Performance of three- four- five and six blade screws for passenger liners and tankers

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ARCHIEF

See note inside cover

NATIONAL PHYSICAL

LABORATORY

SHIP DIVISION

PERFORMANCE OF THREE- FOUR- FIVE AND SIX-BLADE

SCREWS FOR PASSENGER LINERS AND TANKERS

by

T. P. O'Brien, C. G. I. A.

,

M. R. I. N. A.

Reprints from Shipbuilding and Shipping Record October 1966 and

International Marine Design and Equipment 1966

A Station of, the

Ministry of Technology

Lab. v.

Scheepshouwkuncle

Technische

Hogeschool smi, REP.

(2)

Crown Copyright

Reserved

Extracts from this report May be

reproduced.

provided the source is acknowledged.

!

Approved on behalf of Director, NPL by

(3)

Reprinted from "Shipbuilding and Shipping Record", October 7, 1965

Propellers

The performance of three-, four- and five-blade screws.

Effects of variation in diameter-sand rate of rotation in

passenger liner applications

T. P. O'Brien, C.G.I.A., M.R.I.N.A.

Ship Division, National Physical Laboratory

Synopsis. This article refers to two recent publications on

the effects of varying the geometric features of marine

screws, one on number of blades, the other on screw

diameter and rate of rotation. It discusses performance estimates and design calculations for the screws for a

twin-screw passenger liner which is the sister ship of a vessel

already built. For the existing vessel it had been stipulated

that the screws were to have four blades and were to be

designed to operate at a specified rate of rotation.

How-ever, for the new vessel no stipulations were made

con-cerning either number of blades or rate of rotation.

Consequently, the geometric features of the screws could

be chosen and the rate of rotation could be selected to

give optimum performance consistent with the maximum diameter determined by hull-tip clearance considerations. Selecting the existing 4-blade screw as the basis two sets

of estimates were made, one for 3- and 5-blade screws

Introduction

In designing marine screw propellers the choice of the number of blades, the screw diameter and the operating value

of the rate of rotation are significant

factors.

The effects of variation in

number of blades with applications to

tug propellers have been discussed in a

1), and in a

subsequent article (reference 2) the

effects of variation in screw diameter

and rate of rotation have been covered.

The object of the present article is

to apply the methods previously given in making the screw design calculations for a new twin-screw passenger vessel

which is

the sister ship of a vessel

already built. In designing the screws

for the first vessel it was stipulated that

they should have four blades; more-over, tbe value of the rate of rotation

was specified; consequently, there was

no opportunity to attain optimum

per-formance. However, for the screws for the " second vessel no stipulations were

made concerning either number of

blades or rate of rotation. Thus it was

possible to select the number of blades

and to choose a value for the rate of

rotation to give the optimum

per-formance corresponding to the

maxi-mum diameter consistent with adequate

hull-tip clearance. Before discussing

the worked examples it is desirable to

summarise the data previously given. Variation in number of blades, diameter and rate of rotation The effects of variation in number of blades discussed in the article' are based

on research work at NPL given in a

be made using

four-blade standard series data as the bases. The data given

in the paper' are related to screws of

constant diameter, while those given in

the article' cover effects of varying

diameter, in particular change in

optimum diameter due to

departure from four blades.

Optimum screw diameter and rate of

rotation are discussed in the article', where correction factors are derived enabling changes in blade area ratio

and blade thickness ratio due to

depar-ture from basic values of screw

diameter and rate of rotation to be

estimated. A procedure for estimating the resulting variations in screw

effi-ciency is also given.

Performance estimates and design

calculations

It

is required to design a pair of

screws for a twin-screw passenger vessel

which is a sister ship of the one for

which screw design calculations are

given in Section 11.3 of the book

(reference 4). The design calculations

are to be based upon two sets of

estimates, one covering variation in

number of blades, the other covering variations in screw diameter and rate

of rotation. The first set of estimates

are to be made for three-, four- and five-blade screws, all, operating at the same

designed for the basic rate of rotation, the other for a

3-blade screw designed for optimum rate of rotation. The

calculations showed that reducing the number of blades

from four to three resulted in improved performance, but increasing the number of blades from four to five resulted in adverse performance. It was also shown that significant

improvements in performance could be achieved if the

design rate of rotation were selected and the screw diameter

increased within practical limits of hull-tip clearance.

Selecting a 20-75ft diameter 4-blade screw designed to operate at 116 r.p.m. as the basis, for the 3-blade screw

the increase in efficiency would be 2 per cent, but for the 5-blade screw the reduction in efficiency would be 3 per

cent. Moreover, if the maximum diameter (D=24-25ft) were selected the optimum rate of rotation for a 3-blade screw would be 86 r.p.m. and the resulting increase in

efficiency would be 6 per cent.

rate of rotation, following the

proce-dure given in the article'. The second

set of estimates are to be made for

three-blade screws only, the screw

diameter can be increased within prac-tical limits determined by adeauate

hull-tip clearances and the rate of rotation can be chosen to correspond to

opti-mum performance, following the

pro-cedure given in the article'. The

geometric data and performance values

of an existing pair of screws for the

previous vessels are available, and these

data are to be used as the bases upon

which the comparative performance estimates are to be made.

Design data

Hulltwin-screw passenger: length 680ft (207-26m), breadth 90ft (27-43m),

draught (level) 28ft (8.53m), block

coefficient 0-64.

Estimated service speed 23 knots. Speed in knots Vs: 22.5, 23-0, 23.5,

24-0.

Effective horsepower on trial e.h.p.-r:

21,100, 23,000, 25,250, 28,250.

Enginessteam turbine; shaft horse-power s.h.p.=20,000 per screw, rate

of rotation (for basic screws) NF=116

revolutions per minute, subsequent

screws rate of rotation can be chosen

to give optimum performance consistent

with screw diameter.

Stern details single streamline rudder. Shafts enclosed by bossincs,

shaft immersion I= 16ft.

Stipulationsmaximum screw

dia-meter 24.25ft (7-37m), material bronze.

Design conditionservice b.h.p.=

(4)

Reprinted from "Shipbuilding' and Shipping Record", October' 7 , 1965

(2 .:Perl,cent transmission losSes). Basic

-rate_- Of rotation 'NF=115

(1, Per: cent rwake scale effect, see

reference 4,. 'SeetiOn 4.9). Service speed4-- 23 :knots: 7',

' ScieWS blades)t-screw

: diameter D=20-75-ft (632m)'blide area'

'ratio

an=0.67,

blade -thickness ratio 7=0-038, screw

efficiency no=0.69.

The' comparative, .performance

esti-mates and design calculations for; the bask 4-blade screw (screw 1) and the

.3,-'aild54tlade screws (screws. , :2and 3)

wereinade.using the optimum diameterc -

and blade area charts given in the

article' and ;follovihik the pinCedure as

given in table 2 Of that

publication,

The geometric features' and -'. sCreiv

. efficiency Values 'are summarised"

The effects of variation'- -screw, d la-meter

. and rate of

rotation ;Were

.stUdied.nsing the revised charts

-,

(referende 5) and following the:

;-,diire.ag given in table 1 of -the'..artiCle2.

,-The.de-Sign calculations frit screw 4, 'of

niakiniiim diameter and operating at ".optimum .rate of rotation (D24'25ft, 86 :revolutions per minute) were

Made: applying " the correction fictors

derived; In sections ' 4 and l,5 of . the

atfiele!Tair given intable.2 of that tion. The geometric" features and,per-.

--fOrritanee data for screw'; 4 are

- siuntriarised, in table 1 together. ;With

1hose for screws 1, 2 and 3:

4.- Comparison of results

The results of the ealculatiOns.given

in table. 1

show that,

'grittier. of. blades from 'four, ,to;three ,results in improved ;perfOrmance, but

'inereasing the number . of. :blades -frOrn

four to five results -adverse

' .:performance. Moreover, significant

intprOvements can achieved if the

design -rate of 'rotation is .selected and

the', screw diameter, increased ithin practicalAirriitsr.-: of hull-tip -clearance; as summarised below.

For the bigic four-blade screw (serew 1). designed to .40-ate' at a istipulated

rate Of "rotation; (N= - revolutions

per Minute) the diameter- was -20-75ft

and the screw efficiency was -0-69. Tor :the corresponding; 3-blade-.Serew (screw .2) the screw diameter..-ivouldt be

22ft- (6.71m) and the 'icieW"-effieiency

would be 0.705; thus "the',..,increar

e in

efficiency Would be .2;per, Cent.":" For the- corresponding;:5411ade screw

(screw 3) the screW, ilia** Would be

19-Off (6-07'.m),. and the screw efficiency

would be 0 67 thus the reduction' In

efficiency would be 3 PeCeeni:

'II O'BRIEN, T. P. Desiin- of tug'

propellersper,-forthance of three-, four'; ..and five-blade" icrews:;

Ship 'and.

143cinVI. o1T

h..'O'BRIEN.' T.. P.'.'Design' Of.tug'

4optilituni screw :diameter and ,rate of rotation.'

London; ;,Shipand Boinbuilder - International . . 1966; 19. - - ' '

,.3. O'BRIEN P. Some 'effects of variation In

For ,the" 341ade, screw (screw_ 4) cif rnainnurn diameter consistent with

hull-tipclearanCeS. (D =24.25ft) and designed .

to operate.at optimum rate of rotation

(NF=86 revolutions per minute), were Sarew efficiency would be 0.73; thin the increase in 'efficiency, would be 6, per.

cent.- . .

:.It is.significant' that if replacement

screws haying three blades were fitted to the 'existing .vessel this would 'result in

àn:inciàeJn efficiency of 2 per Cent.

MOreoVer, if 3-blade screws designed to

o'perate' at :Optimum rate of rotation

: were fitted:. to " the new vessel the

, efficiency would. be: 6 per cent greater

: than- that of the existing vessel fitted

with- the four-blade sCreVis. -

-Printed in,England by'sTemple_Press printers Linuted,`Bowling Green Lane. London. E.C.1. 2351-65

. ,

Table 1, .

.

. 'Screws 1 to 4 Geometric Features' and Performance Data

Design ConditiOns. . d:h.p. = 19,600 Per 'screwNF =116 revs per minute

. - . . .- .. , - .(basic value)

' V, ----,23 knots,Nii = 191

knots.'--,-. , ',..., -,:-.. ImmeMionI - 16feet

' Rateof_. Diameter No of - Blade - Pitch , Thk. , .Screw Percentage

Screw. No. rotation (rp.m.) 0E40., blades- -area -ratio_, _. .. ratio' ratio (axis) '

efficiency increase In,

effielency, (basic.screw ' NF B aE p Tio screwi)` 116 20-75 4 0.67 1.00

0'0'

- ' (6-32m) ' . . `--116 2200,. 3 ''0'56 . 0-95 0.060, 0.705 ' +21 ' 3.. -116-,- -., -86 . (6'71m).'' 19-90,-,(6-07m) , '24.25 .'-' 3 0-70 -0-50 . 1-07 -c 1-26 0-055 '. 0.658 . .. . 0.670 ". 0.730 +6 ' ' (7-37m) ,. . . _. . , . REFERENCES", ' . .

urn ber f blades. on model screw performance.

Trans. ME. Coast 'Institution of Engineers

, ,Shipbuilders 1965; .

4., IO'BRIEN,7:. R The design of marine. screw,

Lciridon, Hutchinson Scientific and

Technical press; Atli% 1962.

5. WRIGHT, 'B. D. W. The N.S.M.B. standard

seriespropeller dale: and their triplication: 13S.R.A.:

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Reprinted fromInternational Marine Design and Equipment 1966

Propellers

Comparative performance of 4-, 5- & 6-blade propellers

for large tankers

This article refers to recent publications on the effects of varying the number of blades of marine screws; in particular, differences between the performance of 3-,

4- and 5-blade screws under both non-cavitating and

cavitating conditions. It gives the preliminary results of

experiments and calculations which enables the data

previously given to be also extended to include 6-blade screws. It summarises NPL model experiment data, and

comprises correction factors and design data which enable 3-,4-, 5- and 6-blade screws to be designed and compara-tive performance estimates to be made using 4-blade stan-dard series data as the bases. It discusses the preliminary

performance estimates and design calculations for four

screws for a large tanker. The basic screw had four blades while the second and third screws had five and six blades,

respectively. The first three screws were all designed to run at a stipulated rate of rotation (N, = 110 revolutions

per minute). The fourth screw also had six blades, but the rate of rotation was selected to give optimum performance. The four screws all had the same diameter. It gives worked examples the results of which show that for the stipulated

rate of rotation the performance of the 4- and 5-blade screws would be the same, but for the 6-blade screw the

loss in efficiency would be about l per cent. The optimum rate of rotation for a six blade screw would be 100 revolu-tions per minute and for the screw designed to run at this

speed the gain in efficiency would be about per cent.

1 INTRODUCTION

Recent published work (Ref. 1, 2 and 3) based on

re-search at NPL shows that there are significant differences

between the performance of 3-, 4- and 5- blade screws.

An extension of this work to include 6-blade screws (Ref. 4) shows similar trends, as might be expected.

The object of the present article is to summarise the data now available, thus enabling 3-, 4-, 5- and 6-blade screws

Fig. I. Particulars of model screw BN.I0

T. P. O'Brien, C.G.I.A. M.R.I.N.A., Ship Division, N.P.L.

to be designed and comparative performance estimates to

be made using 4-blade standard series data as the bases,

and to apply the results obtained in making the preliminary

design calculations and performance estimates for four

screws for a large tanker. The basis screw is to have four

blades, and the second and third screws are to have five and six blades, respectively. The first three screws are all to be designed to run at a stipulated rate of rotation.

The fourth screw is to have six blades, but the rate of rotation can be selected to give optimum performance

The four screws are all to have the same diameter.

2 MODEL EXPERIMENT DATA

Some of the data obtained in the paper' are summarised

in the

article2, and here worked examples are given on designing 3-, 4- and 5-blade screws and making estimates of

their performance, both under free-running and towing conditions. In a subsequent article3 the designs of 3-

4-and 5-blade screws for liners are discussed, 4-and effects of variation in diameter and rate of rotation based on correc-tions derived in the article (Ref. 5) are included.

The information given in the memorandum4 include

the geometric data, open water experiment results and

performance comparisons for a group of four screws

comprising three (Screws BN 10 to 12) used in the

previous work' and one (Screw BN 13) having six blades.

The screws were all designed for the same operating condition. The ship screw design values were: 27,100 thrust horsepower at 146 revolutions per minute for a ship speed of about 27i knots, and the screw diameter was 20.5ft (6,248mm). The corresponding model screw performance values were: thrust coefficient kr = 0.185,

advance coefficient J = 0.8, and the screw diameter

was 10in (254min)

Screw BN 10 had three blades,

Screw BN 11 had four blades and Screws BN 12 and 13

had five and six blades, respectively. The particulars of

50 -147 4.75 .140 4.5 -149 4.0 1151 ---3 5 I 1.153 _--- ---\ 3-0 1155 -.1k.---2-5 157 .-....,.... 2-0 1.161 ,--- ---,,---'--- 1-165 2 5 .4 1-173 ...---r t OF BOSS -111ffir- - ---"l'LW , DIAMETER 10.0 ins No. OF BLADES 3-R.H.

BLADE AREA RATIO 0.678

MEAN FACE PITCH RATIO 1.154 SWEEP RADII(019 CYLINDRICAL SECTIONS

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010 060 070 007 050 10

Fig. 2. (above) Open; .water, experiment

- - resulte for Screws BN.10-13

Fig. 3 (right) Result's- alsOme Ofl.the -water tunnel experiments forSCreviS:13NA 0-13

Fig's. 4, :5 k6.Jbelow) PhotograPhs indic-ating .tlie extent of cavitation

, t 070 0'401 0-35 0-30, 0-25 0.20 oin' 010 005 ,1 11 111 1 11.1111 111111 1 111111.1 1111.111.111,1.11 1.1 1111,1 B N.10. 3 -_ 4 BLADES. 5'. 'BLADES.

I III 1111:1:1111111 iii II Ill II Ill I1111111111 0-20 43:40 66o, c5R &so 0..H.1011p11)]..1,11.11 Hj IHII1H 111111 J-65:7 J-0.8 10'070 -0'060 0055', 0-050 0.0-45 0'025:. . Q.015 0-0t0 0.03o 0'020

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Screw 4 are summarised in Table 5 together with those for Screws 1 to 3.

5 COMPARISON OF RESULTS

The results of the calculations given in Table 5 show

that there are significant differences between the geometric

features of screws having 4-, 5- and 6-blades. They also show that increasing the number of blades from four to

five results in no change in performance, and that increasing

the number of blades from four to six results in adverse

performance. However, the combined effects of increasing

the number of blades from four to six and selecting the

optimum rate of rotation results in improved performance, as summarised below.

For the basic 4-blade screw (Screw 1) designed to

operate at a stipulated rate of rotation (N5---110 r.p.m.) the blade area ratio was 0.625, the pitch ratio was 0.77,

the thickness ratio was 0.055 and the efficiency was 0.485.

For the corresponding 5-blade screw (Screw 2) the blade area ratio would be 0.70, the pitch ratio wolld be

0.75, the thickness ratio would be 0.052 and the efficiency

would be 0.485; thus there would be no rhange in

per-formance.

For the corresponding 6-blade screw (Screw 3) the blade area ratio would be 0.78, the pitch ratio would be

0.74, the thickness ratio would be 0.050 and the efficiency

would be 0.475; thus the loss in efficiency would be

about 1+ per cent.

For the 6-blade screw (Screw 4) designed to run at

optimum rate of rotation (N,=100 r.p.m.) the blade area

would be 0.80, the pitch ratio would be 0.84, the thickness

ratio would be 0.047 and the efficiency would be 0.490;

thus the gain in efficiency would be about + per cent.

References

O'BRIEN, T.P. Some effects of

variation in number of blades on model

screw performance. Trans. N.E. Coast Instn. Engrs. Shipb., 1965,

81, 233.

O'BRIEN, T.P. Design of tug

propellers-performance of

3-,

4-and 5-blade screws. London, Ship

and Boat Builder International,

1965, 18.

O'BRIEN, T.P. The performance

of 3-, 4- and 5-blade screws. Effects of variation in diameter and rate of rotation in passenger liner appli-cations. London, Shipbuilding and Shipping Record,

Oct., 1965.

O'BRIEN, T.P. Performance Comparisons for marine

screws of 3-, 4-, 5, and 6-blades. Ship Division Tech.

Memo 107, Jan. 1966.

O'BRIEN, T.P. Design of tug propellers-optimum screw diameter and rate of rotation. London, Ship and Boat Builder International, Feb., 1966, 19.

O'BRIEN, T.P. Design of tug propellers. London

Ship and Boat Builder International, April, 1965. 18, 22.

TABLE 4-Screws 2 and 3 design calculations

Basic Screw D=23.25 feet, 4 blades, Standard Type, Rake

50, Boss Ratio 0.167 Correction Factors Screw Particulars (equation 3) k2

=

aE aEl (equation 4) k, = (equation 5) k4 = 12. (equation 6) k6 = (equation 3)

h=k2

aEi (equation 5) p=k4p1 (equation 6)T=k671 (equation 4) n 0=k 377oi

O'BRIEN, T.P. Graphs and Contour Charts and

their applications to propeller design. Oslo, Norway, European Shipbuilding, March, 1965, 14, 2.

WRIGHT, B.D.W. The N.S.M.B. standard series

propeller data and their applications. London, British Ship Research Assoc. T.M. No. 213, June, 1965. VAN .MANEN, J.D. A review of research activities

at the Netherlands Ship Model Basin, Rotterdam,

Holland, International Shipbuilding Progress, Nov.,

1963, 10, 111.

O'BRIEN, T.P. The design of marine screw propellers,

London, Hutchinson Scientific and Technical Press, July, 1962. Screw No.of blades

B

k2 k3 14 k6 Remarks (1) (2) (3) (4) 4 54.5 1.0 1.0 1.0 1.0 Basic screw 5 1.11 1.00 0.975 0.95 Values from Fig. 7 6 1.25 0.985 0.960 0.90 Values from Table 3 No. of Dia.D aE _P T 11 a (5) (6) (7) (8)

Screw Blades (feet) (mm.) 4 23.25 0.625 0.770 0.055 0.485 7.086 2 5 23.25 0.700 0.750 0.052 0.485 7-086 6 23.25 0780 0.740 0.050 0.475 7.086 Screw No. Deliv. h.p. Rate of Rota-tion Dia. No. of Blades Blade Area Ratio Pitch

Ratio RatioThi. ciencyEffi- Percentageincrease in efficiency Remarks d.h.p. NI. D B aE _Pr 7 no (r.p.m.) (feet) (mm) 1 22,000 110 23.25 4 0.625 0.770 0.055 0-485 0 Basic 7086 Screw 2 VI IV 5 0.700 0.750 0.052 0.485 0 3 VP VI 6 0.780 0.740 0.050 0.475 -1+ 4 IV 160 ,, 6 0.800 0.840 0.047 0.490 _tr

TABLE 5. SCREWS 1 TO 4-GEOMETRIC FEATURES AND PERFORMANCE DATA

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where am, fbi, Pr and

71

are the blade area ratio,

efficiency, pitch ratio and thickness ratio for the basic

4-blade screw.

a,, p and7 are the blade area ratio, efficiency, pitch

ratio and thickness ratio for the non-basic screw

The data discussed above are not sufficient to cover

effects of varying diameter, in particular change in optimum

diameter due to departure from four blades. For some

screws improved performance can be achieved if the

diameter can be modified to suit the optimum value for either 3-, 5- or 6-blade screws. For 3- or 5-blade screws this can be done by applying the procedure described in

Section 3 of the article2. This procedure is being extended

to include 6-blade screws and the results obtained will

be published shortly.

4 WORKED EXAMPLES

It is required to prepare the preliminary performance estimates and design calculation for four screws for a large tanker. The basic screw (Screw 1) is to have four

blades and the second and third screws (Screws 2 and 3)

are to have five and six blades, respectively. Screws 1,

2 and 3 are all to be designed to operate at a stipulated rate of rotation. The fourth screw (Screw 4) is also to

have six blades, and for this screw the rate of rotation can

be chosen to give optimum performance. The screws

are all to have the same diameter.

Design Data

HullSingle-screw tanker; length 830ft, breadth 125ft,

draught (level). 45ft (252.98>< 38.10 x 13.72m). block

coefficient 0.8.

Estimated trial speed-17 knots

EngineDiesel; delivered horsepower 22,000 d.h.p.,

rate of rotation basic value N,=110 r.p.m.

Stern detailStreamline rudder.

Shaft Immersion I=30ft (9.14m)

StipulationScrew diameter D=23.25ft (7.09m) rate

of rotation (Screws 1, 2 and 3) lc,= 110 r.p.m.,

(Screw 4) value to be chosen to give optimum

per-formance.

Screw material, nickel aluminium bronze.

Design condition-22,000 d.h.p. Basic rate of rotation

N=0.98 N,=108 (2 per cent wake scale effect see

Ref. 10, Section 4.9) Trial speed 17 knots.

Propulsion factorsWake fraction W=0.43, relative

flow factor G=1.02, hull factor 1.42.

Screw 1 Design Calculations

The design calculations for the basic 4-blade screw

(Screw 1) were made and using the revised BpS charts8,9

and following the procedure as given in Table 2, of the artide6. The geometric features and performance data

are summarised in Table 5.

Screws 2 and 3 Design Calculations

In making the design calculations given in Table 4, for

the basic value of B corresponding values of blade area

correction efficiency correction k3, pitch correction k4

and thickness correction k6 are obtained from Fig. 7 (for

110 t, 1-05 1.0 0.95 0.90 1.10 cce ag, 1-05 10 0.95 0.9 0.85 1-05 1 0-95 1.0

Ti

10 0.9 0.9

JIIIIIIIIIIIIiIIIIIIIIIIIIIIIIIIIIIIIEr[mi_

_ -

_.____

3-BLADES

-

_

--

_{_}

-_ THICKNESS CORRECTION

-

_ _

-_ 5-BLADES

=

--

-_

--

---..._ 5-BLADES

--

_ _

--

-_ AREA CORRECTION

-

_ _

--

-_

--

_

3-BLADES

-

.-.

. _ .:----

-

--

-_ _ 3-BLADES

--

...

-_

---

-_

-

PITCH CORRECTION

--5-BLADES

-_

--

EFFICIENCY CORRECTION

--

-- ,...--3- BLADES

-5- BLADES

-,..._. _

-f

_

1 iiIIIIIII1 111111111111111iilmili11111111-10 20 30 40 50 60 70 80 90 100 Bp

Fig. 7. Correction chart

five blades) and from Table 3 (for six blades).

Each

correction factor is applied to the respective parameter of the basic 4-blade screw to give corresponding values of blade area ratio, efficiency, pitch ratio and thickness

ratio for the 5-blade screw (Screw 2) and the 6-blade

screw (Screw 3).

The geometric features and performance data for screws

2 and 3 are summarised in Table 5, together with those

for the basic screw (Screw 1).

Screw 4Performance Estimates and Design

Calculations

The performance estimates to enable the rate of rotation for optimum performance to be chosen for the additional 6-blade screw (Screw 4) were made using the revised Bp-8 charts8,9 and following the procedure as given in Table 1

of the article. The value of this was found to be N,=100

r.p.m. The design calculations were made using the correc-tion factors given in Seccorrec-tions 4 and 5 of the article 5 and

following the procedure as given in Table 2 of that

publication.

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TABLE 1. Particulars of model screws BN 10 to 13

TABLE 2. Open water performance values and comparisons

TABLE 3-Correction factors for 6-blade screws-constant

diameter

Screws BN 10 to 13 are summarised in Table 1 and a

drawing of the 3-blade screw (Screw BN 10) is shown in

Fig. 1. The open water experiment results for Screws

BN 10 to 13 are shown in Fig. 2. Some of the water

tunnel experiment results for Screws BN 10 to 12 are

shown in Fig. 3 and photographs indicating the extent of

cavitation are shown in Fig. 4, 5 and 6. The open water

performance values and comparisons for Screws BN 10 to 13 are given in Table 2.

3 SCREW PERFORMANCE COMPARISONS

AND CORRECTION FACTORS

The results of the comparisons showed significant

differences between the performance of screws having

varying number of blades both under non-cavitating and cavitating conditions. The open water experiment results

showed variations in thrust and torque coefficients requiring

moderate pitch corrections to obtain equivalent

perform-ance. Moreover, there were appreciable differences in screw efficiency. There were significant differences in performance

under cavitating conditions, as assessed by water tunnel

experiment results and as shown by visual observations. Consequently, large blade area corrections are needed to obtain equal margin against thrust breakdown.

The correction chart shown in Fig. 10 of the paperl and reproduced in Fig. 7 comprises correction factors

for pitch, blade area, blade thickness and efficiency. Two sets of correction factors are given enabling the geometric

features and efficiency values of either 3- or 5-blade

screws to be derived from corresponding data for basic

4-blade screws. The correction factors given in Table 3

were derived using data for 6-blade screws, some of which are given in the memoranda 4 and 8, and comprise a corre-sponding set of correction factors for 6-blade screws.

In applying this procedure, the power coefficient is

evaluated, and for this value of B corresponding values of

blade area correction k2,pitch correction 1(4and thickness

correction k6 _{are obtained and applied to the respective}

geometric parameters of the basic 4-blade screw. Similarly, the efficiency correction lc, is obtained and applied to the available value of efficiency for the basic 4-blade screw.

The power coefficient _{and its related speed coefficient}

are given by

N ei DIEP

( 1 ) Bp =

VA2 sVA

(2) 8 ND

where N is the rate of rotation of the screw in revolutions

per minute VA is the speed of advance of the screw in

knots

e. is the relative flow factor as defined by equation 9 of

the article (Ref. 6)

DHP is the delivered horsepower in British units

s is the specific gravity of the fluid in which the screw

operates (average value for sea water s 1.026)

The blade area correction 1(2, efficiency correction Ica,

pitch correctionk4and thickness correctionk6are defined by

k2 = 2.21

k,

Screw No. BN 10 BN 11 BN 12 BN 13 Diameter (inches) 10 10 10 10 (mm) 254 254 254 254 No. of blades 3 4 5 6

Blade area Ratio 0.678 0782 0.860 0.950

Mean Pitch Ratio 1.15 1.10 1.08 1.06

Blade Thk. Ratio (axis) 0.065 0.060 0.055 0052 Boss-Diam. Ratio 0.20 020 0.20 020 Performance Values Screw J 0.50 065 0.80 0.95 BN10 k r 0.343 0.265 0190 0125 (3 blades) k Q 0.0608 0.0488 0.0375 0.0274 710 0.449 0.562 0.648 0.690 Screw k7. 0340 0263 0.188 0.111 BN 11 k Q 0.0581 00469 00357 0-0241 (4 blades) n 0 0465 0.580 0.670 0.696 Screw k r 0.340 0.256 0.12 0.090 BN 12 k Q 0.0574 00451 0.0327 0.0212 (5 blades) n 0 0.472 0.587 0670 0.642 Screw kr 0.338 0252 0.166 0.0755 BN 13 k Q 0.0567 0.0443 0.0321 0.0198 (6 blades) .9 0 0.475 0.589 0.659 0.574

Performance Comparisons at Constant k u (4-blade screw as the basis)

Basic J 0.50 0.65 0.80 095 Screw k u 1.36 0.622 0.293 0.1P3

BN 11 IT o 0.465 0.580 0.670 0696 (4 blades)

Screw per cent rpm -0.5 0 -0.5 -2.0

BN 10

(3 blades)

above

basic no -2.5 -3.0 -3.0 -1.0 Screw per cent rpm 0 1.0 2.0 3.0

BN 12

(5 blades)

above

basic no

10 10 -0.5 -4.0

Screw per cent rpm 0 1.0 2.5 4.0 BN 13

(6 blades) abovebasic n 0 1.0 05 25 -8.5

Power coefficient

Corrections to basic blade screw values Blade

area Efficiency Pitchratio ness ratio

Thick-a E ?I . P T

-

-azi k2 n01 1(3 Pi Ic4 Ti ka 20 1.25 0975 0.955 0.90 40 1.24 0.985 0960 0.90 60 1.25 0.985 0960 090 80 1.24 0990 0.960 0.90 100 1.23 0.995 0.995 0-90

(10)

7 `_J.7

y 3..

?lima :if grifrrliiAll Ness, R.P617-P922