strategies for improving fuel efficiency in portuguese trawl fishery.pdf

8/13/2019 Strategies for improving fuel efficiency in portuguese trawl fishery.pdf

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Fisheries Research 93 (2008) 117124

Contents lists available atScienceDirect

Fisheries Research

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / f i s h r e s

Strategies for improving fuel efficiency in the Portuguese trawl fishery

Joaquim Parente, Paulo Fonseca, Victor Henriques, Aida Campos

INRB/L-IPIMAR National Institute for Biological Resources/Fisheries Research Laboratory, Avenida de Braslia, 1449-006 Lisbon, Portugal

a r t i c l e i n f o

Article history:

Received 31 October 2007

Received in revised form 6 March 2008Accepted 6 March 2008

Keywords:

Portuguese trawlers

Fuel saving

Energy economy

ModelsFlume tank

a b s t r a c t

The recent rise in oil prices has brought renewed attention to energy savings in the fishing industry, and

particularly in trawling. Coastal trawlers spend most of their time on fishing grounds near the coast. In

such cases, the most successful energy-saving modifications ought to result from changes in the fishinggear and towing conditions. The purpose of this study was to identify the energy-economy potentialfor Portuguese fish trawlers after altering a vessels operating conditions and improving its trawl gear

performance. Two trawlers, namedTricana de Aveiro and Joao Macedo were selected as subjects in thisproject. Both vessels work with gear of similar design and size. Experimental sea trials were carried outto elucidate the actual vessel and gear performance. A model trawl was then built and tested in a flume

tank, which provided the basis for improving the gear design. Full-scale trials were then carried out withboth vessels using the modified trawls in order to assess changes in gear performance. The new trawls

maintained their previous ability to catch species of differentecological groupsand consumed less fuel atthesame commercial trawling speed. An economic study showedpotential increases in the net cash flow

(NCF) of up to 27% over the range of operational navigation and trawling speeds. Having demonstratedtheperformance of thenew trawls, the skippers of both vessels subsequentlyadopted the new designfor

commercial fishing. 2008 Elsevier B.V. All rights reserved.

1. Introduction

Harvesting fish from the sea requires a large amount of energy,although the energy requirements can vary substantially depend-

ing on the type of fishery.Tyedmers et al. (2005)estimated worldfisheryfuelconsumptionat 50 billion (50109) l.Whileconsideredserious underestimates, these figures nonetheless correspond toabout 1.2% of the global fuel consumption. On the other hand, a

tonne of fuel is consumed for each 1.9 tonnes of fish captured, and1.7 tonnes of CO2 is released for each tonne landed. As such, fuelconsumption is both an economical and environmental problem.

Energy saving has been a subject of research since the 1970s oil

crisis, leading to several studies aimed at improving vessel designand power consumption. Special attention has been given to hullresistance and tests in model basins. Benefits were identified fromusing bulbous bows in small fishing vessels, leading to a reduction

in fuel consumption of 1530% during sailing (Kasper, 1983). Gainsin propulsive efficiency between 10 and 17% during free navigationwere also attained using ducted propellers in trawlers (Basanez,1975). Large savings in fuel consumption (up to 28%) could also

be obtained from this type of propeller by towing at lower speeds(ODogherty et al., 1981).

Corresponding author. Tel.: +351 21 3027162; fax: +351 21 3015948.

E-mail address: [email protected](J. Parente).

In addition tovesseldesign,special attention hasalsobeen givento vessel operations. Efficient ship operation is required for long-term fuel economy ofthe vessel, andentailsselectingthe best route,draft and trim; adequate maintenance of the hull and machinery;

and a rational exploitation of the available systems by well-trainedcrews. The choice of the best running point (that is, the vesselsoperating speed that maximizes cash flow), both in trawling andin free navigation, is a major contribution toward energy savings

and must be continuously adjusted according to vessel require-ments.

Trawlers are among the most fuel-demanding fishing vessels.This is due to the high towing resistance associated with the gears;

the netting drag alone typically accounts for 60% of the total gearresistance (Wileman, 1984).Reducing the netting surface by usinglarger meshes in the net forepart (wings and square) may signifi-cantly reduce net drag without affecting the trawl mouth area and

thus the catch efficiency. This is particularly true for those speciesthat display herding behaviour inside the trawl (Fiorentini et al.,1987).

Other possibilities for reducing the net drag have also been

recently investigated, such as the use of knotless netting and thin-ner twine. Ward et al. (2005) compared the drag of twin trawlsmade of traditional polyethylene twine with similar trawls ofreduced twine diameters, and reported a drag reduction of 6% and

an increase in mouth opening of 10%.Alterationsto thegear rigging,such as the number of bridles and their relative length, may also

0165-7836/$ see front matter 2008 Elsevier B.V. All rights reserved.

doi:10.1016/j.fishres.2008.03.001
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118 J. Parente et al. / Fisheries Research 93 (2008) 117124

Nomenclature

A mouth area (m2)

C total catch per haul (kg)

Cw crew variable wages per trip (D)CPUE catch per unit effort (kg/h)

D total distance covered per trip (in free navigation)

F fuel costs per trip (D)K sum of all fixed costs per trip (capital costs, mainte-

nance, insurance, crew fixed wages, administration,other costs) (D)

m 1.05 (margin for lubricating oil costs as a fraction of

fuel costs)

n number of haulsNCF net cash flow per trip (D)

p unit fish price (D/kg)

pf unit fuel price (D/kg)

p(%) crew percentage over the total catch value (0.35 forall vessels tested)

qh hauling fuel rate (l/h)

qn navigation fuel rate (l/h)

qp harbour fuel rate (l/h)qs setting fuel rate (l/h)

qt trawling fuel rate (l/h)

R total catch value per trip (D)

T total trip duration (h)

Th average hauling duration (h)

Tn total time spent in free navigation per trip (h)

Tp total time navigating inside the harbour (h)

Ts average setting duration (h)

TT(tr) total trawling duration per trip (h)

Tt average trawling duration (h)

V free navigation speed (kn)

Vt trawling speed (kn)

f fish density per unit volume of water (kg/m3)

strongly affect the net shape and drag and consequently improvethe overall trawl efficiency.

Recent oil price increases have brought renewed attention toenergy-saving methods in the fishing industry (e.g., Project Green

Fish1; Leblanc, 2005), including the use of alternative fuels andlubricants (such as bio-diesel and bio-lubricants). However, dueto the European Commission restrictions on new constructions,the major opportunities for reducing fuel consumption are chiefly

related to improving vessel operation rather than commissioningnew energy-saving vessels. Fuel-efficient gear design continues tobe a top priority for improving the efficiency of the existing fishingfleet (European Commission, 2006).

The objective of this study was to identify the fuel-economy

potential for Portuguese fish trawlers either by changing the ves-sels operating conditions or by improving the trawl gear design.Coastal trawlers were chosen for study since they spend most of

their time trawling near the coast, and thusmight expect the great-est energy-saving return from changes of fishing gear.

2. Material and methods

2.1. Choice of vessels

The existence of two primary metiers has traditionally been

assumed for the Portuguese coastal trawl fishery: crustaceans and

1 http://www.peixeverde.org/peixe org eng/index.htm(last accessed

2007/10/01).

fish. Each corresponds to well-defined fleets of 26 and 70 activevessels, respectively (DGPA, 2004). According to the statistics ofthe General-Directorate for Fisheries (DGPA, 2004),the mean val-

ues of gross tonnage and engine power for the fishing fleet are183.8 tonnes (standard deviation S.D.,70.9) and 712 HP (S.D.,285.9),respectively. It is a coastal fleet accustomed to short (3 dayaverage)fishingtrips.Themain species landedare horse mackerel(Trachurus

trachurus, accounting for around 40% of the total catch), followedby blue whiting (Micromesistius poutassou) and other semi-pelagicfish, and finally other cephalopods (squids and octopuses;DGPA,2004).

The present study focused on two trawlers from the Portuguesefleet, since fish trawls usually offer a wide basis for gear mod-ifications. The increase in mesh size in the trawl fore is one ofthe measures usually tested to reduce the net drag in these types

of trawls. This is because most fish species, unlike crustaceans,display herding behaviour within the net area. Such behaviourtranslates to a larger catching efficiency despite the larger meshsizes. The two trawlers studied were the Tricana de Aveiro and

Joao Macedo, each of approximately 24m overall length and 600HPengines. Eachvessel landsa diversifiednumber of species, includinghorse mackerel, other species swimming near the ocean bottom,and benthic species such as octopus and flatfish. These catches

belong to a well-defined landing profile (homogeneous group interms of species composition) recently defined in Campos et al.(2007).

2.2. Trawl design

The technical drawings and rigging details for trawl T 1 (of the

Tricana de Aveiro) and trawl J1 (of the Joao Macedo) are very sim-

ilar. Both trawls are reinforced in the lower belly using a thickerpolyamide (PA) twine that is usually found in trawls of Spanishdesign,and differed mainlyin themesh sizes ofthe different panels.The footropes are made of steel wire rope covered with polyethy-

lene and have extra chain weight protection in the bosom, lower

quarters andat the wingends.The technicaldrawingfor J1 , togetherwith the footrope details, are shown inFig. 1.

2.3. Data collected and measuring devices

A total of eight trials were carriedout during thestudy. For eachvessel, an experimental and a commercial trial were carried out atthe two different phases of the project (before and after trawl gear

optimization) in orderto measure fuelconsumption underdifferentvessel-operating conditions. The vessels consumables (water andfuel supplies) at the start of the trials were kept the same in bothvessels to ensure identical testing conditions.

A fuel monitoring systemwas installed in each vessel. The work-ing time duration of the engine, the engine speed, the total fuelconsumption and the instant fuel rate were logged by the sys-

tem. Data on the exhaust temperature and vessel speed (overthe ocean bottom) were obtained from vessel instruments (suchas engine temperature gauges and GPS equipment, respectively).Trawl geometry (e.g., the vertical opening at the centreof the head-line, and the wingend and otterboard spread) and the water flow in

the towing direction were measured by hydroacoustic (Scanmar)sensors. In the commercial sea trials, the catch weight was alsoregistered for all commercial species.

A typical round trip for a coastal trawler consists of several

operating situations for different engine loadings. Fishing vesselswith a controllable pitch propeller have an optimum combina-tion of pitch and propeller revolutions for each operating situation,leading to optimum specific engine fuel consumption. However,

during free running, it is common practice to transfer some

power from the main engine to constant displacement hydraulic
http://www.peixeverde.org/peixe_org_eng/index.htmhttp://www.peixeverde.org/peixe_org_eng/index.htm


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J. Parente et al. / Fisheries Research 93 (2008) 117124 119

Fig. 1. Technical drawings of the J1trawl (FV Joao Macedo), together with footrope details.

pumps and AC generators through power take-offs, forcing themain engine to run at a constant speed. Having this in mind,

changes in the vessel-operating situation were carried out onlythrough propeller pitch variation, although this is not the bestprocedure to optimize both specific fuel consumption and engineefficiency.

2.4. Experimental sea trials

Data on the above parameters were collected at several pro-peller pitch increments up to the maximum working pitch, both in

trawling and in navigation. Foreach pitchincrement, operatingcon-ditions were kept constant for 15 min. Fuel rate was then recordedunder both conditions as a function of trawling speed and engineexhaust temperature.

Gear geometry was recorded over a range of trawling speeds,

including the average speed used in commercial fishing. The main

parameters characterizing the gear performance, namely verti-cal opening, wingend spread and otterboard spread, were also

recorded as a function of trawling speed (as measured by a speedsensor).

2.5. Commercial sea trials

The vessel performance was evaluated at the different phases

of the fishing trip (Table 1).This allowed for a full characterizationof the average trip for each vessel. A phase is defined as the sum ofseveral sub-phases repeated along the trip (e.g., the trawling phaseis the sum of all trawling operations). The relevant parameters for

each sub-phase are presented inTable 1,and include the workingtime Tof the engine, the fuel consumption Q, the average vesselspeed, the average exhaust temperature T(C), and the average fuel

rateq.


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Table 1

Characterization of the commercial trip and parameters registered during the different sub-phases

Phase Description Parameters registered during the sub-phases

Harbour navigation Harbour manoeuvres and conditioned navigation inside the harbour T,Q

Free navigation Travel between the harbour and the fishing ground, as well as navigation between the fishing grounds T,Q,V,T(C),q

Setting/hauling Setting and hauling operations T,QTrawling Trawling operations T,Q,V,T(C),q

Miscellaneous Net repairing, waiting for dawn to set the gear, and other unforeseen events T,Q

Parameters includeT, time duration (h);Q, fuel consumption (l); V, average vessel speed (kn);T(C), average exhaust temperature; q, average fuel rate (l/h).

2.6. Alterations to trawl design

When improving trawl efficiency, either a new trawl can beadopted or alterations can be introduced to the existing gear with-out a radical design change. The first hypothesis seemed unlikely

since Portuguese fishermen, as with fishermen in other countries,are typicallynot receptive to sudden changes in work habits. There-fore, we decided to modify currently used trawls with the aimof decreasing trawl drag without affecting catch efficiency. Model

tests were carried out in IFREMERs flume tank at Boulogne-sur-Mer. A 1:15 scalemodelwas usedbasedon theoriginaltrawldesign,which was then further modified and tested according to the new

(and hopefully improved) specifications.Only the net itself was scaled down, together with a small por-

tion of the sweeps, in order to obtain accurate measurements ofnet drag and geometry. Given the similarities in shape between theoriginal J1and T1trawls, with only small differences in the overall

mesh size and total twine surface area (J1: 98.2 m2; T1: 104.9 m

2),the flume tank tests were carried out over a single trawl modelbased on the J1trawl design.

The main trawl alteration was to increase the mesh size at the

wings and square; this is the optimum method to improve themouth area without increasing net drag. However, an importantpercentage of the total catch for both the J1 and T1 trawls cor-respond to species that do not display herding behaviour (e.g.,

octopuses; see Table 2) and this has tempered the amount of allow-

able mesh size increase. The former mesh sizes used in the lowerand upper wings (80 mm, full mesh) were increased to 100 and120mm, respectively, in the new design. The square was divided

into two sections: the first with 120mm mesh sizesand thesecondwith 100 mm mesh sizes (Fig. 2).

Further changes in the original trawl design included alterationsto the wingends, which were modified into a V-shape by chang-

ing the panel cuttings. By doing so, it was possible to eliminatesome useless netting while also fitting a third bridle at the joininglevel of thetwo faces. The extra bridle decreased thetension on the

upper bridle and trawl headline, which favoured the vertical open-ing. Alterations to the cuttings were also performed in the panels at

the belly section, while the numberof meshes at the codendjoiningrow was made equal in order to match their widths.

Following the tests on the optimized model, a new full-scaletrawl J2 (Fig. 2)was tested at sea. Adjustments were made to the

length of the lower bridle during the full-scale trials in order tooptimize the mouth area. Performance comparisons of the twogenerations of full-scale trawls focused on the following criteria:first, the WS/VO ratio, as a measure of the amount of trawl flatten-

ing over the ocean bottom. High ratios are characteristic of trawlsadapted to the capture crustaceans and benthic fish, while lowratios lend themselvesto catching fish with higherverticaldistribu-tions, such as horse mackerels; second, the mouth area (A), roughly

estimatedas theproduct betweenthe vertical opening andthe win-gend spread(thatis, VOWS);third, the trawl resistance,for whichvalues were obtained during the tests with the flume tank.

2.7. Economic analysis

The average parameter values were used as inputs whencomputing the economical analysis of each vessels trip. These

parameters included the total distance covered pertrip, D; the totaltrip duration, T; the number of hauls, n; the average duration ofhauling and setting, orThand Ts, respectively; the total time navi-gating inside the harbour,Tp; the fuel rate during hauling, setting

and harbour navigation, or qh, qs,and qp, respectively; the unit priceof fish,p; and the unit price of fuel, pf. Based on those parameters,it was possible to simulate the potential for fuel savings and thenet cash flow (NCF) variation, assuming different combinations of

the navigation andtrawling speeds andthe corresponding values offuel rate during navigation and trawling, or qnandqt, respectively.The NCF for a fishing trip is computed as below (see Nomenclaturefor an explanation of the variables):

NCF = R F Cw K, (1)

Table 2

Percent catch, revenues, and CPUE for trawl T1 (vesselTricana de Aveiro) and trawl J1 (vesselJoao Macedo)

Species Catch (%) CPUE (kg/h) Revenues (%)

T. AveiroHorse mackerel 41 31.08 Pouting 30

Pouting 27 20.57 Horse mackerel 25

Skate 11 7.83 Skate 13

Octopus 6 4.71 Atlantic john dory 7

Blue whiting 3 2.55 Common sole 7

Large sc. gurnard 3 2.04 Octopus 5

Total catch 75.54

J. Macedo

Octopus 38 10.45 Octopus 34

Pouting 15 4.18 Common squid 21

Horse mackerel 14 3.74 Pouting 14

Small spotted dogfish 7 2.00 Horse mackerel 7

Common squid 6 1.57 Atlantic john dory 5

Large sc. gurnard 5 1.31 Seabass 5

Total catch 27.69


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Fig. 2. Technical drawings of the J2trawl and rigging specifications.

whereRis the total catch value per trip,Fis the fuel costs per trip,

Cw is the variable crew wages per trip, andKis the fixed costs per

trip. Additionally, we have

R=

C

n

p, (2)

whereCis the total catch per haul. According to Dahle (1982),thiscan be simulated by

C= (

Vt 0

.5147

Tt 3600

A)

f, (3)


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Table 3

Operational parameters under two different working conditions during navigation,and corresponding fuel rate decrease obtained through speed reduction

FVTricana de Aveiro FVJoao Macedo

A B A B

V(kn) 10.0 9.6 9.7 8.6

q(l/h) 96 78 125 92

T(C) 417 362 387 330

Speed reduction (%) 4 11

Fuel rate decrease (%) 19 26

A denotes original condition and B denotes estimated critical speed.

whereVtis the trawling speed,Ttis the average trawling duration,

and fthe fish density per unit volume. Here, fwas calculatedby considering the average parameters forC,Vt ,Tt and Aobtainedduring the second sea trials of both vessels.

The fuel costs per trip (F) and the crewwages per trip(Cw) were

obtained according to

F=mpf [(qt Tt + qh Th + qs Ts)n+ qn Tn + qp Tp], (4)

andCw =p R, (5)

wheremis the margin for lubricating oil costs, Tnis the total timespent in free navigation per trip, andp is the crews (fixed) percent-age of the total catch value.

The values for Tn and Tt were calculated according to the fol-

lowing equations, whereDis the total distance covered per trip infree navigation, Vis the free navigation speed and TT(tr) is the totaltrawling duration per trip:

Tn =D

V, (6)

TT(tr) = T Tn n (Th + Ts), (7)

and

Tt =TT(tr)

n . (8)

The objective was to maximize the NCF through the best com-bination of the navigation and trawling speeds, in order to havecomparable results for both trawl versions. The fixed costs (K) werenot considered since they were assumed constant throughout the

analysis.

3. Results

The first set of experiments estimated vessel performance dur-ing free navigation. Values obtained for V, q and Tare presented

inTable 3for the original working conditions (denoted by A) and

for the estimated critical speed (the speed beyond which the fuelrate increases sharply; denoted by B). A reduction in the navigationspeed alone leads to a decrease in fuel rate of up to 26% for this

phase.

Table 4

Results obtained in full-scale trials for the J1and J2trawls

Speed (kn) VO (m) WS (m) WS/VO Mouth area (m2 )

Full scale J1 J2 J1 J2 J1 J2 J1 J2

3.5 2.7 2.8 16.7 14.7 6.2 5.3 45.1 41.2

4.0 2.5 2.6 16.0 14.8 6.4 5.7 40.0 38.5

4.5 2.3 2.6 15.8 14.6 6.9 5.6 36.3 38.0

5.0 2.2 2.6 15.8 14.4 7.2 5.5 34.8 37.4

VO: vertical opening (m); WS: wingend spread (m).

Table 5

Vessel data obtained from trawling experiments with the old and modified trawlsat the commercial average speed

Vessel Trawling

speed (kn)

Exhaust temperature (C) Fuel rate (l/h) %

J1trawl J2trawl J1trawl J2trawl

T. Aveiro 3.7 367 315 78 64 18

J. Macedo 4.3 390 365 120 104 13

Table 4presents the results from full-scale trials for both the

J1 and J2 trawls. The best results were achieved with a regulatorychain 1.5 m long, resultingin higherverticalopenings(maximumof2.8m at 3.5kn). Wingend spread varied only slightly (between 14.4and 14.8 m) over the experimental speed range (3.55.0 kn) while

otterboard spreadremained constant (between 70 and71 m).Com-paring these results with those from the first experiments at sea(Table 4), we note that the wingend spread of the new gear is lowerwhile the vertical opening is higher, especially for higher trawling

speeds. This potentially favours the capture of species that swimhigher in the water column. Gear resistance measurements are notavailable from full-scale trials; however, tension values obtained at

the flume tank indicate a reduction of approximately 15% in trawlresistance (within the range of commercial trawl speeds) when thenew trawl was tested (Parente, unpublished data). Overall reduc-tions in fuel consumption of 18 and 13% were obtained using thenew trawls for the Tricana de AveiroandJoao Macedo, respectively

(Table 5).These results hold at the towing speeds usually adoptedby the skippers (3.7 and 4.3 kn, respectively).

TheNCF was determinedbasedon themean valueof fishdensityper unit volume (f) estimated with data from the secondcommer-

cial sea trial (Table 6).Both trawls were assumed to fish under thesame conditions, andthe valueoffwas assumedconstantin orderto facilitate the comparison of economical results. A set of param-eters (also assumed to be constant) characterizing the activity of

each vessel was obtained from the commercial sea trial (Table 6).The values A,qt and qn at different speeds, along with the above-mentioned parameters, were entered into equations(2)through(8)in order to estimate the values ofR,F, andCw . Finally, the cor-

responding NCF values were determined.Figs. 3 and 4display thetrends in theNCF asa function of the trawling speed (Vt) for a rangeof navigation speeds (V) within the limits specified inTable 6foreach vessel and trawl version.

Table 6

Characteristic parameters and speed variation for NCF determination

Parameters Tricana de Aveiro(J2 trawl) Joao Macedo(J2trawl)

Used forfdetermination

C(kg) 253.8 158.2

Vt(kn) 3.7 4.3

Tt(h) 2.964 3.516

A(m2) 39.0 38.0

Characterizing each vessel tripD(n.mi.) 71 56

T(h) 28.7 25.9

n 5 5

Th(h) 0.17 0.17

Ts (h) 0.15 0.17

Tp(h) 1.32 1.23f(kg/m

3) 31105 15105

qh(l/h) 50 80

qs(l/h) 60 87

qp(l/h) 80 114

p(D/kg) 359 330pf(D/kg) 30 30

Speed variation

V(kn) 8.410.0 8.19.7

Vt(kn) 3.44.6 3.74.5


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Fig. 3. Net cash flow (NCF) for J1and J2trawls as a function of the trawling (Vt) and

navigation (V) speeds for the ranges specified inTable 6.Vessel:Tricana de Aveiro.

Fig. 4. Net cash flow (NCF) for J1and J2trawls as a function of the trawling (Vt) and

navigation (V) speeds for the ranges specified inTable 6.Vessel:Joao Macedo.

3.1. FV Tricana de Aveiro

For a navigation speed of 10.0 kn, the NCF estimates (Fig. 3)arehigherforthe J1 trawlat lowertrawling speeds,with a highest valuefound at 3.7 kn (D1604). This figure is similar for both trawl ver-sions at 4.1 kn,substantiallyincreasing forthe newJ2 trawl at higher

speeds (whereit decreasesfor theold J1trawl), andreachinga max-imumat4.6kn(D1769).According to theseresults,with bothtrawlsworking attheirbest operatingconditions,a 10% increasein theNCFis obtained with the new J2trawl (Table 7),despite the increase in

fuel costs resulting from the adoption of a higher (+0.9 kn) trawlingspeed.

Table 7

Determination of NCF, fuel costs and percent variation

Tricana de Aveiro Joao Macedo

J1trawl J2 trawl % J1trawl J2trawl %

V(kn) 10.0 10.0 9.7 9.7

Vt (kn) 3.7 4.6 4.3 4.5

F(D) 366 416 +14 482 447 7

NCF (D) 1604 1769 +10 383 487 +27

3.2. FV Joao Macedo

Similarly, for a navigation speed of 9.7 kn, the NCF values arehigher for theold J1trawl at lowertrawling speeds.The best results

arefoundat3.7kn(D 433).TheNCFisaboutthesameforbothtrawlsat a speedof 4.0kn,increasing for thenew J2trawl at higher speeds(whereit decreasesfor theold J1 trawl), andreaching a maximum at4.5 kn (D 487;Fig. 4).The best operating conditions with the new

J2 trawl are attained with a slight increase in the trawling speedrelative to the original speed adopted by the skipper for the oldertrawl, leading to a 27% higher NCF (Table 7).

4. Discussion

This study demonstrated that significant improvement in fuelconsumption and net cash flow can be obtained in the short-termfor two Portuguese coastal fish trawlers. This benefit can also be

obtained without the needfor majorchanges in overall vessel tech-nology. Fuel savings of up to 26% were obtained by bringing thenavigation speed close to the critical speed. However, the latterfigure pertains to the navigation phase alone, and data from the

commercial sea trials showed that the time spent at this phase is

low when compared to trawling. Even though the duration of thenavigation phase may vary substantially, since it depends heavilyon the strategy adopted by the skipper (such as the distance from

the coast and time of navigation among fishing grounds, as dic-tated by the abundance of target species), it averages only 24% ofthe whole fishing trip. As such, the percentage of fuel consumed innavigation will alwaysbe substantially lower compared to trawling.

The trawling phase therefore emerges as the more importantphase for fuel reduction efforts. Simple changes at the trawl level(such as steeper cuttings in the wings and bellies, and mesh sizeincreases in the respective net sections) demonstrated fuel reduc-

tions of up to 18%. Overall, simulations carried out to estimate theoperational conditions that maximized the net cash flow showedthat the NCF could increase up to 27%, with significant dependenceon the specific vessel.

Strong differences were observed in the NCF trends between J 1and J2 trawls. This can be explained by the corresponding differ-ences in mouth area and net resistance, which in turn affect the

total catch per haul and the fuel cost per trip. The J2trawl was moreefficient at higher trawl speeds, while the J1 trawl showed betterperformance than the J2 trawl at lower trawl speeds. In fact, the

J1 trawl mouth area decreased substantially with increasing speed,

leading to reduced catches, while for J2 this parameter remainedapproximately constant within the range of commercial trawlingspeeds.On the other hand, the lowernet resistance associated withthe use of the J2 trawl was the likely cause of the lower fuel con-

sumptions, thus contributing to the increase in the NCF.The efficiency gains in gear drag resulting from the introduc-

tion of technical alterations to the trawls allowed an increase in

trawling speed and therefore in ground coverage. This increasedthe fishing yield at the expenses of extra fuel consumption. Thistrade-off proved to be the best option to maximize the NCF as, inboth cases, the maximum NCF was achieved for trawling speedshigher than those originally adopted. Also, the potential savings

from bringing thenavigation speed close to thecriticalspeedwereovershadowed by the advantage of navigating at a higher speed, 2

thus freeing more time for trawling and therefore increasing catchyields.

2 However, it can be argued that the differences in the NCF (due to the differ-

ence between the critical speed and the navigation speed adopted by both vessels)

is small. This would not justify a higher navigation speed since it might increase

maintenance of the engine due from the higher loading.


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Table 8

Percent catch, revenues, and CPUE for J2 trawl: vessels Tricana de Aveiro and JoaoMacedo

Vessel Species Catch

(%)

CPUE

(kg/h)

Revenues (%)

T. Aveiro

Pouting 37 31.72 Pouting 28European hake 19 16.27 European hake 24

Horse mackerel 11 9.31 Octopus 10

Octopus 7 5.80 Horse mackerel 7Little sole 5 4.12 Skate 4

Large sc. gurnard 4 3.10 Common sole 4

Common squid 1 1.01 Little sole 2

Dogfish 1 0.61 Common squid 2

Total catch 85.64

J. Macedo

Pouting 3 1.25 Pouting 2

European hake 28 12.51 European hake 28

Horse mackerel 16 7.39 Octopus 10Octopus 11 4.78 Horse mackerel 7

Little sole 5 2.10 Skate 1

Large sc. gurnard 8 3.47 Common sole 4

Common squid 9 4.04 Little sole 4

Dogfish 8 3.81 Common squid 24

Total catch 44.99

At about 4.5kn, the optimized trawl presented an overall moutharea only slightly higher than the original one, but favoured the

vertical opening (+13%) over the horizontal opening (7.5%). Thesealterations to trawl geometry may be advantageous at high tow-ing speeds since it increases the trawl efficiency towards smallpelagic, fast-swimming species, while still allowing for simultane-

ous capture of a number of benthic and demersal species. This isevidenced when comparing CPUE results in Tables 2 and 8.Also,the overall CPUE was higher for the new trawl in both vessels. Thenew trawl was maintained by both vessels after the conclusion of

the project, which is a strong indicator of the skippers acceptanceof our modifications. Unfortunately, there was no possibility fora follow-up assessment of the skippers adherence to the vessel-operating conditions necessary to achieve the expected reductions

in fuel consumption.At the time of the reported experiments, fuel consumption

issues were not so strongly felt as today. This has since changeddue to the continuous increase in fuel prices. There is currently

a growing awareness among the main stakeholders of the needto ensure economically balanced fishing companies, as well as agrowing awareness from the public for greener fishing activities.The latter should be perceived not only in terms of stock sustain-

ability, reduction of by-catches and discards, and seafloor impact

of gears, but also in terms of the broader perspective of energyefficiency and vessel emissions. In other words, the environmentalapproach should concern the entire ecosystem, rather than just the

marine ecosystem. Consequently, the time is ripe for the adoptionof short-term,easily implemented,effectivemethodologies such asthosereportedin this study. Theseshould be further complementedwith the use of high strength synthetic fibres and reduced-drag

otterboards, whilealso continuingresearchon fueltechnology (bio-diesel and bio-lubricants) and higher efficiency engines.

Acknowledgements

The authors are most grateful to Jean-Claude Brabant and thestaff of the flume tank at IFREMER/Centre de Boulogne-sur-Mer fortheir contribution to the modelling and testing of the trawl gears.We also thank Peter Stewart, the referees and the editor, whose

comments greatly improved this manuscript. The work was par-tially financed by the European Union under EU project TE.2.408:Fuel Saving in Portuguese Trawlers.

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