Some effects of variation in blade area, blade outline and boss diameter on model screw perfromance

NATIONAL PHYSICAL

LABORATORY

SHIP DIVISION

SOME EFFECTS OF VARIATION IN BLADE AREA, BLADE OUTLINE

AND BOSS DIAMETER ON MODEL SCREW PERFORMANCE

by

T. P. O'Brien

(Excerpt from the Institution Transactions Vol. 84

with subsequent discussion)

A Station of the

ARCHIEF

Lab.

v.

Scheepsbouwkunde

See note inside cover

_{Technische Hogeschool}

_{SHIP REPORT 127}

Extracts from this report may be reproduCed:

provided the source is acknowledged.

-iApprov.ed-on behalf of Director, NPt. by

Mr.

Paffett, Superintendent, Ship:DiYision

SOME EFFECTS OF VARIATION IN

BLADE AREA, BLADE OUTLINE

AND BOSS DIAMETER ON MODEL

SCREW PERFORMANCE

Newcastle upon Tyne Published by

NORTH EAST COAST INSTITUTION OF ENGINEERS AND SHIPBUILDERS

by T. P. O'BRIEN, C.Eng.,

Associate Member

(Excerpt from the Institution Transactions Vol. 84

with subsequent discussion)

THE INSTITUTION

IS NOT RESPONSIBLE FOR THE

STATEMENTS

MADE,

NOR FOR THE

OPINIONS

EXPRESSED IN

THIS EXCERPT

PARTICULARS OF MEMBERSHIP of The Institution will be supplied on adplication,to the Secretary (for address see cover)

PRODUCEb IN GREAT BRITAIN

Printed by W. 11. HOULDERSHAW LTD. LONDON ROAD SOUTHEND-ON-SEA

Some Effects of Variation in Blade

_Area,

Blade Outline and Boss Diameter

on Model Screw Performance

Synopsis

This paper gives the results of experiments for three groups of

model screws, covering variations in blade area, blade outline and boss diameter. It includes comparisons of performance undernon-.

cavitating and cavitating conditions based on open water experiments

and water tunnel experiments. Correction factors_{are derived and}

design data 'are given which enable screws of different blade area,

blade outline and boss diameter to be designed and comparative

estimates of their performance to be made using data for a standard screw as the bases.

Introduction

In designing marine screws and making estimates of their

performance it is desirable to apply correction factors to

make allowance for departure from standard geometric

features. This is of particular importance in making ship

screw performance estimates based on propulsion

experi-ment data for standard type screws the geometric features

of which may differ considerably from those of the ship

screw. Effects of variation in blade section shape, blade

thickness and number of blades have been covered in

previous work at NPL (Refs. 1-4) and the present paper

is a continuation of this work. The object of this

paper is

(1)

_{to obtain performance data for three groups of}

model screws, all having the same basic screw of standard

type and each covering variations in a particular

geo-metric feature comprising blade area, blade outline and

boss diameter, and (2) to derive correction factors to enable

screws of differing blade area, blade outline and boss

diameter to be designed and comparative estimates of

their performance to be made using data for a standard

type screw as the bases.

Particulars of Screws

2.1 Screws BA I to 4 (variation in blade area)

The first group of screws was selected from the NPL

standard series (Refs. 5 & 6) and comprised four

screws

all having four blades, the same diameter (D

= 0.7 ft.),

the same pitch ratio (p/D = 0.85), the same blade thickness

ratio (r = 0.045) and the same boss diameter ratio

(d/D = 0.167). The basic screw (Screw BA 1) had

a blade

area ratio aE of 0.5. The non-basic screws all had the

same blade outline as that of the basic screw and the

variation in blade area was made by applying a constant

factor to the section chords: Screw BA 2 had a blade area

ratio of AE = 0.4, and Screws BA 3 and BA 4 had blade

Ao

area ratios of 0.6 and 0.7, respectively.

Principal characteristics of the screws are summarised

in Table 1 and the main geometric features

_{are shown}

in Fig. 1.

by T. P. O'BRIEN, C.Eng., Associate Member:

8th APRIL, 1968

The basic screw of the series was also chosen to closely

correspond to a full-scale screw designed for a tug

(Screw 1 of Ref. 8).

The particulars of the screw and its design and operating

conditions are given in Table 2.

2.2 Screws BO 1 to 4 (variation in blade outline)

The basic screw of this group (Screw BO 1) was the same

screw (Screw BA 1) as the basic screw of the group

covering variation in blade area. The non-basic

screws

had blade outlines differing from that of the basic

screw.

Screw BO 2 had a narrower tip and Screws BO 3 and 4

had wider tips than that of the basic screw. Apart from

the differing blade outline, which resulted in differing blade

area ratio, the other geometric features were the same for

all the screws of the group.

For the non-basic screws the section chord values

were

derived from those of the basic screw by applying

a

factor of the form

(1) C

CI [1

k (x

0.2)]

where C is the chord length of the non-basic blade

outline at the radius fraction x

C1 is the chord length of the basic blade outline

at the radius fraction x

k

_{is a constant (k > 0 for wide tip, k > 0 for}

narrow tip).

For Screw BO 2 the value of k was

0.5 and the blade

area ratio was 0-41; for Screws BO 3 and 4 the

values of k were 0.5 and-1.0 and the blade area ratio

was

0.59 and 0-68, respectively.

Principal characteristics of the screws are summarised

in Table 1 and the main geometric features are shown in

Fig. 1.

2.3 Screws BD I to 3 (variation in boss diameter ratio)

The basic screw of this group (Screw BD 1) was the same

screw as the basic screw of the other groups. Screw BD 1

had a boss diameter ratio of 0-167 and Screws BD 2

and BD 3 had boss diameter ratios of 0-2 and

0.3,

respectively.

Apart from

the differing

boss

diameter ratio, which resulted in differing blade area ratio,

the other geometric features were the same for all three

screws of the group. For Screws BD 2 and BD 3 the

blade area ratio was 0.46 and 0.43, respectively.

Principal characteristics of the screws are summarised

in Table 1 and the main geometric features are shown in

Fig. 1.

Model Screw Experiments

3.1 Manufacture of Model Screws

Screws BA 1 to 4 (of the same basic outline) were selected

from the NPL standard series.

Screws BO 2, 3 and 4 (of varying blade outline) and

Screws BD 2 and 3 (of varying boss diameter) were

added to the basic screws of the NPL standard series for

the purpose of obtaining the data given in the present

paper. All the screws were made in white bronze using

the NPL profiling machine the prototype of which was

described in the paper (Ref. 1).

3.2 Open Water Experiments

Open water experiments were made for the screws of

basic blade outline (Screws BA 1 to 4), the screws of

varying blade outline (Screws BO 2, 3 and 4) and one of

the screws of varying boss diameter (Screw BD 3). Each

screw was tested over a range of advance coefficient at

a constant rate of rotation (n -= 10 per second). The

open water experiment results are shown in Fig.

2 and

comparisions of the results are made in Table 3.

3.3 Water Tunnel Experiment Results

Water tunnel experiments were made for the screws of

basic blade outline (Screws BA 1 to 4) and two screws of

varying blade outline (Screws BO 2 and BO 4) in the

NPL No.

1

(Lithgow) water tunnel. Each screw was

tested at atmospheric pressure over a range of advance

coefficient at a constant rate of rotation (n = 25 per

second) and at variable pressure at low and moderate

advance coefficients, each over a range of cavitation

number. The low advance coefficient simulating towing

or trawling conditions was obtained by stopping the tunnel

impeller and running the screw at a constant rate of

rotation as described in the paper (Ref. 6).

The moderate advance coefficient

simulating

free-running conditions was chosen to have the same value

(J = 0.56) as that of the full-scale screw, the particulars

of which are given in Table 2.

The water tunnel experiment results are shown in

Figs. 3 and 4. Performance comparisons at non-cavitating

conditions are made in Table 3 together with those of

the open water experiments. Performance comparisons

at cavitating conditions are made in Table 4.

Comparisons of Screw Performance

4.1 Screws BA I to 4Non-Cavitating Conditions

The performance values of the screws of varying blade

area ratio are compared in Table 3 using the screw

of

A E

basic blade area ratio (Screw BA 1,

= 0.5) as the basis.

For free-running conditions the comparisons are made at

constant thrust horsepower (i.e., at constant ku) following

the procedure described in the paper (Ref. 2).

For

towing conditions the comparisons are made at constant

torque.

The results of these comparisons are summarised as

follows:

Free-Running Conditions

Reduction in blade area ratio from 05 to O4

Efficiency: 1% high.

Rate of rotation: 1% low (open

water experiments).

Efficiency: 11% high. Rate of rotation: 1 % low (water

tunnel experiments).

Increase in blade area ratio from 05 to O6

Efficiency: 1 % low. Rate of rotation: 1% high (open

water experiments).

Efficiency: 1% low. Rate of rotation: no change (water

tunnel experiments).

Increase in blade area ratio from O5 to 0.7

Efficiency: 4% low. Rate of rotation:

1 % high (open

water experiments).

Efficiency: 2% low. Rate of rotation: 1% high (water

tunnel experiments).

Towing Conditions

Reduction in blade area ratio from 05 to 04

Thrust: 31% high. Rate of rotation: 41% high

Increase in blade area ratio from 05 to 06

Thrust: 31% low. Rate of rotation: 2i % low.

Increase in blade area ratio from 05 to 07

Thrust : 6/ % low. Rate of rotation: 61% low.

4.2 Screws BO 1 to 4Non-Cavitating Conditions

The performance values of the screws of varying blade

outline are compared in Table 3 using the screw of basic

blade outline (Screw BO 1standard blade outline) as

the basis. The comparisons are made following the same

procedure used for the screws of varying blade area ratio.

The results of these comparisons are summarised as

follows:

Free-Running Conditions

Reduction in chord values giving a narrow tip

(k =

0-5)

Efficiency: 1% high.

Rate of rotation: 1 % low (open

water experiments).

Efficiency: 4% high. Rate of rotation: 1 % low (water

tunnel experiments).

Increase in chord values giving a moderately wide tip

(k

0.5)

Efficiency: 2% low. Rate of rotation: 1 % high (open

water experiments).

Increase in chord values giving a wide tip (k =- 1 .0)

Efficiency: 2% low. Rate of rotation: 1% high (open

water experiments).

Efficiency: 2 % low. Rate of rotation: no change (water

tunnel experiments).

Towing Conditions

Reduction in chord values giving a narrow tip

(k =

0.5)

Thrust: 3 % high. Rate of rotation: 5 % high.

Increase in chord values giving a moderately wide tip

(k = 0.5)

Thrust: 4% low. Rate of rotation: 4% low.

Increase in chord values giving a wide tip (k = 1 -0)

Thrust: 7% high. Rate of rotation: 81 % low.

4.3 Screws BD 1 and 3Non-Cavitating Conditions

The performance values of one of the screws of varying

boss diameter ratio are compared with those of the basic

screw of standard boss diameter ratio in Table 3. The

comparisons are made following the same procedure used

for the other screws.

The results of these comparisons are summarised as

follows:

Free-Running Conditions

Increase in boss diameter ratio from 0167 to 030

Efficiency: 2.5% low. Rate of rotation: 3% high (open

water experiments).

Towing Conditions

Increase in boss diameter ratio from O167 to 0 .30

Thrust: 3% low. Rate of rotation: 1 % low.

4.4 Screws BA 1 to 4Cavitating Conditions

The performance values of the screws of varying blade

area ratio are compared in Table 4 using the screw of

basic blade area ratio (Screw BA 1,AE

= 0.5) as the basis.

AO

The comparisons are based on point of thrust

break-down and expressed in terms of a thrust breakbreak-down

factor FT following the procedure described in Section 5.3

of the paper (Ref. 3).

The thrust breakdown factor FT is defined by

(2) FT = GT

where an

is the cavitation number at point of thrust

breakdown for the basic screw

where GT is the cavitation number at point of thrust

breakdown for the non-basic screw.

The results of the comparisons are summarised as

follows:

Free-Running Conditions

Reduction in blade area ratio from 0.5 to 0-4: FT = 1.17.

Increase in blade area ratio from 0.5 to 0.6: FT = 0.83.

Increase in blade area ratio from 0.5 to 0.7: FT = 0.70.

Towing Conditions

Reduction in blade area ratio from 0.5 to 0-4: FT = 1-10.

Increase in blade area ratio from 0.5 to 0.6: FT = 0.79.

Increase in blade area ratio from 0.5 to 0-7: FT = 0-69.

The foregoing comparisons clearly show that a reduction

in blade area ratio results in adverse performance and

that increases in blade area ratio result in improved

performance.

Cavitating performance comparisons as assessed by

visual observations indicate

similar

trends to those

discussed above. For the screws of reduced blade area ratio

the amount of cavitation was appreciably greater, but for

the screws of increased blade area ratio the amount of

cavitation was about the same as that for the basic screw.

4.5 Screws BO I, 2 and 4Cavitating Conditions

The performance values of some of the screws of varying

blade outline are compared in Table 4 using the screw of

basic blade outline (Screw BO 1standard blade outline)

as the basis. The comparisons are made following the

same procedure used for the screws of varying blade area

ratio.

The results of the comparisons are summarised as

follows:

Free-Running Conditions

Reduction in chord values giving a narrow tip (k = 0.5):

FT = 1.37.

Increase in chord values giving a wide tip (k = 1 0) :

FT = 0-60.

Towing Conditions

Reduction in chord values giving a narrow tip (k =

FT = 1.29.

Increase in chord values giving a wide tip (k

= 1.0):

FT = 0-59.

The foregoing comparisons clearly show that the

adoption of a narrow tip results in adverse performance

and that the adoption of a wide tip results in improved

performance.

Cavitating performance comparisons as assessed by

visual

observations indicate

trends similar to those

discussed above where a reduction in chord values giving

a narrow tip results in a greater amount of cavitation.

4.6 Overall Comparisons and Correction Factors

The performance of the screws of varying blade outline

and those of varying blade, area ratio are compared on a

basis of equivalent blade area ratio as shown in Fig 5.

The procedure for making these comparisons was as

follows:

First, values of thrust breakdown factor FT for the

screws of varying blade area ratioScrews BA 1 to 4

given in Table 4were plotted on a base of blade

area

A E

ratio

The relation between thrust breakdown factor

Ao

and blade area ratio was found to be near linear over the

range covered

(AE

= 0-4 to 0.7), and for free-running

Ao

conditions the divergence was small. Assuming a linear

relation, the results for free-running conditions were

extrapolated to cover the range of thrust breakdown

factor (FT

059 to 1.37) for the screws of varying blade

outline (Screws BO 2 and 4):

this enabled equivalent

values of blade area ratio for these screws to be obtained.

The results of the comparisons are summarised as

follows:

Free-Running, Conditions

Reduction in chord values giving a narrow tip (k =

A E

= 0.27.

Ao

Increase in chord values giving a wide tip (k =- 1-0):

A E

= 0.76.

Ao

Towing Conditions

These results showed trends similar to those for

free-running conditions, as might be expected.

Conclusions

The results of the work described in this paper showed

appreciable differences in screw performance under both

cavitating and non-cavitating conditions due to changes

in blade area ratio, blade outline and boss diameter

ratio.

The performance comparisons at non-cavitating

conditions are based on open water experiment results

and those at cavitating conditions are based on water

tunnel experiment results.

Variation in Blade Area Ratio

5.1 Free-Running ConditionsNon-Cavitating

A reduction in blade area ratio results in a small gain in

efficiency and increases in blade area ratio results in small

losses in efficiency. The effects of these variations also result

in changes in rate of rotation requiring pitch corrections

to obtain equal advance coefficient. Moreoever, variations

in blade area ratio, which result in variations in chord

ratio at the blade root, require thickness corrections to

maintain conditions of equivalent stress. The effects of

variation in blade thickness also results in changes in

rate of rotation and efficiency. Consequently, additional

pitch corrections and efficiency corrections need to be

applied, and the numerical values of these can be obtained

from the data given in the paper (Ref. 2).

Typical values are:

Basic Experiment Results (Screws BA 1 to 4)

Derived Values (corrected to obtain equal advance

coefficient)

Blade Area

Ratio Blade Area Ratio% Increase in % Increase inEfficiency Rate of Rotation% Increase in

0.4

20

i

I-0.5 Basic Screw..

0-6 20

4

_i

0.7 40

4

1

Blade Area

Ratio % Increasein Blade Area Ratio % Increase in Efficiency % Increase in Thickness Ratio % Increase in Pitch Ratio 0.4

20

0 12 0.5

....

Basic Screw

....

0-6 20 0

8

li

07

40

3

15

_2i

5.2 Towing ConditionsNon-Cavitating

A reduction in blade area ratio results in a moderate

gain in thrust and increases in blade area ratio result in

moderate losses in thrust. The effects of these variations

also result in changes in rate of rotation requiring pitch

corrections to obtain equal power absorbed. The

applica-tion of these pitch correcapplica-tions are reflected in changes in

thrust which result in close agreement between the

per-formance of all screws. Moreoever, variations in blade

area ratio which result in variations in chord ratio at the

blade root require thickness corrections to maintain

conditions of equivalent stress.

Since the effects of

variation in blade thickness result in changes in rate of

rotation and efficiency, additional pitch corrections and

efficiency corrections need to be applied, although for

towing conditions the numerical values of these corrections

are lower than for free-running conditions.

Typical values are:

Basic Experiment Results (Screws BA 1 to 4)

Derived Values (Corrected to obtain equal power

absorbed)

5.5 Towing ConditionsNon-Cavitating

A reduction in chord values giving a -narrow blade tip

results in a moderate gain in thrust and increases in chord

values giving wide tips result in moderate losses in thrust.

The effects of these variations also result in changes

in

rate of rotation requiring pitch corrections to

obtain

equal power absorbed. The application of these pitch

corrections are reflected in changes in thrust which

result

in close agreement between the performance of all screws.

Unlike the screws of varying blade area ratio, the screws

of varying blade outline all had the same value of

chord

ratio at the blade root. Consequently, apart from relatively

small changes in tensile stress due to forces on the outer

region of the blades, which could be neglected, the blade

152

5.3 Cavitating Conditions

A reduction in blade area ratio results in adverse

perfor-mance and increases in blade area ratio result in improved

performance under both free-running and towing

con-ditions.

Typical values are:

Variations in Blade Outline

5.4 Free-Running ConditionsNon-Cavitating

A reduction in chord values giving a narrow blade tip

results in a small gain in efficiency and increases in chord

values giving wide tips result in small losses in efficiency.

The effects of these variations also result in changes in

rate of rotation requiring pitch correction to obtain equal

advance coefficient. Unlike the screws of varying blade

area ratio, the screws of varying blade outline all had the

same value of chord ratio at the blade root. Consequently,

apart from relatively small changes in tensile stress due to

forces on the outer region of the blades, which can be

neglected, the blade root stresses can be assumed constant.

Typical values are:

root stresses could be assumed constant.

Typical values are:

(a) Basic Experiment Results (Screws BO 1 to 4)

Blade Area Ratio

% Increase in Blade Area

Ratio

% Increase in Cavitation Number at Point of Thrust Breakdown Free-Running Towing 0-4

20

17 10 0-5

... ....Basic Screw....

0-6 20

17

21

0 - 7 40

30

31

Blade Area

Ratio Blade Area Ratio% Increase in

% Increase in Thrust % Increase in Rate of Rotation 0-4

20

31 41 0-5 ..Basic Screw 0-6 20

31

21

0-7 40

61

61

Blade Area

Ratio % Increasein Blade Area Ratio

% Increase

in Thrust % Increasein Pitch Ratio % Increase in Thickness Ratio 0-4

20

1

5 12 0-5

..

Basic Screw.... 0-6 20

1

3

8

0 - 7 40

1-

61

15

Blade Outline % Increase in

Efficiency % Increase in Rate of Rotation % Increase in Pitch Ratio to obtain equal advance coefficient

Narrow Tip (k = 0-5)

Standard (k = 0) Wide Tip (k = 0-5)

Very Wide Tip (k = 1-0)

1

2

2

1

. Basic Screw 1 1

1

Blade Outline ' % Increase in Thrust

% Increase in Rate of Rotation

Narrow Tip (k = 0-5)

3 5

Standard (k == 0) Basic Screw

Wide Tip (k = 0-5)

5

4

(b) Derived values (Corrected to obtain equal power

absorbed)

5.6 Cavitating Conditions

A reduction in chord values giving a narrow blade tip

results in adverse performance and an increase in chord

value giving a wide tip results in improved performance

both under free-running and towing conditions.

5.8 Towing Conditions

The foregoing comparisons indicate that screws having

reduced chord values giving a narrow blade tip are not

to be recommended. However, the adoption of increased

chord values giving a wide blade tip as an alternative to

Variation in Boss Diameter Ratio

5.9 Free-Running ConditionsNon-Cavitating

An increase in boss diameter ratio results in a moderate

loss in efficiency. The effect of this variation also results

in a change in rate of rotation requiring a pitch correction

to obtain equal advance coefficient.

Typical values are:

5.10 Towing ConditionsNon-Cavitating

An increase in boss diameter ratio results in a moderate

loss in thrust. The effect of this variation also results in a

small change in rate of rotation requiring a small pitch

correction to obtain equal power absorbed.

Typical values are:

Variation in Blade Outline and Equivalent Blade Area Ratio

5.7 Free-Running Conditions

The foregoing comparisons indicate that screws having

reduced chord values giving ,a narrow blade tip are not

to be recommended. However, the adoption of increased

chord values giving a wide blade tip as an alternative to

the standard blade outline for screws of large blade area

ratio (AE >0.5) results in improved performance.

Ao

Typical values are:

the standard blade outline for screws of large blade area

AE

ratio ( >05) results in no loss in performance.

Ao

Typical values are:

Typical values are:

Basic Experiment Results (Screws BD 1 and 3)

Derived Values (Corrected to obtain equal power

absorbed)

Acknowledgments

The work described above has been carried out as part

of the research programme of the National Physical

Laboratory, and this paper is published by permission

of the Director of the Laboratory.

Blade Outline %Increase in Cavitation Number at Point of Thrust Breakdown

Narrow Tip(k

= 0.5)

Standard (k

= 0)

Very Wide Tip (k

= 1.0)

Free-Running Towing 37 Basic

40

29 Screw

41

Blade Outline % Increase in

Thrust % Increase inPitch Ratio

Narrow Tip (k = 0.5)

2

6

Standard(k

= 0)

Basic Screw Wide Tip (k

= 0.5)

2

4

Very Wide Tip (k

= 1.0)

0

81

Designation Blade AreaRatio % Increase inEfficiency % Increase inThickness

Ratio % Increase in Pitch Ratio to obtain equal advance coefficient Very Wide Tip(k

= 1.0)

Standard(k

= 0)

Standard (aE = 0.76) 0-68 0.50 0-76

2

0 Basic Screw ..

19

+ . . 3

Designation Blade AreaRatio % Increase inThrust %Increase inThickness

Ratio % Increase in Pitch Ratio to obtain equal power absorbed Very Wide Tip (k = 1.0)

Standard(k

= 0)

Standard (aE = 0.76) 0-68 0.50 0-76 0

1+

0 Basic Screw..

19

81

....

8

Boss Diameter

Ratio % Increase inThrust Rate of Rotation% Increase in 0-300 0-167

3

Basic 1 Screw Boss Diameter

Ratio % Increase inEfficiency %Increase inRate of

Rotation % Increase in Pitch Ratio to obtain equal Advance Coefficient 0.30 0-167

2'5

3 Basic 5 Screw Boss Diameter

Ratio % Increase inThrust % Increase inPitch Ratio 0-300 0.167

3.5

Basic

15

Screw

References

SILVERLEAF, A and O'BRIEN, T.P. "Some effects of blade section

shape on model screw performance", Trans. N.E.C. Inst., 71,1955

O'BRIEN, T. R "Some effects of blade thickness variation on

model screw performance", Trans. N.E.C. Inst., 73,1957. O'BRIEN, T. P. "Some effects of variation in number of blades on model screw performance", Trans. N.E.C. Inst., 81,1965. O'BREN, T. P. "Performance comparisons for marine screws of three, four, five and six blades", Ship Division Tech. Memo., 107,

April 1965.

O'BRIEN, T. P.

"Design of tug propellersperformance of

three, four and five blade screws", London, Ship and Boatbuilder International, 18,1965.

Dousr, D. J. and O'BRIEN, T. P. "Resistance and propulsion of trawlers", Trans. N.E.C. Inst., 75, 1959.

O'BRIEN, T. P. "The design of intarine screw propellers",London, Hutchinson Scientific and Technical Press, July 1962.

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

Boatbuilder International, 18, April 1965.

= Expanded blade area

= Disc Area

= Expanded blade area ratio

= Number of blades

-- Chord of expanded blade section

= Chord ratio

= Screw diameter

= Boss diameter

= Boss diameter ratio = Thrust breakdown factor = Advance coefficient = Coefficients and ratios

=

Coefficients and ratios

Torque coefficient

= Thrust coefficient

= Rate of rotation

= Geometric pitch

= Geometric pitch ratio

=

Static pressure at screw axis Torque Tip radius 154 IA

=

tID tIC

=

VA

=

VR.7

=

X = r/R

=

7)

p==

=.

a = 2(poPv) =

pv2

aR-2(PoPV)

--pvit72

tP

TABLE 1.

Screws BA 1 to 4, BO 1 to 4 and BD 1 to 3

Geometric Data

Radius of blade sectional element

Stress

Thrust

Maximum cylindrical thickness of blade sectional

element

Blade thickness (equivalent value at screw axis) Thickness ratios

Speed of advance of screw

Resultant velocity at x = 0-7 radius fraction Radius fraction

Screw efficiency

Mass density

Cavitation number (general form)

Cavitation number (resultant velocity at x = 0.7 radius fraction)

Cavitation number at point of thrust breakdown Blade thickness ratio

Geometric pitch angle Rake angle

List of Tables

Table 1Screws BA 1 to 4, BO 1 to 4 and BD 1 to 3

Geometric Data

Table 2Particulars of Ship Screw and Nominal Operating

Conditions

Table 3Screw Performance Comparisons (Non-Cavitating

Conditions)

Table 4Screw Performance

Comparisons (Cavitating

Conditions)

List of Illustrations

Fig. 1Screws BA 1 to 4, BO I to 4 and BD 1 to 3

Geometric Features

Fig. 2Screws BA I to 4, BO I to 4 and BD 1 and 3

Open Water Experiment Results

Fig. 3Screws BA 1 to 4 and BO I, 2 and 4Water

Tunnel Experiment ResultsAtmospheric Pressure

Fig. 4Screws BA I to 4 and BO 7, 2 and 4Water

Tunnel Experiment ResultsControlled Pressure

Fig. 5Screws BA 1 to 4 and BO 1, 2 and 4Assessment

of Cavitating Performance

Screw No. BA 1 BO 1 BD 1 BA 2 BA 3 BA 4 BO 2

B03

B04

BD 2 BD 3 Blade Area Ratio AE

lio

0-5 0.4

0.6

0.7 0.41 0.59 0.68 0-46 0-43 Blade Outline Shape T₀ -tz c 0 cn "E 0 ir,

c

₀ rn 12, -0a_c co cn 12_m -0

_c

03 cn )=. 2

z

a

:-. 4') 70

3

cl. >. j 0.)<1.) > 73

3

V, -ca C CI rin 77 m C 0 on Boss-Diameter d/B 0-167 0.167 0-167 0.167 0.167 0.167 0-167 0.20 0.30 Ratio

Model Screw Diameter

D = 0.7 feet

Number of Blades

Z = 4

Pitch Ratio (Maximum) Pr/D =- 0.85

Pitch Ratio (Mean) Pm/o = 0.837 bA

Blade ThicicneSs Ratio (axis) -- 0.045 D

Rake Aft t.1, = 10 degrees

CrT = tAID 0

=

Symbols AE Ao AE Ao c/D FT = aT J = VA nD KQ

= Q

pn2D5

KT=

T pn2D4 p PID PV

TABLE 2

Particulars of Ship Screw and Nominal Operating Conditions

NOTE: for other details see Ref. 8, Section 8.

TABLE 3

Screw Performance Comparisons

(Non-Cavittiting Conditions)

Screw Particulars Operating Conditions Free-Running Towing Diam. (feet) D 9.0 DHP 1,100 792 Number of blades Z 4 N (r.p.m.) 200 144 Blade area ratio AE Ao 0.5 _Vs (knots) 12-5 0 Pitch ratio P/D 0-853 J 0-557 0 Thickness ratio t/D 0-047 GR

059

GNI

-

1-21 Tu (tons) 11-8 . Free-Running

at constant ku at constant torqueTowing

Basic Screw Open Water Tunnel Open-Water

J = 0-56

J = 0-56

J = 0

BA 1)AE = 0.5

ku 0.515 0-500 ki. 0-350 ji-o BO 1) BD 1) d/D = 0 - 167 '11 0 628 0.609 kQ 0.0407 BA 2 % above N

0-5

1-0

% above

N

4.5 AE

= 0.4

Ao basic 7) 0-5 1-5 basic T 3.5 BA 3 % above N 0-5 0 % above N

2-5

AE

_{= 0.6}

Ao basic T1

0-5

0.5 basic T

3.5

BA 4 % above N 1-0 1-0 % above N

6-5

AE

0.7

Ao basic ri

4-0

2.0

basic T

6.5

BO 2 % above N 5

1.0

% above N 5-0 Narrow

Tip basic -21 0.5 4-0 basic T 3-0

BO 3 % above N 1-0 % above N

4-0

Mod-wide

Tip basic 1

2.0

basic T

5-0

BO 4 % above N 0 - 5 0 % above N

8-5

Wide

Tip basic 7)

2.0

2-0

'

_{basic T}

7-0

BD 3 % above

N

.3-0 %above

N

1-0

X - 0.9

X - 07

OC - 0.5

3C - 0.3

- 03

I I, . .

..,.TABLE 4

Screw Performance Comparisons-(Cavitating Conditions) 156 --7 --4-, -- - -

-BA I

BA

BA 3

BA-4

'

BO

3'

4 .6T3 GT cal BA 2 aE 0.4 0.350' _ 1.40

BA 3. aE

0.6 0.250 .-:0-4o5 _ 0.79 -. BA 4

aE = 0.7

0-210 0.70

-B02 narrow tip

0.410 127.. . BO 4. wide tip 0.180 0:60 '0.300

. _Screw No. Free-Itiamiag:

-Towing , -Basic Screw 1 BA 1) AE 0.5

BO-1) A°

F BD 1) d/D";= 0.167 I -crti =- 0-30 6-51 trf FT FT I

BD 2

BD 3

. GEOMETRIC , -FEATURES _

Fig. I

BO!

BQ

B04

0.50 0.45 0.40 0 35 0.30 025 0.20 015 010 0.05

111111

Ii II II lit ill LIII

11111 0.10 0-20 0.30 0-40 0-50

OPEN WATER EXPERIMENT RESULTS

Fig. 2a

060

070 080

070 0-50 0-60 0-45 0.50 0-40 J

OPEN WATER EXPERIMENT

RESULTS

Fig. 2b

to 4. 0-70 0.50 0-70 0.60 0.45 0.60 0-10 0.20 0-30 0.40 0-50 0.60 0-70 0-80 0.40 0-35 0-30 0-25 0.20 0.15 0-10 B D, 1 B 0.3 7-71 d,- 0.167

Fig. 2c

OPEN WATER EXPERIMENT

RESULTS 0.50 0.40 0-30 020 0-10 0 1 0-10 0-20 0-30 0-40 0.50 060 0-70 0-80

0.50 -IBA 3'

I'

--\

_\\

I 1 I 0.130 0.90 1 -0 WATER'TUNNEL 020 0-30 0.40 0.50, 0.60 .0.70

EXPERIMENT RESULTS7 ATMOSPHERE

Figr3a

. 4 13 0.5

O2

03

B02 0'70 70.40 -9.30 -0.20 I -I I

0-10

10 -so 0.50 -0-60 0 45 1 10.50 PRESSURE -k TOW ING 0 251 NE., - NARROW 'TIP' T-" VERY -WIDE TIP'

. 2.... 0.40 0-35 0.30 0 .20 0-15 010 0-05 0

,'\

0-20 FREE .,RUNNING 060 0.70 0e2 0-70 0-50 o a Jo 0-10

PAESSURE:-BA .BA 2 BA 3' BA 4

025

tr. 020

9.0

0040-0.085

0-030

0-025; 0-25 I i II

020. 0 40

060

680

100

cr;d,

0.20

040

0.60

0.80 b.

WATER TUNNEL EXPERIMENT RESULTS

_CONTROLLED

_PRESSURE

'-Fig. 4a

10 .20

0 -15

0 10

9- 0

7 0

60

0 045

0.040

40 TOWING BO I

BO 2

---4T/4°

4

Q

0-20

0.40

0-60

0.80

1.00

FREE RUNNING BO 1 BO 2 BO 4

J - 056

C:57,1 C311

-0.60

40 I

II

I

020

0-40

0.60

0-80

1-00

WATER TUNNEL EXPERIMENT RESULTS

CONTROLLED

PRESSURE

Fig. 4b

160

0.80

O70

050

0-030

0.025

0.05

0.035

0-030

0.025

0 30

0.25

0-20

015

//

//---I

4

I I

ToDr. Townsin

Dr. Townsin's remarks about the research

effort at Ship Division are most welcome. In

case the present paper gives the impression that calm water performance of ships is the

dominant research topic, I would like to point

out that the field of sea-keeping research is also

very active. Within this context, work has

been carried out on the reduction of catamaran ship motions and the measurement of loads

in the cross structure spanning multi-hulls: However, it was not appropriate to include these most important aspects in the present

paper.

In the past, model resistance tests were largely limited to the measurement of total

resistance, with frictional components being

corrected to the ship scale using empirical

formulae. In the last few years direct measurements of the componentsofresistance

have become increasingly common at Ship

Division. As experience grows, it will

undoubtedly be possible to draw upon this knowledge when considering the geometry and performance of both conventional and

unconventional ship designs.

I am pleased to note that Dr. Townsin

agrees with my comments regarding the

desirability of project studies into multi-hulled

ships. I would like to take this suggestion a

step forward and suggest the desirabilityof

having an advanced projects group, whose

terms of reference were the overall technical

and economic study of novel or

unconven-tional marine transport systems.

Such a

group could certainly help to

pin-point regions where hydrodynamic studies could be most rewarding. The trimarans and separable ships, container carriers, side-wall hovercraft

and vessels for exploring hydro-space that Dr. Townsin mentions could be evaluated

economically prior to any extensive research programme at NPL.

As mentioned in the paper, the extension of the calm water analysis method to sidewall

Hovercraft is now straightforward. In fact, it was the requirement to be able to handle

this case which lead to the development of the new Hovercraft wave drag theory described in reference 10. The use of these methods in any consideration of Hoverships seems to be very appropriate.

ToMr. Parker

I would certainly like to acknowledge the useful discussions I had with Mr. Parker at

the beginning of this study. The interest

of

BSRA in multi-hulled ships has been one of

the major influences in the development of

this programme, and our association has

recently extended to consider structural loads in multi-hulled ships. As Mr. Parker suggests, my lack of reference to this work was quite

deliberate, since it had not

then been

published.

With regard to the trimaran layout, I am

quite sure that favourable wave interference

can be obtained whether the centre hull is

leading or trailing- relative to the side hulls. In fact, within the limitations of linear wave-making theory, forward or rearward motion

of a body will produce the same wave

-resistance.

ToMr. Williams

Mr. Williams' contribution is most interesting;

it

is clear that he has experience of the

complicated hydrodynamics of multi-hulled ships. The influence and relative importance

of viscous and wave resistance undoubtedly varies with Froude number, and these factors,

together with structural and manufacturing

considerations undoubtedly are of importance in deciding hull separation and whether hull asymmetry should be employed. At Froude numbers where wave interference is predicted

as unfavourable, the adoption of some out-ward camber on the hulls probably reduces

total resistance by minimising viscous

resis-tance between the hulls. Where the Froude

number is such that beneficial wave interfer-ence is anticipated for symmetrical forms, the

increase of viscous resistance between the hulls may be acceptable. In this context it

should be noted that frictional terms will tend

to reduce in terms of total resistance, when

extrapolating to ship scale.

In any discussion of the seakeeping of

multi-hulled vessels, a wide range of design possibilities must be borne in mind. With the

large roll stability, inherent in this class of vessel, it would be possible to design with

considerable freeboard and a large cross

structure clearance above normal water level,

albeit, with some penalty in structural cost

and weight. Also the adoption of bulbed forms with fine load waterlines would be

possible, resulting in reduced pitching motions

without unacceptable roll stability problems.

The application of high aspect ratio foils between hulls, which would be difficult to

mount on a single hulled vessel, have already been shown to reduce pitching motions. The field is wide, and I would like to assure Mr. Williams that NPL are aware of the

possibi-lities.

The present systematic study has not been extended to consider propulsive performance; however, specific projects have been investi-gated experimentally. I am inclined to agree with Mr. Williams' points when comparing a

single hull vessel with twin screw catamarans.

As a further point, the wide separation of a catamaran's screws will result in excellent

manoeuvring properties..

The application of catamarans in the Baltic is interesting, and is perhaps associated with

the relatively calm sea conditions which I

imagine are found on the shorter sea routes. The associated project studies at SSPA again

accentuate the difficulties implicit in the phrase

"the equivalent single hulled ship". The pay-load density will vary depending on the duty

and route, and will determine whether the craft working that route are designed from

consideration of payload volume or payload weight. The suggestions that the catamaran total displacement could be less than that of

the "equivalent single hull ship" are most

interesting. The comparison shown in Fig. 1 appears at first sight to be a little unfair to the single-hulled ship from hydrodynamic

con-siderations, since the craft is low in comparison

with the catamaran. However, the effect may

have been introduced from internal layout

considerations. I understand that the

cata-maran data presented in the figures is based

on estimated values. If so, I would be most

interested to know what allowances have been

made for viscous interaction .effects and

propulsive coefficients, since these factors can

be very significant. Our own tank tests on

well designed single hull trawler models and

catamaran derivatives of these forms have

shown that quite large increases of resistance

are incurred by the catamarans at design

Froude numbers, both for symmetrical and

asymmetrical component hulls. Propulsion

tests on the catamarans have not yet been

made in this instance.

ToMr. Eckhard

I am interested to hear that the Calais-Douvre was built on Tyneside. I understand that this

vessel was paddle wheel propelled, the wheels

being mounted between the hulls. Having

observed the complicated wave system gener-ated in this region on catamaran ships I have

great sympathy with the designer in attempting

to achieve a good propulsive efficiency. I

understand he was not entirely successful;

perhaps he should have made model experi-ments first!

At present I cannot give any details of how multi-hulled ships would be connected, nor of how the cost and weight of this structure

would affect the economics of operation.

However, it is evident that the cross structure should have as high a clearance from undis-turbed water level as possible, and should not

extend to the bow if slamming on this

structure is to be avoided.

The correct placing of hulls in line astern is known to allow a saving of resistance when

compared with the sum of the resistance of the

component hulls running in isolation. For

similar hulls, a fore and aft stagger of half a transverse wavelength

is probably

appro-priate, (see Fig. 5 of the paper relating to the

use of stagger in trimarans); this result is probably not unreasonable for dissimilar

forms. The idea of operating one vessel on a line in front of a parent is novel, although on reflection it is closely related to the problem

of a tug towing another ship.

ToMr. Pringiers

Although studies had been made on a number. of projected catamaran ships,it was considered desirable to include in the paper only those for which both experimental and theoretical work

had been completed. Further consideration

had in fact been given to both trawler forms,

and also high speed displacement forms of the type that Mr. Pringiers mentions. (The

basic data for this latter study is contained in reference 8, which has now been extended in

the reference quoted below.) The variation

of wave drag interference ratio with Froude

number has been found to depend on hull type.

Evidence suggests that extrapolation of data for a conventional ship designed for operation at a Froude number of 03, to a design having a Froude number of 0.6, is very dangerous. Theoretical estimation of the wave drag of a hull fitted with a transom stern is also likely

to be inaccurate. A virtue of the proposed

superposition method is in fact its ability to

make wave drag estimates of multi-hulled vessels, provided wave pattern data for the

component hulls are available, regardless of their geometry.

The comparison Mr. Pringiers makes

bet-ween a catamaran and a single hulled ship

designed for operation at a Froude number of 0 - 6 is undoubtedly correct. As he points out,

the sensitivity performance to the loading

parameter M is fundamental to craft designed

for this speed. This fact is due to the rapid increase of wave drag with displacement,

which has been shown experimentally to be of the form

Rw

125

A = M1.85

leading to Rwsp1.62

for a Froude number of approximately

0 65-0 70.

(See references mentioned above). In fact, total'diag results at a Froude number of 0.6 Obtained from these references

supports the 30% saving for the catamaran

that Mr. Pringiers mentions. This conclusion is of course in the absence of allowances for interference effects between the hulls. Results contained in the reference below suggest that

the wave interference effect may be quite small at Froude numbers of 0.6 and above,

provided hull separations are in excess of 20%

of the length. However, experimental

con-firmation of these suggestions has not yet been

obtained. Adverse viscous interactions

bet-ween the

hulls will undoubtedly occur,

although no information on this effect is

available at present.

It may be of interest to point out that the

work of Marwood and Silverleaf which Mr. Pringiers references was carried out at Ship

Division NPL. The hull designs mentioned

in the reference below were in fact based upon an optimum form from this earlier study.

The work by Alexander and Beyer

refer-enced by Raft', is most interesting; unfortu-nately I have so far been unable to obtain the original thesis. In Raff's statement no details

of the hulls appear to be given beyond the

statement that they were symmetrical. Also,

although optimum values of separation are quoted, no statement is made regarding the

magnitude of the wave drag interference. It seems probable that wave drag interference

do'es not vary appreciably with hull separation

at high Froude numbers (except where

separa-fions are very low), and therefore that the

optima quoted in Raff's contribution are not critical. However, a distinction must be made between wave pattern resistance and residuary resistance, since it is clear from the reference

below that these two terms are far from

identical for high speed semi-displacement

craft.

To Mr. Steele

I am grateful to Mr. Steele for referencing the papers by Messrs. Eggers, Sharma and Ward,

although I understand that no results are

quoted which can be used for comparison

purposes with the present study.

The tendency for ships to increase in size in

recent years, and therefore to run at lower

Froude numbers has resulted in wave

resis-tance becoming a relatively small term in

comparison with total resistance.

Con-sequently an increasing volume of work is

being done in the field of viscous resistance,

and methods of measuring this resistance

component are being developed and improved.

However, for specialised duties, and in parti-cular passenger transport, there is a demand for high speed ships which can be built to be competitive with hovercraft, hydrofoils and aircraft. In this speed band, wavemaking

resistance is most important, and methods of reducing its magnitude should be developed.

Viscous pressure resistance induced by flow

separation on badly designed full forms can

be major component of resistance of single hull

ships. A corresponding catamaran having the same displacement, length and draft could be designed to avoid this loss, although clearly a large increase in frictional resistance would be incurred. (In this context, separation effects might be aggravated in shallow water.) In the

case of multi-hulled ships, it is unlikely that an

overall favourable viscous interference drag can be developed for hulls in close proximity, unless one includes the unlikely case of hulls

running in line astern. Experiments have

shown that the distribution of frictional resis-tance and pressure resisresis-tance across a hull is significantly modified in the presence of the

second hull, but the overall effect has been

to increase resistance.

The discrepancy between measured and

predicted total resistance in fig. 11 is

undoub-tedly associated with observed differences

between measured and predicted wave resis-tances (see figs. 13 and 14). The difference

in phase between the curves, which is discussed

briefly in the paper, is not fully understood. Further experimental evidence indirectly

supporting these phase changes has been obtained by reference to wave elevations

downstream of the model (see fig. 15). The

improved prediction

of total

catamaran

resistance at high Froude numbers is undoub-tedly associated with an improved prediction of wave resistance at these Froude numbers.

Some reduction of viscous interaction may

have also occurred at high Froude numbers, although a significant reduction seems unlikely.

I have little experience of the calm water

performance of trimarans, since no models have been tested in support of the present

paper. It seems probable that hull separations of realistic multi-hulled ships will be less than

calm water predictions suggest would be

D34

optimum. It follows that such ships will tend

to suffer adverse viscous interactions, and

those having the largest number of hull will presumably suffer the greatest loss.

Mr. Steele's last question is very difficult to answer, for it is extremely hard to generalise

for wide ranges of design speeds and loadings.

In the paper I quoted that "on balance, it

appears that a catamaran design can offer a

calm water performance improvement only where the single hulled ship has considerable

wavemaking and viscous pressure

resis-tances". However, I deliberately chose not to define what was meant by "considerable". It is possible to make simple calculations as to

how small the purely frictional component

of resistance has to drop to make a

cata-maran design attractive from the point of

view of calm water performance. However,

even such simple calculations

require a

knowledge of the breakdown of resistance into frictional, viscous pressure and

wave-making resistances, and also the likely

improvements available from a finer hull

form after making allowance for interference effects between hulls. Calculations that I have

made suggest that catamaran ships become

more efficient where the single-hulled vessel has a frictional resistance less than approxi-mately 40-45% of the total calm water

resis-tance. As will be appreciated, this criterion

tends to rule out the application of the

catamaran to the majority

of

commercial

vessels, assuming that the "equivalent single-hulled vessel" has the same displacement and length and also that calm water performance

considerations are dominant. High speed craft, and also possibly vessels running in

shallow water, appear to be capable of

economic operation

as catamaran ships.

Reference is made in the paper to a note on the breakdown of the resistance components of high-speed semi-displacement type hulls in

this context (Reference 8). (See also Mr.

Pringier's contribution.)

Reference

1. J. T. EVEREST and D. BAILEY, The Wave

Resistance ofHigh Speed Semi-Displace-ment Type Hulls and its Influence on the

Design of Unconventional High Speed Craft. Contribution to Panel Discussion,

Mr. M. N. PARKER, Asssociate Member:

It may not be appreciated by those that do

not have to pay for them, that in model

experiments it costs about the same amount of money, to make the model screw as it does to make the model hull, and this has been Mr.

O'Brien's main motivation in developing

these corrections that can be applied to 'stock-screw' results and thus save having to make a new model propeller for each ship.

There is another way in which very good use can be made of these corrections, which

Mr. Leathard has already referred to in

connection with the design of propellers from Bp delta diagrams. Normally, when getting out a powering estimate in the. prelirninary

stages of design, people are fairly careful about

evaluating an accurate estimate of the resis-tance properties of the hull using one or other of the sets of methodical series data that are available or some other estimating procedure. Having got so far, they then either assume the propulsive coefficient is going to be 0.68, or they use some relatively simple formula for evaluating it.

We have found at B.S.R.A. that it is very often worth while at this stage, reconstructing the propulsive coefficient (QPC) from

open-water propeller charts and wake and thrust

deduction formulaticins. This is particularly important in the sort of studies which involve parametric variations because here changes in propeller revolutions may not show up if you

are merely using a simple formulation. In merely using a simple formulation. In this sort of work it becomes very important to make the corrections that Mr. O'Brien has provided us with the means to do. On first

glance at the paper it seems to be % here,

there, and one may well ask if this is worth

bothering about. It often turns out, however, that you find most of your propulsive effici-ency is made up of % here and % there, and either they are all going the one way or some of them may cancel out. This is an application we have found for the very valuable series of papers which Mr. O'Brien has been producing over the years, which was, I think, not their

original purpose, but still it has been very

useful.

Mr. M. MOUSSOUROS, Student:

It was mentioned in the reading there was a

mistake on page 151, I think this alleged

mistake concerning FT value is not really a mistake, because although in the abbreviations

in the middle of p. 154 FT is defined as T1

if you look on p. 156 table 4 and you read the values, it should be as in fact the author has written it on p. 151, it is just that the division

is missing.

This paper deals with the experimental results obtained by the variation of certain parameters of an NPL propeller standard family. It is important to note that the case

of retaining the same EAR by changing the

number of blades and the corresponding

chord lengths, although significant, has not been dealt with. It would be interesting if the author would comment on such a question.

Limited computer theoretical calculations,

however, recently carried out at the University

of Newcastle upon Tyne revealed that the

ideal "Lerb" design efficiency increases as the number of blades goes up from three to six, in the absence of cavitation of course.

The FT approach of correcting is interesting

and the author is invited to comment why it

has been based on thrust rather than on

torque breakdown values. This paper has once more experimentally reconfirmed the

advantages of widely tipped blades from the point of view of delaying cavitation inception

considerably (See L. C. Burrill & A. Emerson:

Propeller Cavitation, Transactions of this

Institution, April 1963).

Moreover it is the opinion of the speaker

that it is imperative that a Research

Pro-gramme should be established in dealing with the effect of variations in camber in propellers for which the hydrodynamic pitch has been priorly optimised. Such experiments will be invaluable since they might give some idea of the character of the chordwise load distribu-tion and will almost certainly push forward theoretical methods which are largely in need of experimental verification of the truth of the flow conditions predicted by theory.

Finally, just from the point of view of

interest, I just noticed that Mr. O'Brien had

defined this correction factor C at the very

beginning as C = C1

1 + k (x 0.2) and

I was wondering why in fact this has been based on a boss diameter of 0.2 while the basic screw is having a boss diameter

non-dimensional ratio of 0-167?

Dr. F. I. LEATHARD, Associate Member:

With this paper Mr. O'Brien adds to his

previous work on the effects on marine pro-peller performance due to variations in section shape, blade thickness and number of blades.

Propeller designers will welcome this new data

as they did the results from the earlier

investigations.

The results of the geometrical variations

are particularly useful when designing a pro-peller using a Bp - delta diagram, as so often is the case, the required features of the new design are slightly different from that of the family of propellers constituting to the data of the Bp - delta diagram. The correction factors

DISCUSSION ON

Some Effects of Variation in Blade Area,

Blade Outline and Boss Diameter on

Model Screw Performances*

given allow for such deviations in surface area

blade outline etc, to be accounted for.

With the increasing use of controllable pitch propellers and their associated large boss diameters, data relating to changes in

this particular feature are of current interest.

In the case of propellers for large single

screw ships it

is desirable to stagger the

development of high loading on consecutive radial sections of the blade as they pass into the local high wake regions, if vibration and noise are to be kept to a minimum. This may be achieved by (a) modifications in the distri-bution of blade chord widths or (b) by varying the skewback or throwround of the blades.

Incidentally this latter feature of propeller blade geometry is one on which Mr. O'Brien

has not spoken.

The correction factors given in this paper

are derived from experiments on model

propellers with simple geometric variations from a basis screw.

Unfortunately changes in surface area and blade outline involve variations in other fea-tures, namely the section camber and thus the effective pitch, hence it has been necessary to

make corrections to the basic experimental

results to enable comparisons at equal thrust

coefficients. When planning this series of

tests, was consideration given to the

compari-son of results from propellers designed to

develop equal thrust at the given service

condition. This would have obviated the need to correct the basic experimental result thus allowing a more direct comparison.

In the section concerned with the variation of blade outline, the blade area ratio has not been kept constant, so that in comparing the

results of BO 2, 3 and 4 with BO 1, the

differences in speed of rotation and efficiency are not due solely to outline variation.

Assuming there was reason for not keeping the blade area constant, then why not compare

the results of propellers BO 2, 3 and 4 with the

propellers BA 2,

3 and 4 which have

compatible blade area ratios.

The results given are essentially factual and in the case of the free running conditions the

trend in results agree with our existing

knowledge.

From experiments conducted some time ago

at Newcastle University tunnel, on a four bladed series of 16 in. diameter model

pro-pellers,

it was found that a reduction in

B.A.R. from 0.59 to 0.44 resulted in an

efficiency increase of just over 1 % with the

propellers in a free running condition. In a

cavitating condition the reduction in B.A.R.

resulted in a thrust breakdown factor of

FT = 1 . 29, these figures tend to confirm some

of the results given in this paper.

Presumably the cavitation tunnel results

given are corrected for channel wall effects yet