On the propulsion of ships at sea

NORWEGIAN SHIP MODEL EXPERIMENT TANK

THE TEC NICAL UNIVERSITY OF NORWAY

ON THE PROPULSION OF SHIPS AT SEA

BY

JAN ,ORVIG

i

NORWEGIAN SHIP MODEL EXPERIMENT TANK PUBLICATION N°52

SynopsiS

,, 00000 AD; it* CHAPTER I

1.1 Introduction. P.44 11... .11'111* **

*1,-1.2

The

Ships'

00000

1.3

Presentation.

of

Propulsion Data 444,1.404040..4.:400,404400404000.44.0

Practical Coefficients %.* * 11. 000

The Records 'on* '1*. 111 11..* 11111.1.

CHAPTER II

2.1

Propulsion Theory * d, ol

Previous Methods oo

CHAPTER III

3.1

Characteristics of Ship Elements ... 000

3.2

Engine Torque Characteristics co.: .

3.3

Steam Reciprocating Engines The Engine of the "Simon Bolivar"

Diesel Engines 00000 000000000

The Engine of t he ".Lubumbashi"' 0..

Consumption Tests on the "Lubumbashi" 00000

3.5

Steam

Turbines'

0,6, ar,

atliissei Maru" 'Turbine 000000000 ,0..

The Turbine of "W. Alton Jones"' 00000 o

The "Tervaete" Turbine

References ..

Appendix I; Particulars of Ships 41 4Q

Appendix 114 Tables ...

44.044 .... .0.004604444M0444.

41

Av 44.0,04 .4 38

CHAPTER IV

4.1 Propeller Characteristics; 0000 P

Propeller Sea Characteristics 0, ...a....

23

Change in Propeller Efficiency ... p.

25

4.2 Propeller Analysis, First Step ... ...

25

The "Simon Bolivar" Propeller'

*

26

The "Lubumbashi" Propeller ... 4,44.000. .

26

The. "Tervaete"

Propeller .,P,44,440/4.,4440,040444,

444004044400,

27

The "Nissei Maru" Propeller ....-..., ... ...

27

4.3

The Intermediate Results 4.'0. ..}.

28

CHAPTER V

5.1.

Propulsion Factors

,..

29

5,2

Apparent Wake

29

The

"Lubumbashi".

Wake e,

..

Dor IN-.

30

The "Tervaete" 'Low-I Wake . ... 0, 32

5.3 Propeller Immersion Effect ... 32

The "Tervaete" Data Complete 0,0A *10,4404 4,0.4. 04 W,40,4,0,08.**. 33

The "Lubumbashi" Dats Complete . ..

-4 0.40

A:4,0..44044

34

CHAPTER NI

6.1 Desirable Engine-Propeller Interaction

35

6.2

Conclusions p 4.

...

37

Acknowledgments .... 38 page 1 2 2

4

4

7 9

10

11.

11

12

13

13

15

16

17

20

20

1

1.4

ON THE PROPULSION OF SHIPS AT SEA By Jan drvig

Norwegian Ship Model Experiment Tank Publ. No.

52,

September

1958

SYNOPSIS.

A general consideration of screw propulsion of ships at sea, leads to a new approach to the analysis of seagoing performance, which is regarded as the result of

inter-actions of the relevant characteristics of engine, propeller, and hull. A discussion of torque characteristics of reciprocating engines and turbines is followed by consider-ations of propeller action as influenced by immersion, and by its movements in a sea-way. When the propeller torque and thrust characteristics are also accepted as variables, a simple analysis of known records for the "Simon Bolivar", the "Lubumbashi", the

"Tervaete" and the "Nissei Maru" reveals that the data can be divided into one group with the propeller apparently unaffected by ship movements and waves, and another so affected. The still water type of results allow some new conclusions to be drawn re-garding wake, propeller characteristics and the effect of small propeller immersions, which are also, in part, valid for model test conditions. The results affected by waves etc. when combined with engine characteristics, show a typical difference between

re-ciprocating engine drive and turbine drive at sea.

A few general remarks on the results are concluded by a note on the possible use of the propulsion analysis in further investigations of ship resistance and movements in waves.

CHAPTER I

1.1. Introduction. Present propulsion theories apply essentially to still water con-ditions with no wind and waves; are, used also to approximate seagoing conditions through the introduction of factors of experience. The following considerations of

ship propulsion at sea are directed towards the fundamental seagoing propulsion pro-blems. It is unavoidable, however, to touch also upon the theory of propulsion general-ly; as the ideal still water propulsion itself forms one limit of our field of study and must be recognized as a special case. Thus, our study of the effects of weather upon propulsive performance must include an attempt at a definition of this special case also, which is in itself desirable in order to secure a true comparison with the usual tests on models.

The general case of propulsion, with wind and waves, represents quite a formidable complex of problems. The intricate interrelations of the physical factors are not yet exactly known; and the large scatter of propulsion observations or log book records seems at first an almost insuperable obstacle. In a brief consideration of previously proposed "global" methods of performance analysis it is seen, however, that the statisti-cal significance of their results is poor and not sufficient to prove the scatter due only to errors or difficulties in the measurements, and a further refinement of analysis methods may thus be possible.

Further investigation is necessary, and we may then choose between two main methods of attack, the inductive and the deductive. The inductive, which has been vigorously pursued in later years, aims at the establishment of a complete system of equations de-scribing changes in ship resistance and propulsive factors as functions of wind and sea conditions etc.; the deductive method aims at the same ultimate goal but proceeds by successive approximations based upon the cohclusions which may be drawn from correlation of performance with the conditions of sea etc..

Considering our rather limited knowledge in this field at present, it is believed that the last method offers the greatest possibilities, although elements of both

prin-ciples must be present in our investigation. But it is essential, in the writer's opinion, to proceed upon as few assumptions as possible and particularly to avoid any hypothesis which may distort the real relationships. With this requirement, it becomes difficult to use a purely statistical approach, which of course must be based on some pre-vious knowledge. What we want to know are the effects of seagoing conditions upon the

moving system Engine-Propeller-Hull, or in other words, exactly what is needed to make a general statistical analysis. In view of the many factors involved, it seems right to use a type of analysis which can be applied to each separate set of observations; and

also to distinguish between the changes in hull resistance and in propeller performance occurring at sea. This inevitably becomes more complicated than previous treatments, but is necessary in order to establish separate service performance criteriae for hull

and propeller.

In what follows, except for some remarks on propulsion factors generally, older methods, and accuracy, we shall discuss the application of these nrinciples. We are dealing with Puite large effects, and extreme accuracy of calculations is generally useless; during a large amount of preliminary work a 50 cm slide rule read with a good lens has been found generally to give figures well within the accuracy of all available seagoing measurements.

A vast mass of ship's service records, and some very good measurements exist; but it must be remembered that our scope is at present limited by the complexity and part-ly unknown nature of our field of study. Our arguments are based on ship data as direct-ly as possible, but we shall, of course, continualdirect-ly draw upon many aspects of previous thought, experiments, and seagoing experience. We are here concerned with what is really a first approximation to a theory, and it is necessary to single out the most

significant effects; many points of interest will only be summarily touched upon. Alternative treatments can be used, and other presentations are possible. It is the author's belief that only much further work and discussion can ultimately lead to the full mastery of our present subject, the seagoing characteristics of ships hulls,

engines, and propellers, and their interactions.

1.2. The Ships. We shall here use the Published performance data of 4 ships, the "Simon Bolivar", the "Tervaete", the "Lubumbashi" and the "Nissei Maru", which together cover a wide range of cargo ship operation.

The records of the single screw steam reciprocating "Simon Bolivar" are given by R.J.C. Dobson, J. Gerritsma and J.P. Veldhuyzen in a report to Scheepsbouwkunde Dept., T.H. Delft, May 1953. The observations quoted date from between-the 27th March 1936 and the 24th May 1938, and are of what may he called the service record type, supplemented by thrust measurements. The practical importance of such records, and in this case also the extensive model tests published by van Lammeren in N.S.M.B. Publ. No. 32, 1938, makes this ship a very interesting case despite a change of propeller before the

27th March 1936.

The two next cases selected represent steam turbine, and diesel reciprocating types of main engines respeCtively. Both ships have been the subjects of special measurements of seagoing behaviour, which are given in G. Aertssen's well known I.N.A. papers of 1953 and 1955, and later additional data on the "Lubumbashi" in his paper to the I.N.A. 1957. The "Tervaete" is an AP3 Victory type ship, on which a large international programme of model tests has been carried out, and also particular tests by the N.P.L. for the actual loaded trial condition used. The "Lubumbashi" is a diesel cargo liner of about 9500 tons dw. built 1954, for which model tests in the ballast and loaded trial condi-tions were made by the N.P.L., Teddington, in addition to the original tests run at the N.S.M.B. in Wageningen.

The "Tervaete" and the "Lubumbashi" data represent sets of special measurements, each taken during fairly short periods of time, which are not individually comparable to the "Simon Bolivar" data. These two cases are chosen because of the very high quality of the measurements.

The "Nissei Maru" records were presented in the Report of the Experiment Tank Com-mittee of Japan to the VII Int.Conf. of Ship Hydrodynamics, 1954, containing results of an extensive measurement program during 1951 and 1952. The ship is a single screw cargo vessel built in 1951; of 13,4 knots designed service speed and 13 870 tons load dis-placement, propelled by steam turbine machinery. Shaft torque was carefully measured, but no thrust measurements are given in this case.

In addition to these four cases we shall also touch upon a few cases of service records, as published by others, in connection with a short discussion of analysis

methods; or as taken by the author directly from ship's log books which were kept without any knowledge of a subsequent analysis.

Information on wind and waves is at best rather general, and it is a great pity that current practice in ship's records include only a scale of wave magnitude classifi-cation, but their direction in very few cases. Because of the different effects of wind and waves, the wind-wave correlation problem is important. In this connection we shall also use some of the observations published by Kent; and one case of service records, of the "Vigrid", Bachke & Co., supplemented by careful notes on wave direction taken by the chief engineer Mr. Pedersen for his own interest before any performance analysis were contemplated. The relevant information on the examples is given in Appendix I.

The choice of our 4 examples was largely determined by the completeness of records. It is also hoped that the different types of rropulsion machinery and the different types of records represented will give results which allow comparisons in these respects. However incomplete, we have here the four main types of ship performance records

repre-sented; the trial trip results, the service results covering a watch or a day, the special measurements taken under approximately constant conditions during a time con-siderably shorter than a watch; and finally the average performance during a trip,

which is the practical criterion of Performance most often used in ship operation. The "Simon Bolivar" records refer to separate days; the "Tervaete" and "Lubumbashi" data are special measurements supplemented by loaded trials, and the "Nissei Maru" re-port contains special measurements, including trial trip type series of tests made on

six occasions. Thus, the special measurements predominate.

1.3. Presentation of Propulsion Data. Let us for the moment bypass the statistical significance of seagoing records, which is better discussed at a later stage, and first briefly consider suitable non-dimensional variables for our purpose. For practical reasons the choice is best limited to the systems of parameters already used in the presentation of propeller add resistance data for design, or scientific, purposes. But we have to make sure that our parameters referring respectively to "ship" and "propeller" systems of non-dimensional compound variables are also logically related.

In all normal resistance and propulsion work two such systems are used; the one derived from a set of physical quantities known to appear in the resistance function of the hull, and the other derived from a different set of physical quantities determining propeller action. This applies to Froude's "constants" as well as to Buckingham's 1915 analysis, and to criaffran's introduction in the same year of propeller coefficients of the type TR2n'L4) and Q/(gon2L5). As a rule, no difficulty is experienced in practi-cal design work by the use of- two unrelated systems of parameters, as the "still water" coefficients referring to "hull" and "propeller" are connected by the absolute values of resistance and thrust. Thus, as long as hull and propeller characteristics may be predicted within reasonably close limits, one of the characteristics may be made the basis of an analysis. In the case of still water trials, for instance, the Propeller thrust, or torque absorption, may be assumed known from model data, sometimes empirical-ly corrected for scale and the influence of hull-rudder configuration; and the pro-pulsive quality estimated by this means from the trial records in the usual manner.

Now, the effects of seagoing conditions upon propulsion are considerable, and ob-viously both hull resistance, propeller characteristics, and their interactions, may be affected. Still, of course, we know that the propeller by its action moves the ship, but we can not express adequately the interconnected changes in characteristics by a

This is the weakness of earlier proposed service performance analysis methods. There is of course no inherent disadvantage in the use of two distinct -hull and propeller systems of parameters for the description of changes in hull and propeller characteristics separately, if only the rational connection between them is made clear. Preferably, the systems should be so chosen as to offer an opportunity for the direct formation of one comprehensive expression for the propulsive quality of a seagoing ship. This rouirement may be met in various ways, mostly rather impractical. Fortunately, the Newton type of coefficient: Resistance

/(S)V2L4),

(or: Torque/(9V2L3)) is suited to our purpose, ,and this "specific resistance" concept is also readily converted into the widely used

(c)

"constant". In aero work this type of coefficient is universally used

for propellers, but has been somewhat less popular in the marine field. This is due, in part, to the inconvenience of having very small coefficient values at small propeller loads and very large values, going towards infinity, as V approaches zero. In the case 1 of normal seagoing ships, however, the range of variations is entirely removed from these' parts of the characteristics. The transfer to the standard propeller presentation in the . form of T/(jan2L4) and Q(yn2L5) is simple and well known.

Omitting shape parameters and pure ratios, we may then represent resistances di-mensionally by Froude's well known approximation for still water conditions;

R/(2LN) = Fi(VsLs/N) + F2(q/iL5) and propeller thrusts and torques for similar conditions:

(1.2.)

T/(N1)

= F3(Vt/nLp ; UpLp/0

t1.3)

Q/(PqVp = FOVp/nLp ; UpLp/Ii.

Here, the speeds V etc. refer to the ship and propeller systems respectively, as indi-cated by suffixes s and p. Up is a blade speed. through the fluid when the viscosity term is taken as the conventional Reynolds' Number; the lengths may be any characteristic linear dimension so far as dimensional considerations are concerned.

The increase of hull resistance in a seaway is to a great extent dependent on the iresulting motions of the ship. These motions are very complex in the usual types of. !seaway, and it is not yet established whether we may correctly apply the principle of

super-position to the movements resulting from the various components of the wave systems met with. For a dimensional consideration, however, it is sufficient to know that the periods of encounter will appear in the final function.

Further, we know that motions like those of a ship at sea may affect the fluid motion in the frictional boundary layer, as compared with the "still water" flow.

Accelerations in the main flow around a body, both normal and tangential to the streamlines for steady motion, have a marked effect upon the fluid friction; relative-ly few comparable cases have been solved mathematicalrelative-ly, however. Generalrelative-ly, the friction tends to increase considerably, and by analogy we may thus expect a ship to show an in-crease of frictional resistance in a seaway. In consequence, the wake may change, quite apart from the influence of waves and ship potions upon propeller action, and a change .

in thrust deduction may also be likely.

Now, increased movements relative to the wave surface seems to be quite well con-) nected with increased resistance, whether this follows from friction or from energy lost

in pitch, heave etc., but it appears impossible a priori to distinguish between the con-moments of this sea resistance increase. Our first attempt at an analysis must then be to correlate the total effective increase in resistance at seazMI over the still water resistance at the same speed Vs, with the wave speeds Vw and the angle cc between the direction of Vs, and V. We may write this assumption symbolically as

(1.4) z11/(?1,21) = F4(q/gL5 4 V,..2,/gL5 ;cc).

To describe the motions fully, other parameters must also be included, particularly re-garding amplitude and phase, and couplings between component movements.

The movements of a propeller at the stern, relative to an imaginary still water surface and the ship's course, and also relative to the real wave surface, follow direct-ly from the ship's movements. But ondirect-ly in one, neardirect-ly resonant, pitch condition will the immersions at bow and stern vary similarly. Generally, they are different, and we may obviously have cases of, say, a ship putting her bow heavily into oncoming waves whilst the propeller is moving well immersed below the wave surface, etc. When the

pro-peller breaks surface its mean torque absorption is reduced, and-so is its thrust. Disregarding viscosity effects in this connection as insignificant, we then put the

resulting changes in propeller characteristics symbolically aa

AT/(VIN)

= F5(Vp/nLp ; Nrs/gLs ; V/gLs ;cc)

t1.6) 4sQ/(?Vpp = F6(Vp/nLp 4 q/gLs ; VI, r/gLs ;oc).

From a dimensional analysis with the three quantities speedy length and density of fluid chosen as primary variables, we have the general equation

(1.7) Force/(yL2V2) = F(V2/gL ; VL/yr; V/nL ; h/I0V2 ; r1.; r2 4 ...) where h is pressure per unit area. It is also easily seen that power P appears only in the compound variable P/(pV3L2); displacement ina/L5; and in the case of, say, loss of energy through the shedding of vortices, their circulation

r

will appear in the

combi-

-3-(1.1.)

nationr/VL. All these compound variables, exceptr/VL, are known from Buckingham's treatment, and are used in practical resistance work. We conclude that our hull ana-lysis is best based on these.

To enable us to handle large numbers of observations economically and to consider first only the most important aspects of propulsion at sea, we may also first disregard the variables Vw and,, and discuss their influence as a second step.

The variables which enter the dimensional considerations as ratios only are very 1 ' important in our problem. They range from what may be called external variables, as

type and composition

of

wave systems, wind steadiness, and occasionally depth of water; to internal variables following from the use of the vessel, as mean draught, trims dis-position of weights, condition of engines, and method of steering. Finally, fouling of hull, and fouling and erosion of propeller will effect a gradual change in performance. ' Thus the characteristics of the three elements hull, propeller and engine, which to-gether determines the sea characteristics of the ship, may all change almost continual= ly.

Practical Coefficients: In order to handle the data from records with a minimum of cal-culations, we can best adopt Bonebakker's type of variables given in his N.E.C. Inst . paper of 1951, Vol. 67; in his treatment the data are handled as a "Propeller system" of parameters. For a given propeller, the right hand fraction in the last column, 4 headed "With Equivalent Factor" in the table below, will be a constant when w is

con-stant. Thus, we can omit this constant and consider the variations of the first fraction which is a direct product of measured values, and avoid any assumptions regarding w in the first instance. Va is here propeller speed of advance.

It is clear that we may use similar variables in a "resistance system" of parameters

!

also.

In the analysiS of model experiments the use of such parameters is an old practice. 1,4. The Records. All the seagoing performance data for Our 4 examples are plotted in Figs. 1.1-to 1.4; and further all the reported resUlts of measured mile trial runs, and also progressive trials at sea or during a voyage, carried out under approximately 1 constant and good weather conditions within a day or less, are plotted separately in Figs. 1.5 to 1.7.

The data are plotted as 100 T/N2 and 100 Pe/N3 over speed function V./N as ab-scissa; with thrust T in tons, Pe horsepower, Vs knots and N revs, per mihute. Ex'ept for the unknown value of the nominal wake factor w this is a "propeller system" of variables, and we have

(.1.8) 0.00T/N2) (80640/iD6)

(1.9) X4.= (I00Fe/N3) (189180/r:014

Speed does not appear in. Kt and Kg, which is the reason why these variables are so use-ful,

in

practical work, for the determination of a speed of advance Vp, as in the Froude method. For our four examples the constant terms in the right,-hand brackets of (1.8) and (1.9) are: Shill I D feet _ 80640/9D4 189180/fD) "Sion Bolivar" I. S:W. 0,3525 0,04488 service 18,56 F.W.

03613

_ 0.04600 "Simon Bolivar" S.W. 0,3474 0,04408 _ tank tests 18,49 F.W. 0,3561 0,04518 "Lubumbashi" ' 17,65 S.W. 0,4185 - 0;05564 F.W,

._ 0420 __

0 05 0 "Tervaete" 20,50 S.W. 0;2300 0,02632 -. F.W. 0,2357 0.02698 "Nissei Maru" 17,21 S.W. - 0,06311 , _ F.W. __ _ 0.06469

K= --2--

pe

: Constant. n2D5

9D5

al fn2D5 _N3 T N2 ,., Constant K, -pn2D4-

9n2D4

9D4

V J -P nD

v

(1 - w) Ji - w)N nD nD I

Coefficient Using Ship speed Vs With Equivalent Factor Pp : Constant _ $

=

q

PiD5

9I030. _ 02

NV 2 T 9E13(1 - w)2 Constant Ct _f,v2D2 a pVp2(1 - w)12 - 1r2 ?D2 (1 _ w) 2 -C : -

-These values are taken at constant; with

5, = 1,937

for F.W. at 120C; and for'S.W., speci-fic gravity 1,025,

f

= 1,986. An increase of sea temperature of 10°C will only decrease g by about 0,15 percent.

On the whole our material shows a somewhat high proportion of good weather data; most ships will experience a worse average. This bias is to be expected in service type records; reciprocating engines are most often indicated in fair weather, as the deter-mination of absolute power takes second place compared to other informationgained from the indicator cards. In the case of special performance investigations the bias may be caused by efforts to secure correlation with still water trials and model test results. Perhaps more important are the well known difficulties encountered when readings are taken in bad weather either from indicating or recording instruments, all values are apt to fluctuate strongly and somewhat irregularly.

The general type of variations at sea can be recognized as similar to results oh-tained with the great majority of cargo ships. It is also evident that the range of

torque variation in the cases of special measurements has been deliberately extended beyond the normal operating range of cargo ships.

Further, the usual, very pronounced difference betweeh the ranges of Vs/N values obtained from trials and at-sea is apparent. Progressive runs in good weather, going from very low powers up to exceptionally high ones, give results which are usually not very far removed from the model propulsion tests. The sea results show a variation of Vs/N which may be more than 10 times as large, sometimes with comparatively small power variations.

In trials with artificially increased ship resistance, as in Kempf's tests of the "San Francisco" with barges in tow, the results conform to the still water trial type. In broad terTs, the results when towing are shifted along the expected still water Kt and K curves, but each range of Vs/N variation is still small. There is very little to suggest the distribution of seagoing results, though the increase of resistance due to barges is comparable in magnitude to an increase in resistance at sea. The data are plotted in Fig. 1.8 from Kempf's data in S.N.A.M.E. 1936. , 2

he sea results of the four examples are further plotted as values of 'I/Vs and Pe/NV over speed function Vs/N in FiEs.

1.9

to 1.12. This is a presentation involving both Hull and propeller variables in all terms; the variables are the equivalents of

(1.104

,4,-)2ce

(7-42)

(6389,4/f2Y2)

(1.11)

(f-w)2C9

,(Vvir)(

/9,69/f2)3)

with the same units as in_(1.8) and 11.9). The constant terms ate for sea water at 12cC:

For F.W. all values increase in the inverse ratio of specific weight. Comparing the K and C presentations, the difference is striking.

The K spots are dispersed over a cloud- or fan-shaped area, often with K = const. as an approximate axis, and sometimes giving a rather vague indication of a limiting botndary near the still water prediction. This is more pronounced for turbine driven ships, which

in normal service seem to give less dispersion of results than diesel or reciprocating steam engined ones.

Practical experience shows that whilst the sea results may come near the expected or predicted still water T/N4 and Ps/N-5 curves even in quite severe weather, the pro-portion of results falling into the "cloud" increases strongly with severity of weather. Good weather results never occur here. As a preliminary step, this was checked from the log books of more thin 20 merchant ships; the time selected being about 2 years and in most cases Atlantic service around 1950-1952. Based upon correlations of exhaust tempera= tures and 20 to 25 sets of indicator cards in each case the Kci equivalents could be

followed from day to day, or closer, and the statement above was found correct. Also, it appears that diesel engined ships never produce sea results at low Vs/N values com-bined with high K values corresponding to the upper part of the K predictions, but tur-bine driven ships may do this.

The methods employed in this preliminary work have since been found to be capable of further development and this material is not used in the following.

In the C type presentation, on the other hand, a strong tendency is seen for the results to group together into *arrow bands, resembling in their general trend the C curves of, say, free running propellers. No marked effect of weather can be seen; but among the correlations which come to mind as likely, the influence of propeller immersion is often seen to be important. Indeed, in some cases this can be noticed as the diagrams are being, plotted. In the case of the "Nissei Meru" the propeller immersion varies within three widely separated, but narrow ranges at high, medium and low immersions respective-ly; the results can be seen in Fig. 1.12 to split into separate bands, corresponding to draughts at A.P.

All seagoing experience, shows the propeller immersion to be an important variable, and it is natural to split up the C diagrams into groups which refer to approximately constant immersion as given in Figs.

1.13

to 1.16. The propeller immersion is given as I = i/D where i is the vertical distance from still water waterline to axis of propeller*

Ship D feet 6389.47pD2 14989/9D3 "Simon Bolivar" service 18,56 9,340 1,1806 "Lubumbashi" 17.65

l075

1,4453 20.50 7,656 0,8761 "Nissei Maru" 17,21 10 861 ___ 1 4806

5

-( = "Tervaete"

Immersion

i/o

0,751 0,68o

With an eye to the number of results in each group, the divisions are chosen as given in Table I. In the plotting of such immersion group diagrams, it is further noted that the spots do not depart from a mean curve in a completely random manner within any one group; with a sufficient number of results at hand, both the time out of dock and sometimes also draught variations within the group can be seen to have an influence. But in all cases a definite trend is found.

Except in cases of very serious cavitation, and some small vessels where the type of radial propeller flow, described by Gutsche in J.S.T.G. 1940 seems to occur, the C

characteristics normally conforms to this pattern. The author has no t yet found a practical case showing otherwise.

Compared to the accuracy hitherto considered attainable in service results or measurements, the deviations from an arbitrary mean curve would seem to be of no

impor-tance. The assumption of a mean curve for each immersion group will, however, tend to obscure a detailed correlation later, and make a final test of significance doubtful. It is highly desirable to avoid the use of approximate curves, as sketched in Figs. 1.13 to 1.16, and an analysis based directly on each set of records for one day, one measure-ment period, etc., is to be preferred. The mean curves might conceivably afford a basis for an empirical formulation; to go deeper into the sea propulsion problem on such a basis is impossible. In order to form a more complete picture, we must first discuss briefly the main characteristics of engine performance, propeller action, and hull

re-sistance. Only thus can an approximation be formed which, simultaneously, takes into account the K variations at sea; these show the same type of results within any one im-mersion group as in Figs. 1.1 to 1.4 if only the observations are numerous and repre-sentative.

TABLEI

Grouping of sea results according to propeller immersion. "Simon Bolivar': Immersion Group A 0,662 0,591 0,520 0,609 0,538 0.449 Immersion i/D 0,877 0.8°6 0,646 0,581 0,527 0.457 No. of T observations do. Q do. 26 26 17 17 27 27 "Lubumbashi". Al A2 B Immersion Group Immersion i/D 1,114 1,008 1,061 0,999 0,984 0.973 0,960 0,946 0,905 0,901 No. of T observations do. Q do. 29 18 29 19 16 16 16 16 26 28 "Nissei Maru". A Immersion Group Immersion i/D

₁₀₁

1,050 1,039 1 001 0,996

0 61

0,958

0 22

0,572 0 545 0,538 o 506 0,497 o 20 0,310 No. of Q observations 19 27 36 12 14 21 16 6 No. of T observations 12 8 10 ₉ do. Q do. 13 15

lo

13 "Tervaete". Immersion Group A 1 C

-7-,

CHAPT,IR II

2.1. Propulsion Theory, The basic idea of wake, which forms the mainstay of all practical powering an propulsion analysis work, was introduced by Rankine in 1865. In, 1883 R.E. Froude developed the idea into a nractical method, described in his I.N.A. paper of that year and further discussed in his 1889 paper. The method applies to model experiments in still water, and is based upon the observation that the model screw in the behind condition, when delivering the required thrust, behaves almost as if it was working in undisturbed water but moving at a speed VP lower than the speed Vs of the towing carriage. At the time, R.E. Froude considered his measurements of torque to be much inferior in accuracy to those of thrust. In consequence he established, in 1883, i the practice of using his thrust measurements only, in conjunction with open model

pro-peller characteristics, to predict full scale propulsion. At first, measured torque was disregarded as too uncertain.

The efficiency thus found was divided by Froude into the "Screw efficiency proper" ' - the efficiency of the screw working alone in undisturbed water; and the change in this

open efficiency caused by the interaction of propeller and hull - the "Hull efficiency". The hull efficiency was again divided into two rarts, the effect of the presence of the working screw upon the model resistance, and the effect of the model unon the effici-. ency of the screw working in its wake. This effect of screw upon model can be directly measured as the difference between model resistance alone and in combination with the working rropeller. This augmentation of the resistance by the action of the screw was

expressed by R.E. Froude as a thrust deduction, i.e. as a "factor of the total efficiency, li

thrust minus augment of resistance divided by total thrust".

The effect of model uron screw was taken to be mainly due to the forward velocity comronents at the screw location; and Froude's approximation is, in his own words of 1883, that "comnlex and varied as are actually the motions of this wake water, the net --, effect of this state of motion upon the screw, is Practically identical with that which, would be produced by a mere uniform current, i' The sneed of advance of the pro- _

peller working in the wake was thus defined as the speed at which the propeller model in the open test developed a thrust corresponding to the augmented resistance.

In the great majority of experiments, torque was also measured, and was found to be generally reduced by C to 2;"' in the behind condition corresponding to "thrust identi-ty". R.E. Froude considered this apparently insignificant error acceptable at the time, when more important investigations demanded his attention; and his method would seem

satisfactory even today were it not for grave doubts as to the soundness of the method itself, raised by the works of Fresenius' followed by Dickmann, Helmbold, Horn and ! others. R.E. Froude realized that the structure of wake flow affected the performande .

of the propeller, and in his first paper on propulsion experiments, of 1883, he

al-ready discussed its possible effects but strictly as a case of potential flow. It seems --4.

clear enough that froude himself regarded his method as a rough approximation which

-.I

might be useful only until a better one

will

eventually be produced. He noted that the behaviour of his observed thrust deduction -, and wake factors did not conform to

Rankine's potential flow theory of 1865. This important characteristic is also found in

later test results, and seems to be fundamenta].

-Comparing this to the main components of resistance, which we split into viscous and wave making resistance on the assumption of a very thin frictional layer, it seems as if such an assumption cannot be used in the treatment of wake. Here the viscous layer Is very much increased in thickness, and it is doubtful whether a potential can be assumed for any part of the flow which enters the propeller disc. Along the run and near the nropeller the flow near the hull is retarded, going against an increasing pressure.

--This will strongly increase the thickness of the frictional layer, and at the same time

iincrease

its tendency to separations. The fact that in model tests the stern wave system is normally underdeveloped compared to full scale, and the more so the smaller the model also indicates the rossible absence of a potential in the model wake.

An effect of curvature of flow unon the degree of turbulence in the wake must be

.4

exnected regardless of scale, when the flow along the hull turns into the propeller disc. Near the hull surface, where the velocity increases outward towards the center of curva-ture, the degree of turbulence may be expected

to

increase; further

out

where the

veloci=

ty decreases outward, in the outer rart of the frictional layer, the opposite effect

-may occur. Such effects are treated for instance by Betz in "Vortrfflge aus dem Gebiete der Aerodynamik, 1930", and also by others; in the model scale and particularly on small models a variation of turbulence may play an important part in producing or preventing

I

separations.

It seem, then, as if the difference between the main hull flow and the wake is one of degrees, rather than fundamentals, but that the separation of wake components on the assumption of a thin frictional layer is impossible. It may be that minor wake components, like potential ideal wake, can be treated separately as an approximation, but the main wake flow must obviously be treated as a viscous flow problem. Pressures, as for instance measured by pitot tubes, are not characteristic enough for a description of such a flow. It also follows that R.E. Froude's direct subdivision of hull - propeller interaction is.

-incorrect as far as scale factors are concerned, because they are here included in the potential flow to which Rankine's principle applies. .

From our present point of view, the important question is whether we may use the conventional, or Froude, wake concept, and the corresponding propulsion equation, in our' analysis of full scale propulsion without fear of misrepresentation. With7pi, Vpi and ..,

Ql representing open values, the complete Froude propulsion equation is

(2.111,,

R6

7-1(Pf

tt

2F,V,AQ 7-//p, -

2f44

d

and 'using Taylor's notation. for the Froude wake speed concept, this can be written in the

-usual manner (2.2)

Here the Froude wake speed is Vs - Vp and in Taylor's notation, this is expressed as

(2.3) W = (Vs - Vp) / Vs

whilst Froude used

(2.4) wF

(vs -

vp)

/ vp

the absolute wake speed being the same in both cases, and VP determined by R.E. Froude's analysis.

The essence of R.E. Froude's method is thus the introduction of the nominal wake speed Vs - VD determined by the use of the "Thrust Identity" speed of the tank carriage in the corretoonding open test of the propeller.

In principle it makes little difference whether the alternative "Torque Identity" method is used, corresponding to an equation of the type

(2.5)

.2774/62 7-JV

_ei

27P4Q

Zi

In both cases the relative rotative efficiency'2r, the last term in (2.1) and (2.5), is the correction necessary to adjust the result of the analysis method to the physical facts of the propulsion experiment. This correction does not contain all resulting errors; this fact was realized by Froude and further discussed by Henderson in his I.N.A. paper of 1910 as a simple case of potential flow. If the propeller works at an open speed of advance V,, the effective speed at the propeller itself is (1 + x)V,, and in an homo-geneous wEke this is not changed. But the recoverable proportion of thework done be-hind the hull will vary with the ratio of this intake with the propeller working, to

the mean velocity without the propeller which is (1 + y)Vp. This effect alone will then give

(2.6)

(7'-

₍

fj

7;Axy))

which has led to the general conclusion that the Froude analysis gives too high _behind propeller efficiencies, and too low hull efficiencies.

Baker showed in his I.N.A. paper of 1927 that results of model experiments with propellers behind a plank and in a mixed wake resembling that met with by a single screw behind a hull, agreed quite well with Henderson's ideal case for high slips. The com-parison was based upon propeller performance calculated by Mallock's method, developed from Glauert's theory, and blade section characteristics taken from general aeronautical data. A recalculation with more modern blade section data, as now used in propeller cal-culation, and using a more complete Propeller theory, tends to shift this agreement to somewhat lower slip values. However, this result is Still only of relative value. The important fact brought out by Baker's investigation is that the trend of the theoretical results is markedly different from the R.E. Froude analysis results; and this is equally pronounced when modern methods of propeller calculation are used. Baker finds errors in the Froude propulsion components of up to 9%, but concludes that within the normal slip range this conventional method is still useful for practical work.

As we have pointed out in connection with still water full scale results, the practf',

cal slip range is very limited under such conditions; Ous a "Factor of experience" _may suffice in this range. The enormaously increased range ad slip, and nominal wake, vari-ations found from an application of the Froude analysit principle to sea results seems, however, to res t on no proper foundation.

It must be pointed out that prediction of full scale propulsion and engine power from ordinary model tests is at present, with our still incomplete knowledge of pro-pulsion, a different matter and must aim primarily at a practical bridging of this gap in our knowledge. An example is the practice of running propellers in the open model tests at much higher numbers of revolutions than in the behind condition, contrary to the original Froude method; the effect of the increased turbulence in the wake is rather uncertain and the real justification is the success of predictions.

The expressions (2.1) and (2.5) for the propulsion efficiency are only valid in connection with a definition of wake speed, in casu R.E. Froude's, or various other pro-posals mainly of recent date. Denoting the unknown correct values by index o, (2.2) can be written

(2.7)

as7r must then disappear, and t is known from measurements; the t value found may in-volve a change of resistance due to a change of flow pattern. This is avoided by hull resistance measurement in the propulsion experiment. (2.1) is then

(2.8)

7

.

77/

n/e,

or 4/ C2

with T and Q ab measured in the same propulsion experiment. In the normal wake structure both values will differ from those obtained in the open, and it is thus necessary to introduce a new definition of wake in order to determine _{The wake w in (2.71 seems} to be what Horn (and others) have termed the "true wake", bat no explicit definition has ever been given. For our present purpose we need only to establish a connection between

(/f

i

r

(

(2.9) (2.10)

- 9

-(2.1)

and (2.8), and our choice of wake definition should then primarily facilitate ana-lysis.

Hitherto, the open propeller results have been regarded as a natural standard of reference; it is also possible to use a theoretical propeller, with some advantages in the analysis of both open and behind values, though it is not a necessary step.

Open propeller curves of Kt and Ka are shown compared to behind Kt' and Knt curves in Fig. 2.1. Thrust identity corresponds to 141/Kt 1 and torque identity to ?Kat/Ka =1. In the "Hydrodynamische Probleme des SchiffsaAtriebs", 1932, Horn pointed out thdt

neither thrust nor torque identity gave a satisfactory solution, and put

proposing an analysis according to the formulation

(2.11) V,..,

5

7-1/Pf

n/a/g

t )

1-é)

Cl-

C -

7

A:4

where the wake is determined from the open propeller test speed at which the propeller develops the required behind thrust divided by s, and absorbs the required behind torque divided by q, with an open effic1ency71 equal to the behind efficiency multiplied by

S.

The two factors q and s are sufficient to describe the departures of propeller characteristics, at any given speed, from a chosen standard as, for instance, the open test results. This is clearly demonstrated by Glauert in his treatment of propeller theory. If the propeller is working in a predominantly viscous flow, however, we must expect the propeller suction to have a reduced effect upon the flow in front. The inflow in the behind condition will then be reduced by an amount dependent upon the viscous wake and the behind characteristics measured may then show departures from the open ones, different for thrust and torque, and changing q and s.

From these considerations we may conclude that the method of Horn, although in prin-ciple adequate in a potential flow, is insufficient to establish a more accurate con-nection between the Froude propulsion analysis and the physical facts. A further funda-mental investigation of propulsion is called for, in the meantime it seems most rational, to compare still water results, including trials, with model predictions strictly upon 1

the Froude principle. 1

In practice, predictions will, however, depart from this on minor points. For our present purpose, we regard such departures as based upon considerable tank experience, and for still water it seems right to accept the predictions as given in the published reports in the particular cases of our examples.

2.2 Previous Methods. Most of the previous analysis work on seagoing propulsion has been confined to formulations within either a resistance or propeller system of non-di-mensional parameters.

Examples of a resistance system are the use of the Admiralty Constant by Telfer in his N.E.C. Inst. paper of 1926, correlated on the basis of a rough all-round weather

classification, and Kent's presentation of service data in his I.N.A. papers of 1924, 1926, 1928 and 1931.

An improved treatment of resistance as increased in service was given by

D. Lockwood Taylor in an I.N.A. paper of 1928, where statistical methods were introduced in the determination of loss of speed.

A type of approximate propeller relation which has long been used in practical ship work, for instance in the determination of speed at reduced power, is the assumption of a constant slip. This corresponds to the "normal" type of distribution obtained in

a (Kq ; Vs/N) service diagram:

(2.12)

_VA/3

constant, approximately.

In 1927, Telfer (ibid.) proposed to base the analysis of service performance upon the assumption of unchanging propeller torque characteristics, valid for I > 0,10 to 0,20. This assumption was modified later to the assumption of propeller characteristics varying with immersions smaller than a certain limit; see for instance the discussion to

Silovic and Fancev's 1955 I.N.A. paper on the "Rijeka". The application of statistical methods to analyses based upon such assumptions was introduced by Bonebakker in 1951, the results, however, generally confirm(2.12). As to the possible variants of Bonebakkert analysis and their results, two further points may be mentioned.

,Present practice is to give a calculated standard error as a percentage of mean Pe/N'. The total range of service variations is only a fraction of this numerical mean, however, and a better measure of the fit of the assumed characteristics to the actual

data is the correlation coefficient r:

lE - ) ( )

(2.13)

x

-97)2

It will be well known to investigators that the correlations obtained from service type data is extremely poor, although somewhat improved by a preponderance of good weather data. As a rule, the correlation of results with (2.12) is slightly better, but not satisfactory by any means. (2.13) can be applied, for instance, to observed C and K

-=

2

W-)

values at any V.,./N; then x and y are theoretical values and and recorded values. 1

An interedEing point arises out of the apparently greatly improved precision in some cases, as for instance the "Rijeka". Closer scrutiny reveals, however, that this im-provement is only obtained when an appreciable I variation exists in the data, and the dispersion of results is not much increased by weather. It is found that any reasonable grouping of results according to I, again give very poor correlations for I approximate-ly constant; but as I increases, mean K, will increase and mean

Vs/N

decrease simul-, taneously. What appears as a propeller torque characteristic is thus seen to be largely

an effect of da, and we must accept the conclusion that external conditions also appears to affect the action of the propeller.

This was pointed out by Schmidt in the "Hydrodynamische Probleme des Schiffs- I°

antriebs", 1932, and provisionally described as apparent effects of changes inf.

Although traces of foam-like tip spirals can sometimes be seen behind working propellers* it appears unlikely that, may change much. But a deficiency of feed into the propeller might produce the same effects, and the investigation of changes in propeller character-istics seem to emerge as the crucial point of an improved analysis.

This was already confirmed by Kent's comparisons of ships and models propelled in waves, which showed that Froude's propulsion analysis then leads to noticeable dis-crepancies, corresponding to an unexplained loss in propeller efficiency.

CHAPTER III

3.1. Characteristics of the Ship Elements. A ship which is being propelled forward at I sea, towards a fixed goal and thus with no deliberate change of course, will move under continually changing conditions. This applies to the condition of the ship itself as well as to wind and waves; and also, near some coasts, to the depth of water and the slope of the tidal wave surface which may occasionally affect the performance noticeably.

The propulsive behaviour of a ship must be a function of all such variables. We I

can not, however, express its behaviour in terms of any one system of non-dimensional variables covering "resistance" or "propeller" behaviour alone. To avoid the deadlock I. resulting from such an approach, we must regard the propulsive behaviour of a ship at I

sea as a result of the interaction of the relevant characteristics of its three main

elements: The Engine, The Propeller, and The Hull. _I

For still water And steady motion the 'general features of these characteristics are known

fairly

well; for the hull and propeller, chiefly as inferred from model tests. The determination of engine torque characteristics particularly from shop tests,

but also from tests at sea, can be effected with a reasonable degree of accuracy. But the exact numerical determination of full scale hull and propeller characteristics is fraught with very serious difficulties even in the still water case. In our

consider-I ations of "sea" effects we start off by accepting the steady state characteristics ,

1 as known, and use the predictions from model tests as given in each case. I

We have here made a choice between the uSe of trial results, and of model tests, and it might be Argued that full scale trials, though not always made under completely ideal conditions, are in any case free of scale effects when compared with other

per-formance data for the same vessel. Further, the direct correlation of model test re- 1 I

1

sults and acceptance trials under average conditions is valuable in itself from the

11

shipbuilder's point of view, as well as from the shipowner's. It must be remembered, however, that the interactions of hull and propeller in full scale can only be numeri-cally described if, say, the propeller thrust and torque characteristics are accepted [ as found in an open propeller model test, or at best corrected for an assumed scale effect. We are thus again thrown back on a "global scale effect factor", not very 1

satisfactory. In this situation, a discussion of R.E. Froude's propulsion hypothesis and later work in this field leads to a choice of the model results as the most rational; and this affords the added advantage that no possible effect of waves is present in the still water results. As it

will

appear later, this is the case in some trials.

H.F. Nordstrom in his Study on the Interaction between the Engine, the Screw Pro-peller, and the Ship in 1931 assumes still water and constant wake and thrust deduction and mentions the general effect of following or adverse wind; but does not otherwise touch upon our present subject.

In the following brief description of the element characteristics and their deter-mination from ship records or measurements we shall take the

opportunity

also to digress on some particular points in connection with our examples; we are here traversing old

ground but it is hoped in this way to illustrate our approach. 1 ]

The most pronounced feature of screw propulsion at sea is perhaps that it is in

1

no respect a steady state. We assume as a necessary condition that all movements and fluctuations of forces can be described in their effects by the use of constant ef-fective values. Such efef-fective forces need not, and will not in all cases, be straight

means; neither will these values always be equal to still water steady state values. ,I Take for instance the engine, which may during a time interval at sea with constant

type of wind and gusts, constant wave soectrum, wave type and direction etc. produce, an effective value of power at the propeller:

j e

r

J C (lf ; A 0 de

J

F-; -

°

-with constant engine fuel setting H. Now, under steady running conditions with unchanged engine setting and, say, at the rpm which equals mean N at sea, Pe will differ from the seagoing effective value of Pe by some unknown amount, probably small. This.

by tb=

puts the emphasis on measurements on board. But regardless ofthis, we assume that ef-fective values do exist under weather and ship conditions which can be described as constant in fundamental characteristics.

Concerning waves, we may often have a confused sea with no waves of ecual size and shape met with during the time interval considered; provided the sea can be adequately described by energy spectrum, wave type etc., the work of St. Denis, Manleyand Pierson in S.N.A.M.E. 1953 shows that a description of such a general nature is sufficient to indicate the existence of a definite effective resistance.

At sea, the constancy of weather etc. during a measurement interval of time will very often depend on the time interval. Wind and waves are continually changing in magnitude, type and direction in the normal case. The effect of this is best discussed at a later stage; now we simply assume as a first approximation that effective values of all propulsive factors do exist, and may be treated as if representing a steady state motion. In the treatment of our examples, we also make the provisional assumption that the records may be accepted as such effective values.

3.2. Engine Torque Characteristics. The propelling engine consumes steam, oil fuel and air, or electricity; in some cases also power from auxiliaries, anddelivers a net power on the shaft at the propeller location. We regard the thrust bearing, intermediate shaft bearings and stern tube with glands as parts of the machinery. The torque or power delivered to the propeller is our chief concern here. Over a sufficiently long time interval the work delivered must obviously be absorbed by the propeller; but at any particular moment when the ship is moving in a seaway, the torque delivered from the engine may differ from the instantaneous propeller absorption and these fluctuations may be large.

Disregarding, these momentary values, we are interested in the effective power, or torque, delivered to the propeller, and particularly in the determination of effective power or torque from engine variables only.

In the following notes we shall touch upon some'points in connection with steam and diesel reciprocating engines, and steam turbines, which will be well known to

engineers. Our aim is to review ways and means for the determination of tprque character-istics with particular reference to our examples, but to some extent the remarks may have a wider application in the more general case of ships with no equipment for torque, measurement installed.

The very marked difference between tyPical torque characteristics of turbines and reciprocating engines makes a different treatment necessary; it is also convenient to discuss steam and diesel reciprocating engines separately here.

3.3. Steam Reciprocating Engines. Engines of this type are still in use, although few are now built. In the old days of coal fired boilers, the fluctuations in steam pressure made the determination of indicated mean pressures

pi

very uncertain as a basis for analysis. With oil fired boilers this situation is greatly improved, and the indi-cated pi is more nearly a function of steam admission.

We express the indicated power developed in the cylinders by

L3.1)p C

where c is the usual "engine constant", and the indicated mean pressure

pi

is found from the indicator cards. We can also express the indicated power approximately by

(3.1a)

e

c, N

f (A/ )

where H is steam admission scale setting, often given in per cent admission but seldom to an exact scale. This is possible because a change of vacuum will have a small effect both on pi and effective power; badly fouled condensers excepted. H may however be re-garded as an arbitrary scale of numbers only. (3.1a) may be a good approximation, but in some engines with rather small valve openings etc. the flow restrictions will cause

pi

to decrease for increasing N with H constant. This error may be noticed, but seems unimportant compared to the possible uncertainties in the determination of pi.

A part of the indicated power is consumed by engine friction, attached pumps, friction in thrust- and intermediate shaft bearings, stern tube and stuffing boxes. The remainder Pe is the power delivered to the propeller. We define our mechanical efficien-cy as

(3.2)

7

/2

=

-

(67, )

where pm is the loss in mean pressure and includes all friction. In principle the shaft-ing loses are to be included in the total frictional loss, but as the shaftshaft-ing losses are not large the error introduced by taking this deduction as a percentage of indicated power is unimportant. Exceptional shafting losses may occur in cases of heavy wear due to whirl, very weak thrust bearing foundations, or bending of the propeller shaft in the stern tube.

A number of thorough investigations of the mechanical losses were made in the days of reciprocating steam engines. All the best results show a connection between engine friction and steam admission. Among various expressions developed, the well established Hrabak formula Ls still a good approximation for the engine alone

# B,

V P'e

with effective shaft power P, and mean piston speed v measured at designed normal load. The formula is found in standard textbooks as Doerfel, Dubbel, Bauer etc. in the more practical form

( 3 . 3 )

=

=

(3.3a)

The constants are generally given for condensing engines with designed steam pressures of 140 to 170 lbs/sq.in. and attached air pump, as A = 0,85 and B = 0,40. Higher steam pressures will increase the influence of Pi/v or the equivalent variable pi.

Information on this point is scanty; neither have the effects of size been close-ly investigated recentclose-ly in the case of large triple or quadruple expansion marine engines. Small engines in a laboratory offer the best possibilities for close investi-gations and the most reliable results; these offer examples both of increasing and decreasing losses pm with increasing N; the former predominating in quite small engines. Such effects are partly due toe bearing design features. The small engine investigated by Nordstrom, given in his 1931 study, seems to suffer from mechanical losses increasing with N, but the results are too few for an accurate analysis.

In ordinary merchant service no engine is frequently indicated, and no records are kept of engine setting. Good use can be made of cards from trials, but as time goes some external valve gear types, as Klug and Marshall, are apt to change the valve motion through wear. With only a small number of cards taken, it may be almost impossible to determine f (H) in (3.1a) with an acceptable accuracy after a year or two.

The Stephenson gear does not have a comnarable disadvantage, as wear of its bearing surfaces only transfers power from one side of a piston to the other, with practically no change in over-all performance. In this case it seems possible to put at normal

N = No:

(3.4) =

Vk 14.

.

To give a correct torque at any H and N, the expression would have to be corrected for steam and condenser conditions. Unfortunately, no records have yet come to hand, complete enough to test this properly.

As far as large marine engines are concerned we shall probably do best by assuming a formula like (3.3a) valid at normal designed revolutions No; and simply assume pm constant for all N. We then have the value of 7m at other N for pi constant:

(3.5) -

(No/4/)(/ 7m)

with

7

= the mechanical efficiency of the engine at designed full load rpm.

MQ

The Engine of the "Simon Bolivar". This ship is equipped with a reciprocating quadruple expansion steam engine of about 5000 metric ihp, built in 1927. The designed revs, for this power is given by van Lammeren in N.S.P. Publication No.32, 1938, as about 85 per minute.

Service type observations of indicated powers, rpm, and also ship speed and thrust, are given by Dobson, Gerritsma and Veldhuyzen in their valuable T.H. Delft Report of May, 1953. The observations date from 27th March 1936 to 24th May 1938; no information on dockings is given.

The ranges of revolutions and powers recorded are normal for a cargo ship, and the propeller immersion also varies over a full operational range. No clear connection is apparent between torque, or power, and propeller immersion. Here we may have inaccuracies due to faults in indicating, and due to the indicating mechanism. But our only means of eliminating such errors would be the introduction of new assumptions as, say, probable propeller characteristics; we must thus accept the indicated results, and defer judgment of their accuracy.

Dobson, Gerritsma and Veldhuyzen state that "Aan de indicateurdiagrammen was te zien, dat steeds met geheel opengetrokken schaar is gevaren". This would be most unusual, and is not supported by the observations ouoted. However, only small adjustments are usually needed in service, and the statement probably represents an approximation of sufficient accuracy for their analysis.

Proceeding as if the values of pi were correct, and using the mean numbers of re-volutions recorded for each observation period of one day from noon to noon, we can use (3.3a) to improve the relative accuracy of their estimate of Pe. We take the mechani cal efficiency of 0,86 estimated in the Delft Report as valid at design conditions; 4927 ihp and 85 rpm; and put

- 08001- 0,01065

t/if'/y

Because of higher steam pressure of about 220 lbs/sq.in. and 4 cylinders, the con-stant term is reduced here, compared to (3.3a); the concon-stant B is increased accordingly. The values of'?, are given in Table II, according to (3.5); the variation between 0,854 and 0,864 is not large. If the indicating is good, however, it may be worth while to take it into account.

The corresponding values of delivered horsepower Pp and of torque function Pe/100 N3 are also given, and Fig. 3.1 shows the values of Pe/N plotted against N. Lines of the approximate engine Pe/N characteristic are also drawn for constant Pi/N and very nearly represent constant engine admission setting. It is seen that the effective torque then increases with N, and that the rate of increase is almost independent of engine setting within the practical range of records. Some points appear to fall on lines parallel to P/N = constant, but the intervals used on the steam admission scale are too small com-pared to the errors in Pi to allow us to deduct this scale itself with satisfactory precision.

It is also apparent that the propeller in question has absorbed a hi her torque than foreseen in the original design; the design condition is shown at Pt /N = 57,96 and 85 rpm, only 3 of 51 points are lower in pi and 5 are equal or higher in rpm.

There seems to be nothing in the appearance of Fig.

3.1

which contradicts our =

13

-approximation of7m. A slight tendency of spots to group themselves along lines resemb-ling arbitrary "Propeller Law" curves can be seen, and also a slight tendency of a grouping along lines of Pi/N constant. Accidentally Pi/N for the design condition is nearly a whole number and nearly equal to a normal percentage H.P. admission in this engine. In practice, the engine sett ing is most often chosen at a round figure on the scale, as 60%, 61% etc.; and we have in this case at values of Pi/N of about 60, 62, 64 and 66 some evidence of this.

3.4.

Diesel Engines. A mechanical fuel pump delivers a fixed volume of fuel into the combustion space of such engines, and to a first approximation the indicated mean pres-sure developed will be proportional to this volume. Corrections can be made for differ-ences in fuel heat value, and specific gravity. Further, the fuel quality and N will affect the combustion process, but to a different degree in various engines dependent upon design features and condition of engine.

It is customary to use (3.1) for indicated power, but the determination of pi in diesel engines either from cards or pi-meters is particularly difficult and requires great care. As with steam engines, the difficulties are increased under seagoing

con-ditions. Opinion is divided as to the attainable accuracy both in careful trials and in service. Some recognized Engineers Textbooks, as well as some engine builders, advise service adjustment of individual cylinders to not more than 2 or 3 percent differences in indicated power. On the other hand some engine builders do not even include cam gear for indicator movement in their standard engine specifications. Then only hand cards

can be taken to check the combustion pressures, and for adjustment of cylinders pi-meters must be used.

In the practical running of diesel engines, most engineers will be more concerned about information from the cards concerning the cylinder process, than the absolute power. In that respect the exhaust temperatures are a better guide. The cards will pri-marily be a supplement to the 7..ontinual observations of exhaust temperatures, cooling,

and scavenge pressure.

A great number of other well known possible sources of error in the indicated results exist. Their cumulative effect may be large, but is not always so; moreover, part of this error is of a systematic nature and unimportant if we consider relative powers only. Similar considerations apply to the use of exhaust temperatures. In a practi-cal analysis a number of minor effects must be left out. We will here first attempt a simple analysis of the "Lubumbashi"s engine in order to check the relative accuracy which can be obtained in careful seagoing tests.

The mechanical efficiency qm of diesel engines is lower than the steam engine ef-ficiency, and shows a much more pronounced variation with pi and N. In contrast to what is found in steam engines, cylinder friction is here an important part of the frictional losses. Bearing losses are also increased, and show greater variations with changes in pressure and speed because of relatively shorter pins and journals. A case of special interest in this respect is the Burmeister & Wain type engines with excenter drive from the top and bottom pistons; the short, large diameter bearing surface of excenters at-tains its minimum coefficient of friction at much higher load and speed than other bearings. This minimum may by design be placed near the engine design condition, and the excentric drive losses will then be of the same order of magnitude as other bearings'

or even less.

Most diesel engines will also expend power on attached auxiliaries as blowers, oil pumps, etc.; the total may amount to 2 or 3 times the steam engines losses. As a part of the power to drive blowers etc. may also vary with pi and N, it is clear that an ana-lysis which does not include both may easily defeat its own purpose. From our simple analysis of the "Lubumbashi" engine, with both variables, it appears that the relative accuracy of the measurements is much better than Aertssen's assumptions, or Telfer's in the discussion of Aertssen's paper on this vessel, seem to indicate.

We must expect pm in (3.2) to vary from case to case, not perhaps noticeably in the simpler conventional engine types, but more pronounced in excentric drive and high super-charge engines. No single approximate formula will cover all cases, as even in similar ships the arrangement of auxiliaries may differ.

It may be remarked in passing that only once in a long while are engine tests published, complete enough to be of real use, showing the mechanical efficiency as a function of both pi and revolutions.

The Engine of the 'Lubumbashi". This engine is a Cockerill-Burmeister & Wain 6 cylinder, 2 stroke d.a. type; it is similar in design to the slightly smaller Harland-B & W engine described by C.C. Pounder in his Inst. of Mech. Engrs. paper in 1949, as far as we are here concerned.

The engine is coverless, with top and bottom ristons connected and driving through excenters placed outside the main piston crank webs. The main piston works in 590 mm bore with 1250 mm stroke; top and bottom pistons in 592 and 588 mm bore respectively with 450 mm stroke; and according to Aertssen "developing in normal service conditions 6000 bhp at 112 rpm". This is taken as the design condition; Pe/N = 53,57.

The engine described by Pounder is 550 mm die, with 1200 + 400 mm stroke. He gives the losses of the type approximately as fractions of indicated power:

Main Bearings, Connecting Rod Bearings, Excentrics and Guides 0,06

Main and Exhaust Piston Rings 0,05

Scavenge Blower 0,05

Camshafts, Chains etc. 0101

Aggregate losses 0,17

corresponding to a mechanical efficiency Qm of 0,830.

The seagoing records given by Aertssen include effective shaft powers measured by torsiometer; these values do not alone give any information as to the torque