A review of ship model data

A REVIEW OF SHIP MODEL DATA

B

A. EMERSON, M.Sc., Associate Member

and N. A. WITNEY

A Paper read before the North East Coast Institution of

Engineers and Ship builders in Newcastle upon Tyne

on the 24th February, 1950, with the discussion and

correspondence upon it, and the Authors' reply thereto.

(Excerpt from the Institution Transactions, Vol. 66).

NEWCASTLE UPON TYNE

PLJBLISHeI) BY THE NORTH EAST COAST INSTITUTION OF ENGINEERS AND SHIPBUILDERS, BOLBEC HALL

LONDON

B. & F. N. SPON, LIMITED, 15, BEDFORD ST., STRAND, W.C.2 1950

THE INSTITUTION IS NOT RESPONSIBLE FOR THE STATEMENT& MADE, NOR FOR THE OPINIONS EXPRESSED IN THIS PAPER, DISCUSSION AND AUTHORS' REPLY

PARTICULARS ;Oi MEM111 of The Institution wilFbe supplied on application to The Secretary (for address see cover)

M4D AND PRINThD7N GREAT BRITAIN

A REVIEW OF SHIP MODEL DATA

By A. EMERSON, M.Sc., and N. A. WITNEY

(Communication from the National Physical Laboratory) 24th February, 1950

Smors's.The paper shows the effect of eliminating la,ninar flow errors in the estimation of model residwi,y resistance. Changes in dEsign required to obtain minimum resistance with single screw cargo vessels are described and the use of a bulbous box on such forms is illustrated The alteration in method of obtaining propulsive efficiency and the effect of the combined changes on the ship power estimates are also discussed.

Jutroduction

IN

a paper read before this Institution(') two years ago the Authors

presented results obtained with single-screw ship models during the

war.

These results illustrated "optimum" curves of resistance and

propulsive cfflciency.

Since that time there have been tWo major

changes in the method of estimating ship powers at Teddington; the

first is using the propulsive coefficient at the ship-trial loading instead

of at the model self-propulsion loading; the second is using a trip

wire to obtain turbulent boundary-layer flow over the modEl surface.

Neither change is completely satisfactory. There is some scale effect

on the propeller thrust and torque coefficients and in the wake speed

which makes the simple ship and model comparison inaccurate. With

the turbulent boundary layer the model residuary resistance can be

measured more precisely but the extrapolation to ship total resistance

needs further research.

It is fair to say that the changes remove two of the main inconsistencies of the model experiments and represent a substantial step in ensuring that com-parative model results are correct. With this firmer experimental basis, future

changes in extrapolation to the ship will be less important. In the meantime this paper is intended to show the dEsign changes and by comparisons of old

and new methods to allow adjustment of marginal allowances.

2.The Use of a Trip Wire to Cause Transition to Turbulent Bowdary Layer Flow Using the ink stream method(2) it was found that the boundary layer temained

"laminar" on some model hulls up to Reynolds numbers of over a million

(the Reynolds number is for transition using model speed of advance and

length from the bow to the transition region). In other terms on certain 18 ft.

long models laminar flow persisted over a quarter of the length at 3 to 4 ft. per second. If for example one model design has 7 per cent, less laminar wetted surface than an alternative design there is about 7 per cent. difference in skin

frictional resistance due to the smaller frictional coefficient in the lammar area Unless this difference is taken into account there will be a 10 per cent. discrepancy in the estimated ship resistance. (The ship Reynolds nümberis perhaps bOx the model value and there cannot be corresponding Jminar areas on the ship

hull).

(1) Numbth iefcr to bibliógriphy at the end of the paper.

296 A REVIEW OF SHIP MODEL DATA

The direct way of obtaining the model frictional resistance is to observe the

extent of the laminar flow and then apply the appropriate frictional coefficients.

This is obviously unsuitable for general use. An alternative method, which is

now the standard method at Teddington, iS to make iransition occur at a known

position near the bow. A "trip wire" 0036 n. diameter is fixed round the

model touching the model surface at a station 5 per cent, of the length abaft

the fore perpendicular..

The standard trip wire has been observed to cause immediate transition from a laminar to a turbulent boundary layer on the models tried at the usual experi-mental speeds and at much lower speeds. The measurement of the drag of the trip wire itself on a ship model under test presents some difficulties. The drag

of a straightiength of trip wire was measured on a vertical-sided model.

Apply-ing these results to the 18 ft. model of a ship 400 ft. x 55 ft. x 24 ft. draught,

the trip wire resistance adds about OOl0 to theCc)

value at .VIVE= 055 and

less at higher speeds. There was no appreciable ?Iifference in trip-wire resistance

when it was causing transition, from its resistance with the boundary layer akeady turbulent.

The trip wire in the standard position does not affect the flow on the first

5 per cent of the length which contains perhaps 2 percent. of the wetted surface.

There is, therefore, a mll ambiguity in the results since it is possible for the boundary flow over this 2 per cent, area to vary from completely laminar to nearly all turbulent. At present, since this error cannot make an. appreciable difference in a notmal model comparison, the model is regarded as having a

fully turbulent boundary layer, and the trip-Wire resistance taken to be equal to the reduced resistance of laminar flow forward of the trip wire.

The use of a trip wire increased the resistance of models by amounts

corresponding to the eliminated laminar flow areas.

It has not caused any

measurable change in wake fraction or thrust-deduction fraction as measured

in self-propulsion experiments. It is considered to be a satisfactory means of

producing transition without otherwise affecting the flow over the model hulls.

3. The Change in Resistance by Elimination of Laminar Flow Considering fli'st models without a trip wire or similar device, the area of

-laminar flow is affected by the conditions of testing. This includessuch factors as. model surface roughness and vibration, of travelling carriage.

For any

particular model the laminar area depends on the speed, draught and fore-body shape. Most model hulls tested gave consistent results which could be repeated within ± 1 per cent., if the model was subsequently remade. On certain models

slight roughness caused large increases in resistance and in a few particular cases"two different resistance curves were obtained and repeated.

So that

although the laminar flow area is in general well defined and stable for any size .:and design of model at a given speed the marginal hull designs prevent the use

.of'a correction table for effect of trip wire.

With this reservation a general description of the changes may be given in

..tèms ofchange of © due to a trip wire on the usual 18-ft. model. There is very little mcrease at speeds above VI ,/L

= 0 8 (i e above 6 ft /sec)

At

lower speeds laminar flow persists if the bilge diagonal is convex and may persist

if 'it is straight or slightly hollow. The increase is greatest for deep draughts and usually disappears at the light draught. With the full slow-speed form

having a 'short entrance length the laminar flow extends to the shoulder and the

trip wire increases the () by about 0'06 at service draught.

This addition remains practically constant from VI VL = 05 to 07 and higher.

Model 2935 is Model 2925 with a wide beam. Model 2938 is Model 2923 with a wide beam.

Model 2925 has the same lines as 2751A described in the previous paper';

Model 2923A has the same lines as 275lF and as 2202C, the war-time Straight

frame tanker

The 400-ft. ship' © values'to a base of V/4/L taken from the experimental

curves are given for three level draughts, without trip wire in Table 4(a) and with:

trip wre in Table 4(b). Model

No.. ' Fore Body

F.B.

Cb' After Body' 'A.B.çb ' Gb

2923A

Hollow bow for Vf/rO.l

706 Fine S.F. stern -657 -680 B ,, ,, ,,

' ,

,, -706 Intermediate stern -695 -701

C ,, ,, ',, , ,, -706 Full stern -724 715

D ,,

,, + bulb

737 , -724 -730

2925A ' Convex bow -'743 FitieS.F. stem _{657 700}

B ' 743 Full stern -724 .734'

2925C Straight bow -726 ,, , -724 -725

2923E ,, ,, ± Bulb -765: ,, ,, -724 .745

29511) Fine bow for V/4/L:O.8 -654 Intermediate stern 695 -674

C ,, _{,, + bulb} ,, -699 ,, ,, 695 -697

B ,, ,, ,, ,, 699 Full stern ' -724 -711

A ,, ,, + large bulb -708 ,, ,, -724 -716

A REVIEW OF SRI? MODEL DATA 297

The fluter conveA bows have a longer entrance and at low speeds the laminar flow extends to the shoulder and causes a trip-wire difference greater than 0-06, but the area decreases with speed. In hollow forms designed fOr VI v'L = 0! 7 the fore-body flow dives under the bottom and the laminar area is small except

at the very deep draught The results given later m the paper show the differ-ences obtained with different forms and draughts of normally proportioned ships

The coaster type of htill will have the differences emphasi±ed because of the greater beam and draught. The coaster models are 'usually much less than 18 ft. long and the difference persists to values of VI s/L > O 8.

'It is clear that Since the addition of a trip wire increases the © value of the

convexbow ship 0-03 ta 0-04 more than the increase on thó h011ow bow, then

the old comparison of hollow and round bows must be' seriously affected.

Models were made to carry the hollow-bow form to fuller block coefficients. by increasing 'the after-body fullness and also by holding the fore-body water-line

and adding a bulb.

4. Additional Model Resistance Experiments

There are four different ordinary bows :-2925A, a low-speed convex bo'; 2925C, a straight bow; 2923A, a hollow bow designed for VI '/Eo7o

_{; and}

2951D, a finer, slightly hollow bow designed for VI /L 0-80. Bulbs have been

added to the straight bow, the hollow bow and the fihe bow. Three after bodies of different fullness have been used.

The models are defined in Figs. Ia and lb and in Tables 2 and 3, but 'it' is convenient to list them here.

S

298 A REVIEW OF SHIP MODEL DATA

The displacement added in the bulbous bows was decided arbitrarily. The resUlts show that the fore-body block coefficients have been increased by 004 without appreciable increase in © Value In each case it is probable that the

same © value could be obtained with smaller bulbs and with slightly larger ones.

lie accuracy of the results is not better than ± per cent. on the flat part of the © curve so that the trip-wire differences may contain a measurement error

of .1 per cent, of the © value.

The valuós given fOr models 2923A and 29254 differ slightly from the results previously obtained with models 2751A and 275 IF. Model 2938 tested without a trip wire gave a double-resistance curve, One agreeing with the result previously. obtained with model 2767A and the other much lower.

5. D&gnof Low

ResistanceForms

The results given for a ship 400 ft. x 55 ft x 24 ft. may be applied with

minor modification to the considerable numbei of single-screw ships having LIB ratios between 6 9 and 7 6 and comparable draught. Minimum © values

withOut trip wire, plotted to a base of block coefficient in Fig. 2 (a), represent

the old basis of design. The cUrves are similar to those shown in Fig. 3 of the

previous paper except that with a fuller after body lower values have been ob-tained for the hollowbow forms at block coefficient 0'7l. The bulbous bow

curves have been added Simibrcurves for twin-screw hulls show that above

V/ilL

O'75 there is an appreciable change in interference length but in general the twin-screw stern has less resistance, and the twin-screw form with a 0'04 fuller after-body block coefficient obtains the single-screw © value. The variation in single screw after body lines with propeller aperture has a

similar effect. FOr this reason the minimum curves are given for 400-ft ships with approximately 155 ft. propeller diameter.

The new basis curves, with trip wire, are given in Fig. 2(b). These are defined to a large extent by the results of models described in Sec. 4. The hollow bow of models 2923A, B, C, has a favourable wave interference at

V/i/i = 0-68 and

an unfavourable one at

V/ /L

076.

The finer models 2951 are designed

for V/ilL

0-80 and the straight bow (2925C and 2923E) has a limited useful

range at VI VL=r 0-70, mOre particularly at the deepdraught. The minimum

curve for V/ilL = 0'75 depends on the estimated

improvement in 2923 and

295liftheyweredesigncdfor'IVL0'75. The V/VL0.75and=0.80

curves have been extended to the fuller block coefficient values of forms designed for lower speeds.

The curves in Fig. 2(b) give the general design picture and the comparison with Fig. 2(a) shows the change introduced. The choice of hollow bow in

place of convex bow now occurs at a fuller block coefficient, the extra displace-ment being carried aft. By the addition of a bulbous bow the block coefficient

may be increased by about 0'02 for the same © value. At a speed for which

the normal form resistance is increasing rapidly with fullhess, the useof a bulb affords a considerable saving.

For the moderate speed-length ratios covered in this paper it appears to be

satisfactory to regard the bulb as a means of carrying displacement and the upper

half of the hull as the source Of wave-making resistance. This precludes the use of area curve slopes to define the form and su stitutes waterline slopes as shown in Fig. 5.

A REVIEW OF SHIP MODEL DATA 299

table 5 is intended lo show the type of design required to obtain the (ç)

values shown in Fig. 2(b). Two relations of speed to block coefficient are. given, Ci,

l4)6_v12V1 and Ci,

1.o8_v/2v'z:

It is clear that such a table can be misleading but it supplies information on the centre of buoyancy

position and type of hull which may be reqUired early in the design stage.

Minimum curves are also given in Figs. 3 and 4 for 28 ft. draught and 21 ft.

draught, again based largely on the results obtained with the present models. In using the results as a guide for ships of other

L/B

ratios the breadth-draught

ratio is important. With wide beams it is difficult to hOld the 15° hollow-bow

entrance water.line angle and so keep the good hollow-bow result. 6. The Change in Quasi-Propulsive Coefficient

Because of the reduced skin-friction coefficient on the ship, the ship resistance coefficient is much less than the, model resistance coefficient. For the usual model scale on 400 ft-500 ft. single-screw ships 'the model © = 0'93 corre sponds to a ship (c) ' 0'65. In the old method of seif-propu sion experiments

the quasi-propulsive coefficient was taken at the model self-propulsion point,

i.e. corresponding to © 093.

In the new method it is taken at the ship-'

trial propulsion point corresponding to © = 0'65 + 10 per cent, or 0' 715.

The old q.p.c. values were at thrust loadings perhaps 30 per cent. greater than.

the ship-trial loading and consequently at 4 per cent, to 6 per cent. less

efficiency.

The old empirical formula q.p.c. = 0' 83 (where N400 is the r;pm.

corrected to 400 ft. ship length) was used in the first place for ordirniry single-screw ships and later fOund to have a more general application. Three or four

coaster models at high VI /E values gave propulsive coefficients appreciably higher than the formula value at high revolutions. The scale ratio of these

models is relatively small so that the skin-friction correction is less than that of the ordinary single-screw ships Because of the high resistance the 10 per cent.

trial allowance is a relatively large quantity so that the model self-propulsion

loading is not greatly different from the designed ship trial loading, e.g. of model

© 1'4,ship© = 1.2,ship© + lOpercent. = 1'32,andthemodél-screw

loading is only 6 per cent greater than the ship-tnal loading with an efficiency cOrrection of perhaps I per cent.

It appears, therefore, that the old empirical values over the whole range should be increased about 4 per cent. over the whole range because the new

propulsive coefficients do not contain the effect of overloading on the model.

it is suggested that the new empirical formula q.p.c. = O.86_0 should

now be used where N400= 400 ft. ship r.p.m. = ship trial r.p.m..x

ftv/lthb.

This "increase" in q.p.c. will show almost entirely in the "open" and behind screw efficiency values, the hull efficiency remaining unaltered or slightly

reduced.

The change in estimated shaft horse-power may now be considered. For the slow-speed convex-bow ship the trip wire increases the effective horse-power

figure 10 per cent.; the trial allowance has been increased 2 percent. (from 8 per

cent, to 10 per cent,.); and the q.p.c. increased 4 per cent. The shaft horse-power estimate is therefore increased 10 per cent. ±2 per cent..-4 per cent.=8

300. A REVIEW OF SHIP MODEL DATA

per cent.

This increase is less at higher speeds V/tJ0.6S and 070. At

lighter draughts the increase disappears. For the hollow bow at

there is little change, the .3 per cent. trip wire increase and 2 per cent. increase in trial allowance balancing the 4 per cent. increase in propulsive efficiency.

7. General Remarks

The use of a bulbous bow to obtain minimum resistance at rather higher block.

coefficients has been shown in Sec 5 Any loss in propulsive efficiency due to a bulbous bow is believed to be small. The performance of bulbous-bow forms in service requires a separate investigation In particular the applica-tion to coasters appears to merit early consideraapplica-tion.

Ship-trial results provide. the ultimate answer to the questions of accuracy

of model prediction&

It is not easy to obtain a trial result which has not a

possible 3 per cent error in power measurement, wind resistance and sea allow-ance, or surface roughness variation including change m plate butts and edges

A discussion of this. subject is outside the scope of the paper.

It was intended that corrections should be applied to published data but this

cannot be done satisfactorily. With the simple experimental method available to show liminar flow on models and the effect of artificial turbulence sti±nulation

of different types now in use, an intertank comparison should now be of

con-siderable value. Future published data would then be on a comparative basis.

The use of the trip wire has removed a number of anomalies in model

resist-ance data, and in particular it now seems that it will be possible to obtain

calculated wave-resistance results corrected for viscosity which will apply even

to the type of model considered in this paper.

Finally, it should be emphasized that the changes in method are required to put the model comparison on a sounder basis, and the resultant differential change in ppwer estimate for hollow and convex bow ships of the same size appears to be unavoidable.

The work described above has been carried out as part of the research pro-gramme of the NatiOnal Physical Laboratory, and this paper is published by. permission of the Director of the Laboratory.

A REVIEW OF SHIP MODEL DATA 301.

REFERENCES

1 A. EMERSON, M.Sc. and N. A. WmIEY, " Experiment Work on Merchant Ship Models during the War," N.E.C. Inst., Vol. 64.

J. F. AJ.L&N, D.Sc. and J. F. C. C0NN, D.Sc., "Effect of I.iminir Flow

____

400' % BEAM AT STATIONS

-FP 94 9Y 9 8

8 7V 7 6 5 4 3 2

2 1 I

i2

30' 125 242 745 978998 tootoo 870778 631 411 24' 18'

78 175407648837941lO0

'

14

581bO tOO

6'

89

3' 42 4T.868O482i9I989.7

133 58.

R.FU25

26 2 I8

O (%J

30'

100 tOO 100

9638Ot

24' 100 999 870 91t 81.4 652 431 18'

I

'fl

A

OO 100 99I 94484769485Z3O

12'

'

_{096989376t 58366 1461}

6' IIII978787 626 432 249 96

3'

I9t93l7 173 7I

R.F.

94fl4!5U

30

100 t0O9O

37.5

24'

tOO 100 5i.I 46119.5

18's

_{AS 223 A}

100100100

1211

r

I[bO too

.6'

G87I0 125

3'

91.9 91.9 838 714 562 38I

92 I

R.r. 186 S S

S

-F.

0

CV)

ôô

.

II

24' 1.0 l0O 457 70087.0 9&2 100100

IS' 3I II78619e099t 00100

I..

12' 7.1 17I

l94599-0 too

oo

6'

l06

860l;..

3'

14.4 V.0 36 5O

8o 879 9i9

R F

G'

30' 4. 1630I5I698l tOO 100 tOO

24' 20 I44276522745900975998 tOO 100 t8' 4.0 l&3 5Zt

100 I00

12' S4 3t.9

73958

100 100

6' tl22l53t752.l 695830916961 982982U

3'

R.F/2S

302 A REVIEW OF SHIP MODEL DATA

00

.

% BEAM AT STAT ONS

FR

9 8

8

7 6 5 4 3 2

IY I

/2

o

(1

30'

4OO 682 871 tOO 100 24'

ro

624 828 947 tOO 100 18'

t22783925: 100 IO0AS

_A 12'

69 214 4877I596I 0

6'

365 584 877 982

56'I8689t9U

3t2 487 R.F. 28 147

o

a'

(J 30' 3.1 168 56! 9S9 100100 tOO

24'

134 2&5 525 100100100

18'

101 23288t 975 too too oom

AS 2923 C

12' 6 2 t87 66.5 545 995 tOO tOO

6'

1t.8 34.9,75. 7.595___._._

3'

5.4 47.2

9.9 99

R.F.

it)

30' 49 13.1 2l5382889tO0lOO

24'

96 178 54.2 too too

18' 20 tt2 193 37473-687-9f00 '°°

AS 223 C

12' 7.5 I522374Z-26O8774895IO0 ooN.

6' IO8 8-3

465

942 98-298-2

3' 6I I5O2I25IO66-988O9l99I-9

-R.72 87569.5j

-24' 1.994t7I

tOO tOO

¼. 12' 7.. t48 .9 100 tOO

6' 108 I742484l15747I6982

3' 6-I 1402

0

a'

c'

30'3sl225s37s6887B98IooIoO

_ 24' 8-0 162

539953 tOO tOO

18'

l3-9T69-, 84-3I0o tO0A5

_2O2

B

12 39 II 9 293 47-6

931 tOO 100

79

889-098982

19-5 B7.9 8IO 91-9 919

RF

1

7428

A REVIEW OF SHIP MODE! DATA 303

TABLE 3

MODEL

400' SHIP PARTICULARS

AREA CURVE ORDINATES

IOB d

BMLO dMLD MLD'

A1AL

CI Cp

'iE © F P 9fr 9 8Y 8 Th 7 6Y2 6

5 4 3

3 2Y2 2

I I

'/ A P

2923 A 55 I 24 4 I047 706 657 661 972 700 0 BBF 18 3 I4!a 612 128335 565 765 90? 958 970 972 972 966929854750654478 322 I 60 28

B 55 I 24 4 I078O 706 695 701 972 720 0 06F 18 I 14'k 612 .

_{AS 29 3 A}

972968951 914 833 7C9 549 8G9 177 28

C 551 24.4 !II000 706 724 715 972 737 058A I60I4 6II AS

29 3

A - 9.72 9689639.428767666124-17207

D 55I 244 Il240737 724 .730 972 751 028F 179l3'a6O9 '49 2284236.247.929049569729729-72 AS 2923 C

E 55 I 244 11470 765 724 745 972 766 099 177 19V2603 60286502695840922958972972972 AS 2923

2925A55l 244 IO78O43657700872720 I83FIBI: 256-02

I845l&688209l4939709729-72 AS129j23C

B 55 I 24 4 11300 748 724 734 872 755 03SF Il 8 25 602 .

As 29 5 A

-. AS 2923

C 55 I 24 4 11160 726 724 725 972 7460 05A 179 20 604 168 4046.23 797 9.07 955968 972 972 - AS 2923 1C

2951 A 55 I 24 4 11020 708 724 716 972 737 0 34A lB 0 lI 606 55 210382 555 719 851 93 9-64 972 972 AS 2923 C 9 55 I 24 4 10950 699 724 711 972 731 0 ThA 18 0 II !-'2 606 056 202369542 6.99 827 925 964972 972

AS 2923 C

C 55I 24.4 10730 699 -G85 697 972 7!7 008Ai8-3, 11Y26.06 -AS 29 1 B .

AS 2923 C

D 55I 24.4 10370 '654 695 674 '972 693, 0IIA 184 12 607 I06 2.74 4566-32 787 8-969548-72972 AS

2923

C

2935 647 244 12650

A 2925 A

AS 2925 A

MODEL

400 FT ©

AT SPEED

I.N KNOTS

io

ii

itJ' 12 12 13

l3Y 14 I4b 15

I54 IS

_I6 17

27-8 0605 606 610 616 620 627 632 638 652680 748 826 922 2923A 24-4 0-62! 624 629 639 642 645 647 654 670 698 759 833 915 20-9 0629 633 642 650 655 659 66! 663 675 704 763 833 918 27.6 0-604 605 608 615 622 631 641 652 669 700 765 824 865 2923B 24-4 0-620 6Th 625 63! 638 643 649 662 682710 759 613 857 209 0630 633 640 653. 665 676 68! 683 697 724 766 817. 862 27-8 0622 625 630 637 642 650 659 667 686 724 790 844 870 2523C 244 0-634 638 643 650 657 663 670 677 701 740 790 841 872 20-9 0651 .656 663 673 680 587 669 .591 717 751 21'8 0622 628 646 66. 673 680 687 697 715 749 603 864 912 28230 ?440-648 655 667 678 682 683 685 688 711 752 805 871 911 27-8 0-600 608 617 628 636 647 662 680 707 764 845 93! 2923E 24-4 0-614 617 640 672 689 696 704 717 738 790 875 953 1025 23-9 21-8 0-573 582 592 607 620 638 660 686 717 758 819 910 1-021 2925A 44 0-593 607 620 637 650 669 693 721 746 782 839 925 1-046 20-9 0630 641 652 667 682 703 727 757 791 830 90! 977 278 577 586 597 609 629 651 55! 716 770 852 942 1-OIl 2925524-4 20-9 27-8 2925C 24-4 _{641. 659 673 686696 706 711 729 781 849 907 950} 239 27-8 0.655 658 670 682 696 715 719 720 726 732 740 749 767 2951A 24-40-660 665 610 680 695 710 714 717 719 721 732 74! 755 785 20-9 27-8. 2951B 24-4 20-9 27-8 2951C 24-4 20-9 27-8 608 610 617 634 652 662 669 673 679 684 697 717 747 29510 24-4 618 622 627 636 653 660 661 667 673 683 697 712 75! 20-9 _{643 650 660 671 68! 686 693 695 697 699 702 715 745} 218 0570 578 590 602 620 638 660 688 719 756 805 897 2935 24-40-599 609 620 632 646 662 680 703 733 776 838 914 2090-643 650 660 671 690 711 737 767 798 836 889 963 27-8 0-599 606 612 622 630 637 645 657 679 718 780 853 2938 24-40-622 625 630 635 640 649 661 675 696 727 787 860 20-90-643 650 658 668 677 685 695 705 718 744 794 864

A REVIEW OF SH MODEL DATA 305

306 A REVIEW OF SHIP MODEL DATA

TABLE 4(b) Values with TripWire

MODEL-P

400 FT © AT SPEED

IN KNOTS

10 11

Il'2 12

12 13 13 14 14 15 15Y2 16 16 17 278 0-634 635 639 645 650 654 657 660 668 697' 758 830 926 Z9Z3A24-4 0-642 642 646 651 657 662 667 669 680 708 764 836 20-9 0-650 653 657 661 666 670 676 683 693 714 776 844.928 27-B 0-634 635 636 643 649 656 661 669 682 713 777 830 670 -2923B24-4 0-643 645 647 651 659 664 670 677 691 720 769 825 867 Z0-9 0-653 658 661 668 678 688 692 695 708 733 773 622 863 27-8 0-651 654 660 665 670 674 679 685 702 74Ô 800 849 877 2923C 24-40-658 661 665 670 674 680 685 691 712 751 799 645 876 20-90-672 678 683689 693 699 701 703 725 766 811 855 880 278O-663 668 676 685 9i 697 701 705 719 754 806 869 911 2923D24-4O667 672 681 688 691 693 695 698 716 758 808 874 915 20-90-686 691 699 707 711 715 718 722 732 762 818 872 897 21-8 0-651 660 6O 680 687 695 704 712 732 784 865 9461-007 2923E 24-4 0-668 675 684 697 704 713 723 735 761 808 884 966 1-041 Q.9 0-687 699 715 732 744 754 768 785 812 864 938 1014 1-070 21-60-630 637 647 660 673 690 707 729 756 798 654 940 1.036 29Z5A244o-G3s 647 659 673 687 706 726 751 777 816 876 959 1-059 20-90-655 665 674 689 704 723 747 777 811 845 911 987 27.8 635 642 656 670 686 703 726 761 .816 894 973 l-O9 Z95B24-4o-65I 658 665 676 689 703 724746 787 640 915 992 1.054 20-30-687 698 706 717 727 742 764 796 833 883 949 1Q21 1-086 27-8 2925C24-4 676 694 705 711 718 725 73I 748 802 866 923 965 20-9 685 704 726 739 748 750 759 783 824 879 933 974 27-8 0-668 671 680 690 703 717 719 720 726 73Z 740 749 767 789 2951A 24-40-670 675 680 690 700 710 714 717 719 721 732 741 755 785 20-90-681 684 693 705 717 728 731 733 730 736 741 757 774 800 V-B 0-653 658 670 683 696 707 709 710 714 719 728 9 750 769 2951B 24-4 677 666 695 697 699 701 708 719 731 737 761 20-90-679 681 691 704 712 719 726 728 730 733 737 749 762 782 27-80-651 655 661 673 688 700 702 703 707 712 717 724 735 759 - 2951 C24-4 660 667 677 685 695 697 699 701 702 707 713 725 756 20-90-674 676 663 695 707 717 721 723 727 731 735 739 750 770 27-8 0-650 650 653 663 680 692 697 699 702 704 705 706 724 754 2951 D 24-4 640 646 654. 665 678 682 687 689 690 695 703 118 753 20-9 653 662 670 685 691 694 698 700 702 704 707 720 750 27-60-648 652 658 666 675 688 704 729 759 790 845 922 2935 -40-652 660 669 680 695 711 731 753 782 818 870 934 20-90-671 679 687 700 714 734 758 786 815 846 896 968 27-80-635 640 648 656 666 672 677 688 707 736 784 853 2938 24-40-660 664 670 673 678 682 687 694 710 739 791 862 20-90-663 670678 685 691 699709 716 728 752 799 868

TABLE 5

DESIGN DATA

FOR USE WITH FIG. 2(b)

V/,t

O80

O75

O70

O65

.O6O

0685 O'71 071 0735 0Th5 O76

108 _V//Cb

068 O68

0705 0705 0705

073 073 073 073

O7550755

078

EB. Cb 064 O66 O67 0675 069 070 071 O705O725 07I5 O74 074 073 0.745 O74 078 0785 078 082

A.B.Cb 068 070 O69 0?695 O72 071 07O O7I50695O755O7Z 072 073 0725073 O730725 O74 074

L.C.B.

09%A09%A05%A05%A07%AOz%A0.2%F 0ZA07%F09ZA04%F04ZF 0 04%F0ZZF tI%F I3%F 09%FI8%F

oe

110 _{12° Fl° 13°} 140: _{11° 14° l4Ya° 13V° I4Y I3/2° 250}

_{O 250 14° 23° 28° 29° 29}

TYPE0rLW.L (j) ®

CD ® ©

® ® ® ® ® ® ® ® ® ® ® ®

TYPE OF BOW ORD ORD BULB ORD 3RD BULB BULB ORD BULB ORD BULB 3RD ORD 3RD BULB BULB 3RD ORD ORO

©

O69 0'70 069 0'69 072 070 0.70 O69069 076 O70 0.75 .O75 Q705 O70 O715 072 069 070

308 A REVW OF MOD DATA

STERFSI CONTOUR

MODELS

2923 A.B.0 & D

Fig. 1(a)

lilt_ilir

ILIJIIL In

\\'

\1JfgvjgjIjIg

11I1iiIVJI1)

BOW CONTOUR 2923 A.B.0 2925 A.B.0 2951 0 2923 D.0 2951 A.B.0 BOW CONTOUR 244 rT I0 FP.

2923 A FORE

BODY-

AFTER

BODY-2923 B FORE

BODY -

AFTER BODY

--2923 C FORE

BODY -

AFTER BODY 2923 D FORE BODY - AFTER BODY

MODELS

2925 A & B

FORE BODY SECTiONS 2925 A FORE BODY

AFTER BODY AS 2923 A 2925 B FORE BODY

AFTER BODY AS 2923 C

MODELS 2925 C & 2923 B

FORE BODY SECTIONS 2925C FORE BODY

AFTER BODY AS 2923 C 2923 E FORE BODY

AFTER BODY AS 2923 C

MODELS

_{2951 A.B.0 & D}

FORE BODY SECTIONS

295! A FORE BODY

AFTER BODY AS 2923 C

295! B FORE BODY

AFTER BODY AS 2923 C

295IC FORE BODY ---AFTER BODY AS 2923 B

2951 D FORE BODY

AFTER BODY AS 2923 B

A REVIEW OF SHW MODEL DATA 309

9 93i 9 &4 8

'0 .9 9

4FT

87

SINGLE SCREW SHIPS (SCREW DIATM 155F1 APPROx)

o95_© VALUES FOR SHIP 400X 55 X 240FT.

without

trip wire

064 065 066 067 068 069 070 071 oia 073 0.74 075 076

BLOCK COEFFICIENT

077 078

Fig. 2(b' 095 ..O90 o. 085

t

§ O75 065

SINGLE SCREW SHIPS (scREw © VALUES FOR SHIP 400 X

With trip wire

THESE VALUES MAY BE APPLIED

IN THE RANGE L/B 69 TO 7 6

LENGTH CORRECTION TO CE) VALUE

+030 FOR LBP 220 FT + 020 270 FT +010 330FT

OIO

480FT SBOFT DIM' I55FTAPPROX 55 X 240 FT

___

-4

pp-4

-073 'II,. .60

-_

074

I...

075 076 jrc 130W 077 078

-068 069 BLOCK

__..

010 071 C0(ITICILNT 072 064 065 066 067

Fig. 3

095

090

SINGLE SCREW SHIPS (scREw DIATM 155 FT APPROX)

© VALUES FOR SHIP 400 X 55 X 21 FT

THESE VALUES MAY BE APPLIED

IN THE RANGE L/B 69 TO 76

WIUi trip wii

Q

-W ₇₁.1

'

f--

.frc 'f,

p#

,

- -)

.9AJ

;

U!

,#.:423c

07O 0-60

___

Bow

________________

BLOCK COEFFT

(24 FT OFT APPROXIMATE Cb VALUES - (0.70) (075)

I1g. 4 0.80 8₀₇₅

I.-°

o.o GO

SINGLE SCREW SHIPS © VALUES FOR SHIP 400

THESE VALUES MAY SE

IN THE RANGE % 69

wire

(scREw DIATM 155 FT APPROX)

X 55 X 270 FT APPLIED TO 76

1'

4

,..

--,

.. V .10 ... O65-%ç65-' VffL.6O, %r-55 - .70._ HOtLOW SOW

-rr1

(p.., COH'1Y'

-5(i4 -- -

LS5

BLOCK COEFFT

(4FT OFT) APPROXIMATE Cb VALUES (070) (0.75)

314 A REVIEW OF SHIP MODEL DATA

4O

10

CURVE OF L.W.L SLOPES

ORDINATES OSTAIND FROM STATION DIFFERENCES

OF L.W.L ORDINATES

LOAD WATER LINES scE TA8E

7 8 STATION Fig. 5 9 100 80

DISCUSSION ON "A REVIEW OF SHIP

MODEL DATA

" *

Dr. J. F. ALLAN, Member:

As I was closely associated with the work described in this paper I do not propose to criticize it, but as the changes in method

at. the N.P.L. which made this paper

necessary were introduced under my direc-tion I think some remarks from me might be appropriate and useful. Before I make them, however, I would lik to say how valuable. I think this paper is. It has

involved a considerable amount of work, the results are very ably presented, and it does give very useful information to the designer.. Used intelligently and with dis-cretiôn, he can make a reasonably accurate estimate of the e.h.p. and s.h.p. of ships coming within this range. In addition, he can produce lines for the vessel, and these lines will have . the correct character. Therefore, I congratulate Mr. Emerson and Mr. Witney on the paper, and I recommend it to all who are concerned with the design and powering of ships. The need for this review had arisen because of two changes in procedureat the National Tank at Tedding-ton, and I wish to say something about these changes.

Referring to the first changethe stimula-tion of turbulent boundary flow on the

model by a trip wire or other meansthis change is justified for two reasons. First, because it enables one to avoid unstable flow conditions, which have given ambigu-ous answers in some cases in the past, and secondly, because it enables one to select the best form for the ship in cases where on, the previous basis of working the model test would have indicated the wrong form. The range over which this applies is clearly seen by comparing Figs. 2a and 2b, to which Mr. Emerson has already drawn your attention.

At fuller blocks and lower speeds it is

quite a serious amount, as you can see.

These diagrams also show the range over which a good bulbous bow is useful and

the trip wire indicates that this range is

larger than indicated by previous work. The use of the trip wire is therefore fully justified, in spite of the disturbing feature of the marked increase in optimum e.h.p. which it causes in certain cases.

Due to the second change which has been introduced, namely the determination of q.pc. at 'the point corresponding to ship

Paper by A. Emerson, M.Sc..,Associate Member,

and N. A. Witney. See p. 295ante.

t7

self-propulsion and not at the overloaded point corresponding to model self-propul-sion, this increase in e.h.p. is offset, to.some extent, by an increase ih .q.p.c. There is still, however, an increase in sji.p. of 6 per cent to 8 per cent in certain cases.

At this point the matter affects the ship-builder.

In the past he has probably

powered ships on the basis of previous. tank work and had a perfectly satisfactory answer. He therefore argues there is no

need for this increase. The change in condition Of flow on the model has no effect on the ship. We agree with that, and so an indication of the increase is given in current N.P.L. reports and in addition, by referring to this paper, the designer can estilnate the matter for himself in any 'given case.

The question that arises here, however, is how accurately do we know the perform-ance in the ship? Considering the difficul-ties of measuring speed and power, the

occurrence of wind ,and weather on the measured mile, the unsatisfactory know-ledge of ship roughness, and in addition the "scale factors" in propellers and wake which are not yet fully explored, it is not unreasonable to conclude that the higher estimate of power may be just as correct as the previous estimate. This' does not invalidate methods of estimating power

by applying empirical factors to model data, but it does suggest that we have something more 'to learn in this direction. When one studies the matter it is surprising how few excellent ship-model comparisons are in existence, and I think it is very important to pursue this matter further in the direction of obtaining such good ship-model comparisons. To that end we are

making a substantial effort just now in the Ship Division, and in close co-operation with the Research Association. We hope

by pursuing this thoroughly by careful measurement of power and thrust, and also by pursuing research on roughness and scale effect, to arriveat a better knowledge of these problems. The shipbuilders. and shipowners are co-operating fully in this matter.

Prof. L. C. BURRILL, Member:

In common with many other experiment-tank papers this review of recent ship-model

data presents to us a record of work

ni 12 A REVIEW OF SHIP MODEL DATA controversial matter that can be the subject

of a great deal of discussion.

In this paper we have had presented to

us the

results of some very excellent experimental work and in addition, through the medium of the film that has been shown, we have had presented to us a new technique which shows vividly what is happening with these models in relation to the extent of laminar flow towards the forward end. Furthermore, in presenting the results of

the new model tests,' the Authors have kept in mind the view-point of the ship designer and other users of such data, and the

tables of () for different speeds

corresponding to the ship length of 400 ft. show clearly the effect of using a trip wire.

Mr. Emerson has' stated that typical comparative figures for the thrust necessaiy to propel the model atits own self- propul-sion point and the thrust which is required

at the

point which represents normal propulsion conditions in good weather, are 8 lb. and 5 lb. respectively. These figures

are markedly different and it has been clear for some time to those who have analysed the tank data that the conditions under which the model propeller operates for these two different methods of conducting internally' propelled model tests are very widely different.

Although this is a very short, paper, I am sure it will be judged finally to be a very important one, for it introduces two changes which have recently been made in tank-testing procedure at the Ship Division of the N.P.L.

The first of thde changes arises from the fuller appreciation in this, country of the problems relating to "laminar" and "tur-bulent "flow in connexion.with ship models. It must be clear to all those who . have studied recent work including, the present paper; that laminar flow has been present

in some degree on many models which have been tested in the past, and there seems.no doubt 'that some of the informa-tion which has been given to shibuilders as a.result of such tank tests has, in fact, been misleading. I heartily endorse the

decision that.has been taken by Dr. Allan to fit a trip wire to all models as a standard test procedure, and he is to be,congratulated on' having made this decision and thus

facing up 'to the difficulties arising from

this change. No doubt the shipbuilders

will soon become familiar with the new results, and will adjust their methods of estimating accordingly.

The second matter of great importance is the decision to run the self-propelled tests at a condition corresponding to e.h.p.

plus an allowance of 10 per ceht.and to take off the q.p.c. at that point..

It has been evident to those of us- who have been concerned with ,the analysis of .,ship data and the design of propellers that

the previous practice of running the model at its own self-propulsion point caused the model propellers to be working under different conditions from those under which they would normally be operating at sea. For example, I have known instances where the model, as it was run in the Tank, actually represented a con-dition corresponding to e.h.p. plus 50per cent to 60 per cent, with the result that a propeller designed to work near the peak of the efficiency curve would be working at a much higher slip and appreciably down the slope of the efficiency curve. Such results were very confusing and could be'

quite misleading to those who do not analyse the testresults in detail.

I think the Tank authorities are to be

congratulated on having changed over to this new method, and I firmly endorse the adoption of 10 per cent addition to the

naked horse-power to represent the ship

on trial rather than the older figure of 8 per cent. In my view the 8 per cent was too small2 although I realize, that this figure was to some extent influenced by the low q.p.c.'s obtained by running the model at its own self-propulsion point. I think it, will, be found that this allowance of 10 per cent taken in conjunction with q.p.c. values obtained from the. model tests at the-ship-trial propulsion point, or estimated with the aid of standard systematic-series propeller data, represents a reasonable and practical value for this allowance, and I hope that the Tank authorities have decided that they will on occasion increase this

allowance to 12 per cent or sometimes to 15.per cent in the case of twin-screw vessels.

In reading the paper, it was not quite

clear to me why a change of 7 per cent in the skin-friction resistance accounts for a 10 per cent .discrepancy in. the final, total estimated resistance of the ship, and I feel that this is a matter on which some of us would, like some further enlightenment.

It. is quite clear to me that although the model propeller may be assisted in pro-pelling the actual model down the tank, the energy ,involved in creating the' dis-turbance round the model will remain the same, so that the condition under which the model propeller will be working is still not equivalent to the condition obtained in the actual ship. This is a matter which requires further consideration and I feel that some day soon a proper solution will be found, to do this problem;

At the end of the paper we are told that

some of the decisions made in the past

concerning the adoption of hollow bow water lines and the. use of bulbous 'bows

are no longer acceptable to 'the Tank

authorities. We 'are also told that bulbous bows are now coming into the picture in connexion with faster cargo-ship forms:.. I hope that this is correct, and that we are not being misled again.

Before the recent war, the Germans were very keen on the bulbous bow and a good deal of research work was carried out by them on this subject, but some of the ships

built with this fdrrn of bow were not

entirely successful in service under sea-going conditions.

Mr. D. FRASER-SMITH, Student: I should like, first of all, to comment on Section 6 of, this paper which deals with the new method for self-propulsiOn ëxperi-nients now in use at the N.PL.

As I understand it, there has been no

radical change, hi the actual testing pro-cedure; that is 'to say, it 'is still the practice 'to make a series of runs at one speed and vary the revolutions from run to run above and below those required for self propulsiOn, so as to obtain curves of K, KQ and KR-T against advance coefficient. (The fact that frictional resistance does not follow the law_of comparisons and' the consequent overloading of the propeller at model self-'propulsion point 'is now, however, taken

,into account presumably by using the Kr and K0 values at the advance coefficient .corresponding to KR-r=friction and trial correction, instead of Kit-T=O as formerly. This is, I consider, a long-needed reform and it will enable the results thus obtained -to be compared, with those from foreign -tanks, where the frictional correction has been taken into account in. such cases for 'many years.

-It is perhaps not entirely relevant here, but it may be of interest to point out that,

- as the Authors are no doubt aware, most

-tanks in other countries adopt in some

-form or other what is known as -the

-"continental ". method -for self-propulsion -tests. This method differs from that in Tees-side Branch Meeti Mr. J. LENAG HAN, Member of Council

and Chairman Tees-side Branch:

- The - changes recently introduced by

Teddiiigton for estimating ship powers -confirm more than ever the importance of j-carefully comparing thodel results with - -s&trial - performance. In cases where appreciable discrepancies have occurred

it is in everyone's interest to have these investigated and, in many instances, more -thought could be given to the conduct of -trials -and - the - methods employed for -collecting the trial data. Also, this paper and several others recently produced on the :effect of laminar -and turbulent flow on '-ship models woUld suggest that more 'frequent opportunities should- be given to the Tank authorities 'and the officers of B.S.R.A -to attend sea -trials.

Earlier model data in - many cases pre--Sumably nOw will be reviewed and corrected.

use at the N.P.L. in that the seres of runs covers the same speed range as the resistance -experiments, the model being self-propelled on - each run except - for -a towing force equivalent to the friction correction Thus the results are obtained immediately at,ship -self-propulsion, point and, moreover, for the same testing time and :costs, propulsive coefficients and efficiencies, etc., can be given over a range of speed instead of-merely

at one speedan odvantage which the

shipbuilders of this country would, I think, appreciate. It would therefore seem a logical enough step' to adopt the "con-tinental "method for selfpropulsion experi-ments and it would be interesting to hear the Authors' views on- this question

My second point concerns the results obtained with the bulbous bow models.

It is generally agreed that little, if any, benefit is derived from fitting a bulb o

forms designed for V/.V7j.<o.8 and- this contention is borne out by the results given in Table 4b where it is, seen that Models 2923D and 2923E are not-so efficient as the corresponding normal forms. However, on looking at the design data in Table 5

and the subsequent diagrams, it would appear that even at speeds as low as V/VL 065 the addition of a bulb is beneficial. I assume that these data have been prepared from other model 'results in addition to those givth in this paper but I should like to have this apparent discrepancy explained a little.

VOTE OF THANKS

-On the - motion of the PRESIDENT

(Mr. Mungo Campbell) a vote of thanks

was accorded to Mr. Emerson and Mr. - Witney for their paper.

-ng, 22nd March, 1950

It- is unlikely,- however, - that the higher power figures that follow - will change opinion on a model -or hull form which in the past has given good results in service: as the Authors indicate itis the performance of the ship'itself that provides the\iltimate answer.

In- applying, the changes suggested by Teddington to a ship now building, and for which the model tests withOut trip wire had been carried out approximately- two - years ago,' it was found that the estimated

increase on the

horse-power- made it - necessary, to add a cylinder to the Diesel engine. 'This perhaps was amarginal case, -but as such cases do exist -the possibility of alteration in the engine -size- for certain - repeat ships may have to be-faced.

-The Authors refer to two similar wartime models, Nos.- 275 IA and 2202C, the tank tests of which are given in their previous

0

Dl 14 A REVIEW OF SHiP MODEL DATA

paper" Experiment Work on Merchant Ship Models during the War ". For the present review these model tests have been repeated under model No. 2923A. It is interesting to see that the good results of the earlier models have been confirmed in the later test. The reference to these models in the third paragraph below Table 1 should be correctedthe first 2751 model

is Fand the second A.

The " U" forward section in association with a normal bow- contour will give as - low-resistance values as the adding of a bulb to full models. Will the introduction of a bulb to such forms avoid a recurrence of the slamming troubles to which- "U" shaped fore endsare prone, or is it expected that the bulb, by virtue of its shape, will restrict the upward movement contributary to slamming?

J.4r. W. T. BUTrERWICK, C.B;E., Vice-President:

In the first p1ace,I support what Mr. Lenaghan has said about the model numbers requiring correction. This is a factual paper and it is difficult to get a discussion about it because it is simply a record of what, has happened. The last remarks of

Mr. Emerson seem true, namely,

"If

we- are going to get progress at all and be able to repeat things or to make proper

comparison between form and form, we must have them on the same basis." He has also pointed out that on a 465-ft. type of ship differences are not so pronounced. Recently we have had a casea very interesting case, of a 515 ft. vessel with .75

- block with a speed of 15k knots on trial.

This ship had turbo-electrical propulsion and we had a- Michell thrust meter and were able to get the actual power at the propelling motor. We had ordinary trials on the Newbiggin mile, but fortunately we were able to get the owners to allow us in association with the British Shipbuilding Research Association to have the loan of

- the ship for four or five days. We took

the ship to the Arran measured mile and

she was put through exactly the same

paces as those of the model tests running at five different draughts.

As things turned out we were not able to run at the whole five draughts but did all but the lowest draught, and'these trials were very good indeed. The tendency. especially in - deeper ballast load draughts, was for the actual powers observed on

trials almost to follow tank predictions at lower speeds. but as one got higher along

- the curve, the curve flattened and less

power was required than tank prediction. We had welded butts and bottom shell, but not a welded side shell, (the seams only being, riveted). That may. have had some influence but we have had another series

NE.C. J,,st. Vol. 54.

of ships-560-ft. tankersand have had

four on trial; in each case the same thing has been observed. The powers required at top speeds are less than tank predictions; that is for about a 78 block coefficient. - There must be lots of cases when one is

just on the point of making- a jump to another cylinder and it is a very vital point because this is very expensive.

Mr. F. C. MORLEY, Associate Member: I should like to ask Mr. Emerson if he considers the N.P.L. allowance of 10 per cent for trial is adequate for a North East

Coast trial mile such as Newbiggin. I submit that we have conditions under which trials have to be run on the North East Coast, when at least 14 per cent should be the allowance to give the power actually required on the loaded triaL

Mr. BUTFERWICK:

-I fully endorse trying to get at the differ-ence on the North East Coast. No wise

shipbuilder dare put figures forward on the basis of what we can obtain in Clyde cOnditions. They vary from day to day. On other occasions we have had 14k knota from round about the 6,600 h.p. mark. We - took a ship out last Thursday on the same lines, propellers and loading, yet just due to weather conditions we had to develop

7,000 h.p. to get the same speed. Last Friday we had' gyro-electro steering gear 'and were hunting three degrees each side

of centre-line on account of wind and. weather and the ship falling off her course,, the' wind being twenty miles per hour with gusts up to thirtysix miles per hour. -Mr. MORLEY:

N.P.L. predictions are given for a. measured mile, but it is left to those aboard the ship to decide how far the vessel ha to run before entering the mile.

Would, it- not be a:n advantage if the

N.P.L. could give, along with its speed prediction, a minimum distance

or

approach, so that say 95 per cent of the predetermined speed could be reached on the. engine settings appropriate to the 100 per cent speed, before entering the mile? This would eliminate losses due to turning and give a better average speed over the mile.

Mr. BUTFERWICK-:

On Thursday we deliberately made the ship run at least 2k miles each side of the: mile before attempting to turn, and then

only point at 10° helm. We had 32,000'

tons displacement and that has to be'

taken into account, and you can, if not. careful, get'very different. results.

-Mr. E. H. VIE, Associate Member: Could Mr. Emerson tell us -why . 03&

not have been 05? Also, was there any means of tensioning the wire? Were' any

precautions taken to ensure that' it had

continuous contact round the model, and were there any precautions taken to see

that it'did notcut into the model?

Sir AMOS L. AYRE, K.B.E., Honorary Fellow:

We have certainly reached a period when laminar flow, has come to be respected in this country, and it can be said to be one

of the good things that has come out of

the work of B.S.R.A., where the subject has been persistently pursued.

I do not

think, however, that we have yet got to the stage when, even with the- qualifications given by the Authors, we are able to.

generalize and define its variable effect throughout the speed range, or at a given speed. it is certainly not stable. Many features suggest themselves on close examinatiOn.

On a previous Occasion * I suggested that the Alexander speed, i.e. where Cb = 1 .o -might be the position -at which laminar flow breaks down. ,It certainly has a strong influence either in bringing about an initial breakdown or, in some

circum-stances, is the point at which it has

dis-appeared, the latter apparently applying particularly in the more stumpy forms, say where is less than 17'O. These features would seem to be becoming more apparent as comparative experimental work extends. There is also now reason to believe there is the probability in the finer forms, capable of being run through a wide speed range, that the incidence of laminar flow, instead of being,stable,. varies throughout the speed range, apparently being influenced by the

"hump" and "hollow" speeds. At the normal positions of the "humps" V/.y/L = 78 and 98, a tendency for the effect to reduce would seem to occur and, that the intermediate "hollow ", V!iJZ 86, en-courages a fresh increase. The models referred to in the paper do not, howeyer, cover such a range of speed. I have been able to examine only two of them, 2923A

and 2951D, each at 27 8 ft. and 244 ft. draught, and find thatthe laminar condition is generally of a reducing effect as speed increases and practically disappears at the Alexander speed of about 16 and 16+ knots respectively.

A feature of our belated recognition of the effect of laminar flow is the need for a revaluation of .-many of the conclusions that have. been reached from the work on methodical series in the past. Many of

the models then used were undoubtedly affected by laminar flow. Perhaps 'I may

mention-here that my C2 values-are believed to be free of the effect; all obviously freak models were excluded when these were last compiled.

The references in Section 6 to what many of us have- come' to term the "Emerson formula.", are very interesting. The change' from 83 to, '86 is certainly necessary and, if every care is given to the positioning of the propeller within the aperture, I am sure that a further increase, even up to about 88,' can be made. This applies particularly when the distance from the propeller to the propeller post is made as reasonably large as possible by adopting a long boss

on the latter. As the Authors -know, -I consider that this simple but valuable

formula can be improved to a small, but desirable, extent by introducing some measure of propeller diameter. In fact,

tentatively, I prefer to use it as

x

qpc

and, as just stated, x might be as - high as

88

Even with this recognition of dia-meter, there still remains sOme effect when D is either very small or large relative to the breadth and the form of the vessel which it is driving.

Those of us who for so long had searched for a formula for what was termed the.

missing link between e.h.p. and d.h.p.

should be grateful to Mr. Emerson for his valuable contribution.

-Mr. H. BOCLER, Member:

-It appears at first somewhat disconcerting that recent researches on laminar flow around ship models impugn to some extent the validity of inferences drawn from much ship-model testing previously carried out and that earlier ship estimates of power from model experiments as issued from the N.P.L. are to a varying amount open to suspicion, especially in

the case of

low-speed full forms. Incidentally I men-tion here that it is rather surprising to me

that the laminar flow detected should

apply more to fuller than finer forms as I would have thought the steeper angles of entrance more liable to break up such flow. On the other hand, from the practical point of view of ship designers using model results for guidance in estimates of power for actual ships, such phenomena-as have now been detected have, in fact,'been more -or less covered f-or by allowances which designers have found necessary to bring the N.P.L. predictions from model' experi-ments into line with the results of actual trials. In my own experience I have found an increase of 7+ to 10 per- cent

commonly required above estimated ship figures- as hitherto supplied by the N.P.L. and the power so increased being a net' figure, 1 have 'in new designs added a

ril6

-A REVIEW OF SHIP MODEL DATA further allowance for contingencies

depend-ing upon the circumstances of the case and the stringency of owners' 'requirements. With the later N.P.L procedure referred to by the Authors the extra' allowance I have previously made *111 be reduced. Of course, there are other factors than laminar flow which may enter into my 7+ to 10 per cent allowance.

As pointed out by the Authors and also 'by Allan and Conn in a recent paper read

before the I.N.A.* the main issue is to

have proper comparisons made between actual ship trials and the corresponding model tests'. As Allan and Conn put it'

"the crux of the matter is the question of obtnining trial data of stifficient accuracy to permit an adequat6 analysis to be made."

A consideration arising from the new aspect of affairs is' that. relatively a' model found best' with' previous testing practice may not be found the best under the new procedure. Especially may this be the case with full models having the centre of buoyancy well forward of amidships and it may' be some desirable change in practice is indicated here.

'The Authors' results

appear to

be favourable to bulbous bows. While such may be better from the resistance point

of view, there is also the question. of

slamming "to consider and the bulbous sections shown in the paj,er now under

discussion appear to me to be of a shape contrary to that recommended by-J. L. Kent in his paper read before this Institution

in l949 as desirable to avoid slamming, Mr. Kint's view in this respect having the support of contributors to the discussion on his paper.

Another point. which occurs to me is that the Maier 'bow, 'which is the very opposite of the bulbous bow, has 'been favoured in certain quarters. Do these

later findings about laminar flow indicate

,that the Maier model obtained unduly favourable results on this account? Mr. F. G. BRYANT, Member:

it

is very encouraging to learn that research has been extended following the

'recent paper by Drs. Allan and Conñ. The present paper at once explains and indicates the disturbing fact that there

could be a discrepancy of 10 per cent in 'the estimated ship resistance' by neglecting laminar 'phenomena, which is a very serious matter, especially if one depended entirely upon model tests and as hitherto reported.

So far as I am concerned, model data in the past have been used more as a guide, Trans. LN.A., Vol. 92, p. 107: "Effect of Laminar Flow on Ship Models," by J. F. Allan,, D.Sc., and 3. F. C. Coon, D.Sc.

"Causes and Prevention of Slamming of Ships in a Seaway," N.E.0 Inst.. Vol 65.

Loc. cit.

and data from other ships have' dictated required and reserve margin power output of machinery to be installed in the 'ship.

From very recent experience, I agree

'with the Authors that one cannot' as yet unaided assess quantitatively the effect of

laminar flow; aótual model test with a turbulence-producing device is necessary. In some models I recently had tested at deep draught with and without trip wire, the'e.h.p; with trip wire was increased some 3 to 4 per cent in one model, and only 1+ to 2. per cent in another model (models of fairly similar ships). These results also confirm that laminar flow may disappear

or be. negligible at lighter draughts, at

least for the models tested

In the circumstances and in the light' of present-day knowledge on the, subject, we are now into a new era in model experi-ments, and it is therefore'too early, to say much as we have a tremendous amount

yet to find out.

In the meantime, the

change in the q.p.c. is noted and one will be guided accordingly.

I am, however, in entire agreement with the Authors' statement that ship-trial results should 'in due' course provide the ultimate answer. I would add loaded ship trials and many of them. On a loaded-ship trial on the measured mile with a tank-tested model which took, place some few years ago, I was very satisfied with the results at the time. I now think that had the model been tested with a trip wire the correlated results ,twixt model and ship would have been more in agreethent. (Members on the shipowning side will readily agree that it is not an easy or simple matter to arrange for loaded-ship trials for vessels other than tankers, owing to insurance clauses for deviation, etc.) Other loaded trials where owners have been keenly interested in the matter have taken place at fairly regular intervals since, and as the Authors say, on inalysis of results a possible discrepancy of'3 percerrt or possibly more is not unusual. I have found it advantageous for trials to fit a thrustmeter as well as a torsionmeter

for this and other reasons. Be it said, however, on 'two fairly recent loaded trials with tank-tested' 'models (with trip wire) power prediction over 15 to 20 knots range gaye less than + per cent difference; but as regards prediction of propeller revolutions the actual ship r.p.m. were 2 r.p.m. less than tank prediction over the given speed range. This matter of propeller speed, tank v ship, requires further elucidation, as other loaded trials have evinced' similar discrepancy. It is of interest to note that

the Authors now say that there is some scale effect which 'possibly causes maccu-ràciesfurther research in this direction is desirable and essential, as we should

hear more of this aspect

I do not, however, lose sight of the fact that actual ship propellers as made' and

finished may differ in pitch -from designed values; -the error may be of the order of designed ,pitch 0 1 per cent to 0-5 per cent but I do not think that this factor, while contributory, -is entirely responsible for such: discrepancy.

I am also in agreement with the Authors' statement relative to the effect of condition

of ship's

underwater surface and in particular the effect of recent changes

from riveted to welded butts, etc. In the past, allowances have no doubt been based on ship performance with all seams and butts riveted, and I have found that some correction is necessary where welding has been substituted for riveting, which is somewhat analogous to what, I discovered some years ago, that power for speed was less than -normal calculatiOns indicated for vessels built with a fair amount of flush plating forward (by fitting inside and out-side doublers) for working in ice.

The various tables, design data and

graphs are very useful, although, -as the

Authors, point out on p. 299 a propos Fig. .2(b), Table 5 may be somewhat mis-leading but has other merits.

Dr. KENNETH S. M. DAVIDSON and Mr. J. THOMAS TOTHILL:

The adoption by the National Physical Laboratory of turbulence stimulation in resistance testing, and of the ship-propul-sion point in self-propulship-propul-sion analysis, are notablesteps-forward towards more uniform international practices and also, we believe, towards reliable ship predictions.

The paper. discusses the effect of these changes in text technique on the design

practices evolved at the N.P.L., and the

Authors' main conclusions appear to be that in the slow-speed -cargo-ship field con vex bows have a smaller range of usefulness, and bulbous bows a greater

range of usefulness, than was previously supposed.

The reason for this would seem to be that full-bowed forms are more susceptible to laminar flow- than fine-bowed forms, as has been noted -in several - other recent papers. - The Authors' results provide considerable confirmation of this trend, as is shown in Fig. 6, which is a plot of the

.05

0

.22

0

differences- with -and -without. trip-wire to a base of the half angle of entrance. The differences 'are averages taken over the whole, speed range for each model at the- 24-4 ft. draught, - and show -a rapid rise

with entrance angle in the range

covered bythe tests. -

-The derivation of Figs. 2 to 4, which the Authors state are defined -to a large extent by the model tests described in the paper, is not clear to us. Perhaps some descrip-tion of the reasoning invOlved in this step could be furnished in the Authors'

reply.-A. sinfficant feature of the comparison between Figs. 2(a) and 2(b) is that turbu-lence stimulation appears to have

over-thrown the former concept that at low speeds a full-ended convex-bow form

actually had less resistance than the equivalent fine-ended form. The, new concept on this question is substantially in lin with the results of Taylor's.standard-

-series, which showed -that minimum resistance is given by very fine-ended forms of about 0-53 prismatic, even at low speeds. The question arises: "How did TaylOr's standard series stafid as regards turbu-lence?" The' models were about 20 ft. long and there seems to be very little doubt that the fuller-ended forms would have shown a higher resistance if turbulence, stimulation had been used. - This questiOn should clearly be investigated, on -a few sample standard series - models; it' does not necessarily follow, however, that the whole series should be re-ruti if discrepancies are found. The mairi value of the series for

many years has been its function as 4 yardstick against which to compare practical forms, and a great deal of evidence exists to show that it does ,fulfil that function admirably, in that it accounts for the main trends in proportions and prismatic coeffi-cient, Whether the Taylor curves do or do not truly represent the resistance characteristics - of the Taylor forms is immaterial to this particular application. With regard to the new self-propulsion

point, we assume that Froude's friction

coefficients are still being used for both model and ship. Perhaps this could be -confirmed in the Authors' reply? At the 1948 International Conference of Ship Tank -Superintendents,, general agreement was voiced that a formulation more in keeping with modern concepts of skin friction was desirable, and the Schoenherr formulation was adopted as an acceptable alternative to Froude's. If a change is to be made, the present would seem to be a most -opportune time, because the whole subject of ship resistance is under review and previous data obtained under mixed-flow conditions (including Froude's plank experiments) are now known to be unre-liable.- Since a fresh start is being made in any case, a new friction formulatiOn can A REVIEW OF SHIP -MODEL DATA Dll7

5 IC - IS 20 00