED

KNOTS t I .. 12 13 14 15 16 17 18 Fig. 7.

Propeller horsepower from tests with inward turning interlocking propellers.

-/

3 0 000

20 000

10 000

12

-II

---- - -

INTERL. PROPS. OUTW. DIST. 0,68 0 D. P No. 5915 DIST. 0,77* D, P No. 5915 0 .1 DIST. 0,87* D, P No. 6533 //'

////

./1

1

/

/7)

///'

0

/7

/0 '

/

/

4/7

/ /

/,-

i

/7

*

/

/0

0

/

FULLY LOADED ,

A0

/

0

0/

/

0 ..

ry

i

//

,i'

7'.

.77.

e

'

/z//0

/0

Z0

Ar° BALLASTED .

,---

....--

----..- ...

....0

..-- ---SERVICE

SPEED

KNOTS ).. a 12 13 14 15 16 17 18 Fig. 8.

Propeller horsepowers from tests with outward turning

30 000

20 000

10 000

Fig. 9

Comparison between results from tests with inward and outward turning interlocking propellers.

it-'

A

INTERL. PROPS. INW.

.

OUTW. DIST. 0,68. DIST. Oy .D, DIST. 0,77.D, D, P No. 5915 P No. 6137 P No. 5915

/

/

/

/

/

/

/

i /

//

i

/

/

/

/ '

/

1/

/

/

/

FULLY LOADED

,'

/ ,'

/0

/

BALLASTED 4 4.

/

z

/

+

7

/

0

//'

//

V

/

/

7

--'"--°

7

/.7*

/

z

V

,./

/

_...-.,...-° ...----° --°

...

---...

.----.

" e

----""-.../.

,.--- ' ..---..""

/

0 SERVICE SPEED KNOTS D t 2 13 14 1S 1E 17 In

/

RPM 170 160 150 140 130 120 110 100 90 80 70 60 50

NORMAL SINGLE SCREW P No. 6507

--xx., NORMAL TWIN

SCREW P No. 5915

INTERL. PROPS. INW. DIST. 0.9 xD, P No. 6137

. _{0.8 xD, p No. 6137} . _{" 0.7 xD, P No. 6137} 0.68 'DIP No. 5915

0-0-

* M OUTW. . 0.77 XD. P No. 5915 . . . 0.68 xD, P No. 5915 11 0,87xD, P No. 5915

--- LARGE SINGLE SCREW P No. 6535

0

14

-FULLY LOADED BALLASTED

IL

Aim

Ilitw

Pit

0

---SERVICE SPEED 12 13 14 15 16 17 18 KNOTS

Fig. 10.

Revolutions versus speed for the propellers.

o

* 4

.40. 0

-.4 k1 10 0

0_

SERVICE SPEED 1

-...

--

-Fig. 11.

Model torque wake coefficient.

- NORMAL SINGLE SCREW P No. 6507 NORMAL TWIN SCREW P No., 5915

INTERL PROPS. IINW DIST. 0.9 DP No. 6137

PII lc W j! 08 .0, p No. 6137 1.1 ix rill al 0.7 .D, P ,No. 6137 _r_--11-=- X -,-11.--. !,k .! 111 0,68 00, P No..5915

,;,_.p.

ii OUTW. .3 077 .DP No. 5915 .., T.., 0,68 D. P Nlo. 5915 0,87 A D., P No. 5915,

---LA ROE' SINGLE SCREW' P No. 6535

Wm --- o---s o .4_ 0 X SERVICE SPEED 0 _ I k 12 13 14 15 16 17 113 'KNOTS BALLASTED 18 12 13 14 15 16 17 FULLY LOADED .5

Table 4. Wake coefficient corrections used for analysis of the

self-propulsion tests

Ship, wake coeff. model wake coeff. - wake coeff. corr.

Propulsion arrangement wake coeff. corr.

Ordinary single, screw

Large single Screw Twin screw

Interlocking propellers, dist. btw. axes 0.7 D'

Interlocking propellers, dist. btw. axes O.& D

Interlocking propellers, dist. btw. axes 0.9' D

RI

NB. The wake correction is calculated as the difference

between the velocity it the boundary layer integrated over the propeller disc for model and for ship.

16=

SINGLE SC 16 KN.

Fig. 12.

INTERLOCKING PROPELLERS ,1 PROPELLER AWARD 13 KNOTS 1 Fig. 13.

'Propeller blade stress recording.

0.140 0.120 0.010 0.097 0.086 0.074

-INTERLOCKING PROPELLERS 2 PROPELLERS OUTWARD 116 KNOTS

Fig. 16.

INTERLOCKING PROPELLERS 2 PROPELLERS ARD 1 1 1

Fig. 14.

INTERLOCKING PROPELLERS I PROPELLER OUTWARD 116 KNOTS _

4

Fig, 15.

Propeller blade stress recording.

18

-Discussion of the results A. Interlocking propellers

It is seen from Fig. 6 that the interlocking propeller system with the propellers turning outward is the most favourable one as

re-gards total efficiency.

The comparison shows at the service speed that the interlocking

propellers require 20% less horsepower than the small single screw, 15% less than the twin screws and 6% less than the large single screw.

None of the propellers were optimum propellers. A calculation shows that if the propellers had been given the optimum numbers of re-volutions by changing the pitch, the interlocking propellers would require 17% less horsepower than the small single screw, 16% less than

the twin screws and 5% less than the large single screw.

As stated above the diameter of the interlocking propellers was

chosen as 90% of that of the small single screw. This corresponds to only 74% of that of the large single screw. Had the diameter of the interlocking propellers been increased to 90% of that of the large single

screw, the horsepower would have been 11% less than required for this

propeller.

It is therefore obvious that part of the gain obtainable is due

to the adoption of optimum revolutions, which in many cases can be

ob-tained only for geared shafts. But a considerable part of the gain is

solely due to the large disc surfaces placed in the high wake and is

therefore inherently connected with the adoption of interlocking or

overlapping propellers. A saving of ab. 10% in horsepower is well

worth considering.

From Fig. 11 is seen that the mean wake is higher when the pro-pellers are turning outwards than when they are turning inwards, and that the mean wake for each interlocking propeller is decreased by the action of the other, most pronounced for the inward rotating

pro-pellers. The differences in total efficiency are caused by the dif-ferences in wake, a higher wake giving a lower propeller efficiency,

A. small shaft, distance is

in

this case not advantageous. As the propellers are moved inwards the wake and thrust deduction in

Crease, but the interaction Of the propellers will, increase too, and decrease the resulting wake. This additional inflow velocity is de= pendent on the propeller loading, and as the resulting wake has a

great influence, on the total efficiency, the optimum distance be

tween the propeller axes must be a function of the propeller loading.

A general optimum distance' can therefore not be given at, present. Of the three shaft distances considered the intermediate distance, was Clearly the best.

The differences corresponding to inward and outward rotating' interlocking propellers' are due to the wake components in the propeller disc plane. It is possible to get an Impression of this component if

the normal force :Oil a blade in a, certain position. is compared to the normal force, on the blade in the same position, but for the propeller

turning in the opposite. direction. The difference between the normal

forces is due to the differences. between the tangential velocities as shown in the velocity diagram Fig. 17. The tangential wake velocity,

Wt2i

is

seen to change. the resulting inflow velocity from V1 to V2,

when the direction of rotation

'is

altered, and this largely influences,

the normal force.

Fig. 17.

Velocity diagram showing influence of

102*

0

8ir

2ff

20

-If the stress curve is looked upon as a curve of normal force as earlier mentioned, and the curve for the normal right-handed single screw is copied upside down, the copy will give the normal force for

a left-handed propeller. When the two curves are placed together as shown on Fig. 18, their mean value represents the normal force in the case of no tangential wake component, and the difference between the mean curve and one of the other curves gives the additional normal

force caused by the tangential wake component. Fig. 18 then indi-cates that wake components are present as shown in Fig. 19.

PROPELLER TURNING RIGHTHANDED PROPELLER TURNING LEFTHANDED

PROPELLER TURNING RIGHTH. OR LEFTH. AXIAL WAKE ONLY

6w 4ir . . .. . . s ..../ ..-._. . , , _, ... -... . .

\

.

/

I ,,, . . . . , . . , .

.

. . _.,

_.

. 4

ANL

-ROP FROM TANGETIAL

All

TURNING RIGHTHANDED, WAKE COMPONENT.

ANL

ADDITIONAL NORMAL

AIL

FORCE AdOL

,41.

4ir

2ir 0 TURNING

LEFTHANDED

6fr

8ir

1011 TURNING

RIGHTHANDED

Fig. 18.

Normal force curves for right-handed and left-handed single screws.

The stresses measured in the cases where one of the

inter-locking propellers was removed, are in the same way compared on Fig.

20,

showing the curves representing mean normal force and additional

normal force. The transverse wake components are indicated on Fig.

19. The normal force is at the same speed and number of revolutions

Fig. 19.

Wake components in the propeller disc plane.

/PROPELLER TURNING OUTWARD .PROPELLER TURNING INWARD

LY. : IS\_X X N X X X

.

...

'

1 / / / /

II

/

/

- --'

.

.

\ 1 N. \ X

\

.

_.

.

ROPELLER

.

_.

_{__.}

TURN _

/

,

i

I , / 1 1 / 1 1

,I

/ /

.

.'.

/ /

.

4

NO INW. OR OUT

-.

/

, I 1 \ / 1 ' \\ / 1 ' X

/ /

.

.

,

_/

_/

/

.

_.

.

.---4

. AXIAL % ,

\

\ \ %

.

%

.

_.

.

_.

.

41IAIKE I

'

1 I , / _/

/

/

_/

/,,

-II 1

Mb..

0 21r

_Lir

_6ir

_{fisr RIGHT PROP}

OUTWARD TURN.

81T 651" _4ir

2r

_{0 RIGHT PROP}

INWARD TURN. PROP TURNING OUTWARD. ADDITIONAL NORMAL FORCE

FROM TANGENTIAL WAKE COMPONENT.

Fig. 20.

Normal force curves for only one interlocking _propeller,

22

-higher for outward than for inward rotating propeller. This

dif-ference will, when the model is self-propelled, result in a lower number of revolutions and a higher wake for the outward rotating

pro-peller, in accordance with what is well-known from ordinary twin screw

ships. The rest of the difference between wake for inward and outward

rotating interlocking propellers may also be explained. The velocity

diagrams for a blade in the overlapping position, rotating inward and

outward, are given in Fig. 21 and show that the induced velocity from

one propeller is much higher in the case of inward rotating propellers and therefore the wake for the other propeller i much reduced.

Fig. 21.

Velocity diagram for a propeller blade in the interlocking zone, inward and outward rotating.

This is in good accordance with the result of a comparison of

the stress curves for a propeller rotating inwards or outwards when

it is working alone and when the other propeller is also working. It is seen from Fig. 22 that the influence from the other propeller is

much greater for inward rotating than for outward rotating propellers.

The results of the stress measurements on a single screw and

on the interlocking propellers are scaled to the same mean normal force on Fig. 23 to show the relative stress variations. These are higher for the outward rotating interlocking propellers than for a single screw, but the vibration-generating forces will be smaller as the mean force for an interlocking propeller is only half of that for a single screw. The comparison is, however, not quite correct as the wake scale effect is not taken into account.

8,r

6ir

4cr

RIGHT PROPELLER TURNING INWARD

1 PPOPELLER

2 PROPELLERS

Fig. 22.

Normal force curves for an interlocking propeller working

alone and in combination with the other propeller.

B. Overlapping propellers

This modification of the interlocking propeller arrangement

did not give rise to any measurable difference in the total horse-power. The wake coefficient for the forward propeller was increased

by 0.06, and for the aft propeller decreased by 0.04. _{The two}

pro-peller loadings therefore were different, so if this arrangement should

be preferred to interlocking propellers the two propellers should have different diameters and pitch.

I I I I

.

_,

-RIGHT

I t t I

I

PROPELLER TURNING

I I t t t_.

.

_,

-.

OUTWARD t t t t t

_.

... erir

20-

0

IN

-

24-The stresses too were nearly the same as for interlocking

pro-pellers, (Fig. 22). The influence of the aft propeller on the stresses of the forward propeller was small, but the influence of the forward propeller on the stresses of the aft propeller was, as could be

ex-pected, more pronounced.

.

FL

1

"CREW

11.1,

1.00 1.09

I

TERLOCKING PR SPELLER OUTWA D TURNING

JIPPIPPI

100

078

I

TERLOCKING PR PELLER INWARU TURNING

851-

6tir

411"

2gr

0

- ----

NORMAL FORCE MEAN LINE

Fig. 23.

Comparison between normal force curves for the

different propulsion systems.

Conclusion

The results of the test series show that the interlocking or overlapping propeller systems with outward rotating propellers and with

a distance between the shafts of about 0.8 times the diameter, is the most advantageous for the type of ship tested.

The reduction of the required horsepower, when one of these

arrangements is used instead of a single screw, depends on the loading

of this screw. Compared to a large slowly running single screw the

reduction will be about 10 per cent, whereas the reduction may be up to 20 per cent if the diameter of the single screw is small due either

to restricted draught or to a high number of revolutions. Compared to twin screws the reduction may be as much as 15 per cent.

No unexpected effects take place caused by the close position

of the two propellers, and the vibration-generating forces on a pro-peller blade is smaller than for the single screw. The angular

posi-tion of the blades of one propeller in relaposi-tion to that of the other may be chosen to give favourable interference between some of the forces or, in case of overlapping propellers, the number of blades may be different for the two propellers to prevent severe interference

phenomenae.

The choice between overlapping and interlocking propellers

depends purely on the wishes of the owner. One large engine may be used for the interlocking system, and the shafts connected by a gear.

The two propellers have similar dimensions, and the entire propulsion

system acts as a unit. When overlapping propellers are used, the

pro-pellers will be different and may be separately driven, which is

26 -Nomenclature Propeller diameter Non-dimensional radius Wake Resultant wake

Tangential wake component _Wt

Axial wake component Wa

Number of revolutions per sec.

Ship speed _Vs

Inflow velocity V

Velocity induced by the propeller itself V._is Velocity induced by the other interlocking propeller

V0

Resultant force normal to the blade root section

Effective horsepower Eli?

Propeller horsepower PEP

Additional resistance coefficient _CA

References

1 Pao C. Pien and J. Strom-Tejsen: "A Proposed New Stern

Arrangement", Report 2410, Naval Ship Research

and Development Center, May 1967.

2 C.W. Prohaska: "Analysis of Ship Model Experiments and

Prediction of Ship Performance", Report No.Hy-1, Hydro- and Aerodynamics Laboratory, Lyngby, Denmark, December 1960.