The wake peak and its influence in marine propeller design

3 SEP. 1984

ARCHIEF

SYMPOSIUM ON

"HYDRODYNAMICS OF SHIP AND OFFSHORE PROPULSION SYSTEMS"

HØVIK OUTSIDE OSLO, MARCH 20. 25., 1977

"THE WAKE PEAK AND ITS INFLUENCE IN MARINE PROPELLER DESIGN"

By G. Patience

Stone Manganese Marine Ltd., London

SPONSOR: DET NORSKE VER ITAS

Ref.: PAPER 19/2 - SESSION 2

Lab.

y.

Scheepsbouwkunde

Technische Hogeschool

Deift

b

H

i

ABSTRACT

This paper covci.s some aspects of wake variation

from the viewpoint of practical design to prevent

cavi-tation erosion. A simple two-dimensional approach to

the problem is used to illustrate the e.ffect of the wake

peak ori the selection of the propeller geometry. Three

main variables are discussed: the profile type, its

width and its carìber, and when considered within the

competitive requirements of both weight and efficiency it is shown that with computer application the proposed

design approach can adequately account for the variable

nature of the wake field.

1.

INTRODUCTION

Cavitation, and in particular its effects, is a

problem that the propeller designer has learned to accept. Like many other problems it tends to occur in cycles,

provoking periodic intensification of research programmes into the phenomenon until the immediate problem that has arisen is satisfactorily understood and resolved. In this respect the understanding arises from research, while

the solution and it. _{future development is usually in the}

hands of the designer. For practical design purposes

2

complex and sophisticated analysis procedures employed in research can prove unwieldy, time-consuming, and

therefore expensive design tools. The application of

the high speed digital computer has served to empiasise this aspect rather than assist in closing the gap. So

that, as a consequence, the designer strives to approach the development of the solution of the problems that arise in as simple a manner as possible, consistent with the

e.

achievement of the required results. As an example, the

application of finite element techniques to the stressing of propeller blades has shown to date that the much simpler cantilever beam theory, properly applied, is an adequate representation for commercial design practice.

The problem considered here concerns the variable nature of the wake field and how it influences propeller

design when rninimising the effects of cavitation. This

is not a new

problem but one which has become of greater

importance over the last ten years, due not Only to the trend towards full bodied hull forms but also the increase in power transmitted by a single screw configuration.

In this respect a major step forward in propeller design was

achieved some twenty years ago with the publication of various design methods often referred to as wake adaption

procedures. In such methods provision _{was made to}

camber and pitch the individual _{sections of the blade in}

accordance with the circumferential mean of the wake at

that radius. _{Propellers designed in this way proved}

successful from the cavitation viewpoint but Only so far as an adequate balance between the extremes of face and

back cavitation were concerned, _{and the cases of erosion}

that did occur were attributed to an exceptional wake peak or some other external cause.

Nowadays the use of model wake _{surveys has become}

commonplace and the designer _{is so much more familiar with}

the characteristics of the wake flow at the propeller disc. As a result design methods are now being introduced where-by consideration is given to the _{influence of the wake}

peak itself. _{This paper describes such a method which}

was established not only with the objective _{of achieving}

the required result but to _{do so in as simple a manner as}

possible for practical application and tailored to conform

to existing design methods.

As a final comment, whilst accepting that the _presence

of the wake peak should be _{taken into account in propeller}

design, it must always be remembered that the wake distri-bution is a function of the hull form and as such provides another path to the solution _{of this particular problem.}

4-2.

CHOICE OF PROFILE TYPE

When considering the type of profiles to be adopted for marine propellers the influence of the fluctuating wake and the magnitude of the wake peak, prticular1y in

the region of the tip, can be a significant factor. _For example, it is appreciated that a relatively large leading edge nose radius is a desirable feature for operation in

a flow of varyin incidence. So that, for design purposes

it is of interest to examine the characteristics _{of the}

pressure distributions of available profiles as regards their suitability for propeller applications in _{a wake.}

In theory the most suitable profile from the

cavita-tion viewpoint is that having a uniform distribucavita-tion of suction along the chord, the magnitude of _{which is less,}

by some appropriate safety margin, than the _local

cavita-tion index. _{In practice, however, such an arrangement}

is not possible except in the particular _{case where the}

flow field is circumferentially _{uniform. A reasonable}

approximation to this type of flow in practice is that of

the wing screw configuration where, _{because of its more}

open position, the propeller operates in wake _{of relatively}

lower magnitude and which _{more importantly remains fairly}

5

wake shadow that exists behind a skeg or strut.

Pro-viding that the associated wake peak is not excessive it therefore follows that the ideal 'uniform suction' type

of profile - e.g. the NACA a O8 camber with NACA 66

(mod) thickness (Reference 1) commonly adopted for

pro-peller applications - can be successfully applied in such cases.

For the mora general case of the single screw ship with its characteristic fluctuating wake field the magni-tude of the wake peak has a much greater effect and it can therefore be argued that consideration should be given to

other profile types. The variation in incidence of flow

to the blades caused by the variations in wake gives rise to a continually fluctuating magnitude of suction across

the profiles. So that for a profile with a uniform suction

distribution characteristic any increase in incidence can immediately promote the formation of a cavity across a substantial portion of the profile chord. This feature

can be avoided by incorporating greater safety margins, e.g. by increasing the widths and so reducing the potential cavity length at the leading edge, but with a consequent

detrimental effect upon the efficiency. Therefore, from the cavitation viewpoint, a profile having a suction

6

distribution characteristic that reduces towards the

leading edge at normal angles of incidence _{may be preferred.} Thén as the incidence _{increases in a region of higher wake}

the resulting increase of suction over the leading portion

of the profile chord does _{not immediately exceed the local}

cavitation index and provoke _{the development of cavitation.}

A profile having this characteristic _{is, for example, the} NACA 65 mean line with _{an appropriate thickness body.}

e.

Accordingly, it is contended that the choice of profile type should depend upon the nature of the wake field. _{For those cases where the flow}

approaches

unifor-mity in a circumferential direction, profiles having a uniform distribution of suction may be preferred as being

as close to the ideal as possible. _{For the more usual} case of fluctuating wake flows, howe'er, profiles having a reduced level of suction _{towards the leading edge at design}

incidence should prove _{more suitable.}

As an illustration, it is _{interesting to note that the} type of profile currently employed by the Author's _Company, and which has evolved over many years' experience of

successful propeller design, _{has a suction characteristic}

basically of the second _{type given above. With Only minor}

modifications, this profile _{type has been used in the}

-7

of propellers for more than thirty years and is even now still considered preferable for application to the wake fields of modern single screw vessels.

3.

CHOICE OF SURFACE AREA

From the viewpoint of commercial design, the selection of the required surface area of a marine propeller is a

compromise between the conflicting requirements of

cavi-tation and efficiency. The cavitation properties of a

propeller can be improved but usually at the expense of

some of its efficiency. As a result the general design

philosophy is to provide a minimum surface area sufficient to avoid the harmful effects of any cavitation that is

present. In this respect the influence of the wake peak

is critical for, although the presence of some cavitation is unavoidable and can be tolerated without detriment, at some point the cavitation is such that erosion, noise or vibration become unacceptable. Yet it is only recently

that methods have been proposed which- take account of the

presence of the wake peak, for example References 2 and 3.

The method adopted here (Reference 4) is based upon

a simple quasi-steady two-dimensional approach whereby the

IM 5E VÍ FI .

4E&E

8

pressure distributions of a given propeller geometry are

ca.lculated and assessed ori a comparative basis with

pre-vious experience. A notable disadvantage of this type

of approach is that it is ari analysis rather than a synthesis

procedure and can sometimes prove to be a time-consuming process when applied as a design tool. For practical

design purposes, therefore, the method has been refined

and simplified tQ consideration of the pressure distribution at 075 radius. Using certain assumptions regarding the detailed procedure of wake adaption and with computer application a rapid method for evaluating the propeller surface area for a specified calculated cavitation length

was developed arid a criterion established from full scale

data for estimating purposes. This criterion is shown in

Figure 1, the calculated cavity length at 075 radius for the most onerous cavitation loading condition being plotted on a base representing a measure of the wake fluctuation. An interesting feature of this diagram is that the proposed

criterion is apparently unaffected by the wake fluctuation as presented, except possibly when the maximum wake is

extremely low.

Using the method outlined above, the influence of the magnitude of the local wake peak upon the required surface

/ 9

area is illustrated in Figure 2, which represents the case

Fi Z

of a six-blade propeller of a large bulk carrier at ballast

draught operation. Using the above criterion, it is

readily seen that the effect of the increasing local wake can be significant in terms of area and therefore propeller weight and cost. Furthermore, to attempt to significantly reduce the amount of cavitation that is present on the

blades by an incréase in surface area alone would prove

more prohibitive as the magnitude of the wake peak increases.

4. FATIGUE STRESSING

The cyclic nature of loading upon the blades of a propeller working in a wake field can be considered as

superimposing a fluctuation of stress about the mean design stress and this must be taken into account in propeller design to ensure that a blade failure in service due to

fatigue is avoided. The method employed by the Author's

Company has recently been published (Reference 5), in which the influence of stress fluctuationsas a result of wake variation have been demonstrated. The magnitude of the stress fluctuations is estimated from consideration of the maximum and minimum extremes of wake, and thence thrust using the open water characteristics of the propeller.

10

-In association with the propeller weight this establishes the required allowable stress to be used in the determination

of the blade thicknesses.

Since the magnitude of the wake peak influences the level of stress fluctuation its effect upon the blade thicknesses can be readily established in a given case. As a simple illustration of this the difference between a high and low lev1 of stress fluctuation can represent as much as 10% difference in the root thickness of a propeller

blade. For more detailed considerations of this aspect

reference should be made to Ref. 6.

5. CHOICE OF SECTION PARAMETERS

Once the basic particulars of a given propeller design have been established, i.e. diameter, profile type, surface area and thickness, more detailed consideration can be

given to finalising the section parameters, taking into account the wake field in the própeller plane. Current

practice, because of standardisation in design procedures with regard to width and thickness distributions, is to wake adapt the basic propeller geometry in terms of maximum

camber and pitch to suit the circumferential mean of the nominal wake for the radius under consideration. _However,

this ptocedure can now no longer be considered adequate if the wake peak is to be accounted for satisfactorily. So that, following the normal wake adaption process, the

section parameters must be investigated within the anticipated extremes of the wake to ensure that the resulting cavitation will be acceptable in service.

For such an investigation the method adopted here examines in effect the two parameters of width and camber, since the local pitch is established from consideration of the resultant camber and optimum effective pitch, and the section thickness from consideration of the resultant width

and basic section modulus. Only the sections outside O6

radius are investigated since the problem of cavitation rarely occurs for the inner sections of a propeller for merchant application. Commencing with the basic propeller as determined from wake adaption, i.e. using standardised width and thickness distributions and optimised camber and pitch distributions, each section is investigated over a range of camber and width by calculation of the pressure distributions at the extremes of maximum and minimum wake. By suitable variation of camber and width the form of the resulting pressure distribution can be controlled and

restricted withi: specified limits for the required inflow. For current design these limits are specified as follows:

FIG,3

HERE

12

-Back cavitation:

Either a maximum calculated cavity length of lO% or less of the chord from the leading edge, or a maximum midchord suction peak magnitude that does not exceed the local cavitation

index.

Face cavitation:

A maximum suction peak magnitude that does not exceed the local cavitation index at ½% of the chord from the

leading edge.

In this calculation procedure, which has been computer-ised, each combination of camber and width also specifies the thickness and pitch by reference to a constant section modulus and a reassessment of the wake adaption process

res-pectively. Typical results are illustrated in Figure 3 for

an outer section of a 6-blade propeller working at ballast

draught. Referring to this diagram, the boundary limits

for the three types of cavitation are presented together for a range of maximum and minimum wakes. Therefore for given extremes of wake the intersection of the appropriate boun-daries will finalise the section design within the limitations

13

-required. Furthermore, since this type of presentation

illustrates the 'safe' combinations of the section para-meters at a glance, the designer can optimise the section design not only from the cavitation viewpoint, but also from considerations of efficiency and weight. This

feature is particularly useful when fairing the blade for manufacturing purposes as it is unlikely that the first choice of section parameters will combine to present an

e.

acceptable blade form. Finally, since Fig. 3 in this

particular case covers a range of wake extremes, the

influence of the wake peak upon section design can be readily

appreciated.

6.

DESIGN EXAMPLE

As an illustration of the method described and to demonstrate the influence of the wake peak upon propeller

geometry,

a design

exercise was considered for a variation in the magnitude of the wake peak together with a propeller designed by the conventional approach. The principal design particulars were as follows:

14

-Ship type Large bulk carrier

Delivered horsepower 32,000 h.p.

Rated revolutions 86 RPM

Blades 6

Taylor wake fraction

045

Crown of boss ratio 0.167

Shaft from base

60 M

Loaded speed 155 knots

Loaded draught

205 M

Ballast speed 17 knots

Ballast draught

120 M

The radial distribution of circumferentially averaged nominal wake is given in Table I. The propeller was

7sL

I

MERE _{designed basically to suit the rated machinery at the}

loaded draught condition. For the conventional approach

the wake peak was ignored but to illustrate the new approach four cases were considered with the magnitude of the peak wake

fraction of 06, 0.7, 08 and 09 for the blade positioned

in the top of the aperture. In each case the minimum wake fraction was taken to be 01, occurring at a position 1200

from the upright.

JN5ET

a)

Conventional desiqn

approach

Using propeller design charts the optimum behind diameter was established at 8500 mm. The surface area was determined using the Burrill diagram, taking into

account the ballast draught operation, and amounted to 4041 M2 in association with a standard outline form in

current use. The root thickness was calculated using

simple cantilever beam theory and amounted to 324 mm, also in association with a standardised distribution. This

basic propeller was then wake adapted to determine the

optimum camber and pitch distributions for the given radial distribution of wake. The principal geometric parameters of the resulting design are shown iñ Table II.

b) New desiqn approach

This required four separate designs taking into account the magnitude of the wake peaks of 06 - 09 as specified. In each case the optimum diameter was maintained as before for convenience, although in fact the diameter should nor-mally be reassessed with any change in surface area.

The surface areas were determined for ballast draught operation and the appropriate wake peak in accordance with

15

tNEeT FJc 1. N5ET 'rAstE5 ill, -) Y)'I PERE

-

16

-the approach outlined in Section 3, using a calculated cavity length criterion of 16% in this case. This

pro-vided the estimated surface area for use in the normal derivation of the blade thicknesses. At this stage in the design procedure all four propellers have the same family of outline and thickness distributions as the

conventional design above. The thicknesses were then

reassessed to take into account fatigue in accordance with

section 4. In this exercise only the cases where the

wake peak exceeded O7 were found to be affected and the thickness distributions modified accordingly. Each

pro-peller was then wake adapted in the normal way to provide

the basic propeller design.

The sedtion design was then finalised in accordance with the method outlined in Section 5 for the sections

outside 06 radius, the inner sections remaining unmodified. Figure 4 illustrates the case for the maximum wake peak

of 07 and shows the final sections in relation to the basic wake adapted sections. The principal geometric parameters of these finalised designs are shown in Tables

INSERT 7A81 ir

1JEE

17

-c) Desiqn comparisons

The pitch and width distributions of the five-pro-pellers are shown in graphical form in Figures 5 and 6.

F145 _{These are of interest in that they reveal a significant}

movement of area distribution from the root towards the more critical tip sections, which would indicate that the standard outlines in conventional use should be recon-sidered as the wake peak increases. Also, in this case,

the pitch distributions reveal a trend away from the conventional pitch reduction at the tip with an increase

in the wake peak. It is also interesting to note from

Tables ii to VI that the new design method has resulted in a significant reduction in camber at the tip in com-parison with the conventional propeller design, indicating

less risk of the more harmful face and midchord types of

cavitation.

For completeness the five propeller designs were analysed for efficiency which together with blade area and propeller weight are given in Table VII. This

reveals the comparative effect of the influence of the wake peak in this particular example. _{It is seen that}

for the same theoretical cavitation characteristics the effect of an increasing magnitude of the wake peak imposes

18

-a pen-alty in terms of both efficiency -and propeller weight,

anç3 therefore cost. Furthermore, from comparison with the conventional design it is apparent that the safety margins incorporated in the conventional design procedure, to account for such factors as the wake peak, are comparable in this case only when the maximum local wake fraction does not exceed a value of O7.

Finally it should be noted that the results illustrated here are peculiar to the particular case under consideration

and do not necessarily apply generally to all propellers. It can be appreciated from Section 5 that it is the magni-tude of the maximum and minimum extremes of wake fraction that decide the final propeller geometry and this feature is of greater significance than the details of the results

so obtained.

7.

CONCLUSIONS

A method has been described which can be employed in the practical design of propellers to take into account the variable nature of the wake field, and in particular the magnitude of the maximum and minimum extremes of local

19

-Application of the method to a particular case has illustrated the influence of the wake peak upon the propeller geometry, both in detail and in terms of the more competitive aspects of efficiency and weight.

8.

ACKNOWLEDGEMENTS

This paper represents some aspects of the research and development into marine propeller design by Stone

Manganese Marine Ltd. The author is grateful to the

directors of the Company for permission to publish this work and for the assistance provided by the staff of the

Hydrodynamics Department, in particular Mrs. A.J. Connor for the preparation of the manuscript.

20

-REFERENCE S

ABBOTT, I.H., and VON DOENHOFF, A.E. Theory of

Wing Sections. _{New York, Dover Publicatior, 1959.}

VAN OOSSANEN, P. _{A method for minimising the}

occurrence of cavitation on propellers in _{a wake.}

I.S.P. 18(1971) : 205.

LINDGREN, H., and BJARNE, E. _{Studies of Propeller} Cavitation Erosion. _{I.Mech.E. Cavitation Conference}

1974 : Cl73/4.

PATIENCE, G. _{Minimising cavitation erosion: a}

pressure distribution approach to the design of

marine propellers. _{Trans. N.E.C.I.E.S. (1974)}

SINCLAIR, L. _{Propeller blade strength. Trans.} I.E.S.S. (1975)

WEBB, A.W.0., _{EAMES, C.F.W. and TUFFREY, A.}

Factors affecting design stresses _{in marine propellers.} S.N.A.M.E. Symposium _{- Propellers 75, 1975.}

DI2GRAMS

FIGURE 1 Cavitation erosion criterion - wake

fluctuation

FIGURE 2 Influence of the wake peak upon propeller

surface area

FIGURE 3 Typical cavitation design diagram

FIGURE 4 Design example: cavitation design diagram

FIGURE 5 Design example: finalised width distributions

FIGURE 6 Design example: optimised pitch distributions

TABLE I

RADIAL WAKE DISTRIBUTION

nR

w

_r

09375

O374

0875

0398

075

0458

O. 625

0530

O5

O626

O375

0736

025

O842

TABLE II

CONVENTIONAL DESIGN

is the thickness/width ratio

Vc

is the camber/width ratio C/D is the width/diameter ratio

/D is the pitch/diameter ratio

r/R t/c C/D

_P/D

09375

O-0233 0-0141 o- 1741 0-7519

O-875

₀₀₂₉₀

0-0167 o-2187 0-7639

O-75 O -0432 0-0199 o-2585 O'7731

0625

Q-0619

00242

Q-2671 O -7738

O-5 O-0874 0-0305 Q-2565 o-7596

O-375 O-1277 O-0415 O-2299

07368

0-25 O-2020 C-0691 0.- 1887 O-6975

TABLE III

NEW DESIGN APPROACH

Max wake peak = 06

ri

_'R

09375

0875

t,

_fc

0O27O

00300

00115

O0148

C/D

O1599

O2159

07722

07738

075

o 0478

o 02 11

02438

0'7726

0625

O0740

o 02 73

O '2412

07726

05

01O46

O '0346 O '2 2 98

0'7600

O375

O' 1528

00480

O '2059

O '7 386

025

O '2418

O'0952

O' 1691

O '7 124

TABLE IV

NEW DESIGN APPROACH

Max wake peak = 0.7

r/R

4

t/c

C/D

O9375

O0261

OO112

O1641

O7716

0875

00285

O0150

02242

O7692

O75

OO444

0.0212

0.2567

0.7674

O625

00687

00258

02547

07731

05

00971

0.0327

O2419

O76O7

0375

O1419

00449

O2167

O7380

TABLE V

NEW DESIGN APPROACH

Max wake peak = 0.8

ri

_/R

ti

_'C

_{Y/ C/D}

09375

0.0229

0.0102

01786

07685

0875

0.0249

00146

02455

07618

075

00372

O0209

0-2835

07596

0625

00580

0025O

02818

07648

05

O0906

00310

02594

07602

0375

01327

00423

02269

07380

025

02 110

00725

01862

07013

TABLE VI

NEW DESIGN APPROACH

Max wake peak = 0.9

r,

_IR

t,

_fc

_{Y/ c}

C/D

09375

00208

00095

01896

07660

0875

0.0217

00142

02684

07562

075

00317

00190

03094

07596

0625

00516

00235

03029

07636

05

O0825

00294

02724

07578

0375

01244

00399

02378

07376

025

01983

00658

01952

O '6978

TABLE VII COMPARISON OF DESIGNS Design Ar e a (M2) Weight (kg s) Effi-ciency Fol_/0 Conventional 40.41 53,360 43.81 06 max wake

3697

52,161

440l

07 max wake

3883

53,238

4385

08 max wake 41-96 54,651

4344

0.9 max wake 44.80 55,575

4308

o

z

w -J

/// '/t-y-,/, j,

_{'/ //, '/}

,

_/

_/W,

-

_5 aD

-0

>-L) w

o

-J IO

MAX. LOCAL WAKE

O.75R MEAN WAKE

L) I 2 3

4

5

i

- PROPOSED

/0/

,CRITERION

p47Eit

FI

i

L)

_EROSION

4O

o

o

Q NO EROSION

.

tA

OOS

BALLAST CONDITIONS

w

w

o

D

J,

6 BLADE PROPELLER BALLAST CONDITION

.8

16°M CRITERION

IO

20

30

40

CALCULATED CAVITY LENGTH AT .75R °MC MAX WAKE AREA M2

.6

35.3

50

.7_.8

37.2

_39.4

MAX _.9

_42.0

6 BLADE PROPELLER

.9375R

BALLAST CONDITION

BACK CAVITATION LINES FOR

O

BLADE POSITION

FACE CAVITATION LINES FOP

1000 BLADE POSITION

Kc.Ci. c

K.Kd.

MID-BLADE

CAVITATION

IO

FACE CAVITATION

.09

_.2 MIN

.06

WAKEBO BASIC SECT ION

.05

_IO°j

_BACK

CAVITATION MAX WAKE .8 ./7E,/CF 3

I.0

1.1 1.2 CHORD BASIC CHORD

o

L)

4

LL w

4

L)

.08

.07

SECTION DESIGN BALLAST CONDITION

BACK CAVITATION SHOWN - MAX

WAKE .7 AT 0

FACE CAVITATION SHOWN MIN WAKE .1 AT 1200

.625R

.75R .875R .625R FINAL SECTION BASIC SECTION OFALL RADII .9375R FINAL SECTION

--

-

875R FINAL SECT ION .75 R

.VFINAL

SECTION LO 1.2 CHORD BASIC CHORD

WIDTH DISTRIBUTIONS .9375R .875 R .75R .625R .5R .375 R .25R CONVENT IONAL DESIGN DESIGN FOR

MAX WAKE=.6

(MIN WAKEu.I) '.2 '.3 WIDTH I DIAMETER

/47A(CE FX 5

.9375R .875R .75R PITCH DISTRIBUTIONS CON VENTIONAL DESIGN .

/

DESIGN FOR

MAX WAKE-.9

I.8\

j

.7\.6

PITCH / DIAMETER