Aspects of marine propulsion and applications to high powered vessels

-7-;.7,4, .1 Awn

A Station of the

Ministry of Technology

'

,

:SHIP REP. 111

December 1967

Uit

NATIONAL PHYSICAL

LABORATORY

SHIP DIVISION

ASPECTS OF MARINE PROPULSION AND

APPLICATIONS TO HIGH POWERED VESSELS

by

T. P. O'Brien

Crown Copyright Reserved

Extracts from this report may be reproduced

provided the source is acknowledged.

Approved on behalf of Director, NPL by

ASPECTS OF MARINE PROPULSION AND

APPLICATIONS TO HIGH POWERED VESSELS

BY

T. P. O'BRIEN

Aspects of marine propulsion and

applications to high powered vessels

In this article, T. P. O'Brien, CGIA, CEng, MRINA, of the Ship Division, National Physical Laboratory, refers to topics which need to be

considered in designing marine propellers for high-powered vessels. It also summarises results of research work at the NFL on effects of

varia-tion in operating condivaria-tions and geometric features of convenvaria-tional single screws. It discusses research work at DTMB on comparative per-formance of twin-screw and single-screw vessels, the former propelled by conventional screws the latter by both conventional single screws and contra-rotating propellers. It indicates some of the factors which need to be considered in designing contra-rotating screws for

turbine-driven vessels. It gives some comparative propulsion data for two turbine-powered passenger vessels, one with twin conventional propellers the

other propelled by a single pair of contra-rotating screws. These results indicate that while the contra-rotating screws would show a reduction

in propeller efficiency of 23 per cent there would be an increase in propulsive efficiency of 14 per cent.

Recent developments in the field of marine propulsion include those relating to hull form, power units, conventional screw pro-pellers and alternative propulsion devices. For instance, the increas-ing sizes of modern tankers and the correspondincreas-ing higher powers needed to propel them has stimulated research on: reduction in hull

resistance and improvement in hull performance; increase in operating powers of conventional power units; availability of

alternative power units; increase in operating range of conventional screw propellers and availability of alternative propulsion devices. Similarly, the increasing speeds of modern passenger vessels has

stimulated research not only on the foregoing topics but also on the comparative performance of single- and twin-screw vessels.

Recent work at NPI, on the design of hull form and propulsion of merchant ships is discussed in the recent paper by Silverleaf and

Dawson (Ref. 1). The results of recent work at DTMB on the

propulsion of single- and twin-screw vessels propelled by conven-tional screws and alternative propulsion devices, including contra-rotating screws, are given in a paper by Hadler, Morgan and Meyers (Ref. 2). Developments in modern marine power units, including work by Abell and Butler (Ref. 3) on direct-coupled diesel engines,

by Moriarty and Schowalter (Ref. 4) on medium-speed diesel engines, and by Jung (Ref. 5) on marine turbine and gearing are discussed in the proceedings of a recent symposium held in the United States of America. A research on tanker propulsion at present being undertaken by the author was discussed last year

in an article (Ref. 6) and some of the results obtained, including

those for conventional screws and contra-rotating screws, are

given in a subsequent article (Ref. 7).

The object of the present article is to discuss aspects of marine propulsion, propulsion of single- and twin-screw vessels powered by alternative power units and propelled by conventional screws and contra-rotating screws, and to give some comparative propul-sion data for two turbine-driven passenger vessels, one propelled by

twin conventional screws the other by a single pair of

contra-rotating propellers.

Aspects of marine propulsion

A marine screw propeller operates by converting the greater

part of the power transmitted to it from the power unit (the delivered horsepower dhp) into a thrust horsepower thp. The delivered

horse-power is transmitted in the form of a torque Q at a shaft rate of

rotation n. The thrust horsepower is applied in the form of an axial thrust force T when the propeller operates at a speed of advance VA in propelling the vessel at a speed Vs. The delivered horsepower and thrust horsepower are defined by

27mQ _TVA

(1) dhp and (2) thp

550 550

where the powers are in British units:

is the rate of rotation in revolutions per second

VA is the speed of advance in feet per second

is the torque absorbed in pounds feet is the thrust applied in pounds

The screw efficiencyn is the ratio of power applied to power

absorbed. It is given by:

thp TVA

1

dhp 27mQ

The speed of advance of the screw VA and the speed of the hull Vs can be linked by the Taylor wake fraction WT which is given in the form

VA =-(1 WT) Vs

A given marine vessel requires a certain amount of power to

propel it at a specified speed. If the screw were removed and the hull were towed instead of being propelled, the force required to

tow the hull at a given speed would differ from the thrust that

would have been applied by the screw at the corresponding speed of advance. This is due to the fluid flow around the stern of the hull

affecting the performance of the screw. The power required in towing the hull is termed the effective horsepower ehp which is

defined by

RVs

ehp =

550

where R is the force in pounds required to overcome the resistance of the immersed hull and the air resistance of the superstructure

Vs is the speed of the hull in feet/sec.

The thrust horsepower and effective horsepower can be linked by the hull factor 414 defined by

ehp =411 thp

The propulsive efficiency Tip (or quasi propulsive coefficient QPC)

is the ratio of effective horsepower to delivered horsepower: it is

given by ehp RVs p =

=

dhp 27mQ also thp

11p=}1=tH

n dhp

The screw efficiencynowhen operating in uniform flow in open

water generally differs from the screw efficiency TIE when operating

in non-uniform flow behind the hull. This difference in efficiency can be expressed in the form of a relative flow factor tR defined by

TIB= 4R 10

If it is assumed that the thrust has the same value in both uniform and non-uniform flow then the difference in efficiency is reflected in a difference in torque which can also be expressed using the relative

flow factor as follows: tR QB=Qo and

R dhpw=dhpo

also, equation 8 can be re-stated in the form

T1P=tx 11B=

where QB and dhpB are the torque and delivered horsepower (in non-uniform flow behind the hull)

Qo and dhp, are the torque and delivered horsepower (in

uniform flow in open water)

4p is the overall hull factor linking screw efficiency go and

propulsive efficiency lip

Various charts and coefficients are available which can be applied in selecting the geometric features of marine screws. These include the B-6 and the k-J systems.

The B-6 charts are of a form convenient for selecting the most suitable diameter of a screw to operate at specified power, speed of advance and rate of rotation, and when this is done the pitch ratio

and screw efficiency can be determined. The coefficients are given by

ND

TABLE I.PERFORMANCE OF STEAM, DIESEL AND TURBINE-POWERED VESSELS

can be manufactured. Until recently the maximum value was about 25 feet. However, propellers of up to 30 feet in diameter can now be made and it is probable that manufacturing facilities will shortly

be available for larger propellers. For heavy-duty screws (those

designed for either high-powered or heavily-loaded conditions) the blade area ratio (ratio of developed area of face of blades to area of disc defining screw diameter) needs to be large from cavitation

considerations. Moreover, the relation between the blade

thick-nesses and blade widths needs to be assessed with regard to strength. The number of blades can vary from three to six. Consequently, it

might be impracticable to locate either three wide blades or six narrow blades in relation to the available size of the boss. Most

modern propellers are made in nickel aluminium bronze and some

of them are of manganese bronze. Since the former is the more strong and durable its use leads to thinner (and lighter) blades.

Consequently, this factor is significant in relation to any

manufactur-ing limitations.

Stipulations concerning geometric features ofthe screw.These

are diameter, number of blades, polar moments of inertia. Clearly,

these topics are closely related to those already discussed. For instance, the diameter may need to be selected to suit the shaft speed of the power unit and it may need to be restricted from manufacturing considerations. Moreover, it may also be limited due to aperture size (single screws) or hull-tip clearances (twin

screws). It may also be restricted in relation to its polar moment of inertia and to effects of vibration. Similarly, the number of blades

may also be chosen in relation to manufacturing considerations

and it may be stipulated due to effects of vibration.

The effects of variation in number of blades, variation in screw diameter and rate of rotation have been studied as part of a research on the propulsion of large tankers. The object of this work which is discussed in the article (Ref. 6) is to provide propulsion data and propeller design data for both conventional screws and alternative propulsion devices. Results so far obtained include design calcula-tion and propulsion estimates for seven convencalcula-tional screws and a

pair of contra-rotating screws. The basic screw was designed to

absorb 22,000 dhp at a rate of rotation of 110 revolutions per minute.

The screw diameter D, rate of rotation N and number of blades B were stipulated and the respective values were D=23.25 ft, N=116 rev/min, and B=4. The other conventional screws comprised two groups and a single screw. The first group of three included varia-tions in number of blades and rate of rotation. The second group of two included variations in number of blades and small variations

in diameter. The remaining screw was designed to operate at a

lower rate of rotation and had a larger diameter than those of the

basic screw. The geometric features and performance data are

listed in Table 2, overleaf

Propeller-hull interaction effects(A)Basic values of

pro-pulsive factors (wake fraction w.r, relative flow factor tB, hull

factor tH, overall hull factor p and propulsive efficiency rip)

for a marine hull propelled by a screw propeller. (B)Effects of

variation in propulsive factors due to variation in underwater hull form and fluid flow around the hull, location of screw relative to hull, geometric features and operating conditions of screw. Some

typical values obtained from the results of research on tanker

2 Type of vessel Machinery Delivered horsepower, dhp Rate of rotation rev/min Screw dia. (feet) 10.25 Screw effi-ciency -no 0.665 Steam 1,100 140 Tug Diesel 1,100 160 9.00 0.627 Diesel 1,100 200 9-00 0.618 Trawler Steam 1,455 136 11-5 0.70 Diesel 1,455 250 8-4 0-625 Tanker Diesel 22,000 110 23.25 0.485 Turbine 22,000 75 29.50 0.575 Passenger Turbine 39.200 116 22-00 0.690 Turbine 39,200 86 24-25 0.730 (13) 6= V. N

RdhpB N

dhp.

(14) S V. Va2A V.2- S Va thp

Bu=---\ S Va

where N is the rate of rotation in revolutions per minute

V. is the speed of advance in knots

S is the specific gravity of the fluid

The k-J coefficients are widely used for the presentation of open

water experiment results. Two of them (ku and J) are of a form

which enables the optimum value of the rate of rotation for a screw of given diameter operating at a stipulated speed of advance and applying a stipulated thrust. The coefficients are given by

VA

J=

nD

ku---r A2D2

In making performance estimates for a screw under cavitating conditions it is desirable to use two coefficients, one related to the forces loading the screw the other linked to the pressures affecting

the fluid around the screw. The effects of the forces can be expressed

in terms of the thrust loading coefficient ku since for a given speed of advance its value is directly proportional to the thrust per unit

area. The effects of the pressures can be expressed in terms of a

pressure ratio or cavitation number defined by

p-e cra =

VA 2

2

where p is the static pressure at the screw axis

e is the saturated vapour pressure of the fluid

Conventional single screws

Significant factors affecting the ultimate performance of a marine screw propeller operating behind a marine hull include: (1)

restric-tions in the operating condirestric-tions of the power unit; (2) propeller

manufacturing limitations; (3) stipulations concerning geometric

features of the screw; and (4) propeller-hull interaction effects.

These topics and some typical comparative performance values are summarised below.

Restrictionsin the Operating Conditions

of

the Power

Unit

Type (e.g. steam, diesel, turbine). Transmission (directly driven or

geared). Limitations in shaft speed (stipulated range for geared diesel, minimum value for turbines). Some typical performance

values as given in the publications 6, 10, 11, 12 are listed in Table 1, on this page.

Propeller manufacturing limitationsThese are screw

dia-meter, number of blades, blade width, material and weight. For some

large vessels, in particular large tankers, it may be necessary to

TABLE 2.LARGE TANKERSSCREWS 1 TO 9GEOMETRIC FEATURES AND PERFORMANCE DATA

propulsion (Ref. 6) are listed in Table 3 below.

In designing marine screw propellers and making estimates of their performance it is necessary to include propeller-hull interaction

effects. This is of particular importance in studying effects of

variation in geometric features as some of the results obtained from

the research on tanker propellers6 have shown. For instance, in comparing the performance of Screw 1 (of moderate diameter operating at moderate rate of rotation) and Screw 5 (of large diameter operating at low rate of rotation) on a basis of screw

efficiency, a difference of 18 per cent was obtained. However, due to the variation in geometric features, in particular the screw dia-meter, there was a corresponding variation in propulsion factors, in particular the hull factor. Consequently, the increase in screw efficiency of 18 per cent was accompanied by an increase in

propul-sive efficiency of only some 10 per cent. Single and twin-screw vessels

For some high-powered single-screw vessels the limiting conditions

are now being reached at which the maximum power can be absorbed by a single conventional screw. This has stimulated

3

Delivered horsepower 22,000 DHP

research not only on the comparative performance of single- and twin-screw vessels but also on alternative propulsion devices. The results of work at the David Taylor Model Basin, given in the paper

by Hadler, Morgan and Meyers (Ref. 2), include resistance and

propulsion data for single- and twin-screw ships and performance

data for conventional single screws, tandem screws and contra-rotating screws, some of which are reproduced in Figs. 1 to 6.

Resistance and propulsion data for a twin-screw vessel and a

single-screw one, both propelled by conventional single-screws, are shown in

Figs. 1 and 2.

Significant aspects which need to be considered in assessing the comparative performance of single- and twin-screw vessels include: Performance of hull alone. Change in dimensions,

displace-ment, hull form, effects of appendages (e.g. shaft bossings or brackets)

due to fitting a twin-screw machinery installation in lieu of a single-screw one. It is not proposed to consider these matters in the present article but it should be noted that they can have a significant effect on the ultimate results obtained.

Performance of screw alone. For the single-screw vessel all the

power would be absorbed by one screw. For the twin-screw vessel,

TABLE 3.LARGE TANKERSPROPULSION FACTORS AND BASIC SCREW PERFORMANCE DATA

Hull Screw No. of blades Rate of rotation Trial

speed Geometric features

Diam. D

(ft)

Blade area ratio

aE Pitch ratio P Thick ratio T B NF rev/min (knots)Vs I 1 4 110 7 23-25 0-625 0-770 0.0550 1 2 5 110 7 23.25 0-700 0-750 0-0520 1 3 6 110 7 23-25 0-780 0-740 0-0500 I 4 6 100 7 23-25 0-800 0-840 0-0470 2 5 4 75 7k 29-50 0-530 0.820 0-0490 1 6 3 110 7 24-00 0-515 0.720 0-0595 1 7 5 110 7 22-50 0.690 0-790 0.0545 3 8F 3 110 7 20-80 0-350 0-675 0-063 3 9R 3 110 7 18-70 0-390 0-810 0-060 Performance data

Screw Propulsive Percentage increase

efficiency efficiency above Screw 1

in in TI 0 TI P 110 11P I I 4 110 7 0.485 0-703 1 2 5 110 7 0-485 0-703 0 0 I I 3 4 6 6 110 100 7 7 0-475 0-490 0-689 0-710

1k

1 1

If

i

2 5 4 75 7k 0.575 0-770 18 10 I 6 3 110 7 0.500 0.725 3 3 1 7 5 110 7 0-485 0-703 0 0 3 8F 3 110

71

0.495 2 3 9R 3 110

7f

Hull Screw Series Dia. D (ft) Trial speed Vs (knots) PROPULSION FACTORS

Wake fraction Hull factor Relative flow factor Overall hull factor

WT 1-1 I'l

4

1 A 23-25 17 0.43 1-42 1-02 1-45

2 B 30.00 17 0-38 1-28 1-05 1-34

2 C 30-00 17k 0-38 1-28 1-05 1-34

SCREW PERFORMANCE DATA

Rate of rotation Screw efficiency Propulsive efficiency

Available effective horse/power NF 10 Tip EHP1 (rev/min) I A 23-25 17 110 0-480 0.695 15,300 2 B 30-00 17 75 0.575 0-770 16,950 2 C 30-00 17-k 75 0-583 0-180 17,150

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would be reflected in a reduction in propulsive efficiency. Some

typical performance values as given in the paper (Ref. 2) arelisted

in Table 4 below.

The above comparisons clearly show in which way the changesin

screw efficiency and hull interaction effects are reflected in changes in propulsive efficiency. In particular, for the twin-screw vessel the

screw efficiency rio and propulsive efficiency 1p were0.66 and 0-65,

respectively. However, for the single-screw vessel the corresponding

values were n=052 and np=0.685. Consequently, althoughthere

was a reduction in screw efficiency of 20 per cent this was

accom-panied by an increase in propulsive efficiency of 5 per cent. Contra-rotating screws

In addition to conventional single screws, alternative propulsion devices are available for propelling marine craft. One of these is the contra-rotating screw propeller which, as the name suggests,

con-sists of a pair of screw propellers both rotating about the sameaxis

but in opposite directions. The sciews are situated a shortdistance

apart and each one affects the performance

of the other. For a

conventional single screw energy losses occur in the race and the

rotational losses are significant. The addition of a rear

contra-rotating screw results in some of the losses being regained and for

SPEED COEFFICIENT J

Fig. 5: Characteristics of the pairofcontra-rotating propellers illustrated on the next page

TABLE 4.-PROPULSION DATA FOR TWIN AND SINGLE SCREWVESSELS

the corresponding pair of contra-rotating screws the rotational

losses are small.

There are, of course, practical problems associated with the arrangement of co-axial shafts and gearing and these need to be

considered in relation to the problem as a whole. A shafting

arrangement for a turbine-driven contra-rotating installation is

shown in Fig. 7.

Significant aspects of the comparative performance of

contra-rotating and conventional screws are summarised as follows: For moderate sizes, power and speeds the adoption of a pair of contra-rotating screws in lieu of a single conventional propeller would probably result in a small gain in screw efficiency but the

transmission losses would be greater. However, due to more

favourable hull-propeller interaction effects the corresponding gain in propulsive efficiency would be greater.

For large sizes, high powers and speeds the loading conditions are such that for a single screw the diameter, blade area ratio and

weight of screw are restricted by practical considerations. An

alternative conventional twin-screw installation while offering a practical solution would result in higher operating costs, adverse hull-propeller interaction effects and resulting loss in propulsive efficiency. The adoption of a contra-rotating screw system offers an

COUNTERROTATING PROPELLERS 4105 & 4106 14 MAY 1964 I , , Re = 4. 37 x 10 e KqF

_=4.142j

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, Vessel Propeller Screw dia. (ft) Rate of rotation (rev/min) Wake fraction w-r Hull factor 41 Screw efficiency 110 Propulsive efficiency 11P

Twin-Screw Convent'l screws 22-0 115 0155 0.995 0460 0.650

Single-Screw Convent'l screw 25-0 102 0.360 1.330 0-520 0-685

Contra-roeg screw 22-5 1021 0 440 1 470 0-608 0.735

210

97f

alternative solution, however, since the loading would be.haled;

and accordingly the resulting diameters, blade area ratios and weights

would be reduced and he Within, practical limits. Further, gain's in propulsive efficiency Would result from the improved hull propeller interaction effect.S. Clearly, for such vessels contra rotating screws

offer advantages.- -_

: Before discussing the comparative performance of contra

rotating and conventional single: screws it is desirable to define in Some detail some of the fundamental differences between the way in which they operate, and this can be done by considering the way in which a blade sectional element of a screw propeller operates. In addition to the nominal axial and angular velocities which have the

same values as the speed of advanceVAand speed of rotation wr

(where W is the angular velocity and r is the radius of the blade sectional element), there is an induced velocity which can be

resolved in 'axial and angular directions in the form of axial and angular-velocity factors.

Fora pair of contra-rotating screws the induced velocities of one

affect the performances cif the other and the way in which each screw

is affected is as follows:

For the:frontserew the axial velocity is increased but-the:angular velccity is unaltered. Accordingly, the speed of advance is greater than that of a cOnventional single screw.

For the rear screw -both the axial and angular velocities are in-creased. Accordingly; both the speed of advance and rate of rotation, are greater than those of a conventional single screw.

A convenient procedure for designing. contra-rotating screws is

Fig.6: Contra-rotating

propellers,4105-6, run in

the water tunnel for the

heavy displacethent propulsion condition

-that given by van Manen and SentiC (Ref. 8) and in applying this procedure the appropriate values of speed of advance and rate of

rotation are modified by applying contra action factors. Typical

Values obtained in making the design-. Calculations for a pair of

chntra-rotating.screws for a large tanker (Ref: 7) are listed in Table ,5

'beldw...

TABLE 5.BASIC ,DESIGN PARAMETERS FOR CONTRA ROTATING SCREWS Screw Conventional Single Contra-Rotating f Front Pair Rear Speed of Advance . knots 10.2 13.7

Resistance and propulsion data given in the Paper (Ref. 2) for a

single-screw vessel propelled by a pair of contra-rotating screws, and

the-appropriate screw performance data-and visual observations, are. reproduced in Figs. 3 to 6 and in Table 4.

Significant aspects of the comparisons made in Table 4 are: as

follows:. Rate of ' Rotation rev/min 108 108 115 Fig. 7: Shaft arrangement for turbine-driven contra-rotating propeffes

21 Knots

22 Knots!

TABLE 6.-COMPARATIVE PROPULSION DATA FOR TWIN SCREWS AND A SINGLE PAIR OF CONTRA-ROTATING SCREWS

For the twin-screw vessel the screw efficiency -no and propulsive

efficiency Tip were 0.66 and 0.65, respectively. For the conventional

single-screw vessel the corresponding values were no =0.52 and

np=--0-685, respectively. For the single-screw vessel propelled by

contra-rotating screws the corresponding values were tl0=--0.608 and

1p=0.735. Accordingly, the adoption of a pair of contra-rotating screws in lieu of a conventional twin-screw system would result in

a reduction in screw efficiency of 8 per cent and an increase in

propulsive efficiency of 13 per cent. However, if the contra-rotating screws were adopted in lieu of a conventional single-screw there

would be an increase in both screw efficiency and propulsive

efficiency and the corresponding increments would be 17 per cent and 7 per cent respectively.

Comparative performance data for high-powered vessels It is required to make a preliminary assessment of the performance

of a turbine-powered passenger vessel propelled by a pair of

contra-rotating screws. This vessel is the sister ship of a twin-screw vessel for

which the design calculations and propulsion estimates are given in Section 11.3 of the book (Ref. 9). The propulsion factors are to be derived using the data given in the paper (Ref. 2). The screw per-formance estimates are to be made using the data given in the paper

(Ref. 8) and following the procedure given in the article (Ref. 7). The

geometric data and performance values of the pair of screws for the first vessel are available and these data are to be used as the bases upon which the comparative performance estimates are to be made.

Design data

Hull-passenger: length 680 ft., breadth 90 ft., draught, level 28 ft.,

block coefficient 0.64.

Required service speed 23 knots.

Engines-steam turbines; shaft horsepower 20,000 per screw, rate of rotation NF =105 revolutions per minute.

Stern details-streamlined rudder, shaft immersion 1=16 ft. Stipulations-maximum screw diameter 25 ft.

Design conditions-service shp =20,000 per screw, corresponding to dhp =19,200 (4 per cent transmission losses). Rate of rotation N=0.98 NF (2 per cent wake scale effect, see Ref. 9, Section 4-9).

Service speed 23 knots.

Geometric data and performance values for first vessel

Basic screws (four blades)-screw diameter D=2075 ft., blade area ratio aE =0.67, pitch ratio p =1-0, blade thickness ratio

-=0.058, screw efficiency no=0.69.

Basic twin-screw vessel-wake fraction w.r =0.17, overall hull

factor t p=0.99, propulsive efficiency gp=0.683

Performance assessments for contra-rotating propulsive system

The propulsion factors were derived from those of the basic twin-screw vessel by applying corrections based on the data given in the paper (Ref. 2). The screw performance estimates were made following the procedure given in the article (Ref. 7). The value of propulsive efficiency was derived from the corresponding value of

screw efficiency. The results are summarised in Table 6 above. Comparison of results

The results of the calculations given in Table 6 show that for the

basic twin-screw vessel the screw efficiency no and propulsive

efficiency Tip were 0.69 and 0.685 respectively, and for the single pair

of contra-rotating screws they would be no =0.53 and 0.78 respec-tively. This indicates that the adoption of a single pair of contra-rotating screws in lieu of a conventional pair of twin screws would result in a reduction in screw efficiency of 23 per cent; however, it

would also result in an increase in propulsive efficiency of 14 per cent. Acknowledgements

The illustrations (Figs. 1 to 6) are reproduced by permission of the David Taylor Model Basin, Washington, U.S.A. and of the Society

of Naval Architects and Marine Engineers, New York, U.S.A. Fig. 7 is reproduced by permission of the Stal-Laval Turbine

Company, Sweden.

The author is grateful to Dr. Wm. B. Morgan, Head of the

Propeller Branch, David Taylor Model Basin, and Dr. Ing. I. Jung,

Chairman, Stal Laval Turbine Company, for providing some

additional information.

References

SILVERLEAF, A. and DAWSON, J. Hydrodynamic design of merchant ships for high-speed operation. Trans. Royal Inst'n of Naval Architects, June, 1966. RADLER, MORGAN and MEYERS. Advanced propeller propulsion for high-powered single-screw ships. New York, Trans. Soc. of Naval Architects and Marine Engineers, November 1964.

ABELL, T. W. D. and BUTLER, J. F. The future of the large direct-coupled diesel engine. Philadelphia. Soc. of Naval Architects and Marine Engineers, Spring Meeting 1966. Paper No. 10.

MORIARTY, J. M. and SCHO WALTER, C. H. Applications of medium-speed diesels to marine propulsion. ibid. Paper No. 12.

JUNG, I. Swedish marine turbine and gear development. ibid. Paper No. 6. O'BFUEN, T. P. Some aspects of the propulsion of large tankers. London, Shipbuilding and Shipping Record, August 1966.

O'BRIEN, T. P. Contra-rotating propellers for large tankers. London, S. &S. R.., International Marine Design and Equipment 1967.

VAN MANEN, J. P. and SENTIC, A. Contra-rotating propellers. London, Trans. R.I.N.A., 1956.

O'BRIEN, T. P. The design of marine screw propellers. London, Hutchinson Scientific and Techniral Press, 1962.

O'BRIEN, T. P. The performance of three-, four- and five-blade screws in passenger liner application. London, S. & S. R., October 1965, 106, 481. (Reprinted in Ship Division Report 87).

O'BRIEN, T. P. Design of tug propellers. London, Ship and Boat Builder International, April, 1965, 18, 22, (reprinted in Ship Division Report 75). DOUST, D. J. and O'BRIEN, T. P. Resistance and propulsion of trawlers Trans. N.E. Coast Insen. Engrs. and Shipb'rs, 1959, 75, 355.

Vessel Screw dia. (ft) Rate of rotation (rev/min) Wake Fraction wT Overall hull factor tp Screw efficiency no Propulsive efficiency TIP

Basic (Twin Screw)

Single Screw (Contra-rotating)

2075. 20.9 116 105 0.17 0-45 0-99 147 0.69 0.53 0.685 0.780