The power loss factor in ship service performance

THE 1WERLOSS FACR IN SHIP SERVICE PERFOEMANCE

by

Prof. E.V. Telfer, D. Sc., Ph. D.,

Tre ndJae im.

The following note has been prompted by a study of Prof. Aertssen's recent I.N.A. paper oi the service performance of the Victory ship TERVAETE.

In this paper Prof. Aertssen showed that propulsive loss broadly was directly proportional to such disturbing influence as wave height, wind. pressure, pitching anglo and so on; and further that the loss was greater the smaller a vessel's relative power. In two papers read by the present writer before the N.E.C. Institution in

_1926-27

and in

1934-35

these basic facts of the sea behaviour of ships were previously demonstrated, As this earlier work is now capable of further useful development a brief

recapitulation may be excused.

In the

_1926-27

paper it was shown that by rearranging a vessel's service statistics into the four distinct weather groups nautically labelled fine, moderate, heavy and very heavy and using the generalised power diagram then developed to determine from the group average speeds and propeller revolutions the corresponding true average indicated horsepower, it became possible to calculate the average admiralty constant

2/3

V3/SHP for each weather group. Applying this analysis to the data for a large number of ships it was found that the loss in admiralty constant between successive weather groups was substantially constant. This basic statistical fact at once made possible the develoment of a simple numerical scale of weather intensity, since it implied that loss in admiralty constant was directly proportional to increase in weather intensity, By adopting the scale, zero

for fine weather, 100 for moderate, 200 for heavy and 300 for very heavy, the average weather intensity on agiven voyage was obtained by adding the total

time percentage of moderate weather on the voyage, to twice the corresponding heavy weather percentage, to three times the corresponding very heavy weather percentage. The total f igire placec.i the mean weather correctly between the

constituent classes; and when voyage average admiralty constants were plotted to a 'oase o±' this weather number, a lin relation was usually found.

Vhich, with

= KW/C0

This shows now that the power-loss ratio was directly proportional to weather intensity; and so long as all the constitutent physical disturbances of

weather increase linearly with our weather number our expression I agrees with the first of Prof. Aertssen's findings.

-Such diagrams were generally based upon constant power statistics, but assuming for the moment that the diagram held good for wide ranges of power it can also be interpreted on a constant speed basis, Thus let Po be the shaft horse power required in fine weather to produce the assumed constant speed and. P the corresponding power required for the same speed in weather of intensity W, then the relative power loss is given by

,

2/3

A2/3

v3 1/Po - ip P - p P P o 2/3v3/p _{= C} , can be itten o o

,2/3

_v3/0

_{1/po P}

If we construct a normal speed and power curve (fig. 2) we see more clearly

the significance of (P_P/

The value (p-p ) is lost power so far as

o

transport is concerned; and. the power-loss ratio is a measure of the Z relative power wasted in overcoming weather.

Dur earlier finding that admiralty constant loss was directly proportional to weather intensity, i.e. that

¿/3

. v3/

/3

v3/p

= KW reduces to

2/3

₃

_{P - P}

( C = KTV

3

3. In between the 1926-27 and. 1934-35 papers a shipping slump had set in.

For this very reason the author in his professional practice carried out a larTe amount of investigation work into the inefficiency of reduced. speed-running of merchant ships; and. inter alia, found to this then consternation that the liner loss in admiralty constant ..'ith weather intensity certainly no longer a:plied, It was boon evident, hovever. that for a given weather intensity the loss was greater the lower the power with which a given weather was faced.; and. that the greatest departure from the previous full-power

relation was always at the heaviest weather intensities. To interpret this experience fig. 1 had. to be developed to the form hon in fig. 3. For each

constant power a separate line was required. All lines radiated. from the same value at zero weather but had. a greater slope (i.e. loss) the lower

the power, Jhen this slope variation was plotted to a base of power for a large number of ships, the plot for each ship IJaSof a clearly hyperbolic natue, a fact which suggested. that the power-slope product would be constant. This was found. to be the case, so that instead of' the admiralty constant loss being solely proportional to weather iatensity it was also found to be

inversely prorortional to the absolute power on the voyage. To use this fact statistically the somewaht artificial concept of weather per 1000 IHF was introduced.; and this as once stabilisod and unicuely linearised the voyage statistics.

We now see that instead of' the expression 1 above, our expression ':ith variable power running shows that the relation

(r-r)/p = K1TT/CP ₂

is required., In principle, this again agrees with Prof. Aertssen's firflings, It is evident, however, that this expression still further _{simplifies to}

(pp0)

= ₃

In other words we see that for a given ship meeting constant weather the power loss is ind.ependant of the absolute power. It thus follows that all iso-weather

(isonduir T) power-speed curves are naturally paralle' to the basic zero weather relation, implying therefore that the speed-loss in _{given weather is} much less at high power than it is at low, _{a wellknovi fact in ship experience.}

which L is ship length in feet. This expression being dimensionless obviously reruires the weather intensity also to be dimensionless. If weather were

measured by wave-height in relation to the length of ship, which could for xample be an acceptable definition for model experiment purposes, then we could say that

P-p o = K . (H/L) or calling (p-r), \ P we have = Constant H

in which H is the wave-height in feet, This relation can be conveniently called the power-less factor due to waves. It obviously would be extremely useful in reducing to a single value the results of model tests over a range of ship speed (clear of resonance, of course) and. over a rango of wave height. If model

geosimos were tested one might reasonably expect the power-loss factor to be independent of scale. By varying the wave-length for a given height the

influence of A /L can be examined, The behaviour of the factor over a

sufficient ). /L range would supply an acceptable criterion for one of the

important weatherly cualities of a ship form. For our present purpose it is sufficient to state that model experiments do endorse the power-loss factor concept.

5. When weather intensity is measured only by relative wave height the idea of a dimensionless weather intensity is simple to comprehend. When the weather, however, is descriptively defined but numerically interpolated as in

the authors system, the physics behind the statistical treatment is not so - 4

§ 4.

How can we best study the implications of this power-loss constancy ? ilst in our earlier work we used the admiralty constant, we could eçually well use other pseudo-dimensionless presentations. As the power-loss is

vidently independant of speed it is probably simpler to adopt a dimensionless presentation which does not involve speeds A suitable form would appear to be

(p-r)

w

\,

5

obvious. Provided, however, that the ships! officers give a true description of the weather and sea conditions and one which is not subconciously influenced by the size of ship which they are in, then the resulting weather intensity may be regarded as directly proportional to some equivalent wave ight : and

expression ₅ could be retained for stastial analysis in the form

= Constant 6

and correlation with the model experiment in waves rould determine the eçuiva-lent wave height in terms of weather intensity. Actually, moderate weather

appears to correlate with a wave height of 6 to 7 feet and the length dimension

of weather may therefore not be too unacceptable.

e can call (6) the statistical power loss factor. On any particular voyage. the actual average power can be determined by the author's generalised power diagram. The zero weather power corresponding to the average speed can be derived from a suitable analysis of load trial performance extended to embrace a range of draught. The power loss is thus known; and hence the statistical powerloss factor can be calculated from the known weather on the voyage.

Actually for numerical convenience, the statistical powerloss factor is best

expressed by

P

= Constant 1000

A

This factor can be presumably applied to all ships independent of size and will be a criterion of weathert-1 merit, independ.ent of poiiTer, size and weather

intensity. For a given ship the steady increase in the value of the factor with time out of dry dock, or its rapid increase on actual fouling of the hull,

should serve elso as an excellent indicator of hull surface condition.

It is submitted that the use of these two powerloss factors, the wave

powerloss for models and -the euivalent wave or statistical power loss factor

for ships, should prove extremely useful research instruments, assisting the better understanding of the sea behaviour of ships; and hence contributing ultimately to their better design and performance

400

30e 200 100 o o F M f00

200

300

WEATHER INTENSITY

FIG. I.

o VH O lOO

200

3oo

WEATHER INTENSITY

F M H

FIG. 5.

SPEED

FiG. 2.

H VH

i

P

I

w o Q-cl,

o

400

3oo

AC.

₂₀₀ too

AFXS (I

I1

s P/îi CQPAiSON

by

I. J. P.ALLAN, Superinterzient, Ship 1)ivision,

atiorl ¡ysicsl Labors tory.

-= - -== =-= -= .. - -=-=-=-= - -

--The subject of this Syziiposium is "Trials of ships at Ses.

This is

taken to refer prLrnarily to Measured Uile Trials, with goî coriittions of wirxl,

weather az

_{ship, an-i also to service porforuioe so far as that can be assess ed.}

Airoest all ship deai

today are based on the resulte of nodel teats

both for resistance ani propulsion.

This practice infers tIt the recuits of

a1el tests can be interpreted accurately on the full scale both jualitatiely

azzi quantitativoly, arzi it wi].1 be of ntcrest to cousider in seine detail the

fact ors

nvoIved.

The coerparison of the resulta of tests on ship models with the results

of full scale trials is one receiving particular attention in recent

years, chiefly

because of a geneml desire to sxplore azz3 urderstaM tho "factors of iiorance",

azi. also because of an increasing appreciation of the eff eat

on perforaanoe of

hull roughness, both structural arzi due to fouling.

This is true both on a

national ani en international basis,

_{Special research on this subject is}

certainly going on in U.S.A., U.., Ibilani azxì Sweden, sui

we have the work

recently carried out by the Centre kielge do Recherches navales

on the "Torva ate"

sl ro1xsed for another vessel.

In a'ì-ìtion, the matter has been cLiscussed

from various angles at the International Conferences of Ship Tank Superinterdenta.

Letailed results of various investigations in hazii have not yet been published

so full discussion is z,t possible at this stage, but the various aspects

can be revi awed.

Th. ship owner suite rightly considers the perforìenoc of nia ship

in service sa the real couercia1 criterion, but it is difficult

even on a

statistical analysis basis to asacas service perfonnce

_accurately.

The

increase for

average

_{service perforsnce over}

_{seured mtl. p.rformenoe varies}

from some l5

to 2O

_{on the iost favourable m* routes to some 1}

_to

451

on the least favourable routes.

_{In view of that has been sd above euch}

percentages cannot be other than very rough arzi it is ccnclz1&

_{that the ship/}

eadel oorçarison should in thc first place be based

on a measured nile

perfonce.

_{thile accepting that position for basic OOflFi$E}

_{cene should}

not lose sight of the importance of

_{desipp for scaIiiilinevs, a quality which}

is not tested on the eured

_{1 e trial axi which has not rec4vi sufficient}

attention in the past.

The probln of correlating the nodel

_{resu1t$ to the ship result}

was first suocoesfully solved by the Frc*x3e

_{method in the 1rtter pert of last}

centu2.

_{The basic principles of that method continue in use today, although}

some departures in detail have been

_introduced.

_{Gne of these departures is}

the universal adoption of self-propelled

_{tests in place of the earlier tests}

i'vith propellers supported from

frames behiz

_{the hull model.}

_{Another is the}

use Of a different method to extrapolate

_{the model resistance result to the}

ship resistance result, i.e. the

_{surface friction correction.}

It may clafify

_{thoughts if we consider that there}

_{are two}

approaches to this

_{tter (1) from the designer's point of}

_{view, and (2) from}

the research worker's point of viow.

_{AS regards (1) we re only concerned}

with a rcaonabie prediction of the ship's performance, and so long as s useful

set of correlation factora has been ,orIced out from xperiere it is not

_iportont

which basis of extra>olation is used.

_{In other words so long as the}

_{extrapolation}

method was reasonably in line with

_{the physical faots, the correlation}

_factors

would look after the rest.

_{The method is effective in dealing}

_{with normal}

designs but does not give such oonfidce whc one is faced with unusual designs.

It should probably be adiitted that ifl the pest research workers have to a

o.aidereb1e extent been content with

_{t his approach.}

_{As regards (2) it is}

important to ezilain and assess in detail the various parte making up the

reels tance aM propulsion

_{picture both of the model and of}

_{the ship.}

_igeinst

this bnckground the control

_{of flow conditions on the modele the accuracy of the}

metha a of extrapolation of resistance and propulsion, and the correct

_asseesuent

of the ship performance melding

_{knowledge of the roughzims effects}

_{on the ship,}

ail assume

_{eat isportance.}

_{The interpretation of results end}

_{the ultiate}

value of the deductions

_{depes on the success achieved in these directions.}

Taking the

_{del ed first it will probably}

_{not be diauted tha.t the}

resistance (including speed) is

known to within

_{T1 and the propulsive efficiency}

to within 2.

The hull analysis factors

_{are not de ¡irìed with the same}

accuracy.

_{It isay be assumed that the xx1el hull and propeller aro technically}

5(500th, i.e. they áo not

oase in frictional resistance due to

roughness effects.

It is isportant

)

I1&U]

that the bouniar7 layer flow

is t'bulent, aM this is g emily a

bievecì by fitting suitable

stilatore

at or

ar the boy,

th

al.

so far as the model

propeller is coricerz

zio special preceutiozie are

taken except to evoii the use

of very sii%dl propellers

(roto critoriozi tìuoptet by I.C.T..)s

It is assumed that flow coiiti

on the

propeller are turbulent.

Root researches izïiicate that this may

not be an

6itirely

tisfactozy assumption, although the effect on force measureenta of

a limited

eiflOUXlt 0f laminar flow ta prcbably within the accuracy

of the experiments.

If the normal 1rode extrapolation

coefficients are coepard with,

sr, the ckoenherr

line for eaooth turbulent flow, it will be founì

that the

ente give

exoOsE over the

ohoenherr lino which varies idth

the size nd apev

of the vessel, the excess being of the oror

of lQ

for

horter 1engtia

v1

f GD log!er length.

It is i,lear, therefe, that the

Ñ'oe CfjL

_:iits

.l1 lot L4v

_{consistent rult for all lengths az1 speeds}

of vesse1a.'

f ri0t1i liae

the above dim

rwi at low noi4

sae evjj erxe to show that the true

inimnwn turbulent

h,jbe Ateeper than the 5choeztherr

et.nd if that is true

4'

11 be j flcrcssed.

The epecif

of

odel

sp

3d13 Ue above the

herr 1ir,

_{alzi This incresc}

zwy be dcribed as fm resistance arising partly froa

VelocJty e1f0t

_{on tho}

friction azI partly frocs unbalanced preseure effects.

_{The question of the}

crrect way of dealing with this "fozei effect" arr! whether or not it is subject

to søale effoet is a matter of importance which is being investigated

_{at the}

present ti.

Other methods of overecnirig these difficulties he#e been

_proposed

but they will not be discussed here.

It is not general practice at the mOEsont to make

any adjustment of

ax,del pr.dietions for scale effects in propeller thrust azzl torque,

or in

ke

factor' aM thrust deduction factor.

These are known to exist in varying

degrees aM until satisThctoxr methods of allowing for thes

are worked ont

the dtsiled comparison of ship sod model results reterru to earlier cannot

be achieved.

As regarrs prptller faotor

tie cre i.ti

cts

e svoidul by

arznging the scale BO tht a oerte.tn rnixLRv.

r

ia eeed. on the blades

but

there is still a good deal of iwrsrLce regardthg slow

seals effects which xY

take 1ace between this point aM the full scale.

Efforts

re biflg je to

explore this field.

tts wake azrl thrust dIuatiorL scale

etfect it is probable

that the 1&tter ja negUgible, but it is krovn

that the f oziner is

sterial1

w

the writer has propos

a

ethcd of dealisg with this in a z'sct paper

to

the North East Coast Institution of flgineers &

thipbuilders.

There is always

an e1cent of doubt reerding the

effect of ship's roughness whith

texas to

offset the reduction of wake factor on th. ship vs

coçred with the sod el.

The application of this iske correction to a abcr of good trial results

iïLtoat* a good areenimt between torque coefficients on ship aM

del but

a considerable eesa of ship thrust

coefficient canpared with the rodel.

In gere1, t1efore, frc

the nodel

mi, *hile the baLto rults

are known to a good degree of aceursay,

the abolute value of the ship preijotions

is

xzr in

ø doubt for the varis reesons stated.

Turnuzg now to the ship sido of the picture, it

is beccxning increasingly

clear that there are very nany difficulties in

obtaining ccpletely satisfactory

ahip trial results.

Ordinary eoercial trials are subject to a number

of

possible errors, and the cxIparison of the results

with the predictions from the

model teste is to that extt fortuitous.

it is important that the ship surface is snooth a:i

clean.

Experience

S LIII'?

has shn that a surface which looks clean but

has a

feeling can increase

the pcy,ior to a material extent.

ilecent research has also shown the important

influence of structurel roughness ori resistance so

that anart frc

the saving

S

steel weight, the adoption of flush welding on the

shell offers a substantial

saving in power if the hull is kept clean.

In any given case the effect of

tise out of clock depenla on the location end xvemcnt of

the ship in the

intervening period, the time of the year, crxl the nature aM

quality

of the

*int or other finish used.

The only completely satisfactory method

is to

dock the ship i&miately before trial aM to Meavour to assess

the rghnezs

by

aaureiìcnt befc*'e unlocking.

Considerable interest attaches to the study

of the thic1Qes of the bouzxìary layer s

to the

velocity distribution

in the bowzary.

It is probable that with the

accumulation of infortion on

these points it '411 be pructicable

to assess roughness increases

from a

aìeasurenent of the bourilarj layer.

If the trial is conduot&1

in anything but reasonably good

weath

it

is difficult to correct for the

wiM resistance, and almost impossible to correct

for roui sea effects.

In any case, the wind direction azz intensity should be

measurad si

sccc attempt made to aases&

the sea cozlïtioi.

Assuming a clean

paintod ship si1 good weather

conditious it is important to ensur

steady

conditions on the mile runs by having an

adequate approach ron and maintaining

Steady engine conditions.

If these arc achieved the speed results should be

reliable ar

they c an also be j

from e tidal analysis, vthioh

should give

a roesonable curve in

relation to the local coeiitic'ns.

The measurement of revolu. ons can be made accurate by continuous

recording but a count of total revolutions over the

duration of

sh run is

also a reasonably accurate method.

The measurement of torque and thrust la more

difficult.

Torque

derived from indicator cards is goncrally unreliable,

especially conaideririj;

the correction necessary for the mechanical

efficiency of the enire.

This

is

articulariy true of diesel machinery.

Torque from a good torsionzeter which

has bei calibrated on the shaft should be correct to

provided the zeros

arc satisfactory, but torque

from a torsioineter fitted to an

uncalibrated shaft,

even assuming satisfactory zeros, may

be in error by

5. ,

especially if the shaft

is stiff in relation to the power transmitted.

The mnsuremcnt of thrust is of

cat value but it is difficult and

relatively expensive to do this riith u' etui accuracy.

Existing instruments

such s the i4ichell and the Kingsbury, even 11 they measure the thrust both

on the ahead and the astern

side, are probably not reliable to less

than

The checking of zeros is difficult arti

i

generally confin

to a chocking

of the accursey of the pro

ure zcouring instrument.

The coxcluaion one arrives at from theac

cirations is that

for re*lly useftil

n.lysis of Bhip triMls in relation to

1)Cael

teats, the

orctin*ry ccxxmercia1 trial 1

of vexr liiuîted v1ue, a

the c&j

to mke

a ueeful

vince in thiz rnatter is to nake a particular

study of e limited

number of ves

is, whith would be required for triel purpos es. ror roas

onab]. e

periods of time.

Various E1Îs efforts are being direetett in this direction ax

useful results should be obtsined in the not dIstant future.