Analysis of model experiments trial and service performance data of a single-screw tanker

ANALYSIS OF MODEL EXPERIMENTS, TRIAL

AND SERVICE PERFORMANCE DATA

OF A

SINGLE-SCREW TANKER

By J. W. BONEBAKKER, Member

SYNoPs!s.In Section 1, complete data and regression equations are given for the model and for the ship under trial conditions.

In Section 2, the regression equation for average service conditionsvessel30

weeks out of drydockis computed from a limited number

of observations. 990 sets of records are grouped according to prevailing weather conditions (wind direction relative to the ship's course, and wind force).

In Section 3, service performance allowances on d.h.p. (tank) are ascertained

for each weather group. These allowances comprise four components; scale effect, difference in roughness between model and ship, difference in surface

roughness due to fouling, resistance due to the ship's motions. An attempt is

made to segregate these components.

In Section 4, ships' service performance analysis is considered from two angles: the scientist's and the shipowner's They have one thing in common; trialspeeds under ideal conditions. The scientist aims at an accurate prediction of trial

speeds based on model experiments; for his investigations a high degree of

accuracy is absolutely necessary. The shipowner is interested in a quantitative

comparison between the performance of individual ships, as expressed by the difference in power required for a certain speed under ideal trial conditions (the tank prediction), and under well-defined service conditions of weather and fouling. In his case, the abundance of records is expected to compensate theirunavoidable inaccuracies.

I. Model Experiments and Trial Analysis

Routine self-propelling model tests were carried out at the Wageningen Tank. The results are tabulated in Table I, and d.h.p. is plotted against speed in Fig. 1.

The apparent slip values SA are based on the propeller's mean virtual pitch

H' = 4803 m (about 15 ft. 9 in.).

The other characteristics are

4 blades; D

5750 m (about 18 ft. lin.);

FAIF

= 0400. The model's

regression equation is

d.h.p./(01 N)2 = 0050

SA

+ 450

(1)

Its calculus is shown in Table 2.

One of the fundamental assumptions of the method is a constant

wake

fraction r; this assumption will hold over the small range of speeds selected,

i.e. 12 to l45 knots.

The tank regression equation is plotted in Fig. 2. For = 100 per cent., the value of d.h.p. /(0 IN)8 becomes 950. This extrapolation has no physical

meaning. The assumed linear relation between d.h.p. 1(0 1N)3 and SA is valid

only over the range of propeller loads occurring under

service conditions.

But the intersection of the straight line with the ordinate for 100 per cent. apparent slip Qoint A in Fig. 2) has an important feature. If we substitute

various r values in the regression equation (see Appendix), we get a fan of straight lines, each with its own particular constant value, but all passing

through A.

Trial results are given in Table 3. The vessel rolled and pitched slightly.

From these data mean values of d.h.p. ¡(O- IN)3 and SA are computed in

Table 4.

Though against the Author's principles set forth in previous publications, let

it be assumed that the ship's regression line passes through the spot representing the trials' means and point A of Fig. 2. Obviously this cannot be quite correct, because we are shifting now from model to ship, and consequently scale effect

comes into the picture.

Butas stated beforewe are only concerned with SA values occurring

under service conditions. _{Even an appreciable difference between the values}

of d.h.p. /(0 IN)3 at 100 per cent. SA for ship and model will entail only small

inaccuracies over the vessel's slip range (say from 10 to 30 per cent).

The ship's regression equation for trial conditions is

d.h.p./(O'1N)3 = 0056 SA + 388

2. Service Performance Data

I

First of all, the relation between wind force and

wave height was investigated. In Table 5, frequencies

are tabulated, the abscissae being wind forces, the

ordinates wave heights in metres. In the majority

of cases the directions of wind and waves coincided;

so we may speak of" the direction of the weather." The direction of the weather relative to the ship's course is grouped according to Fig. 3:

Fig. 3

The vessel's service performance data include the following records, taken

during each watch; in total 990 sets of observations are available: I. Ship's course;

Wind direction and wind force; Wave direction, height and period; Ship's speed through the water (V); Propeller revolutions per minute (1V). Only eleven power records were taken (Table 6).

In Fig. 2, a straight line is drawn through spot A, having coordinates SA =

100 per cent, and d.h.p. ¡(0

_{i N)3 = 9-50, and the mean of the Il records of}

Table 6. _{This is the regression line for service conditions, the average time}

out of drydock being about 30 weeks. _{The service regression equation is:}

d.h.p. (0- i N)3 = o-052 SA + 4'31 _(III)

The eleven sets of records of Table 6 are included in the total of 990. By

substituting the other 979 records of N and S in equation (III), we will get a fairly accurate figure for d.h.p. during each watch.

The next step has been the grouping of all records according_to:

Weather direction (I - II - III - IV, see Fig. 3);

Mean wind force _{(zero, I and 2, 3 and 4, 5 and 6, and 7 degrees Beaufort);} Average time out of drydock (weeks) (see Table 7).

ANALYSIS OF MODEL EXPERIMENTS OF A SINGLE-SCREW TANKER 477

3. Analysis of Service Performance Allowances on d.h.p. (tank)

In the last column of Table 7, service performance allowances are given as

a percentage of d.h.p.

(tank). Each allowance comprises the following

components:

Variations in surface roughness due to fouling.

Resistance due to the ship's motionscaused by wind, waves and

steeringand air resistance.

Difference in surface roughness between ship and model. Scale effect.

Our first aim now is to eliminate the iniluence of component I for weather

group zero (flat sea, no wind). It is believed that it must be possible, without

undue trouble to the ship's personnel, to take at least once a week accurate and

Simultaneous records of engine output, propeller revolutions and ship's speed. This would enable us to compute the ship's regression equation for periods of

(say) IO weeks.

Such information not being available for the tanker under consideration, an

estimate has been made of the influence of variations in roughness on regression

equation (III).

Substituting 5A 15 per cent, in equations (II) and (III), we find

d.h.p./(OIN)3 = 473 (trial conditions);

d.h.p./(0lN) = 5'1O (service conditions, about 30 weeks out of

drydock).

Hence it is estimated that, at constant SA, the values of d.h.p. ¡(0' lN) will vary about 8 per cent, on account of variations in ifr (wake factor), due to the

degree of fouling, which is proportional to the period out of drydock.

For all records of N and V, taken simultaneously under weather conditions zero, the corresponding d.h.p. has been calculated by substituting N and Vin equation (III). These d.h.p. values are then corrected for differences in D (weeks out of drydock).

At D = 30 weeks the correction is zero.

The

corrected d.h.p. values are compared with the d.h.p. tank values for the same speed, and the differences are expressed as a percentage of the latter. See

Table 8.

The percentage fine-weather allowances are plotted in Fig. 4. Table 7 shows

that for zero (fine) weather conditions and the ship l94 weeks out of drydock

the allowance on d.h.p. (tank) is 27 per cent. For all the other weather

classifications, this allowance can be split up into a fine-weather allowance

corresponding to D, and an additional percentage due to weather direction and wind force. This analysis is shown in Table 9. The corrections for variations

in D are small, (see column 5 in Table 9). The allowances of column 6 are

plotted in Fig. 5, giving a clear impression of the influence of weather direction,

relative to the ship's course, on speed.

An allowance of 27 per cent, covers the gap between d.h.p. (tank) and d.h.p

(ship) in fine weather and the vessel about twenty weeks out of drydock. For

D = zero, this allowance should be sixteen per cent. (see Fig. 4). Incidentally,

this sixteen per cent, is about the same as the "normal" allowance on d.h.p.

(tank) as applied by Continental tanks when converting model results into

fine-weather trial predictions. This percentage includes, as mentioned above, the difference in surface roughness between ship and model, and scale effect.

A very accurate assessment of this allowance and of its components, should be left to the experts of the model tanks, because this information is primarily

In Fig. 6 a comparison is made between the weather allowanoes of the motor

tanker and a passenger steamer. Their leading characteristics are given in

Table I O. The difference between the two vessels is obvious, and as might be expected. With the weather head on, the fuller and slower tanker requires

slìghtly larger allowances. In a following sea, the finer and faster passenger steamer gains speed under wind forces up to 45 degrees Beaufort; the tanker does not show any benefit.

In the preceding pages, an example has been given of the method of analyzing ship service performance data. The principal developments since the Author's

1951 paper are:

I. Average regression equations are computed for a limited periodsay 10 weeksfor displacements corresponding to i 0 to 08 loaded draught.

Each equation represents a straight line in a diagram, where d.h.p. ¡(0 iN)3 is plotted to a base of apparent slip SA. Only two spots are required to locate this straight line : _{spot "A" for SA = 100 per cent, taken from}

the model, and spot "B" having the co-ordinates of the average of a few accurate records of V,N and d.h.p. taken simultaneously (see Fig. 2).

2. The introduction of Telfer's weather factor has been abandoned.

It

has been found that high correlations exist between wind direction and wave direction, and between wind force and wave height or state of the

sea. For the purpose of performance analysis, weather conditions can be characterized by wind force and wind direction (Fig. 3). The same

procedure has been advocated for many years by other authors. For further development, co-operation with the meteorological institutes is

essential.

The'regression equations of the three straight lines of Fig. 2 are:

The propeller's virtual pitch Hp' being 4803 meter,

/

Vxl852

_\

V

S4=IOOl

_{60XNXHv)10037}

Substitution in the above equations transforms them into

1. Model d.h.p. '

= 0'0095 N3 - 003215 VN

2. Ship, trial d.h.p.

= 0'0095 N3 - 003615 VN2

3. Ship, service d.h.p. = 00095 N3 - ØO3335 VN2

These three equations enable us to plot the relation between N and d.h.p. for constant speed; see specimen for V = il knots, Fig. 7.

Suppose this speed is to be attained under the following circumstances: wind direction 2, wind force 3.

For weeks out of drydock zero 26

From Fig. 4:

Fine weather allowance 16 % 300/

From Fig. 5:

Weather allowance 22 % _22%

Total allowance 380/ _52%

From Fig. 2:

Tank d.h.p. for V = il knots

2,050 2,050 Total allowance, horse-power 780 1,065

Total d.h.p. 2,830 3,115

From Fig. 7:

R.p.m.

841

845

Corresponding r.p.m., model 81 8

839

1. Model

d.h.p. ¡(01 N)3 = o050

+ 450

2. Ship, trial d.h.p./(0 IN)3

0056 SA + 388

ANALYSIS OF MODEL EXPERIMENTS OF A SINGLE-SCREW TANKER 479

The same equations enable us to compute the speed losses, due to adverse weather, for constant horse-power.

This is illustrated in Table li.

Table li is based on over 600 sets of observations in which the d.h.p. records

ranged from 3,240 to 3,355. These values are considered to be "constant ": 3,300 d.h.p. plus or minus i 8 per cent.

4. Concluding Remarks

It seems to be appropriate to formulate a general survey of the subject under consideration.

In the Author's opinion, ships' performance analysis covers two distinct

fields of

interest:-I. That of the scientist, or model-tank experimenter, who aims at an accurate

prediction of the full-size ship's actual performance under ideal

conditions: ample depth and width of water, no currents, no wind,

no waves or swell, and a ship's bottom of specific smoothness.

2. That of the shipowner, who is interested in a quantitative comparison between the sea-going qualities of different ships, as expressed by the

difference in power, required for a certain speed, under ideal conditions and under well-defined service conditions of weather and fouling.

Both interests have one thing in common: the vessel's speeds under ideal

conditions.

The scientist has to deal with scale effect and the difference in roughness between model and ship; fouling and weather conditions are the owner's

concern.

For the scientist's investigations a high degree of accuracy is absolutely

necessary. It is hoped that his main problems will be solved by Wageningen's Victory programme of research. These experiments, with paraffin models of

a Victory ship, on scales = 18 to 60 should disclose the laws of scale effect. Similar experiments with the D.C. Endert Jr. ( = 6) and with actual Victory ships, and experiments on surface roughness of paraffin models and actual

vessels, will bridge the gap between model results and fine-weather trials. For

merchant vessels, the time available for such trials is restricted to a few days. Hence, records can never be numerous, and so they should be accurate and

reliable. This necessitates, on the part of those collecting data, full knowledge

of the recording instruments and experience in their handling. This can only be achieved by experts, and not by the ship's personnel. This view seems to be generally accepted, as witness statements on behalf of the British Shipbuilding

Research Association and such authors as Brard and Jourdain, Aertssen,

Kempf, and others. The ships' personnel, on the other hand, can collect quite numerous data, though not of the refined accuracy just mentioned. These data

are restricted to:

1. Technical records:

Ship's speed through the water, Propeller revolutions,

D.h.p. (b.h.p. minus 3 per cent friction losses), Fuel consumption.

2. Nautical recôrds: Ship's course,

Wind direction and wind force,

ANALYSIS OF MODEL EXPERIMENTS OF A SINGLE-SCREW TANKER

The ship's personnel are well acquainted with the items enumerated above. What is lacking is the Conviction that nowadays this information can be, and shonid be, reliable and accurate to a degree, and that it pays to collect such

information.

In the bygone days of the coal-burning steamer it was impossible to figure out fuel consumptions during short periods. Indicator cards were only taken

to check the proper working of the engine's steam and exhaust control

mechanism. Log readings were useful to make a guess of the ship's position

after a period of steaming during the night or in a fog. Under such conditions it was quite unnecessary to record r.p.m. in tenths! Nevertheless, the old-time sea-going engineer has always been interested in apparent slip (and so in r.p.m.

and ship's speed); for SA reflects the external conditionsweather and fouling

under which the ship is sailing.

To-day, the majority of merchant vessels are propelled by Diesel engines, and

steamers are oil burning. Accurate and reliable recording, during short

periods, of fuel consumption and r.p.m., does not present great problems.

Ships' logs and torsion-meters are being improved, and propelling power

measurements can be checked with simultaneous records of fuel consumption,

exhaust-gas temperatures, and reports on the main-engine's shop trials. So it

is quite possible to ascertain, during the first months of a vessel's life, her

"standard performance ", by computing her regression equation. It repre-sents the "exact" relation between V, N and d.h.p. ruling during a period of

(say) ten weeks. In conjunction with shop-trial records on the relation between

fuel consumption, r.p.m. and engine output, the "standard performance"

serves two purposes:

L It is the only means of checking accurately simultaneous records of fuel

consumption, r.p.m., horse-power and speed as to their reliability, and of judging the fuel consumption per h.p. per hour.

This should be of

primary importance for an economic exploitation of the vessel; undue speed losses and excessive fuel consumptions will become apparent

without delay.

2.

By comparing horse-power and speed with the nautical conditions

prevailing at the time, we can properly assess the loss of speedor the

excess of horse-power required to maintain speedon account of the

weather.

Strictly speaking, a ship's regression equation will be valid only under the restriction that the wake fr remains constant, which implies constant speed, constant draught and trim, and constant roughness. None of these conditions is fulfilled in service. However, for the practical purposes with which we are concerned, the resulting inaccuracies are tolerable, provided that the" standard performance" is computed from records collected during a period of limited duration, and for a small range of speeds.

During each successive period, the ship's regression equation should again

be computed, and compared with her "standard performance ".

Analyses

of service performance data of several vessels have shown that the lowering of

their performance due to fouling, and its improvement after docking, comes out clearly. The left-hand end of the regression line, shown in Fig. 1, is raised at the end of each successive period; it drops again after docking, but never to

its original level. This phenomenon covers also the effects of deterioration of the propeller's surface, and its distortions. In this way, the development of

a vessel's performance can be easily supervised during her lifetime.

As to the main engine, it will be borne in mind that its fuel consumption is bound to increase, and its mechanical efficiency to decrease, due to wear and tear of all moving parts.

Turning to the nautical records, it will be agreed that the ship's course can be stated fairly accurately. The direction of the wind will be less constant,

ANALYSIS OF MODEL EXPERIMENTS OF A SNGLE-5CREW TANKER 481

-and is related to six sectors of the horizon.

Assessing the wind force is a matter of training and experience.

As for a true description of the

sea-direction, height and length of waves and ¡or swellit is more often than not a matter of guesswork. However, do not let us be too pessimistic. On the North Atlantic the wind force is below six degrees Beaufort during two thirds

of a year.

This is one of the roughest sea routes; many others are much

better. Under such conditions, the daily assessment of wind forces and wave heights, by innumerable officers on watch all over the world, should yield

fairly accurate average values. At all events the accuracy will be sufficient for

correlating speed losses and weather conditions, and for classifying different vessels accordinglyexcluding heavy and very heavy weather. This will be of interest for medium and large ocean trading merchant ships.

In any case, technical and nautical records are collected on board every merchantman at sea. In the past, these records were often incomplete and

taken in a haphazard way; nobody asked why they were collected. Nowadays,

it is possible to get a wealth of information about the performance of each

individual ship under service conditions, which can be of reasonable reliability

and accuracy. It should become a tradition to collect records systematically, at regular times; they should be complete, and taken simultaneously. This

cannot be achieved unless the officers and engineers of the merchant navy understand how and why service performance records are collected, and what

their analysis may disclose. The analysis iteslf should be executed by one or two qualified members of the shore staff. They should have spent some time at sea, and should be acquainted with the fundamentals of ship propulsion and

statistics. In this way the benefits of the analyses are immediately and directly

at the disposal of the owner, who learns to know his ships individually. The shipowner might take the opposite view, and leave the whole matter to the scientists and the shipbuilders.

No doubt their experts will be much

better equipped than the ship's personnel to collect information of wider scope.

Their data must be much more accurate, because they cannot be numerous; but it will take a long time before the shipping community can profit from the

resulting general conclusions.

Acknowledgment

The Author wishes to record his appreciation of the investigations carried

APPENDIX Symbols:

D = screw diameter m

d.h.p. = delivered horse-power 75 kgm.sec-'

H

= pitch

m

Km = torque constant of the screw

-M = torque on the screw kgm

N revolutions of the screw mm-'

S = apparent slip percentage

n = revolutions of the screw sec-'

= apparent slip

-y = ship's speed m sec-1

ve = intake velocity for the screw m sec-'

A = velocity coefficient

-= wake fraction Çraylor)

-p = density of fluid kgm-4sec2

Derivation of the Regression Formula

It is assumed that a linear connexion exists between the torque constant of the

Screw:

M

Km

-pD1n2 and the velocity coefficient:

nD This linear connexion is represented by the equation

Kmrao+aiA

(1)

a0 and a1 being constants (see Fig. 8)

Whether this assumption holds good can be verified by considering the diagrams of the Wageningen B series of screw propellers.

The interval with which we are concerned in actual practice is small (about 20 per cent.) in comparison with the total range of the diagrams.

We can write: 2 ir Mn 75 d.h.p.

d.h.p. = ---- or M

-2 n (metric units) in which Further: or By substituting this in Km d.h.p. Km = 0 (0-IN)3

v(l)

nD nD

A=A'(l -)

63x75 -= 2 ITPDT is a constant y y H H

in which: A'=

₌

. - = (1 Sa)

(2)

M

ANALYSIS OF MODEL EXPERIMENTS OF A SINGLE-SCREW TANKER 483 By substituting (2) and (3) in (1) and (4) we get:

d.h.p.

(0lNj

=ao+a1A'(l lr)

(5)

and d.h.p.

_{=a0+a1(1 Sa) -}

_{(1 - i/r)}

₍₆₎

or d.h.p.

_c153+c

₍₇₎

(0 IN)3 in which:

i H

20irpD'H

c1

= -

a1(it)

_{Tb = -}

_{x 75}

.a1(1 fr)

i

_[ao+aii

fr)H]

2OOOiTPD&[+(I _r)i]

C

_{- 603x75}

Equation (7) will be referred to as "the regression formula". The applicationof this formula to overload propulsion tests shows that for model-experimentsit can be assumed that c1 and c are constants.

Hence, variations in iii can be neglected in this case. The Influence of Variations in the Wake Fraction 1r

Under service conditions, a ships wake will not be constant, on account of changes in draught, fouling of the bottom. If we substitute various values in formula (7), we get a fan of straight lines, each with its ownparticular value. All these lines intersect in one point A, where Sa = 100 per cent. (see Fig. 9; also Telfer,"Marine Propeller and Propulsion Miscellany," N.E.C. Trans. Vol. 68).

This can be seen with the aid of formula (7) by substituting Sa = 100 per cent.

we get: d.h.p. 2,000 ir p D5 ₈

(0lN)5 60 x 75

This value is a constant and independant of

l-So, ail the regression lines with different sfr values pass through the point A with co-ordinates:

d.h.p. 2,000 ir p D5

(0 iN)3

60 x 75

00

S = 100 per cent.

Analysis of overload tests on a model with three grades of hull roughness and with variable draught confirms the accuracy of this thesis.

It must be stated that point A has no physical meaning in itself. The assumed linear connexion between Km and A is only valid in the normal performance range of the screw and cannot be extended to Sa = 100 per cent.

Estimating the Co-ordinates of Point A Case i.

From service performance data only. Three conditions should be fulfilled: I. Variations in draught to be small.

Variations in hull roughness to be negligible.

The range of Sa values to be large enough to determine the slope of the regression line.

With the aid of regression analysis* the position of the regression line can be determined.

Extending this line to Sa = 100 per cent, gives the ordinate of point A. Case 2.

From the results of overload model tests. Apply the same method.

Case 3.

From open-water tests of the screw propeller.

Estimate Qo as shown in Fig. 8, and substitute in equation (8).

When we use the methods (2) and (3) it is clear that a certain amount of scale-effect affects the position of point A. However, scale-effect influences only the slope of the regression line. It is easy to see for instance, that, an error of about 5 per cent. in the correct position of A gives errors of only 0's per cent. or less in d.h.p. at normal

slip values. (0' iN)

TABLE i

s" The Application of Statistical Methods to the Analysis of Service Performance Data ". See the Author's previous paper, N.E.C. Trans., Vol. 67.

V

knots Corrected for RA Without correction

d.h.p. N SA% d.h.p. N SA% 8-0 804 54-2

5'l

8-5 971 57'8

54

9-0 1,154 61-3 5-6 9'S 1,351 64-8 5'7 10-0 1,565 68-3 5-9 10-5 1,792 71'S 5-5 11-0 2,041 74-9 S-5

ll'5

2,322 78-2 5'4 12-0 2,650 81'9 5-8 4,055 91'4 15'6 12-5 3,035 85-7 6-2 4,644 95-6 15'9 13-0 3,481 89-6 6-7 5.298 99-8 16'2 13-5 3,999 93'8 7-5 6,000 103'9 16'S 14-0 4,615 98-2 8'3 6,782 108-4 16'9 14-5 5364 102-7 9-2 7,656 112-8

l7'4

15-0 6,244 107'5 10-3

ANALYSIS OF MODEL EXPERIMENTS OF A SINGLE-SCREW TANKER 485 TABLE 2 337-3678

=

66-16

=

0-6643 = 2052-07

;x2 = 152-5

xy2 = 263-13 XY= 789-243

xy =

13-135 d.h.p. (0 IN)* cl =

=0.050

= 5-089

= 11-73 c

=Tc1=4-50

R = O-993

f

= 0-0282;F=0-55%?

TABLE 3

= 0-050S4+4-50

Windforce, degree Beaufort Relative wind direction.. Depth of water Wave height Draught aft

..

..

Draught forward

. Tv

6 20° Port bow 21-0m 2-0m 8-92m 8-31m 6 50° Starboard stern 21-6m 2-0m 8-92m 8-31m V(KN) dh.p. N (OiN)3 SA

(0iP')

12-0 2,650 81-9 549 4-83 5-8 12-5 3,035 85-7 629 4-83 6-2 13-0 3,481 89-6 719 4-84 6-7 13-5 3,999 93-8 825 4-85 7-5 14-0 4,615 98-2 947 4-87 8-3 14-5 5,364 102-7 1,083 4-95 9-2

is-o

6,244 107-5 1,242 5-03 10-3 12-0 4,055 91-4 764 5-31 15-6 12-5 4,644 95-6 874 5-31 15-9 13-0 5,298 99-8 994 5-33 16-2 13-5 6,000 103-9 1,122 5-35 16-5 14-O 6,782 108-4 1,274 5-32 16-9 14-5 7,656 112-8 1,435 5-34 17-4

Speed, knots

..

.. V

Il-80 1254

B.h.p. minus 3%..

.. d.hp.

3,425 3,475

R.P.m.

..

..

- - N 89-67 91-50

mean

TABLE 4

TABLE 5Relation between Wind Force and Wave Height

Selected observations are

d.h.p. (01N)3

(flJj

V N SA% 3,425 721

475

1180 8967

154

++

3,475 766

454

12S4 9150 119 465 137 Beaufort . degr.

fre-wave.

height.m..

0 1 2 3 4 5 6 7 quencies

<025

66 17 3 86

025<075

6 31 110 81 1 ₂₂₉

075<125

8 14 105 218 71 _{2 418}

125<175

4 2 38 209 114 26 393

175<225

2 3 18 54 77 27 1 182

225<275

8 2 10 16 33 18 3 90

275<325

12 14 15 18 60

325<375

2 5 8 6 ₃ 24

375<425

1 6 2 ₆ iS

425<475

2 1 ₄ - 7 Grand total 94 69 284 593 317 103 30 14 1,504 Percentage 6 4 19 40 21 7 2 1 100 Selected observations 66 45 215 427 191 45 24 10 1,023 Percentage 6 4 21 42 19 4 2 1 100

Meanwave 044 062 098

130

172 235 308 396

1,504 Height,m

000 065

075 124

170 220 313 420

1,023

ANALYSIS OF MODEL EXPERIMENTS OF A SINGLE-SCREW TANKER 487

TABLE 6Service Performance Data including b.h.p. Records

r No. D B b.h.p.m N V d.h.p. SA (01N)3 1 33 3-4 IV 3,700 885 122 3,592 518 113 2 34 2 IV 3,710

888 122 3,602

514

117 3 6 0 0 3,817 90'8 123 3,706

495

129 4 16 1 I 3,476 891 122 3,375

476

120 5 23 4 IV 3,548

886

120 3,445

494

129 6 28 3 IV 3,704 895 120 3,596 501 138 7 34 3 IV 3,673 893 116 3,566

500

165 8 39 2 II 3,669 879 114 3,562

524

166 9 32 1 III 3,688

889

118 3,580

509

147 10 38 3 IV 3,650

893

120 3,544

497

136 11 45 4 IV 3,686

887

112 3,578

512

188

mean 298

3,666

890

119

504

141

D = weeks out of drydock; B = Beaufort degrees (wind force);

d.h.p. = b.h.p. X 097

b.h.p. N V d.h.p./(01N)' SA

Range 3476-3817

879-908

112-123 4-76-524 113-188

TABLE 7

TABLE 8-(see Fig. 4)

wind force

weeks out of drydock number of observations r.p.m.

= average w.f.

= average time out of drydock

N = average r.p.m.

i)0

B

-

D n eN e V

-

N

-

V S4 d.h.p. ship d.h.p. tank Difference

dhp. %

O O 19-4 69 6013-4 817-7 87-2 11-9 12-2 3,275 2,580 695 27 I 1-8 21-4 61 5204-4 688-2 85-3 11-3 14-8 3,155 2,210 945 43

I 33 13-7 228 19364-6 2488-9 84-9

10-9 17-4 3,189 1,995 1,194 60

I 53 22-7

16 12803 149-7 80-0

94 24-4 2,857

1,290 1,567 121 I 7 39-0 2 164-5 18-0 82-3

90 29-7

3,258 1,154 2,104 182 II

18 26-5

72 6263-2 835-5 87-0 11-6 14-3 3,328 2,390 938 39 fl 3.3 25-5 153 13073-1 1697-3 85-5 11-1 16-5 3,231 2,100 1,131 54 II 5-2 32-9 18 1538-5 188-9 85-5 10-5 21-O 3,375 1,795 1,580 88 III 1-9 21-0 35 3048-6 412-0 87-1 11-8 12-9 3,292 2,510 782 31 III 3-2 22-4 68 5926-3 786-2 87-2 11-6 14-5 3,355 2,390 965 40 III 5-6 34-7 21 1809-6 228-8 86-2 10-9 18-7 3,384 1,995 1,389 70

N 1-9

17-5 72 6218-1 845-2 86-4 11-7 12-9 3,212 2,455 757 30 IV 3-3 23-1 163 14240-5 1897-0 87-4 11-6 14-7 3,387 2,390 997 42

N 5-3

16-2 12 1O356 135-5 86-3 11-3 15-8 3,299 2,210 1,089 49 D n

-

N V SA dh.p. ship Roughness correction % correcteddh.p. d.h.p.tank Fine weather service allowance% 0<10 18 89-7 12-3 11-8 3,552

-4-1

3,406 2,870 19 10<20 17 89-0 12-1 12-6 3,497

-2-5

3,410 2,720 25 20<30 22 82-8 11-2 130 2,829

09

2,804 2,155 30 30<40 6 88-4 12-1 120 3,407 +0-7 3,431 2,720 26 40<50 6 89-0 11-9 140 3,553 +2-3 3,635 2,580 41

B =

D =

n =

N =

ANALYSIS OF MODEL EXPERIMENTS OF A SINGLE-SCREW TANKER 489 TABLE 9 Weather direction Wind force

-

Ti Allowance on d.h.p. (tank) n

Total For roughness

corresp. to D For weatherand sea

O 0 19-4 27 27+0 =27 0 69 I 1-8 21-4 43 27+1 =28 15 61 I

33

13-7 60

27-3 =24

36 228 I 5-3

227

121

27+1=28

92 16 I 7-0 39-O 182 27+7 =34 148 2 II 1-8 26-5 39 27+3 =30 9 72 II

33

25-5 54

27+2=29

24 153 II 5-2 32-9 88 27+5 =32 56 18 III 1-9 21-0 31 27+1 =28 ₃ 35 ffl

32

22-4 40

27+1=28

_{11 68} III 5-6 347 70 27+6 =33 37 21 IV 1-9 17-5 30

27-1 =26

4 72 IV

33

23-1 42 27+2 =29 13 163 IV 5-3 16-2 49

27-1=25

23 12 (1) (2) (3) (4) (5) (6) (7)

TABLE 10

TABLE 11Speeds maintained with 3,300 d.h.p.

Motor tanker Passenger steamer Hull

L(bpp)

..

..

..

148-74m=487'8"

128013m=420'0

L(onlwl)

..

..

152-31 128-587m B ..

..

22-25 17-932m Draught ..

..

..

8-57 7-315m Displacement

..

..

21,111 m3 11,906 m3 Block coeffcient

..

..

0-744 0,706 (1w!) Propeller D (diameter)

..

..

5-75 m 5-658 m P (pitch)

..

..

..

4-68 5-692 m

P(0-7R)

..

..

..

4-415 5-180m P(hub)

..

--

..

3-25

-P(virtual)

..

..

..

480

5-914m Number of blades -.

..

4 4 Fa/F

..

..

..

0-40 0-481

Speed, designed

..

..

125 knots 135 knots

vj,/Ejt

.. ..

..

0- 565 0660

1. Model regression dhp./(0- iN)3 d.h.p./(0- 1N)3=

equation..

..

..

0050sa ± 4-50 0084S5 + 660

2. Ship trial regression d.h.p./(0- iN)3 =

-equation..

..

..

00562s ± 3-88

3. Mean service regression d.h.p./(0- IN)3 d.h.p.f(0- 1N)3=

equation..

..

..

00519s + 4-31

OO7Só5a +6-838 Weather direction Wind force 1 3 5 I 117 11-10 100 II 11-8 11-35 10-6 HI 11-9 11-60 11-1 IV 11-9 11-65 11-3

SS 2500 2000 3-j 500 f000 50

ANALYSIS 0F MODEL EXPERIMENTS OF A SINGLE-SCREW TANKER 491

L j ji

r

Fig. i 9.0 9.5 O.0 f03 V (.K4) as f j .0 80 LS

a.

I

'3 _l0 O IO 20 30 4050 60 70 80 90100 O .34 MO TORTAN KER UNES OF REGRESSION

FIOIIRES NEAR SPOTS INSiLATE NUMBER 0F WEEKS OUT CF DP.Y20C1(

MEAN VALUE I I I AO.50 45 10 5 20 25 30 s Fig. 2 (For Fig. 3, e p. 476)

60 50 40 30 uz 20 o I0 o

-FIGURES NEAR SPOTS INDICATE NUKIBER OF OBSNVATIOND ON WICH THE AVERAGE

IS BASED

.

Il 22 6 IR MOTORTANKER

-TOTAL PERCENTAGE OF ALLOWANCE

-IN F-INE WEATHER PLAT SEA)

o I0 20 30 40 50

WBEI<S Oui OF DkYDOCK-.

ANALY1S OF MODEL EXPERIMENTS OF A S1NGLESCREW TANKER 120 -loo'. 20 -4

-

MOTORTANKER

FIGUREG NEAR SPOTS INDICATE

60 - NUMBER OP OBSERVATIONS ON WHICH THE AVERAGE IS BASED

40

-I) ALLOWANCE PERCENTAGE DUE TO

WIND, WAVES & STE EWING-RESISTANCE

8'.' ANGLES 0F INCIDENCE X_T 2) WAVE HEIGHTS Fig. 5

2/

I-/

/

/

/

/

6 7 BEAUFORT_ -E z t, C W

TT

ANALYS:S CF MODEL EXPERIMENTS 0E A SINGLE-SCREW TANKER 495

P MOTOITANKER SBPASSENGER STEAMER

IWEATHER HEAD ON

UrWEATHER FOLLOWING

FIGUEES NEAR SPOTS INGICATE NUMBER

OF OBSERVATIONS ON WHIC4 THE

AVERAGE IS BASED Fig. 6

/

/

/

/

/

/

/

/

3 4 5 6 7 BEAUFORT -.-. MEA DON FO L LO W IN G

4000 50 0 3000 ₂₀₀₀

a4 DHP

(0.1 N)

t

o

NORILL RAN$E 0F PROF'FLLOR 0AD

i

A

Fig. 8 Fig. 9 tyo(.= -Q1 loo e. o o 75 BO es 90 N (REV/MIK___.... Fig. 7 û. X o 2500

DISCUSSION ON

"ANALYSIS OF MODEL EXPERIMENTS, TRIAL

AND

SERVICE PERFORMANCE

DATA OF

A

SINGLE-SCREW TANKER "

Prof. G. AERTSSEN, Member:

It is an astonishing fact that on a basis

of usual engine and nautical data Prof.

Bonebakker has successfully given such a sharp-lined picture of the performance of his single-screw tanker. Even with not very accurate but numerous records his statistical method leads to very important conclusions, which are of great interest not

only for the ship owner, as Prof.

Bone-bakker says, but also for the scientist. I

think, however, that not all

sea-going engineers will give voyage figures that are sufficiently accurate to make the analysis

proposed by Prof. Bonebakker. On the

other hand it is very difficult to have a good figure for the speed through the water by means of a usual log.

The general features of the conclusions of Prof. Bonebakker are very near to those of the Tervaere work of Ceberena, if it is taken into consideration that a wind force 3 ¡s a mean weather condition for southern routes of the Atlantic, or northern routes in summer (Table 12). The relation loss of speed-weather of Prof. Bonebakker is

TABLE 12Comparison of allowances: Prof Bonebakker's single-screw tanker and "Tervaete"

again very near to the relation loss of speed-windspeed of the Tervaete (15 per cent, for the tanker in weather force 5 against 12 per cent, for the finer Tervaete).

* Paper by J. W. Bonebakker. Member. See

p. 475 ante.

27

Dr. J. F. C. CONN, Member, and Mr. R. E. CLEMENTS, Student: Professor Bonebakker's aim is to popu-larize statistical methods for the analysis of ship performance data. In this, his second paper on the subject, he has made further developments on the original presentation given in his 1951 paper.

There can be no disputing the value of statistical methods in the practical problem

of ship voyage analysis. The time is

surely long passed when shipowners were

satisfied with the arithmetical means of

day to day readings, whether of distance

run, power or fueL The statistical methods

proposed by Professor Bonebakker are comparatively simple and can be used

by those who have no profound knowledge of statistics.

Since the 1951 paper, efforts have been made at the British Shipbuilding Research Association to make a serious study of ship voyage data. After testing various meth-ods it was concluded that Professor Bonebakker's methods were the best

available. Knowledge has accumulated slowly and it may be encouraging to the Author, although a little disappointing to

us, that the developments given in his

second paper have already been anticipated and adopted by us.

When applying the method put forward

Fouling Weather

Single-screw tanker s.s. Tervaete ..

30 weeks 8% one year 12%

Wind direction 2 wind force 3 : 22% Atlantic southern route: 19% Atlantic northern route summer : 24%

(t

in the 1951 paper, for obtaining a regression line of dhp/N3 against apparent slip, it was immediately obvious that change in wake due to fouling could produce a completely false regression line. Also, unless the speed is measured accurately, the wide scatter of values of apparent slip could again produce a false regression line. For this reason we resorted to the method, now suggested in

Fig. 8, of using open-water propeller

characteristics to determine the relationship between dhp ¡N3 and apparent slip, taking the tangent to the dhp/N3slip curve at the ship trial value of dhp/N3.

We agree that a different dhp /N3S0 line

should be used for ship trial and mean

service conditions. If heavy fouling is experienced in a particular port, it has been our experience that it may be necessary to take two lines for the service condition, one for the outward passage, the other for the return. We also agree with the Author's statement on p. 480 that the left hand end

of the dhp/N3Sa linearly drops to the

original trial line after drydocking. There is an obvious lesson here, which might well be noted by marine superintendents.

The estimation of the effect on

perform-ance, of direction of encounter of the

weather shown in Fig. 5, is an interesting development on the earlier work. Here again, Professor Bonebakker may be

encouraged to find that our results, derived from sea trials on a passenger-cargo vessel,

bear a striking resemblance to his own,

at least for curves i and iv. The curve of wave height for a given Beaufort wind force is also very interesting and is similar to the one we use based on meteorological data. This curve and the relationship between increase in resistance and wave height and

direction of encounter, lead us back to

the suggestion that, in unrestricted waters, the increase in resistance can be obtained

from a knowledge of the existing wind

force and direction, data which, as shown on the sea trials of the s.s. Tervaele,* the ship's personnel can usually estimate with reason-able accuracy.

Our present aim is to develop the regression equation to obtain the effect of time since last docking, weather conditions, etc.

This appears to us to be of

con-siderable practical importance, although the Author will appreciate the amount of labour involved in such developments.

The adoption of new ideas and new

methods in shipbuilding is always a slow process, and we trust that the Author will not be discouraged if all his findings are

not immediately accepted and used in

practice.

a 'Sea Trials on a Victory Ship. A.P.3 ". Prof. Aertisen, LN.A., Voi. 95. 1953.

Mr. J. LENAGHAN, Member of Council: Shipbuilders are very interested in

power predictions following model ex-periments, and particularly so, when these have a big bearing on the firm's contractual

obligations for a new ship. Therefore, it is of the utmost importance for this

purpose that figures for power, presented

either by the tank

authorities

or the

shipbuilders' own staff, ensure there is no

doubt that the

specified requirements, both as regards speed and fuel consumption, can be fulfilled without providing excessive

margins.

The correlation of model results with

service performance is never too happy,

unless there is complete understanding

between the three parties concernedthe

scientist, shipbuilder and shipowner. The scientist and shipbuilder can get together

and make comparisons between model and trial performance, but broadly, in many respects, this does not carry the matter far enough.

As the

Author suggests, without the goodwill of the shipowner and his personnel very little progress can be

made towards a proper analysis of the

service performance of the ship and estim-ates from tank results.

It is true to say that a "regression"

formula is only valid when all the factors remain constant, but is it not contradictory

to state also that some instability and

inaccuracies in these factors are tolerable? Recently in some research work carried out, a few inaccurate observations on sea trials gave an entirely false picture when attempting to correlate ship with modeL The importance of accuracy in the collection

of information, whether it be on board

ship or in other fields, recalls some words of Lord Kelvin which emphasize admirably

the thoughts of the Author" When you can measure what you are speaking of

and express it in numbers you know that on which you are discoursing, but when

you cannot measure it and express it in numbers, your knowledge is of a very

meagre and unsatisfactory kind ". It would be of inestimable help and of

great benefit to shipbuilders and

ship-owners alike if some arrangements could be

made to ensure the proper collection of

accurate data on the performance of a ship at sea, particularly in its early stages.

Mr. D. I. MOOR, Associate Member:

The Author bases his analysis of the

effect of weather conditions at sea on the

assumption that for a clean

ship the

increase in power required to maintain a

given speed may be considered to be

dependent only on the Beaufort number and the relative direction from which the

ANALYSIS OF MODEL EXPERIMENTS OF A Sll'IGLE-SCREW TANKER Dl 83

wind is blowing. This assumption is based on the premise that in the majority of cases the direction of wind and waves coincide, and that the height of the waves depends only on the strength of the wind blowing at the time.

While it is true that normally the wind and the waves it is creating coincide roughly in direction, it is not necessarily these seas

which form the major part of the wave

system meeting the ship, since they do not include swells remaining from previous disturbances. These swells may easily run in a completely different direction, and for the comparatively low Beaufort numbers

considered by the Author will almost undoubtedly be longer than the seas. In such circumstances they might quite well have a greater effect on the ship's motion and performance than the seas created by

the wind.

Only the height of the waves is considered in the Author's analysis and it would be

very valuable to have tables similar to

Table 5 showing the length and steepness of the waves. The relation between the mean wave height and Beaufort number obtained in Table 5 agrees well with figures

published by other observers, but it is

suggested that the frequencies apply only

Reply to Prof. Aertssen

To get the required nautical and propulsive data, special forms are given to the ship's officers, together with a written instruction. The instruction explains why it is necessary to get reliable observations, taken simultaneously. In this way the

ship's officer is prompted to greater care in recording data than he is used to devote to

filling in his time-honoured log book: Special attention should be given to the

measurement of the speed through the

water, and logs are still to be improved. The influence of fouling, being 8 per cent. after

thirty weeks out of drydock as

mentioned in Prof. Aertssen's Table 12, for the tanker is probably too small. From the Author's Fig. 4 it would appear to be

about 14 per cent., but this is not too

certain on account of the small amount of

data available at "zero weather" and

"more than thirty weeks out of drydock".

Reply to Dr. Conn and Mr. Clements It is particularly gratifying and heartening that Dr. Conn's and Mr. Clements's views on the analysis of service performance data

to the particular conditions under which they were collected. It appears that the

weather tabulated is somewhat calmer than average. The mean wave height, about I

metres, is lower, and the frequencies of occurrence of low and high waves are

respectively greater and lower than might be expected. Is it correct to infer from the difference in numbers of observations

recorded on Fig. 5 that the weather and

propulsion data were collected at different times, so that it is not really fair to plot the two together?

lt may be dangerous to accept that there is a sufficiently high correlation between

Beaufort number and the prevailing sea conditions to warrant the abandonment of

a separate criterion for the latter. The use of Beaufort number alone might give an indication of sea conditions (and there-fore of ship performance) sufficiently false to invalidate conclusions reached from the analysis of the small number of spots which may reasonably be expected to be obtained on a short observation voyage. The larger the number of readings available, the less the error is likely to be. and the Author has been particularly fortunate in being able

to obtain so many observations, from

which he has succeeded in making a very convincing and useful analysis.

AUThOR'S REPLY

are in full agreement with the Author's, and he fully endorses their final remark that we are not to be discouraged if all our findings are not immediately accepted and used in practice. It is all to the good that we are always exposed to the criticisms of others, who are also confronted with this fascinat-ing subject.

Reply to Mr. Lenaghan

Mr. Lenaghan rightly draws attention to the importance of powering ships without providing excessive margins. Reliable and accurate service performance data can be very helpful in avoiding this.

It is essential that all service data should always be strictly scrutinized by experts as to their reliability and accuracy; instabili-ties and obvious inaccuracies should be sifted out, and an explanation should be sought for their occurrance. Otherwise,

the service data collected by the ship's

personnel cannot and need not have the same standard of accuracy as required for information collected by tank experts.

The quotation from Lord Kelvin should always be borne in mind.

Reply to Mr. Moor

The conditions of wind and sea

en-countered by the tanker were recorded in accordance with the international weather code system. This method makes it easy to eliminate those cases where wind and swell, though coinciding in direction, make it impossible to estimate wave periods, and also the cases where two or more clearly distinguishable wave systems are running in completely different directions. Con-sequently, in our analysis only those cases are taken into account where the directions of wind and swell coincided, and where there was either only wind or only swell.

The influence of wave length on power allowance percentages is difficult to deter-mine. The introduction of wave length as

a parameter increases

the number of

variables to be correlated with power allowances, and a larger amount of data than was collected on the tanker would have been necessary.

Secondly, the effective wave length, Le, may differ considerably from the actual wave length, La, on account of the size of the weather direction sectors (60 degrees). If the wave direction relative to the ship's course comes within sector I of Fig. 3, then the effective wave length, L, may vary from

= L0 to L0

The relation between mean wave height and Beaufort number given in Table 5 was prepared from observations made on the tanker. In some cases the corresponding propulsive data were incomplete; this is

the reason why the total number of

propulsive data is not equal to the total

number of weather observations in Fig. 5.

The Author agrees

with Mr. Moor

that a high correlation should exist between mean wave height and Beaufort number if we are taking the latter as a criterion for sea conditions. In the case of the tanker a high correlation was found by using the "selected observations" of Table 5.