TRAWLING PULL EXERTED BY A TRAWLER - A METHODOF ESTIMATION FROM PROPELLER DIMENSIONS AND

ITS COMPARISON WITH SEA MEASUREMENTS

T. D . GOPINATHA KARTHA * AND R. L. ROY CHOUDHURYCentral Institute of Fisheries Technology, Ernokulom .

The proper matching of the pull exerted by a trawler and thesize of trawl is important for maximising the catching efficiency . Theavailable pull is more dependant on the propeller and its workingconditions than the installed engine power . The normal practice isto directely connect net size to the installed power in the boat byformtila.e without reference to the nroneller dimensions or the avail-ahle trawling pull and this, is not adequate to find out the optimumcombination . By the method outlined in this paper, the accuratecalculation of trawling pull is possible by taking into account onlythe propeller diameter, pitch and r . P . m. The predictions by themethod are compared for trawlers with powers between 30 and 60H . P . and agreement is found to be within ± 5% . The powerabsorbed by the propeller in trawling condition can also be calculatedby this method for checking whether the engine is being overloaded .

INTRODUCTION

The proper matching of installedPower in a trawler and the net size isimportant for maximising the efficiency ofa trawler . This becomes more importantin the case of small trawlers where relati-velv high installed powers can be justifiedif they are effectively utilised for fishingoperations . Basically, this problem willmean estimation of the trawling pull whichcan be exerted by the trawler, so that, the*Now with Geological Survey of India,Hyderabad .

146

net size can he designed to utilise thisavailable pull effectively, or, if the netdesign for a particular set of conditionshave been optimised by other considera-tions the trawling pull or in other wordsthe required installed power can be deter-mined . The state of loading of the engineand the trawling pull are largely dependanton the propeller dimensions and its condi-tions of working. Accurate estimation ofthese quantities are necessary to ensureoptimum combination of net size andmaximum utilisation of engine power and

avoid overloading . Normally used formulaeof the type S = 0 .105 P + 4 (Miyamoto,1959) and S = 0 .095 P 0.56 (Koyama,1962) where 'S' is the area of one otterboard in ft .a and 'P' is the installed horsepower, are based on fitting regression linesto data collected from existing vessels . Thenet dimensions are related by a set ofempirical relations based on dimensions ofotter board . Such formulae, however, donot yield information about the powerutilised during trawling, pull exerted andnet resistance and so it is difficult to knowwhether they are matched properly . Thesecan be calculated by considering only thediameter, pitch and r. p. m . of propellerin trawling condition (Roy Choudhury,1962). In this paper, the method has beenextended with some modifications, and thepredictions compared with sea measure-ments .

CALCULATION METHODS

Calculation of the thrust delivered andthe horse power absorbed by a propellerwill require considerations a set of para-meters defining blade geometry and alsopropeller r. p. m., diameter, pitch (or pitchratio), advance co-efficient (J = n~ where'v' is the velocity, 'n' propeller revolutions/sec., 'd' diameter of propeller, all expressedin any consistent system of units) . Thepossibility of reducing the number of thesevariables was examined (Roy Choudhury,1962) . It was found that effects of thechanges of parameters of blade geometrywere negligible for the small boat propel-lers, and Troost series 3 .50 could be usedfor the calculations of other 3-bladedpropellers . Further the analysis of datafrom fishing vessels (Table 1) shows thatJ = 0.175 can satiafactorily describe theworking conditions of the propeller whiletrawling . With thess simplications thenumber of variables are reduced to only thepropeller r. p. m ., diameter and pitch .Fig. 1 . is prepared from Troost series 3 .50

147

for simplifying calculations . The valuesof thrust and absorbed horse powers for400 r . p . m . are readily available by enteringthis chart with propeller diameter andpitch ratio . For other r. p. m . the thrustand power values thus obtained are multi-plied by a factor proportional to squareand cube of ratio of r . p. m . to standardr. p. m . of 400 . Fig. 2 gives these cor-rection factors against r. p. m. The calcu-lation method is shown in Appendix 'A'

The delivered thrust (T) thus obtainedfrom the chart after applying r. p. m .correction is balanced as

T=R+'t 1 '+ pT (i)where 'R' is resistance of vessel in trawlingcondition, 't 1 ' is the resistance of the gear(including warps) to forward motion andis actually the pull required to tow thegear . 'p T' is a resistance augmentationof the vessel due to hull propeller inter-action and generally is known as thrustdeduction (instead of increase in resistance) .

'R' values were estimated from modeltest results . The values are rather smalland can be estimated by a co-efficient4 lbs/ton of displacement of boat derivedfrom these model results . In the calcu-lations presented here, however, actualmodel test results have been used .

The condition of working of propellersof tugs are very similar to trawlers . Avalue of 0.10 for L T/T is taken on thebasis of the results of tug propulsioninvestigations (Dawson and Parker 1962) .The buttock flow form 'B' and 3 knotsspeed are nearest to the working conditionsof fishing vessels . The actual value 0 .07is increased to 0 .10 considering the ratherwide sternposts of wooden fishing vessels .

In practice, the warp tensions areonly measured . The warps are inclined tothe direction of motion and as such thetotal warp tension (t) is greater than the

gear resistance (t,) . This difference isanalysed with reference to Fig . 3. AB is asmall portion of one warp near the pointof suspension `A'. CD is the directionof forward motion parallel to X - axis .BC is parallel to the Y -axis (transversedirection). ACD is the vertical plane .Angle « and /3 are explained in the I'll-lure .If the two warps are suspended from 'A'then angle 'i3' is the half angle betweenthe two warps at `A' .

From the figuretlt = Co;~ (llS ac COs 13

From measurements `/3' is found to bearound 3 .50 (2/3 = 7°) and ' ~' varies from15° to 30° . Substituting these numericalvalues in (ii)

t = 1 03 to 1 .15 t1 (iii)for a = 15° to 30° respectively .

This relationship is important becausein practice `t' is only measured (withoutand 3) and this relation shows the limitsof accuracy to which the measured tension(t) and predicted pull (t1) can be compared .

MEASUREMENTS

The measurements were carried out in8 boats in 3 size groups 30', 32' and 36'shrimp trawlers . The reduction ratio,propeller diameter and pitch were noted ineach case . The boats operated from Cochinand used shrimp trawls. The 30' and 32'boats operated generally in depths 6 to 12fathoms and 36' boats in 8 to 16 fathoms .The sea conditions were generally calm .During trawling operations, the tension inthe warps, the r . p. m . of engine weremeasured and the trawling speeds wereestimated . The tensions in the warps weremeasured by a portable tension meter,which could be clamped to the warpbeyond the towing point . The tension ineach warp was measured separately by thesame instrument and the total tension

148

obtained by addition. The time intervalbetween the measurements on the twowarps was kept to a minimum, about 2 to3 minutes. The r. p. m. of the engine wasmeasured on the forward end of the crank-shaft by a tachometer two to three timesduring each of the trawling operationsunder observatian . In some cases however,the forward end of the crankshaft was notaccessible and the disc attachment on thetachometer was used on the propeller shaftdirectly and speed measured by comparingthe diameters of the disc and the shaft .The trawling speed was estimated roughlyfrom the time taken by a floating object topass along the length of the boat .

RESULTS AND DISCUSSIONS

The particulars of boats, their redu-ction gears and propellers are given inTable 1 . The measured tension, propellershaft r . p. m. and the estimated trawlingspeeds for the different boats are given inTable - If . As can be expected, for thesame boat many of the measurements i . e .set of r . p. m. - tension - speed values areidentical . From the table, it is seen thatthe estimated advance co-efficient (J) variesgenerally between 0.130 to 0.210, the meanbeing about 0.175. The trawling conditioncan thus be satisfactorily represented byJ = 0.175. There are some advantagesin reading the values corresponding to J =0.175 from Troost series for the prepara-tion of Fig . 1 and so J = 0.175 is choseninstead of the mean 0.170. The thrust andabsorbed power vary + 4 to 7 % for theabove range of variation of J from 0 .175 .

Table III shows the calculated valuesof the pull exerted (t1) and the powerabsorbed by the propeller by consideringpropeller dimensions of the different boatsand the r. p . m. as presented in Table 11 .The calculations were carried out with thehelp of Fig. 1. and the steps shown inAppendix `A'. The available s. b . p. of

rt

440

CACA

QrnPitch/ diomejsr rot, o

044

0

Fig. 1 . Trawling thrust and S . H. P. Diagrams for 400 r . p, m .

O~ta

CD

S^

r- --

R~ '. / rO

10 . 9

IdVON-am

i/ /~!Pp~;- WWW_ .°

/. ~ ©\

o®®.. ~~ ': O-

~0HIM

rNO~dmo'~Z\ ∎o' r UR_-amMp _

'/'s 0.- I a I

0 I 181aa ujrppp.~~S EM I~NAPE

~/PAIN i0-

11

0•0

Imm NNrMEN

Mlm%MMIM∎ mlOMENS

ONE

∎

URU

ShUM

IN

∎

O

kM ME NG.MM

~ME%" ME

~.1~M

Mi .~.y00

wC%0

0cn

Power corr. factor (rJ ----K~N

Q4

NC)1Thrust corr. factor (rte) -©

Fig. 2 . Thurst and S . H. P. correction factors

150

0

N)

Y

z

A,

151

x

Fig . 3 . Diagram showing the difference between warp tension (t/2) and resistence toforward motion (t 1 2)

C.

152

TABLE I BOAT PARTICULARS

Boat No . I I III I V V VI VII Viii

Length 30' 32' 32' 32' 36' 36' 36' 36'

Breadth 9'-3" 9'--6" 9'-6" 9'-6" 11'-6" 11'--6" 11 1 ---6f/ 11' --6"

Displacement(tons)

7 7.42 7.42 7.42 12.25 12.25 12.25 12.25

Max BHP / 33 43 .5 43.5 36 62.5 62.5 62.5 62.5r . p. m . 1200 1500 1500 1500 1500 1500 1500 1500

Reduction ratio 1 .61 :1 2 :1 2 :1 2:1 3 :1 3 :1 3 :1 3 :1

Propeller Dia / 23 25.51 25.5 26 35.3 35.3 36 _36Pitch (inches) 18 18 19 .5 16 25.5 25.5 25.6 25 6

Pitch ratio P/ D 0.782 0.706 0.765 0.615 0.722 0.722 0.711 0.711

TABLE II WARP TENSION AND R. P . M . MEASUREMENTS FROM

I

1d = 23

IId = 25 .5

111d = 25 .5

IVd-26

Vd=35.3

VId=35 .3

VJ = 1217 X Ndn i = N (col.2) X Red. Ratio .

VIId=36

VIIId == 36

153

THE BOATS

Boat No . Prop . Measured warp Estimated tra- EngineProp. dia . RPM tension (t) wling speed J r. P . M .

(in) (N) lbs . v knots (n1)

2 3 4 5 6

1 . 670 792 2.04 0.161 1081)2 . 670 851 2.22 0.175 10803 . 663 834 2.02 0.161 10654 . 663 830 1 .62 0.129 10655 . 645 792 1 .87 0.153 1038

6 . 552 845 1 .58 0.136 11047 . 552 870 1 .53 0.132 11048 . 563 880 1 .58 0.134 11269 . 552 900 1 .62 0 .140 1104

10. 555 835 1 .72 0.148 111011 . 555 835 1 .89 0 .163 111012 . 540 785 1 .82 0.161 1080

13 . 550 710 1 .80 0.153 110014 . 550 710 1 .80 0.153 1100

15 . 342 1275 1 .24 0.125 102616 . 352 1223 2.52 0.246 105617 . 356 1192 2.49 0.241 1068

18 . 364 1135 2.22 0.210 109219 . 362 1162 1 .99 0.190 108620 . 356 1129 1 .99 0.193 106821 . 362 1130 1 .91 0.182 1086

22 . 356 1012 1 .48 0.140 106823 . 358 1019 1 .42 0.134 107424 . 356 970 2.13 0.202 106825 . 347 1047 1 .62 0.158 1041

26 . 387 1107 2 .27 0.204 116127 . 366 1114 2.28 0.210 109828 . 382 1135 1 .99 0.176 104629 . 370 1115 1 .97 0 .180 1110

154

TABLE III CALCULATED VALUES OF POWER ABSORBED BY PROPELLERAND TRAWLING PULL

Boat No .s.h.p. max .

Calculated pull(t 1 ) lbs .

Calculateds . h. p . (ab) . s . h . p. (n 1 )

1 2 3 4

I 1 . 806 27 2730 2 . 804 27 27

3 . 790 26 274 . 792 26 275 . 747 24 26

I I 6 . 788 24 267 . 789 24 26

39 8 . 821 25 279 . 788 24 26

III 10 . 796 24 2639 11 . 794 24 26

12 . 752 22 26

IV 13 . 626 17 1932.5 14 . 626 17 19

V 15 . 1033 25 3856.2 16 . 1062 28 40

17 . 1091 29 40

VI 18 . 1149 31 4156.2 19 . 1141 30 41

20. 1102 29 4021 . 1143 30 41

VII 22 . 1180 31 4056.2 23 . 1195 31 40

24 . 1164 31 4025 . 1117 28 39

VIII 26 . 1382 39 4456 .2 27 . 1229 33 41

28 . 1352 38 4329 . 1266 34 42

1400

1200

U)zQ

Z 1000W

800

CALCULATED PULL (t,) lbs .

Fig. 4 . Comparison of measured tension (t) and calculated pull (t 1 )

155

6

o ,+

f®

OO

c

CtI 9 where C-

® Propeller was

!-02c to 1 , 436 (me

damaged; 3o coicu

n 1 . 071

ted

terse ons are he€gher.

500

700

900

1100 1300

I

the engine for the particular engine r . p . m .(Col . 4 . of Table III) is estimated byassuming a constant torque i . e ; s . h. P .

(trawling) = n '-X max. s. h. p .1

`n1 ' is trawling r . p . m. of engine fromCol. 6 Table 11 .

`N 1 ' is continuous rated r. p . m . ofengine from Table I

Max. s . h . p.=0.9 X Max. BHP fromTable 1 .

The s. h. p. absorbed by the propellerare calculated and shown in Table III(Col. 3) . Comparison of Col . 3 & 4 showswhether the engine is overloaded or under-loaded and by how much. The differencesbetween the available (Col . 4) and utilisedi . e . absorbed s . h . p . (Col . 3) are small inthe case. of 30' boats and very large in thecase of 36' boats. So possibilities exist forthe 36' boat to increase the catchingefficiency by increasing net size, increasingspeed of trawling if that is beneficial, usingbetter net configuration (which mayincrease resistance) or even the use of twonets .

In Fig. 4, the measured tension (t) isplotted against calculated pull (t 1 ) and itis seen that most of the points lie withinthe lines t = 1 .020 t 1 and t = 1 .136 t 1 .This is in good agreement with the relationt = 1 .030 to 1 .150 t i derived from equa-tion (iii) . However, some of the pointsmarked specially, are completely outsidethe range and calculated values are toohigh . These are from tw\36' boats VII &VIII for which the propellers were severelydamaged by erosion . The damages werenot due to cavitation and later it wassuspected due to stray current effects . 76%of the remaining points lie within the twolines mentioned above . The warp tensionpredicted by the mean time t = 1 .070 t lwill have + 5% accuracy . However, whiledesigning the net, the gear resistance

156

should be equated to pull (t 1) and notwarp tension (t) as explained earlier .

Another factor affecting propellerperformance is onset of cavitation . Thecalculation of the condition for cavitationcannot be simplified . But generallyit is found in case of these small trawlersthat if the propeller r . p. m. is around 400to 500 corresponding to full rated r, p . m .of engine and if the propeller pitch -diameter ratio is between 0.6 to 0.8, thepossibilities of cavitation and thrust break-down are negligible .

ACKNOWLEDGEMENT

The authors are thankful to Dr . A. N .Bose, Director, Central Institute ofFisheries Technology, Ernakulam, forpermission to publish the paper .

REFFRENCES

Choudhury, R. L; 1962. 10th IPFC . Proc .,Sec. III pp. 87-93 .

Dawson, J. & Parker, M . N ; 1962 . Trans .of Royal Inst. of Nav. Arch ., London,pp. 252 .

Koyama, T; 1962 . Bull Tokai Reg. FishRes. Lab . 33 : pp; 32 .

Miyamoto, H . 1959. "Modern Fishing Gearof the World ." Fishing News (Books)Ltd ., London, E . C . 4 . pp . 248-250 .

APPENDIX `A'

The method of calculation of thetrawling pull (t 1 ), absorbed s . h . p. duringtrawling and available s . h . p . duringtrawling are illustrated by takihg observa-tion No . 17 from boat No. V ref. TablesI & II .

From Table I Prop . dia. = 35.3 in . - (a)Pitch ratio = 0 .722 - (b)

Reduction ratio

= 3:1Displacement

= 12.25 tons

From Table iI

Prop . r. p . m- whiletrawling = 356 - (c) (observation No . 17)

Corressponding engine r . p . m . (n 1 ) _356 x 3 = 1068 - (d)

R (estimated at 4 lbs ./ton of displacement)= 52 lbs . A T = 0.10 T = 127 lbs .

1 .57

So t i = 1091 lbs .Measured warp tension (t) = 1192 lbs .t/t l = 1 .09

Calculation of available s. h. p.

28 .6 x 100=51%56.2

(where 3 is reduction ratio) .By entering Fig . I with prop. dia . (a) andpitch ratio (b) the absorbed s . h . p. andthrust for 400 r . p. m .

From (d) Engine r, p . m. (n l ) in trawlingcondition = 1068

Full engine r. p . m . (N 1 ) from Table I= 1500

Full B. H. P. from Table I = 62 .5Full

h,

= 0.9

62.5

56.2

s. h . p. = 40.5Thrust = 1610 lbs .

Actual prop . r . p . m . (c) is 356 .

From Fig . 2 Thrust and s . h . p correction

s .

p.

X

==s . h. p. available at trawling condition

ni

1068full

h .

56.2 = 40factor for 356 r . p. m. are 0.792 andrespectively .So for trawling condition

0 .705 x

s . P . = -i-506 XN 1

So % utilisation

of available

s . h . p .

during trawling =25.640 X 100 = 71 .5%Thrust (T) = 0 792 X 1610 = 1270 His .

Absorbed (utilised) s . h. p . = 0.705 X i . e. underloaded40.5 = 28 .6 - (f)

Calculation of pullfrom Eqn . (i) T = R + t i + A T Percentage utilisation of full s, h . p .