kite tranung

.tD/LN/5e
Septenber
1985
BASIC IC,{OWLEDGEOF TRAI{L AND TRAWIIING
.Bundit
Southeast'Asian
CHOKESANGUAN
Fisheries
Development Centex
FOREWORD
lihts conpllation
lE intended to serve as a textSook for
TOISEAFDECtraineea.
Its al.In ls to gtve the gGieral prlnclpleg,
of
tlanl anal provlale solutions to the problenE aitstng tn trawl fishlng,
Descrlptlons
of equipnent €nd llggi-ng, as w€l1 as ttre tlesLgo and t_he
rEthofl for calcl ating the dinensidrs of a trasl are alBo gLven.
It is hopeal that thl-s materlal wlll also be usefu1 to fiEhlng
tecthologlsts
and to those who have statt€dl to enqacreln tlald, fLBheries
antl need a basic knordledge of Che trawl net and iis opclatlons.
-rrt-
co{ltsNTS
9age
Introaluctioi
I
cla,Esiflcatlon of tirawl . . ... ,.. .
1
Conatluctton of trasl
4
.........
3 , 1 . : n r eg e c t t o n s o f a t r a i r l n € t . . . . . . . . . . . . . . .
3.2
4
M o u n t l n go f a t r a w l n e t . . . . . . . . . . . . . .
6
3 . 3 T r a w l ! 1 9 9 1 ! r g. . . . . . . . , ,
Il
3.4
sneetrFlln€s
.....
3.5
rlilependent pleces andl back6tlops . . . . . . . . . . . . . . . , . . .
3,6 warps
.3,7 otter board pelformance
4,
6
11
.......,..,
12
.......,...
14
T l a n l i n g t e c h n l g u e s. . . , , . . . . . . . . . . . . . ' . . . . . . , . . . . . . . . . . . . . .
t
4.I
TranUng tedDtque of slaletla$lers .......,.....,...
19
4.2
f r a w u n g t e c h n t q u e6 f s t e l r r t r a w l e : . s . . . . , . . . . . . . . . . ,
21
4.3
T r a l r 1 t n gt e c h n l q u eo f p a l r t r a w l e r s . . . . . . . . . , . . . . . . .
14
4.4
! . t L a l - r y a tter ra w l l n g . . . . , . . . . .
25
4.5 Trasl nouth ............
.......
27
...........
4.5.1 Ftshtngheight of a trawl ,...,....,..........
4.5.2
F L E h l n gs p r e a d o f a t r a r r l . . . . . . . . . . . . . . , . . , ,
4 . 6 T o r d . n gE p 6 6 i l. . . . . . . . . . . .
4.t
Acctalents and difficultL€s
-Iv-
..........
durlng trawllng ..........
21
.
29
32
33
Pa9e
Tralrlers
anal their
deck nachinery
5,1
T;awlers
5.2
Trawl winches ..........
37
5.3
Fishing
39
deck arrangetEnt
Design of tra\
s ...........
4I
6.1
Aspects of the design of trawl
6.2
llle selection of the prototlFe treofl ..................
43
6.3
S h a p eo f t - h e t r a w l n e t . . . , . . . . . , . . . .
44
6.4
Determinatlon
of the characteriEtics
of net materlal
€rd the speclfic requirernents ...,............,,.,,.,..
45
Calculation
48
6.5
6,6
7,
fishlng
of dinension
6.5.1
Effective
6.5,t
cear allag fotce
nets
of trardl net by horse pc4rer ..
horse pov€r
4A
(cea! resistance)
ceneral nethod of destgning the optllqlrlt trawl
Flshlnq
eifLciency
Fi6h behaviour
Referaences
of tLawl
in relation
net
to trawl
49
....,..,.
50
51
, .. . .. . ,.
53
56
t.
TNTRODUCTION
It is dtfficult
to establish
exactly where and wh€n ttal'ls
were filst
lntloduced
practice.
into fishinq
T:hey ale lEntloneat.
honever, as early as the l6th century ln offlcial
alocurpnts publish€d
gear that has
in Englanal. To alate, trawls have been known as a flshlng
a high catching efficiency
anal plays ah inportant
role in the fighelLeg
of the worLal.
Professor F.I. Barianov deflned the difference
between tranl end other
gea! of the filtering
ftshing
class, as follor's:
"ff the length of the
path of the gea! e:.eeds the length of the gear itself
ftshlng
by e fec
tinea (no! over 5), then it is conslalered selne q'pe gear, ,hether or
not the gear in action reaches tfie surface of the water.
If the tength
of t-be fishinq
path of the gear is several tens o! even huldlreals of
(up to I0O0) 1onger than t}Ie gear ltself,
tirEs
this is trawl type gear'.
In nor€ simple tehrs, a ttawl is a forrn of fishing
gear consisting
of a net bag that is toweal through a nass of water or along the botton to
trap any flsh ln tts path,
2.
CIASSIFICATION OF TRAW],
Consldering the areas of operation and th€ posj.tion of the fj-shinq
gear j.n rel.atior! to the sea botton when flghing,
gear can
trawl flshlng
tte grouped lnto fou. najor categories:
Surface
-
Sulfac€
-
Mid-water
-
Botton
-
Other towing net (bdach selne, boat seine, etc.).
tfa\rl
(tro-boat
trawls
trarrls
tralrls
or floating
ttll)€) t
(one-boat
and two-boat
(oDe-boat anal two-boat
ttpes),
t)pes) t
trawl
To catch sfiall fish such a6 sardlne and anchovy, snrface trawl
nets are sdretines useal, because these groups of fish often appear
near the surface anal slrin slower than the Larger pelagic fish.
lnd-\ratei
trawl
To catch pefagic
neans the water layer in
and the filst
fe!, netres
sonetimes used. UsuaLly
seas.
Mld-water tlawls
(nialdL€ layels here
fish in t-he rdddle lat€rs
beta,een the fLrst ferd fi€tres below the suraface
above the sea-beil), nitl-!,rater tralrl€ are
mitl-water trawllng
ls catrieat out on the h19h
can be subdlvided lnto one-boat anal tno-boat tvDes.
-2-
bottom
lhe following
trawling:
are the main differences
bebteen ddd-water
ard
1. Mid-water tr:awlinq is aimed trawling.
The search for and
aletection of fish schools, detennination
of thei! swimning alepths,
the vessel to its shooting position,
bringing
dragglng through schools,
anal aluration of draqs, are aI1 operations which are calrieal out according
to reaalj,ngs of hyalroacoustic instruiients.
AlI elements of the trawlerrs
nano€uvres during fishing
are deternineal by air€d trawling.
2. Becairse the nid-water
trawl noves at a consialerable dista.Ilce
fish are abLe to escape by
flon both the sea-bed and the sea suiface.
trawl nets
swinming in all directions.
For this reason, the nid-water
are strtnnetxical (theix top and bottom panels ar.e equal) antl desiqneal
without the over hanging squares characteristic
of bottom trawl netss,
or reverse over hargs.
3. Ihe mouth area of a nial-water trah'f and the towlng speed
hence the
exceeal corsialerably
those erployed in bottom trawlihg,
increased resistance
and incrcased power r:equlrenents of the main
engine for to\,ring nial-water trawls,
Botton
trawf
A variety
of trawl nets erist
anfl teking lnto consialeration
the fishing
melhod anal construction
of gears useal, t-his type of trawl
can be subdiviaed into three groups:
1.
Bottorn bean trawl
2.
Botton
3.
Bottom pair
otter
trawl
trawl
Botton beam tr.awl
lhe beam trawl was the forerunner of afl trawf gear alesign as
we k4ow it today.
Tlle principle
of thls equlpment was slnple anal
consisted of a beam t hose purpose it was to spreail the nettlng whlch
Sornetimes
was held above the sea-bed by t\,/o netal or concrete shoes.
run
over
a hea1,y beam is supported by steel shoes at each enal which
grounal
rope
the sea-bedr the heda rope is connected to the bein, the
being fas-tened loosely bet$reen the base of the shoes.
The coal end is
formed in tie usual manner, the strap having a Lazy line attacheal to
The towing bridle
is foirrneal by two ox three ropes: one from
each shoe, anal another from the center of the beam. These come together
to the to$ing wa4,
arld are shackled directly
-3-
Bottom otter
trawl
This kind of trawl net is basically
a large bag naile of nettlng
i{hich is dral|'n along the sea-bed to scoop up fish on or near the botton.
Depending on the rnanner in which the gear is constxucted anal xigged,
its operating characteristic
can be altereal for use on various tl'pes of
bottom and for nany species of fish.
fhis net is a large bag-shapeal net, oiale at one enal, the nouth
leadinq to the body of the net i,uhich tapers to the closed enal, where
the fish that enter thr:ouqh the inouth are trapped in the "coal end".
The mouth is of an oval shape when viewed fron the front with
two wings stretching
out to increase the area swept and to guide ftsh
in the netrs path do\n-nto the cod eittl.
Around the upper edge of the nouth runs the "head roper' to
which a nunber of floats are fixeal and around the bottom of the nouth
is. the "grounal ropel which is in contact $rith the botton anil is lrej.ghted.
weighted
The cornlcined effect of the floats on the heail rope anal the
qround rope keeps the mouth open vertically.
fi1e grounal rope may be weighted with a chatn, leail, or nerely
wlre when the net is beinq operated on a clear botton, when useal on
a rough bottom, iron, r.Doden or rubber rollers
are riqqed to assist
The horizontal
spread of the mouth is attaineal by the ,'otter
boards" towed ahead of the net a]lal set at an angle of attack to the
towing alirection,
so providing
the outward force necessary to splead
the t ings to lr'htch t}ley are fastened.
TtIe otter boaril may be connected
directly
to the wings or separateal from them by a length of urire ot
rope kno&yl as the "s!reep-1ine":
T'he sweep-1ines are connecteal to the
otter boaral by a backst?op and to the net by a brtdle or "dalt1eno,
arrangement.
Bottom pair
trrwl
Two boats, each tolt'ing one wa4), are useilr the net nouth is
kept open by the out{aral pull provided by the correct lateral
spacing
of the fishing boa€s, so that no ottet boards ard requireal.
By utilizing
the conibined towing pult of the two boats. and
since no otte! boarals are needed, a lar:ger net nay be useal than
l,'oulal be possible by one boatr alternativefy,
two boats of 1o!r horse
power, which could not underta](e single boat otter trawling,
can
conibine to use this method efficientlv.
- 4-
As no otter boards are necessary, the arrangement of the gear
is simplified,
the warps being connecteal dixectly
to the slreep-lines
or bridles
from each v.ino.
3.
CONSTRUqII ON OF TRAWL
Accoraling to the accepted classificatlon,
tra!,rls belong to the
class of filters,
a.nd to the group of dragged fishing
equiprnent.
Ille
principle
of trawling
is the novement of a tranl net fiftering
the water
throuqh the lr€sh netting,
the fish to escape nor
lreither permitting
qilting
then.
Tralrls can be nial-water, botton, or senibottoh.
The
construction
of a botton trawl will be qia.en as an examDfe.
3.1
The sections
of a Trawl net
A bottom trawf net is a horizontafly
asymretric netting
bag.
Ihe upper part is larger than the lorer part, thus creating an
overha]}q of netting,
the square.
The square ts alesigned to prevent
the fish fron escaping utrMards.
- the upper part of the trawl n€t consists of two upper
wings, an isosceles trapeziurn-shaped square, balting,
upper panel of
the lengthener,
anal the upper. panel of the cod end.
lower panels
added.
Figures
- The lower part consists of two lower wings,
of lengthener,
and cod end.
In a four-sean or six-sean net,
ltte details
of the sections of a trawl
1, 2 anal 3.
Lacing and Joininq
belly.
the side baiting
will
net can be seen in
be
Trawl Sections:
l,lhen preparing the upper panel of the trawl net, the bases
of the upper wings are joined to the wider base - the fore edge - of the
square.
The space bebreen the points of attachnent of the wings to
the square is called the upper bosom. The ends of the wings adjacent
to the boson are reinforced
by wedges or grssets, nade of double ttrine
netting,
pointing
towards the square.
Iower wings are joined with their rear ealqes to the fore
ealge of the belly to create the bottotn pait of the triawl net,
The
space between the wings along the fore ealge of the belJ.y is ca1led
the lowel boson.
If the sections of a travrl are maale of different
naterials'
then the square a]lal the wings, the top ajld botton part of the cod end,
as well as the baiting
anal the belly must be naile of one naterial.
-5-
Figure
1.
A
1
3
4
^F
trawl rigged:
- sweep-l-ine, 2 - butterfly
alanLenot
- legs of mainlines and lastridg€
lln€st
- floatr
5 - quarter roper 6 - lazy line
+ho
f^r.
4h1i++ind
c+r^n.
?
-
f^r-
splitting
strop or fore halving beokett
8 - lazy line of the after splittihg
stropt
9 - cowhides r 10 - after splittibg
strop,
11 - chaffing pieces of hula skirt,
12 - body of the traurl net o! belly and
baltingr
13 - be1ly line and lastridg€
l-lne t
14 - foolrope wtth bobbinsi 15 - footrope
legr 16 - netal bc,bbin r 17 - shackles.
Figure
Figure 2.
Otter tranrl:
1 - waq) t
2 - tra\rl boardt
4-wing;5-footropei
6-squar.er7-boaly
(baitinss and berry);
I - be1ly linest
9 - flapperr 10 - cod end.
3. Bea$ trawl:
I - trawl heaals,
2 - beamt
4 - \tarp,
5 - footrope.
-6-
3.2
Mounting of a Trawl Net
After all the parts have been jolned, the trawl net is
nounted, set on its mainlines,
lastridge
line and belly lines,
anat
fitted
with different
strops facilitating
the handlinq of the tra 1.
A vegetable cord, ca11ed a bolsh line (hanging line or
bolsh line)
is attached by double Urine to the fore ealges of both
panels of the trawl net.
The netting
is stapLeal to the bolsh line, one
nesh at a tine, with clove hitches.
the length of the bolsh line alepenils
on the size of the trawl net.
The head line of a trawt is made of conibination rope.
the enals of the head tine ar:e spliced in eye splices,
into which thinbles
are sonetimes fitteal.
A narker is naale at the center of the head line
nhere it indicates
the point at which the center of the upper bosom
shoutd be seizeal.
The botsh line is attached with even slack to the
head line with seizings spaced 20-30 cln a part.
when vrorking a trawl,
care should be taken that the maikers
on the head Iine correspond exactly lrith the center and the ends of the
boson, otsherflise t])e trawl will noE funclion properly.
The fishing line is also made of c.rnbination rope.
The bolsh line is seized every 30 cm to the. fishiDg line, lr'ith eventy
distributed
sf-ack. The tolrer wings are otten flynesh-stapled
to the
bolsh line.
Ihe fishing
line is sometirnes called I'ground rope".
Figures
The detatls
4 and 5.
3.3
of the. nountinq
of a trawl
net can be seen in
Trawt Riqging
(the footrope arrangenent,
Coyrect rigging of the tralrl
the rigging of the heaal line, the cod enal, the various parts anal the
trawl boarals) is very important for the snooth opeiation of the traw1,
- Footrope.
lhe footrope is alesigned to protect
the
lcfier netting panel of the trawl net and the fishing
line against
breaklng anal prenature wear, to facilitate
the trawl passing over
obstacles
on the bottom, anal to prevent stones and sponqes getting
into the trawl net.
a ballast
At the same time, the footrope
preventing
the tiawl fron rising.
do
not
use
footropes:
{Snall trawls
the fishinq lines are ballasted with sections of chain.)
in two forns:
The footrope consists
soft oi rigld.
of several
sections
anal appears
-7
-
l.
A soft footlope consists of severaf sections of steel
gire rq)e, with tle ends of each secti.on eye-splj.ceal (oLal rope is often
taed) anal then a length of vegetable cord.
Soft footropes are still
n€ed hcr'radays o! light grouids.
lhe footrope ls attacheal to the flshlrlg
lilre eitler
by nears of becLets made of steel uire rope strantls or by
E@s of toalalle chains,
2. A rigid
footrope has either t/roodenr noetal o! rubber
bcbbins reeveal on to the steel s{re rope.
To cotr$rensate for the
buoyancy of hoLloo netal bobbins. anil to regulate the pre6suro, of the
footrope on the sea bottom, cast-iron
beckets are used. llhe becket
bc,bbins are attached to the fishinq
line with todalle qhains,
{see Flq.6)
8igure 4.
Figure 5.
Eangiid netting (boson sectia$) heaal llne:
I - head liner ? - bolsh U,ne, 3 - staplingst
4-nelting:5sei2in9.
seizing lastridqe to a line!
I - lastrldqe line or belLy linet
2 - seizing: 3 - nettingl 4 - lastridge.
-8-
a
m
0
fril
ljLl
Figure 6.
Becket
bobbins
While reevinq footropes,
care should be taken that the
becket bobbins are evenly distributeal,
that their quantity
is adequate
and that they are properly attached to the fishing 1ine.
- Rigqinq the heaal line:
The head line of a trawf
rigqed with floats to provide. tie trawt net with fishing height
raise the heail line above the sea bottom,
is
or to
Al1 sphericat
floats have a ccrnnon disadvantage:
tieir
lift
remains ;onstant with the change of the towing speett. Hyalroalynanic
floats
are superior in this respect, as the Iift
increases uith speed.
aesides spherical
floats anal trawl-planes,
other lifting
devices are used in trawling.
ltrese ilevlces are based on application
of the hydroalynamic lift
force.
X'he wooden kite is an exarnple.
Nor"/adays, hydrodynanic trawl ftoats,
providing
both
hyalroalynarlic and static
lift,
are the rnost populat floats.
their
hydroalynanic llft
is provided by the collar $'hich, in action,
assurps
a posi"tion at a certain angle of attack to the direction
of the notion.
- Cod end rigging:
The lor,rer panel of the cod end,
especially
its rear part where tie catch concentrates,
is protected
from abrasion on the fishing
qround by chaffers and cowhides,
sometines
floats are helpful in preventing strong abrasion.- (see Fig. 7)
- Danleno rigging
(I€g ri.qging) : Before sueep-lines were
lntroduced to trawling,
the trawl net wings, and the 1e9 of the nain
lines were attacheil directly
to the rear edges of the trawl boald.
leal to the introduction
Ahe intloaluction
of sweep-lines
of another
-9-
alevice called
lesiatance
of
trawl on the
botton enal of
the "danleno".
Danlenos ale designed to transfer
the
the trawl net to the slreep-l.ines.
Itte efficlency
of a
sea bottom can be judqed by the extent of lrear of the
the d8nlenoItrele
are three
t.
tl4)es of danlenos:
Butterfly
Pipe spreade!
3.
wooden Epreaaler (for snalL trawls)
Ihe b\rtterfly
helps leguLate the notlon of the tralrl,
since
It is a free-seinginq
yoke.
Its only disadvaitage
in conparlson with
the pipe spreader is that it is shorter.
Sometines the end of a butterfly
is connected !.ith t-he bobbin and tl|e other enal with the sweep-line
by
Gans of a shackle and svrivel.
Ihe legs of the head rope are connccted
(see Ftgrs. I and 9)
tdith the upper end of the butterfly,
a
Fig'ure
1
Rigging of a tra{,1 cod enal.
I - hauling leg of the aft€r. splitting
strop;
2 - strengthening
stlop,
rop€: 3 - after splitting
4 - three-link
chainr 5 - belly line;
6 - cod end lock, 7 - sarne as 21 8 - cod Line
-t0-
danleno with
a metal bobbin
Figure 8.
Butterfly
Eigure 9.
Rigging of 25-n trawl i,tithout quatter ropes:
danleno with-bobbin,
L - butterfly
piece naile of coral quartex
2 - intermetliate
roper 3, 6, 7 and 9 - shackles, 4 - ring;
5 - pendant, I - swiveli 10 'a pair of links
11 - stopper ring.
3.4
S'.€ep-Unes
S€ctaotis of steel wire rope are thserted betwe€n the trarrl
boards and sirlgs of the ttaul net 1n oriler to wldeb the flshihg path at
tlle tral'l.
These sections are calleil swe6p-11n€s (briatles, sweeps,
hand roDe).
the sweep-lines are maile of st6e1 wlre ropes.
llhe dl.uEter
of the $reep-Iine
is seJected according to tlte type of trasl€r
and the
Elte of Che tranl net.
Its breaking load sho\rlal be 759 of th€ breaklng
Ioad of tle earp.
ttis
is aLlolred because sone 251 of the pull in the
,a4ts is due Co tJle resistance
of trarjl boards.
itoreover, in the ca6e
of a fastener,
the load sharply increases ahd, if tlle ea4) is not paid
c[t,
1t may part.
If a sueep-!.lne parts insteaal, the tra$l net defofirs
less, and it is easie! to haul the ilalnageil t:.awl and repai! it.
lli€ length of the slreep-lines varies fron 20 to 60 n and
alepends upon the fishinq
conditions.
Sonetin€s, 1OO m sweep-lines
are used,
llhere trawls are \rorled at to\rlng speeals exceedlng 3 knots,
lt is aakisable to increase the Length of the sw6ep-l-ine.
At lowet
towing speeds, tong slreep-lines (exceeding 40 m) sag badly and are dlaggeal
over tlle gr.ound, and thus wear out fast.
For particularly
rough bottoms,
lt is recotmended tha! the s'r€ep-tines be shortened to 20 n,
Eye
spLices, 250-300 rm long, are spliced at both 6nds of the sweep-libes.
Ihe sueep-lines
eust be exactty the sa.lrer the difference
in length r.n
a pair'of
sDeep-lines should not exceeal 0.1 m.
there
are bro kinals of slreep-1in€s
syeep-line
type
Double sueep-line
tyPe
Single
(as shown in figure
l0):
Ille fishirg operatio!! of a tlarrl using the single hand
rope nethoal is silEler and less alangerous, lrhtl€ tJut usihg the atouble
hard rope tt€ttlod is effective,
but nrore dangerous.
3.5
lnal€pendlent pieces
and backstr,rps
Independ€nt pieces connect the sneep-lines nith the warps
olren the tratrl boarals aie Clisconnected frqn the gear during the hauting
(Eigure 11).
oPeration
the length of the inilepenalent pieces differs
for different
types of t].aul boards.
Boths ehds of the wire rope, of
which.the
independent piece is 'nade, are spliceat itlto the eye splices.
Wt'en toeing, the independe[t piecE aloes not carry a loaat
and hangs slack.
It calries
the heaviesl- load d\rl.ibg hauling,
after
the trasl board has been di-sconnected fron the gear,
A steel oire lope
inteltnediate
piece is irserted
bet\.reen the sarp ahd the board to
facllitate
aid male safer the handlinq of the trawL board.
Backstrops, ,irhich are eye-spl"iced sections of steel r'ire
rope, ser\,.e to pass the load from the sweep-1ines to the trawf boaral.
lhe rear eyes are shackleal to the swivel of th6 s!€ep-Iines,
ard the
fore ones to the backstrop rings at the rear outer edge of the trawl
board.
During towing, paying out !ra4)s anal hauling, backstrops
carry the load orring to tie ttatrl Det resistande.
They are released
fron this load during tfte shootinq aJId hauting of tie sweep-lines.
when one warp breaksr the trawl gear is hauled by tneans of the rernaining
wa4), the tral,ol board being hauleal by its backstrop.
In such a case,
ttre backstrops Nrst'sustain
the naxima foaal.
3.6
Aiareter
trawler
wa4)s
Steel wire iope warps are used for toning trawls.
ltle
of the warp is chosen according to the towing power of the
(Table 1.)
and the q.pe of tr.arrl.
To facilitate
control of the warp scope and the even
paying out of the warps, the latter
have markers attached every 50 n.
To prevent defonnation of t}Ie tralrl net tlle distahce between the
markers should be periodicatly
checkeil.
New warps are checkeal at least
two to three times during one fishing
cr\rise, arld used ones once per
At the end of every fishing
single
cruise,
the \{arps are lubricated.
hand
]aying wire
Figure 10.
Kinds of s!reep-l-1ne.
Ptgure
11.
Table 1.
Standald
!2
Tra*l boartl attaattrants!
I - $reep-ltne,
2 - l(ellyrs
eyet 3 - slrivBlr 4 - bacl€trops,
5 - stotr, linkt
5 - shacLlesr
7 - lntlependettt
plecei I - intetrEtllate
pl-6oe of tlawL boardt
9 - trai,{l boaial couiecti[g
hookr 10 - wa4);
1I - Etrop.for hook.
Breklng
(nB)
strenqth.of
Blaking
wlrB rop6.
weight(kgl200m)
Btrength(ton)
114 0
8.17
v
11,1
14.5
220 2
l8
18.4
256
20
22
456
25
494
35.4
38.3
2A
44.4
620
30
51.0
712
32
810
34
914
r,o2s
t,144
38
40
90.7
,lIS No.13 6 x Fi(29) stranal stluctule
Outsiale allainete! of bare lrire 0.058D
1+
7 + 7!1 + 14
-14-
't ?
n+FAr
eharrl
h^rr.l
n'in-ihrac
n./r^frxtce
^a
^++ar
!'.^ay^
;vhrdi-<
An otter board has no power of its ovrn to move and to
spread the connecting wires to which it is attached.
Its abitity
to do
so is aleriveil from the interaction
of eltternal forc€s.
Iio\,r well it
spreads the connecting wire and, hence, the net, depends on its shape
anal size and how well the foxces are harnesseal.
The tnain forces to be
considered are: the tor.ring force, the drag of the net, the hydrodynam.ic
force of the water floir, gravity
and bottom frictiob.
t.
Towing foice
The towing force or pull. of the i:rawler is tr.ansnLitted
to the otter board through the warps.
the
In ihe totring condition,
\tarps leave the trawl gallows or slip hook at arl atlgle r{ith coq)onents
of declination
and divergence,
the latter
owing to the deflectinq
Throuqhout
of
actl.on of the otter boards,
their fength, the direction
the run of the warps is modified by gravity,
the irater forces against
thern inDa:rtinq a certain
curvature,
2,
Drag of the ne!
The drag of the net coq)rises
the hydrodynamic forces
acting on tfle letting,
the net appendaqes'such as hoaal tine, ground
iope, the hauling ropes and the bottom friction
wh€xe there is gIound
contact.
The naqnitude of these forces alepehds ob the anount of
netting
in the net, sizes of twine anal meshesr bunib€r, shape anal size
of all tne appenalagesr owerall shape of the neti and towing speeclhe total alrag of the net is tran$nitteal
or sweepthrough the bridles
lines which connect the net to the otter boarals. with the otter boarals
deflected/
these ropes or wires. adopt an angle of attack to the direction
of tow.
3.
Hyilroalynanic fo.ces
In the towing condition,
Che waler forces act on the
to the in-pull- components of the tension
otter boarals in opposition
The naglitude
in the warps anai brialles,
of these forces depends on
the density of wate!. the towing speed, Ule size end shape of the otter
purposes. the density
boards and their Engle of attack.
For practical
of sea watex is constant, independent of depth, tenperacure or salinity.
ltle size of t}}e otter boards deterndnes the aroa of the waLer that rneets
them, and changes in trovrinq speed valy the volurne of water hrhich neets
the otter boarals per unit of tine.
The relatiobship
of spreaal, drag
anal vertical
cofirponents in the resultant
hydroalybamic force depends on
-15-
the olientation
of the otter boaril, of wtrich th€ ehgle of attack is the
prlnci.pal
angular cdrponent.
In sone cases, th€ angle of heel, and tilt
nust also be considered. (see Fiq. 12.)
4.
cravity
Ttre force of gravity
acts on th€ nass of all the otter
board ccnE)onentg. lhe resuLt of all gravity
forc€s acts downsald
through the centre of gravity
of tie ott€!
boald.
the positlon
of the
center of g0cavity, and hence it.s contrlbution
to tlle alegree of stabilit!'
of a. otter board, delEnds on tle nass ana alistribution
of th€ different
structural
cor[)olrents,
5.
Botton frlction
the strength
of the folce of friction
alep€ndg on the
nature of the botton and the sole of tlre otte!
board, \rhich are in
contact,
anal on the force pressing then togetier.
In the case of
botton trawling
otter boalds, the forc€ ls the resultant
of their weight
and the vertical
component of the uater force wl ch nay be reduc€d by
an qrwatd alirected
c€oponetlt of the rrarp tension.
of tlle
The naqnitutle
force of friction
is notialependent rl)on v€locity,
that is th6 force
lequired to ovelcore frictlon
ls the sase, ho,eve:a fast the otter
boards are movlng.
Interacti.on
of, forces
In Fiqure 13 the forcas acting on a botton
are Shown viewed fron its end elevation.
Itle horlzontal
'An shoes these forc€s
thelr drag and in-pull
Behaviour
of otter
runnlng
otter
board
folces desctlbeal above ale shown in Flgure 14.
in relation
to tJle whol.a trax.l gea! anal "B"
or out-pull
ccnpoDents.
board
It is most iq)ortant
in trawling
that th6 pair of otter boarals
are in the correct position
while they ar€ being pulleal, othe4rise
they
rnay fall
out of position,
thus decreasing the fishing
efficienc!'.
I?re behavioul of an otter board can be Judged fron the degree of abrasion
of the iron shoe,
The ideal behaviou! of the otter boaral is when twothirds of the il.on shoe touches the sea bottom horlzontally
and that
eear occurs slightl.y
more on the inslale than on tlle outside of the 6hoe.
-16-
libasurea
to coirect
- Ir! the event
of the otter
I.
llake
2.
Attaclt
- In Ur6 event
the behavio$
fte
otter
Uake tlte *arp
2.
Make the Io$er
nhtc'
floatlbg
otte!
up
to tjr6 otto!
boald
idcllnlng
ltfiards
boald
longBr
boaral ar€l
fengti
itl
plat€
a welght
f.
+ha
board
wa4) tonger
of the
of an ott€r
in
lenqth
p€ndant IonEFr than
^n6
the puuing
3.
Shlft
4.
Make the lower bri.dte chaitt lohger
one, if a vertical
!6gu1atllrg tl?€
being uBed
5.
lrnfirist
the I'a4) sufficieDtly
boaxd turibles
down
- In ttte event of th€
being insufficient
fulcrlb
developing
t.
PUI1 tfte net fast€r
2.
wide$ warp angle
3.
Make the net
snal1e!
high€r
pder
Ln aLz€
tf
than the upper
otter board is
tJre left
of tha ott€r
ott6r
board
Direction
Angle of attack
I
h|,-\t/
Outwaitl
Ifiraral
Posltive
Figure
12.
Negative
Definition
of otter
boald attitutles.
1.
Tension in the towlng wa4)
2,
Total hyttrodlmanic foro€s
3.
crounal reactioB
4.
weight acting thfougt! centet of gravity
Ftgnrre 13.
(c of G)
llhe rhaln forces actirlg on a4 otte!
boald vie{ed from the leadlng end.
I
Tenaion in tlle toning $arp
wa4) ln-puU
Tolring fols€
lensioll
in the bliaLLe
2 a Briille
in-pu1L
Net drag
liotal hyalr.oa\masi c forces
acting
on the otter boartl
spreading
force
3b Dt'ag
eround sheai
4b
Grormal
tliag
(1b = 2b+3b+rtb
la+2a - 3a+4a)
tr'lgule 14.
Ihe maln horizontat
otter board.
forces
acting
on an
-19-
4.
TRAWI,ING TECHNIgIJES
4.1
Traeling
technique
Irbthod of shooting
of side tlawlers
(setting
the gear)
Itle towing \rarps from the vessel ehd in a short length of
chain that is connected to the towing bracl<ets on tbe ott6! boards, by
(6-Unk)
a flat
link
conbinati-on so that it nay b€ easily €ngag€d anil
disengaged.
importanc€ tn takltg aboardl and setting
the
Of particula!
net on a side traawler is the connection betw€en otter board6 and t'he
To begin setti,ng t.he gear, the cld €nd is placed in tlle
the bullrope
to the head line, anal the quarte! ropes
water,
is lelied
fasteneal once dore at tl|e enil uirg.
lvltre and the toouth of the net taken
aboal.al are put out, tbe quart€rs of the .et reloasedr anal the quarter
ropes al-l.oued to run out.
$hife this i3 dgne the vessel is laintaineal
beam-on to the rlnd with very little
or no iray-olr to ensure 6h€ drifts
away flon the gear, ldninizing
the likelihood
of fouling the plopeller.
The vessel nor tlpves sL@rly ahead, tuaing
to the siale at
which the net iE being workeal so tiat a straj.n ls plac€d on tJr€ glound
cabl.e!.
the glound
lhe brakes on the einch druns are r6lea6eal, allqring
roFs to run out through the kelly's
eye ultil
tjr€y are hetd by the
[email protected]
stopf,e!
in the eye.
*re towj.trg puU Is tlog taken by the otter
boatds,
which are still
hookeal to ttre gallous,
and the ra4)E ars
clipFd
to the otter
boalil toging brackets.
tll€ nlbch alluda now heave
boards off the aqg
ln tlre warps sltqhtly
to take -Ole pul.l of the otter
chains, etrich Day then be unlockedl froD the ott€r board.
At thls 9oiht,
tjte pull of the trawl gear is tlansferred
to tlre wa4)B rhlch
entllcl,y
just above
aie then run out frdr the winch so the boaldE ar6 flying
tlre water surface and all is ready to set th6 otter boarils.
speed i9 now increaged, the vea6€l put into a nbr€ acute
turn so tbat the ne! $rj.ngs out, at a 91wn BlgnaI tjte winch alrtl'n
brakes are released,
out tle earps so
the aEag of the gear pulllng
onc€
that the otter boards enter the uate! and open the netts Douth.
t}le uarps are running out smoothly,
and
the veaael ia atraightened-up
tlle \rarps rrrx out evenly from the drr&s,
l{tren Diarks on the *arps
inaicate
the requireal lenglh is out the einch brakes ale appl,ied and
the earps must be placed i! the towing block.
conditions
the shooting trEthods used under dlffErent
can be €een in Figures 15, 16 and 17.
weather
-20_
,.{;as
II
,&",,"F
\
II
Figure
15.
Brl.nginq the tra\rler
aro\rrlal
I - shooting the tra\dl netr
trawl boaralsr IlI - starting
IV - warps stoppered on the
\,rhile shootibg the qear:
II - slacking a\day the
to pay out srarps t
towittg block.
"laa
III
tEH\l
'u\(,,
'ts l*
Figure f6.
\'--.
lwina
Shooting a trawl
without circular
T
-
ch^^ri
hd
+ha
txavrl net, slacking
away sweep-Lines anal
tra\rl boarals i II - staf,t
of paytuIg ou! warps;
III - stoppering the
lrarps on the towing block.
Figure
17.
Shooting trawl in
gale lrith the stern
to the llinal:
I - shooting the
trawl net;
II - slacking away
the sweep-lines t
III - slacking asray
trawl boards,
nanoeu\rre; IV - paying
out lrarps;
V - stoppering
the
waq)s on the tolrlng
bto*.
Method of hauling
the gea!
When the tine cones to haul \rp the tra$rl gear, both warps
are released fron the towing block and hauled in €ven]y, by tne winch,
until
the otter boards reach tne gallows.
The lrlnch is thetr stoppeal
anal the otter boards hooked to the gallows by rnaans of "doq chains"
fited
to each gallo$s for this purpose,
During this tine the vessel
has been sl.owed o! stoplEd and turned across tlr4! einal with the working
side to qrinalward, so tnat the vessel is bloifi off the g€ar, to prevent
getting the net aboatal ovet the siale.
foulinq
and to simplify
lhe warps are alisconnecteal from tbe ott€r boards arld the
winch again hauls in the warps.
No!r, the i.ndependebt cables pull the
stopper out of the kelLyts eye anal the grounal cabl€s xun freely through
the latter
until
the wings reach the gallows sh€ave,
At this point
hauling is stopped and the winch braked, securing the rlet itl this position.
To prevent the trawl from comitrg unaler the ship's hu1l, it
i9 hauled frorn the weather side, i.e.,
with th€ trawf,er's working side
turned to t-he winil.
The warps can be hauled wheb the ship is in any
position.
Torards the end of the alrag the trawler is brought nith its
stern lrindi.ard.
Ihe details
of haulinq
can be seen in Figures 18 and 19.
4.2
'Trawlino
lbthod
tectmioue
of shooting
uder
alifferent
of stern
trawlers
(settinq
the gear)
weather
condltions
lihert the gear is to be prepareal, th€ boat steans along
the desired cou.rse, into t}te wind, if desirable.
the cod end is thr:olrn
positions
out and the workinqr r:opes retied in thei! coilect
on the
head line and wlngs, the rnouth is ]ifted
out ldth assistance from the
overheaal pulley,
as necessary, the net is strearBal out astenr and the
6weep-l,ines ale run out.
The otter boards are cofflectefl to the na4)s
and unhooked frcm the gallows, all ts reatly for setting.
:fhis nay be accomplished by releasing the winch blakes
sirnultaneously
thus allouing
the otter boards to drop into tie water
and spreaal while "on the run" or altematively
be
tle warps may first
eased out until
the otter boards are just beloe the surface anal seen
to be spreaaling satisfactorily
befole the warps ar6 rel-€ased, then
stopped to a11cn tire inboaral erld of the indepenalent cables to be
disconnected frotn the winch anal clippeal to the wa4).
AII is now ready
to set the otter boards and run out the warDs to their lecruireil ext€nt.
-22-
tlre vessel
in order to reduce the
lhe winch is no\d brakeal
pattern can be seen in
is often slo\r6d bofole tllB ldnch is braked
sudalen shock as the warps take U|€ ttet alrag.
sedrrely
and to\ding cdro6bcetl.
llhe shooting
Figure 20.
wind
^IV
F
t--
s"-=L#
\II
Fi$rre
18.
ttalrling
the trawl without
tia\tler
aro.rnd:
bringing
towing
haullng
walps t
I
releasing
the
blockr
II
- hauling sweep-linesr
Iv - hauling the catctr.
rll
-l\III
IV
(r
,^^..-.;-\!
*j5l{
l-}-
E,
l/ \ I
Y
ll
'
'f::7
rLi
."+->-----;ind
Figure
19.
Eauling in a circufar
nanoeuvle:
r - releasing the warps from the towing blocl< t
rv - blinglng
tlle trawler
rl - hauling warpsi rrr,
itt
arounal while the tra\rl boarilE ar€ approaching
v - hauling sweep-lines r Vr - hauling the catch.
-23-
t.
5.
of crelt
Positioning
hooking t]!e guys.
of crev when the
Positioning
stopp€r quys ale unhookeal.
il-----s*ri
D.1=a
-=--Y'
2,
Positioning
of ctew during
paylng out of the svteep-lines.
Positioning
stoppering
of crew durlng
of the trawf-net
hook rope.
,y'
z\
a
3- Positioning of
tralr] is taken on the stoPpet
guys.
1.
Positloning
of clew alurlng
alischarging of catch f,rom th€
cod-end: a - derrlck whips,
b - cargo winch.
8.
Hauling a heavy catch on board
a BIt{RT-c1ass stern-trawler
:
I - spans; 2 - relnforceal strops:
3 - runnelsi 4 - snatch blocks.
FlE
I
of crew
Positioning
narPs are taken off t-he bfocks.
Figure
20.
the shooting Pattern
ster:n trawl '
of
-24-
l'lethod of hauling
When hauling,
the otte! boards ate hove up to the 9a11ows
anal left hangirg on the warps, being clanp€d if noc€ssary to prevent
slannlinq.
The independent cables are noir unhookeal frorn the wa4)s and
clippeal to eyes on the wlnch which then uinals th€n \rp urtil
the pull
of the trawl is taken, at this point the backstrop is disconnected from
the ground rop€.
cround ropes, wings and bellies
of the trawl are novf
wounal on the Clrrm, leaving the lengti requireal to get the cod end aboaral
for enptying.
with
side
Stern
trawlers:
traslers
offer
two striking
advantaqes when conpareil
(a) As the gear is !{orked over the stern the vessel is
naintained
on a straight
course while hauling and setting,
the pull
being along the direction
of motion.
In nost cases th€ vessel can
therefore
be headed dixectly
into the wind, and will be able to continue
fishing
under nore severe conditions than the side tratrler,
thus pelhitting
of the voyage tin€ to be utilized
for fishing.
a greater proportion
(b) lhe working area at the aft of the vessel is nuch
steadier,
being less affected by pitching,
anal the forward deckhouse
proviales protection
and hence qreater safety and comfort for the crew,
4.3
Trawting
Technique of pair
tral'lers
(see Fig. 21). altiough perhaps in tnore cotunon
Pair trawling
hse fox mid-nater trawling,
is also practiseal with a botton Uaw1.
lvo boats, each towing orle warp, are usealr the net nouth is kept open
by the outward pu11 provided by correct lateral
spaclng of the vessels.
so that no otter boards are required.
By uLjlizing
lhe cqnbineal towinq pull of the two vessels,
anal since no ottel boarals are needed, a larger net may be worked than
woulal be possible by a singte vessel, alternativoly,
two vessels of
Iow horsepc'ser, L'hich could not undertake single boat otter trawling,
can
conibiile to use this method efficaently,
ch.hri
n^
lcots+in^
rL6
rc,'l
vlhen selting,
the vessel i{ith the net aboald sets it out
with the bridfes
hetd at the qauows.
Ihe pulI of ohe brialle is t-llen
tlansferreal
to ttle other boat by mearls of the heaving line anal fiEssenger,
and the second vessel then comects its warp to that bridle.
evenly
Both vessefs then stean ahead togeCher, paying out t}|e warps
to their r:equireal extent,
and fishing
connEhcss.
r
-25
-
(m) (u)
B
aG
fldE\
.-rB
tl
(El
e7^
oB
It
!f,
\**7
,-----*
@?pA
tqs
OEE
"*"-'
V
-..-=-o
- ditto
" (i------
3t!t1e
V
-
"1T"
r\ll
. o------l
2-1,a4)
\
otrt€ratiol!
Settlng
Ftr
\iV
U,
'M
F:I
dt
3-wa4r 6tyls
operatlon
(popul-ar type)
d"{
t\
I
V
coF
figure 2I.
Pair tranl operation of 2-wa!p styl.
3-!rarp style .
anal
qI
-26_
Method of hauling
When the time cornes to haul- back, both vessels haul on their
warps until
the bridles
reach the gallows.
llb6 vessels then converge
until
they are a safe distance apart, anil a heaving l-ine is useal to
transfer
a nessenger line fastened to tll€ enil of the brialle on one vessel
to the other; the first
boat t-hen discoffrects
the bridle
from its wa4)
so that the other may heave in the messenger on the winch throuqh its
second gallc'srs b]ock, \rhen tiat bridle
is hove up to the gallows,
the net may be brought aboaral and the cod end enptied in the usual
nanner for the arrangenent aboard,
4.4
for
flcm
trasl
and
and
Mid-irater trawling
conalition
"Symnetry in the whoLe system is an essertial
efficient
operation of the nlid-water trar,ol.
ceneral syiunetry results
the symnetry of vra4)s and brialles,
equal alinBnsions and weiqht of
boards and their conforrlity
to the specifications
of the designs,
s]rynnetry anal even distribution
of the floats alohg the head line
the ballast along the fishing line.'l
M i d - w a r e r L r a wl i n g i s , a s a rule, ained and carried out
with the use of fishfinding
equipnent.
Ained trawling
consists of
t$/o main operations.
].
Positioninq
the tra\rler
over tha densest part
2.
Bringing the trawl to the fishinq
school is densest.
of the
depth at which tie
After t'he trawfer has reached an area where, according to
prelininary
information,
there are vatuable cornner:6ia1 aggregations
of fish,
and after all pieparations
have been completeal, the sear.ch
for fish starts.
llre search is carried o\rt using both an echo sounder.
and a sonar sirnultaneously.
The siinuftaneous action of boti instrunents
is necessary, for sona! alone is not always able to detect anal record
deep-si.rinfi ing schools.
The fishing
depth is sel.ected according to the vertsical
profile
on the echo sounalex. The warp scope
of the school registered
is decideal upon by the officer
of the lratch trho consufts the ranging
tables and graphsr choosing the one which best fits
the prevailing
conditions.
Taking into consideration
that depth of the school may
chanqe, it is good practice
at the beginnibg to pay out less warp than
necessary for the selected fishing
depth.
-27
During the draq,
tranler
is approxinatefy
equal
t-he school sotrE tifiE after the
tirE depending on the fishing
-
the distance
betneen th€ trawl net and t-he
to tlle riarp scqp6!
The lrawl reaches
tlal'1er has passeal over it, the exact
conditions.
Hanallj.ng rlid-{rater trawls on boald 6iale tlaelers
is slrlilar
t'o the procedure ptactised
on boald this typ€ of tf,awle! when fishing
with botton trawls without quarte! ropes.
4.5
Trawl mouth
4.5.1
fishing
beight
of a trawl
lfte catch of trasl. nattrrally
dep€nds chi€fty
on
correct trawl opelatj.on, !!anply, the shape antl position
of the gear:
in the water.
Ihere are hany proposals for hproving
the shape of
trawls.
The most iq)ortant
one is on trprovlng
the tailoring
of the
netting
of tranls.
Yet fjiom the vielrpoint of fishinq,
the trarl
can be
divided into teo parts:
the front,
aaljace[t to tJre ttawl nouth i and
the rear part (baitings,
be1ly and cod end).
It has been noted that th€ flshing efficioncy
of
a tralrl depenals, in particular,
on the shap€ anil position
of t-h€ front
part or, to be rnore exact, the nouth itseu.
The trawl riouth is formed
by the head line and ground rope and by the spreaalers (or the ott€r
boartls if no sweep-llnes are used) . ttte fishtnq
effici€bcy
of a trawl
afso alepenals irlnrarily
on the position
of these parts.
Naturalli,.the
nettlng of a trawl ibdilectly
affects
its fishing
efficiency.
First of all lrB cai speak here of
the selective
ability
of the rEshi ther6 is also tfie possi-bifity
that
sorE fish caught by the trai'rl nouth escape bact<lraid, etc.
But afl
these consialeratlons
are secondary, and the maln influellde
of th6 netting
on the flshing
efficiency
of a tralrl consisls 1D the fact that by
consti-tuting
a loaal on the trael nouth, t}le netting
affects its shape.
therefore
a1I work air€al at ifiproving tralrls must
concentrat€
chiefl-y on inproving the Shape of ths trawl mouti, its
fishing
spread and fishing height.
ItIe fishing height of a tj.awl is deternined chiefly
by the height of th6 ends of the head line abov6 tl|6 bottom, the height
of the otter boaial or the spreader, anil th€ posltion
of ttre spreader
are also of irportance.
Ihe inclination
of th6 spteader nay be vieireal
in two alirections:
l)
tlle upper end of the epr€ader may be incuned
forwarals (or backrar&) , or
_28_
j.hwarils or outwarils with
ir xnay tilt
2)
tne
Elg'lre 22 sho$s the position
sweep-Iines.
resp€ct
trawl.
of the spreailer
in e ptane lrith
the
ltre fishing
height of a ttawl, is the vertical
distance betr,reen the r44)e! and lo,rer bosons of the trawl net.
Ihe
fishing
height depenals upon nany factors,
of whlch tlre nost irpoltant
f.
Ttte heiqht of danlenos to $fiose upper enals
the heaal li.ne is attached (itr botton tra\dls)
2.
Ihe length
3.
The design of the wings
4,
The action of the lifting
of the nain
line
are!
leqs
devices
(floats,
kite6)
1\,Io systems of parall;1
forces adt on the trawl
head line:
the buoyancy of the floats and the trawl 4et resistance.
ltlese folces detertrline the fishing height,
For the pu4)ose of
siq)Iification,
lre shalL ignore the hydroalynanic lift
of the netting,
though under certain conditions
its effect rnay be significant.
we
shall likewise assunE that the equivaLent force of buoyancy and resistance
acts on the center of the head line at the sl'rfietry
plane of the trairl
net; the resistance of the net is directed horizontallv.
Iihen the _trawl. notion is stable,
the heaal line is
Iocated on a plane conmon to the resultant
of buoyancy anal resi"stance
and to the heail line enils attached to the ilanLenos. The angJ.e of this
plane and the horizon is thus equal to that of tlle lalter
and the
resultant
force.
Fron !'igu!e 23, the buoyancy of the floats e acting
along the arln I creates a nonent which strlves
to turn the h€ad line
clockwise alound an axis passlng through the points at which the head
llne is attached to the danle[os.
The resistance of the tratrl n€t R acting along the
arn Z creates a ndrent in the opposite alirection.
Equilibriun
of the
head line, with stable motion of the trawl,
requir€s the s\fil of the
mdnents appfieal at any point on tlte head line to be €qual to zero.
tr.rA=o
Pz2 - QL = o,
,, - ?L
tne fishing
height
z = Zr + z2 and zr = L,
+ L9^
de SeL
-29-
An analysis
of these equatlons
results
in the follosingt
to Ute ila.tlleno
the fishing height is proPottional
height,
the r,ring length, anal the buoyancy of the
floatsr
it is inversely propoltional
to the
trawl, net resistance;
if all floats ar€ taken off the LrawL net. i.e,,
lrhen I = O, the second telttl of the equation will
becorE zero.
In this case the fLshilg
height is
equal to the heiqht of tie danleno,
when the trawl is not i.n motion, i.e.,
when R = O,
the fishing
heiqht wiII reach the naxinum value
intentled fo! the trarvl net;
lengthening of the trawl wings or their
gives a bette!
flshittg hetghtt
legs
- if the trawl not reslstance
increases as a tesult
of increased towlng speeil, t-lle fishing height
will decrease to a mlnltn\rn, i.e.,
to the danleno
height if regular (not hydrodlmanic)
trawl floats
afe used.
Ino.easing the hej-ght of th€ danl€nos is not advisabte
is difficult
!o balance 1on9 danlsnos.
ltoreover, a traxrl r,rith
alanlenos is very clrlibersome in action.
rcthods
Th€ nost rational
of inproving
the trawl fishlng-heiqht
arer introducing
hyalrodynamic
floatsr
longer leqsi and abolishinq pipE-spreaders,
as it
'Iong
4.5.2
Eishing spread of a trawl
(th€ distance between
The ftshing
spiead of a tralrl
the wing enals and the otter boards) depenas on the dirEnsions of the
tra\rl,
its resistance.
t$e operation of the ott6! boards (their shear
and resistance),
the length of the sir€€ps anal warps.
Figure 24 shows a dj.agran of trawl notion.
The
traeler
is at polnt A, the otter board at polnt B, Iir}€ ttg is the
projectlon
horizontal
of the wa4), anil line BC the s!r6ep. Its
continuation,
sholrn by alashes, intersects
at point D the tine along which
the trarrl is tctured. Itre alc CCI repr€sonts tlr6 nairl lille of the ttavl.
The calculation
of spread of a trawl can be calculated
by utilizing
the value of warp angl"e a'ld th€ lair of slnilar
triangle
(see Fiq. 25).
-30-
Figure
32,
Arraflgenent of tha
sDreailer of a tra\d, .
hctght
naln
Vlew of the tiawl
Ilnes from above.
of a trawl.
Figure
23.
Ftshing
I'iglre
24.
Diaglam of th4 force8 actlng
on the !,srp, the otte! boaral,
the sweeps, anil the maitr IiDe
of tfte tra\dI.
-31
-
I
9r,
I
T.
601
i.l
+lN
o xl+
Itl
g?
6'
l{E
r0
E9
99
-32-
4.5
Bowing speeal
Towing speeil is one of tha trporeart
factorE that lndicate
tJre flshlng effLclency of the trawl, b€oaus€ towlng speed has a alLrect
effect on the spreadling of the net noutlr and ltre resistance of the trawl,
lhe gpleaaing of the net douth can be divided into tno t]rpe8:
one ls the vertical
spr€ailing (helght of llgt douth), the other ls th€
horlzotttal
apleading.
In genelal, an inclea€e of torrtng gpeedl causes a alecteas€
irr tlte helght of ttre net ncirtb.
or Uie cottt!a!i',
Bhen tlle Bp€6tl
ilecteasea, tne height of tlle let rcuth Lnc!€aa€B because of the liftlng
force of the floats.
Bilt rrith so@ devl.o€s such as a kite,
a hydroilynaDlc
tytE fLoat snal other tiftinq
devtces rrhlch ale ettachetl to t]re net, $hetr
the totdng 6peed increases tjre !€actl.otr of ttr€ tlet is tliffe?ent.
The holizontal
spraAing of ttte het tlepentls on the spreadlng
of the otte! board. :Ihe increaBe of towlng Bp€€d causresan lncrsase
ln the horizontal spreading lmtlI the lrlileBt polnt is reacheal and lt
alecreases when t-he resistance of the n€t lE bl99€r than the spreaalLng
force of tlle otter boa-rds.
!€quirea
Accoriling
as follc'ns:
to the kinil
of trarrl,
tcirltlq
slEeil ia,
Bottco bean trawl
l-2.5
Bottd
2-4.5 knot
otter
I'titl-water
surfac€
trawl
t.aFl
trawl
qeneraUy '
kDot
3-a
knot
L-2
knot
the optinurn Ep€ed of the tra$rl lr€t is ileternined ln
accoldance wLth the catch 6pecie6, flshlnE groutlil sondition, net flesigrr
etc.
Illese factors have to be kept lh ldnd wheb \de consialer the tddlng
Epeed.
Deterninatidt
of opt.ielml trawling
Itte catch per haul
chalacterLzetl by the coefficient
fish gchool alensity ci:
Q =
Q =
spe€il
(9) alepanils on Ul€ trasl ileslgrn anil LB
o, tlavled apace_or volr@ V, antl
ciov
ciqFvt
F = trawl opening areai v = tralt1llrg speeal? t - tra\tlLng tln€.
-33-
Schoo1 dellsity Ci should b6 constant dulihg the tlanling
perlod.
The value Ct charactelizlng
the ilegree of trawl perfections
is
often calleal a coefflcient
of the trawls catching capaoity.
According
to Ionas (1967) it can be expresseal as:
-=-l F V
Where B value is
of a particular:
fish species.
4.7
Accidents
aD appro)<inat61y constant
and Difficul-tles
chalacteristic
atrr:.inq Trartinq
Vatious accidents anil nishaps may occur aluring trawling:
alefornation of tle tralr1r foul gear, parting of warps, s\reep-Iines and
strops, etc.,
fastening,
fouling a trawl with that of abother vesset,
fouling
a trawl on the propeller,
Trawling operations are catried out unaler vety diverse anal
cotlplex conditions.
Obviousl.y, however detaiLed it nray be, no textbook
can foresee all potential
difficutties.
Deformations
in
trawl
nets
Strictly
speakinq. the defomiatl.on of a net does not
happ€n by itself,
but is rather the consequence of an accident.
DefortnatioD of netting
and slippage of hounteal s€Ivedg€s on ropes are
rnainly caused by incorrect
rigging of the trawl Dot, and by incouect
action on the part of the fishernEn during shootlng and haulLnq the trat,ls.
Flrst of all,
aleformation rnay occltr when brlnglng the
tranler
arormd.
During haulinq,
after the trawl boatds have been
disengageal, anal the fole ilanleno approaches the v€ss€l,s side, the fore
sweep-fine should be hauleal up so that both wings catry an equal loaal.
With heaw catches especially,
the circular
nanoeuvFe shoutd be carried
out at half or slq, aheaal, since, when tll6 trawl ls unaler gteat sErarn,
slippag€ of t-he netting
is liable
to occur.
when a new tlawl
first
slacked away till
it is
is equal on both wings.
i-s shot, tha for€ ttalrl board should be
abeam the tra\rl ninch, so that the s!!ar-n
Distortion
of the tra$l catt also occlrr if the wat ps are not
paid away evenly.
Xhis ts liable
to occu! particularl-y
in a new trawl
net.
llhen the warps are paial away, the tiawLerrs
speeil should be adjusted
according to the weathe! conditj-ons, and to tJre tlirection
of the winal.
Another reason for defornation
in a new trawl net nay be changes in the
course duxing to!,ring.
The first
3-4 drags with a new trawl net should,
therefore,
be carrieal out on a stxaight
course Lo give the knots of the
netting
and of tie hanqinq tine, to tighten.
tihen heaw catches are taken, they shouf,d be properly
distributed,
so that there is no excess strain on the lower sections
of the trawl net.
For this pu4)ose, th6 hooked fal.ls of the span-blocks
should be paial out as fat as the water surface.
To prevent deformation of trawl nets, the vrarps shoulal always
be naintained
at the correct length.
brhe'l new warps are used for a long
period,
the war? at the erorking siale stretches rnore than the other one.
Therefore,
new warps shoulal be worked fron both sides of the Lrawler.
When the wa4)s are evened out on the towing block without s1olring down
the txawlerts
speeal, the warp which is heaveal sustains increaseal tension
and stretches.
Markexs on the w'arps shouLal be checkeal after a fastener
has been encoutereal or after the trawl has becon€ foul of t}}e tra{rl of
another vesset.
Sometimes the markers or new warps are incorrect,
therefore
new. warps shoutal be remeasureal on the qray to the fishing
grounds.
Foul gea.r
Foul grear usually occurs owing to igborance ib operating
the traw1.
Vlhen the reasons for foul gear are known, there is always
a solution.
Cases of foul gear can be divided into thr.ee groups: full
turn, when the trawl boards becone foul- of eactl other: anal the warps
crossi foul wings (separately) r anil the coil 6ntl bedcnung foul of a wing
or another section of the trawl net.
One of t}le warps becomes foul of th€ opposite tiawl boaral.
hten the trawler has t^'ay on a circular
couxse during a trawl shooting
opelation
anal the trawl boards are released, tbe after boaral may nove
away fron the trawlerrs
siale anal the fo!6 one cotne up close to the
txawlerrs siale.
Itle occurrence. of foul gear can iftiealiately
be iletecteal
the
from
appearance of the warp - they cross on the toring bl-ock, they
vibrate
anal the angle betlreen them is v6ry acute,
-35-
wa4)s which are twisted several tiir6s.
This only occurs
duriag shooting because tdhen the trav/l is on t-he ground it can[ot
levolve several tines.
To avoid fo\rl gear, the warps mrst be paiat orlt
One of the sweep-llnes is foul of the opposite tralrl boald.
A sweep-line nay beccrne fouf of the opposite trawl boaxal aluring shooting,
when there is an abrupt chanqe in the towing co\rrse, on fastenersr
when
two tiawls becone foul of each other, and when hauJ.irlg. Sweep-tines
can be twisted togetier
only as a result of disehtangl,ing
fout txawJ.
boards.
Foul \rings.
Ihis is a very cofimon occurrence 1n trawling.
It occurs frequently
in calm weather when th6 tra\r1 net hangs vertically,
or when, owing to the sea current the trawl gets unaler the hull.
To
prevent foul wjngs the trawl musr be shot correctly.
During hauling,
the wings Inay foul in two ways: (a) when
the trawler
starts a circular
manoeuvre while th€ warps are pointing
towaral its bowr (b) when a circular
nanoeuvre has been started while
the gear is still
on the qround.
Foul cod end.
Iri tfie course of tlarrrl shooting, the cod enat
rnay become foul of a winq.
Ihis can be easity aletecteil from the
appearance of the head line;
it sags while its legs stretch.
When
tcturing the cod end over-board,
the direction
of its tlrift
shoutd be
aleternlineal. To prevent the cod end frqtl fouling a lring in stlonq
currents,
the txawler should not be brought around sharpl-y.
In caln
weather, the sweep-lines should be paicl out slow]y, idhile the trawter
perforns
its clrcular
nanoeuvre,
Easteners
!'here are two t]'pes of fasteners:
- the trar.,l boards diq into
!}le qrountt
- the tra$rl net or the traqrl boards done fast
(!ocks, wr:ecks, etc.)
wderwater
obstacles
on
TrawL boards can dig into the ground-before the warps are
bfockeal anal aluring a drag.
If Che trawl boarals are stuck in the
sea-beal, the rudder should be put over tordarals the gear, the brakes of
the warping dnlms xeleased, and the vrarps paid out until
the ttavrler
cor.es round to a course opposite to its pxevious (towinq) course.
If the trawl boalds oi tlra tranl n€t cdte fast on an obEtacle
otr tbe Eea-betl, th€re are sevel.al n€ttrodg of, g€tttng c16a!.
Itre above
desclLbed riethod nay be useal. Anoth€! rEthod to E6t cJ.ear is for the
t!a\rle!
to nake sterryray towalda th€ tralrl.
In tltLs cas€ tll€ warps a!€
not slaciked away, but thelr slack is h€av€d Ln at once. Tlris nEthod I5
1es6 effectlve
and ie nainly eryloi€d ln g!€at il6ptha.
lhe tiird
retlroat of gettLng clear of a fast€mr
on th€ 6pot. llfie procealule 1s
to knock out tlle sarps, stop the engln€. rna atlrt
h6avlng the \rar!ra.
Foulljrg
the qear ld that of anot}er
trawler
ltte best way to prevdnt foullng of g€ar in Urat of another
tranle!
!s to strictly
abserve €h6 rul6s of fLBhlng bavigatiod anal the
regulations
for fishing iJr a gloup.
!{hen tnr'ing on intersectlng
coursesr the tlawler whtch
passes b€fore the bow of the other trawlei ahould not retltrce its speeal,
whlle the tra\d€r which passes b€hind th€ 6t€rt1 of the ottlei tra\rler
ahould not lnclease its speed, Nev€lttralgBs if th6 trawls alo becdne
foul of each otier, all measuleE shoulil b6 taken bo avoid damaqe.
Foulino the propeller
Ihe trasl net o! tlle walt a can b€cd€
ln the folldlng
says:
1. wlrcn tlre trarrl
steans o\rer the trawl;
foul
of the propeller
is shot from ths l-essitle antl the traeler
2. lltretl the tlalrl is shot ln cahr $eather n:ithout bringlng
the tlarrI€r around, tlte warps hang t€rtLcaUy
arrd the trarrler aalvan@g
at slon aheatlt
3. If. j.n orale! to accelerate tll€ paylng out of the
srre€p-lines in caln \deathe!, slow ahead ls glven when the after danleno
Is 8t111 at" the 9a11ows, rdater curlents nay puBb the ttanl net on to
the propeller.
- 37 -
TRA}TI,ERS AND THEIR
FISHING
DECK MAC:TINERY
5 .1. Tra\rlers
the principal
dinensions of a ftshing v€ssel are th€
Lengtft, breadth and depth of the vessel, w'rlch are usually expressed
Trawl boats ha\,.e long L, slightly
narro!,, B, anal deep D.
as I.B.D.
Longer L signifies
an advaitage for naingainitrg high speed and so aloes
Deep D
the narro\d B, thouqh this results
in un€aslri€ss in stability.
gives enough pqvrer to drag the net and good stabllity.
The center of
gravity
positioned
as low as possibl,e.
c is, therefore,
Ihe propeller
should be large in size to give enougl
thrust for dragginq the net.
consj.alering tlre above, a low-speeal engine
pitch
propeller
with a controllable
is recqff€bded.
For a gooal trawler,
the fornular
be:
woulal
GT
l'XBXD
are classified
= 0.24 - 0.30
Accoxdinq to the nethod of handling the gear, the trawlers
in two gxoups 1 side trawlers anal stern trawlers.
the
side trawlers have a characteiistic
silhouette.
by
freeboaral height anidships aloes not exesd I m. This is stiputated
on boaral
the specific 'conditions
of the hauling and shooting opelations
siale trawlers.
Ihe working 6pace is locateal amidships anal along the
passaq€s at both sides of the superstruct[r€.
slightly
both aft
arnidships
stern tla{r1exs have a farge duperstlucture
shifted
Ihe fishing
aleck is aft, and the holals are
forewaral.
The freeboard height reaches 4-5 n,
and in the bot, part.
5.2
Traw1 Winches
If
The trawl winch is the rnain pi€ce of aleck machlnery.
is designed to shoot anal haul the trawl, and to perforn various cargo
operations.
l'he selection
of the winch and of its drive alePenalson
tne fishing
deck arrangenent anal gear hanalling syston of the vessel,
on the type of naln engine, anal on the dinensions of ttle vessel anal 1ts
gear.
fishing
Most tralrl winches have two wa4)lng al!\rlns, set freely
on the nain shaft of the winches.
Each alrun has an lndepenalent clutch
and a hand brake,
so that the \rarps can be ploperf,y coileal on ttle
drutns, each drum is equippd with either. a hanil-operateal or nechanical
warp coiler.
one or two pairs of barreLs (qLpsy heads, whipping druns)
-38-
ar€ a part of evely tlawl winch and are
opetations
invoh.ed lrith trawl hanatlino.
the cod end, and any cargo operations.
w1lrcnes, one pai! of barrels is replaceal
designed for a\rxiLiary
ilischalging
the catch fron
In sore cas€s, on very strong
with a pai! of srnall warping
ltle chalacter of the winch drive d€pehds
on
-"ffr-"i1""r,
the type of
the naln engine. usuallv, st€m trarrlers have *ir.t
..
u.i".,
while dieset powered
traiters
have electric
$rinches or hydra;i;;'
einches.
rn sralter
nessets, the winch is alriv€rr ly ,,..i"-"i-l-"rr.in
-;;_;*".
transmission
or by neans of shafting frotn th6 nain;.nil,
winches can also be hydraulically
drlven.
vrinch is
the warping
deternined with
=
L
I
is
the length
drun capacity {in netres of wa4)) of a trawl
t-he help of the follo!,ring fornuLa:
V q( D ! - - D 2
of drun
(n)
Dl is
the dianeter
of t}le tkun flanqe
D2 is
t-he alianeter
of t}|e drun (m)
d
the irarp dtametex
ls
(n)
(m)
and V is the warping drun capacity coefficient
(for druns
with hanal-operated coifer,
=
0.66,
for
druns with
{r
nechanical coifer, U = 0.85)
A distinction
is nade between the fult and !,orking capacity
of the walping tlrum; the fult capacity is the anount of warp of a
given diameter which can be colted on a drum. The working capacity
less tian half the forner-
is
Maintenance of trawl winchesr A tlawl wi.nch shoulal alwavs
be ready for acrion a.hd, thelefore, fts techrical
condiai;; iJ iiqu..rrv
checkeal, speciar artention is Daial to the relrabfiit;
oi-thl-.iint.nrnq
details. (bolts, nails, pins, et-c.r; were of atetailg
rhich ale subiF.r
to friction,
lublicant revel irr trre reiluction
e...,
,
Iids and plugs for filting
and draining the lubrlcant "..ii"q-"r-Jjii"
fr"*-tt.
of the reductioh gear and i,arping itrums, surfaces
"""rtr""
of the brale bancts;
clean condltion of the rrawl wincl ana the condirion.r
er"l"iii
ln.the trawl l'lnches with separate drive, the
"",.,
conilition
ti1"
frlcLion clutch"f
""r.*,
-39-
5,3
Fishing
Deck Arranqenent
Side-trawl.ers
(see Fiq.
tr""t
-.
"
gallows which "."t"."
are designeal to facilitate
the trawL gear. tFig. 27)
26)
s equLpnent also incfuiles tlawt
th6 sltootlng airal hauli.ng of
xtre trawl galfows are haile of profiled
netal and installed
on aleck n€ar the bulwark. in the shiprs bc'ws and stern, a pai! of
gallows alepenals orl the sire of the vessel.
Olt the other hand, the length
of the head llne of the tra\dl net useal by the tranlers
of a given class
depenals on the alistance between tlte gallows.
The gallows are fitted
wit}l dog chains for hanging up the trawl boalds while shooting
and
hauling the trawl.
then
ttle gallows are installeil
so as to leave a space between
ln and ale lashed when
and the bolwalk where the txawl boards fit
The towing block is a alevic€ for hooLing up both warps
at one point during the alrag. Itle simplest to$'ing bLock consists of a
chain anal a pelican hook.
Besiiles the latter,
tlr€re are nEchanlcaL
towing blod<s, and automatic tq{ing blocks.
Stern-trawlers
oDly traril
fhe fishing-deck
equipnent of stern traltL incLudes not
(see Fig. 28)
!,rinches but also hanging and guidi.ng blocks.
Eanging blocks for warps are suspenaleal fron extension
arns at the siales of the stern.
Special openiltgs ale cut in the ch€eks
of the blocks for the wa4)s anal the slveep-lin€s.
at the lralls
lheaves from warps are fiolltlteal
cuiding
of the stern ranltr).
on bases installed
Ihe stern ranp is designed fo! ahootlng the trawl and
to the
for hauling it on the deck. _It is a curveal surfao€, inclined
40".
waLfs
both
sides
horlzontal
at an angle of
The stern ranF has
at
and a gate at the entrance to the fishing
deck.
Its length slightly
exceeals the width of the raq).
st-opper guys are attacheal to a rlng at the rarp wall6
quidingr
near the
sheaves.
They a.re fitted
vdth hooks anil axe used
to stopper t}!e traul
whilst
the warps are passeal fron the blocks on
to the rare.
-40-
Flgu!€
25.
side
tlar1e!
fiBhing
deck alrangeren!:
I - trarl
,i!ch:
2 - center botlaldsr
3 _ siit6 boltalilsl
4 - fore qaUow6! 5 - hanging btockt 6 - tower Eheave:
? - afte! ga11os6!. I - hanginq btoclc: 9 _ lorer 6heav€!
10 - qr,alter-lorE
botlald
with sheave: tt_ guidj.ns b1ock6
on 5up6!st4cture:
12 _ e€Esehqe!_lope failleailt
t3 - g@raIe-rcunted
sheaver t4 - toslnq btock.
,A\
lh
t/*ll lf
41e)
cauds
llqute
26.
sten
1 al ? 9 rz,
16
16
*t!h
tlsler
hanslng block (fot .1i1. tt!,le!s)
f1sh1.9
.
deck aEdq€m.nt:
ulnch. 2 _ ha.glDg blockE! I - gultll.q blockst
t!s1
sterD larPt 5 - stopEPr guva: 6 - ilanteno 6toPlE!6,
dos chaln6! I - i€s6€n9e! !oI,€ fo! Ehootlng t!aw1 n€t:
penatantEt 10 _ haullns glryg ! ll _ speclat Pnddt'
14 - 9uv rlngt
bt@kst 13 - stelr gahtlv!
i: "nat.lt
- sahson
!.? - 1'arp_@a6ulihg bLock t
Pst!
- st€m-latq)
tlouqh.
-41 -
stotEGrs fo! the nain bobblna ale guarter lope chains
fltted
wLth pelican hool€.
they are shactdeil lnto speclal openingB
in the base of th€ sLnclt.
Short and loDg dog chaLna are uaeal fo! hanglng traw!
boarilg,
lltte sholt tlog chalns are uEed In calD weather.
Each chain
ls attach€il to an eye bolt ln the post of th6 stefi brldge.
fbr hanglng
a tiawl board, the hoot< of the dog chaln l. hooked lnto th6 link of a
to Ure ottEga shap€d 6hact<1e JolDlhg t$e blacL€ts of
short chaL! fttted
the tlawl boalal.
fire shootLng rotE is plepalsil f!@ a at€61 r'ite rope antl
ls approxLnately 60 d lolrg.
One endl of ttts shootttrq rope ls passeal
to tjre irlnch balrel,
and a releasing hooL lB shaatkled to its othe! end.
Ille rel.€asLng hook is Ehackl6d by
irlth a loop of fla!*tedrp rop€ connectoil to lt.
passetl through a snatch-blocL ana instalba
on
a snatch-block
tn6tall6il
at tJre po* llila cargo
lnstaLled on a stanchion at the sq)eratrlcture,
port stale balrel of tlte trawl winch.
6.
nean6 of a short chain
ttlts Ehootlng rope is
tlte stem bttdge, through
post, through a block
anil ftlraUy to the
DESICN OF TRAI{IJS
6.I
Aspecta of tlre desiqn of tranl. n6ts '
s'ore aspocts
of trarl
ale
srrrmarlzeal
net
il€Elgn
fo!
th6 sel€ctlon
in
-tne foUoiring:
l.
Ess€ntlal
1.1
factols
Klnds of fishee
Surface
[tal-layer
(or orustac6rnE)
slrimtling
setdrtng
Senl.-lElaqic
Den€rsal fish
fl8h
fLsh
fish
of t.|e type of trawl
to be caught
Sulface
llitl--rat6r
trawl
tlawl
Elgh openlng botton
trat'l
Bottom tlawf,
Botton shrlq) trawl
lnclualing il nble ri99e!'
bsa trawr., snall otter
traitL
1.2
conilLtiotrs
grourril
of ftghtng
Bottolr nature
Flat,
Depth of the sea
10-20 Gtr€B
snall
20-,10 Gtr€E
rEdiun tlalrl
Sandy ltrd
org tirn
1.3
trawl
40 metr€s Big trae]
Siz€ of net
lltre size of net dlepends on the flshiDq ground.
ilepth of vate!, ktnd of ftsh aDal slze of school,
klnda of trarrl, sLts of boat and type of trawl boat
1.4
2.
Design of tr.aw1 h6t
Baalc constructLon
2-s6!I|, 4-6eam or 6-8 aeah
Resi5tanc€ of net
ltat€rl.al of net. tI ine
size, lpsh size and size
of n€t
D€819n of trawl net
Cl1ttlng pattern for each
pan€1. hahg-tn anil nunber
of Bsh
Basentlal factols
fo! construction
2.1
Assedcling of shap€d webbLng
2.2
Jolning net slth
2.3
crounal rope constructiob
n€t, Joini[g
of trawl net
net witl
rope etc.
lfl.le is put thlough a tyre, 6t6el bohbin, lubber
rhe ttztr)e
rolle!, wooden s1hk6r, or lrolr sinker.
of naterial is selecteal depending upon tlle condlt'rtotr of the sea bottom.
t.4
Shortening
cenerally tha
Ehorteningr altlrouEh
or less ir alifferent
to tiis shortenlngr
trairl tt€t has 15 p€r cent
this valuo changes nole
palts of the net. oning
tjre natural shape of the tret
-43-
is thought to have a longitudinal
pe! cent of the stretched length
lahd+fi
^f
<n
h-r
Hang-in valves
length of 85
and a latitudinal
-anl
comnonly used in the parts
Wlng net,
fore part
Rea! part
of net boaly
The extleme rea!
of net body
part
of net.
20i
15r
of net body
Cod end
IOt
ni]
Lolr speed net
jn
It is best to have rather large sholtening
the net, then there will be less influence
in
the shape of the net in trawling, and a good
effect enhancing net height and proper 6!,elling
in the net.
High speeal net
It is best to have rather snal1 shortening in
the net.
If lalge ahorteiing
ls given, net
al"acking erinkles
are prduced,
having an
advelse effect oq the net shape. Ordinarily,
with 10-15 per,cent in heaal rope and ground
rope, 50 Per cent in the boson. 85-90 IE! cent
2.5
^f
l_hc f^l-:l
+^
iho
Implovenent
r i-i
lahdl_h
n^
^f
the
net
irill
be
attached
lihoc
of net shape
Through model net expeliments
in a tark the
shapes of the net can be examined case by case and,
by varying the nu[iber of meghes in each panel fron
the original
design, the best shape can be obtained.
6.2
The Selection
of the PrototvDe
Trawl
The selection
of the prototlt)e
is the ncst irnportant
stage $hich detemin€s
to a lalge extent the success of failure
of the
new desigh.
The sefection
of the pxototl'pe can tiot be based on theoretical
calcufationsr
the suitability
of the gear for catching the different
fish
species i6 only obtaineal flon flshj.ng results.
On the other hand it must
be borne in mind that th€ efenents of the design of the existing
trawls
uere derived from practical
experience with fish b€havlour and on the
gearB. Selection
basl€ of long-tern
natural selection
of the more efficient
of the prototype trawl shoulal take the fo1lowing considerations
inLo account:
1) If the target fish for the designed Lralil is a
ftsh trfiich was already the object of trawling,
we select the best of
the tralrfs useal just for this fish speciest
2) fre same fish nay behave differently
in different
regions.
Therefore,
the t!aw1 prototlpe
should be selecteil from the
trawls used in the region, irfiere *re new design is to be enployed;
3)
If the trarrl has to be useal for a known fish but in
a new region, then after selecting
the plototype,
we nust perform
exhaustive tests of the fishing
efficiency
of the trawl in the new legion,
4)
If fo! the same region and the sane target fish
sevelal trawf alesigts are used and it is difficult
to determine which
is best, $e use as prototype the trawl which has operated on the vessel
rrihose por,rcr is closest to that of the ship on lrhich the new travrl is
to be useal!
5) When a trawl for a nevr target fish and a nei{ fishinq
region is to be alesigned, lqe can be guided only by data frotn trawls
used on any of t}le ships with a power siInilar to that of the ship to be
useal,
6) Having selected t}le prototl4)e lre should compare the
characterlstic
features of the regions nhere the prototype is used and
found must
tftele the deslgned gear j"s to be opelateal.
The differences
be taken into account in the designing process.
6.3
shape of the trawl
net
Exanination of the special features of the tailoring
of a
trawl shows that we cannot judge the shap€ of the trawl net aluring
parts fron vrhich it lras
trawling
fron the shape of the individuat
parts relate
in
the
shape
of
the
net
dLssenibled. Changes
to the technology
in
of assedbLlng the trawl, w}lile
operation the trawl behaves as an
entity.
Neveltheless,
inventors of new trawl frequently
related the
supposeal advantages to the special shape inparted to the net and its
parts.
Ihe chatacteristic
"bag" like shape of the rpving trairl is
detenlined by the sistem of the folces actj.ng on it: water lesistance,
weight, folces induced by parts of the rigging,
etc.
Mid-water trawls
are usually st'nanetric in shape while bottom trawls have a mole complex,
ast4nnEtric shape with a smaller fishlng
helght.
Observations unden,atet
shon that the cross sectlons of botton trawls are elliptical
while those
of nlidirater
tranls a.re n€ar1y circular.
-45-
. llode1 t6ats wlth trawls ln vrlrtl tunbals lndlcate that
,the openlng of the Fsh in 1t6 entir€ty
oglreaporalF to the cdwentional
hangtng stth coeffLcLonts of 0.5 tn tia transn€lsll
dril 0.s7 tn the
lotlgrltudlnal
alilectLon of notLdr.
1t6!€f,or€, Lt La rcre cqrvenient
hangl.bg
to lepresont tbe tlawl on ikawlnqs with the tentatlve
coefflclents
O.5/O,A7, Actually, tlle h.trgLng co€lflcients
dlffcr 1n
vad.ous palts
of the n6tr the lEsh la stlr.tqht€r
tlle
dlalEtral
lti
pllne of t.lre trasl tltan nea! the eatqesr tiLs Dust tr€ taken Lnto accorrnt
In tbe calculations.
6,4
!d.th trro
glU fiEh
aa aDall
therefore
leta-ining
Determlnation of th6 charactorlstLcs
and tJle Elreciftc requireGnts
of aet llat€rial
llhen s6lecting the oeah sit€ of a t!a$I tret raa aet out
tlre tranl n€t lust neltlts! 1et through no!
nain requlrerBntsr
of th€ tlaw1 n€t lnrst be
of c@GlIaX slzet the resist$ce
po66ib1e,
ftrete leguirerFnta
anil
a6
ar€ contlldictory,
Ure solutior! dust be a coaErcnLa€.
considellng
the t!aw1 net flon thc pol.nt of vl€w of
flsh it can be tent tLvely dtvldad tnto thr€e seotions:
(i)
(tl)
(Lli)
repeUing s€ctIon,
Ure zone of actlon
tll€ trawl moutht
shlch plevcnts fi6h caught in
of tho trawl flot| escaplng:
dttectinq sectlon, rhlch ilir€cts the ftsh Lt|to
cotl entl of tlle trarl:
tl|g prnal,6t
retaLntng sectlon. whlch k€€ps ttl€ fish
tll€ net! tlle ccd entl.
caught in
Ite Eeah aize nu6t be snal.l€st tn the ooil enalt in tbe othe!
parta it ltay be [uch larger, slnce in th€ r€pe11tn9 anil ill.r€qting
€ectionE the flsh nove alolg th€ net nal,Ls 6nd do not atteryt to paBa
through IJreD. An Lncr€ase ln tlre Esh aLze brlnga &out a ihcreas€ in
tlre total tralrl re€Latance, a r€duction Ln s,eight ol the rlggdng anal
an Lnclease ln tbe tlawllng
apeetl.
t$e follqd.ng
?o prevent
tnequaitty
the passage of flBh tjrrough the coil enal mesh,
must b€ satisflodl,
4eo < Po
llhere ao ls the lEsh Eize in the cod entlt Po 1.3 the p€rlhete!
latgest cro65 sectlon of tb€ fLBh boaly.
of tlre
-
Specific
requireiEnts
for
trawl
46-
netting
varns
!'or trawls tlle netting yatnE shouldl colriblne hlgh wet-knot
breaking strength at snallest
possibl€ twine iliarn€ter, hlgh abraslon
resietance,
relatl\rely
hlgh extensibtltty
uraler flshlng
conatitl.ons,
good elastlclty
for withstanding
shock loaals, 6nil no knot slippage
or inversion
of knots.
1.
Breaking
strength
A conparison of the net knot breaking strength of
yarns r0ad€ of different
tt.fll
synthetlc
ftbres wtth the sane runnage
shows a d€creaelng sequence frotn pA anal pp to pE and pVA. This gr.ves
sone hint \rhy PA-netting yarns are preferable
for big trawls operateat
at htgh towlng sp€eds such as in certaln miil-water trawling
by powerful
stern tralrlels
and why PE, pvA-netting
is sufficierrt
for snal1 otter
trawls,
or bean trawls operated by srnall vessels.
2.
Extension
a.rld elasticity
In this respect PA-nettlnq yar:ns are considered to
have the best properties
because t-hey are more extensible
thao pp-yarns
anal the penranent elongation
bf pA is lower (Klust, 1964).
Netting
yarns .nade of PE ate better than nanila.
pp ahd pES netting yar:ns
have a very low extension.
This seerns to be a disaalvantage for trawts
of big vesselg.
3.
Abrasion
resistance
lbrasion
resistance,
whtch is palticularly
inportant
for botton trawls,
ls influenceil
by the type of haterlal,
kind of fibre,
coistructlon
of netting yan and by its dliarnetet (Klust, 1958).
Itle tests of
shovtn a sequence of incleasing
over najlila,
PES. PE anal PA.
met-hoals, fcnmd a l"ord abrasion
yarns are not well suiteal for
4.
netting yarns \rith the same runnage have
abrasion resistance
from pVA anal cotron
Other autJrors who, r{ith other: testinq
resistance
concluded that pp-netting
trawls.
Towing resistance
Towing resistance or filtering
ability
of a traldl
alepends not only on towing speed and cobstruction
of ttle gear, including
shape of heshes and angle of attack of th€ n6ttinq,
but also on th6
diar€ter
of netting yaln anal si?e of knots.
-47-
lhe fiLtering
ability
of a net wl1l, increas€ as the
dianeter of the yarn, anal along with it the siz€ of th€ knots, decrease.
Stronger naterials
naturally
allow nettlng yarns of snaller dianeter
- one of the first
with loner to*lng resistance
ailvantages noted for
PA-fibles.
Netting yarns of PA and PP of the sane R-tex values
have alnost tlle sane wet-klot
breaking 6€!ength.
sinc6 specific
gravity
of PP is much l"o(re! and specific
glavlty
ahd voluhe are
reciprocally
proportional
at the sane R-tex, pp nettibg yar$s are about
15 to 20 per cent thickei
in dianeter than pA-yarns.
Constancy of nesh size and knot slippage
In genera1, nost natural fible6 shrink artl nost
dynthetic
fibres extend in water.
Ilte ilegreE of chahges in J.ength
depentls also on the construction
ya!n.
of the nettitlg
Knot slippage gelrerally increases with a highe! degree
of twiat in the netting yarn.
Most netting yafns nad€ of synthetic
fibres require knot stabilization
by net-stretqhfurg
followeal by heat
setting
andlor dippj-ng in a knot-bonding ag€nt,
By thls preparation,
knot slipping
and change in mish size can be none or l€ss avoialeal.
6.
Specific
gravity
and buoyancy
Ihis ploperty
influences
the slnking speeal of netting
materlal,
but it has neve! been ftentioneal that sinking speeal is of
Itportance
for tralrls,
not even where nld-lrat€r
trawls a!6 concerned,
lio\rever, with slmthetic
gravity
netting yartl with spedlflc
of less than
one, a better sbape of trawl was expecteal.
During one early
it \ras shown that, in conparison
of a nylon travll, oh'ing to l@rer
opening.
this was al-so the case
gravity
me low specific
ard the
should allow a reduced nurnler of
expdrim€nt wlth trawl-s made of PA
$ith nanila trawls,
the upper panel
gravity,
gave a higher
specific
for tf,awls rnade of PE-netting yarns.
fact that it dld hot absorb any water
floats leaalinq to realuce drao.
when to\ring is stoppeil for hauling or atly other r€ason.
it is desirable
that the aft part of the trawl should collapse to
prevent fj-sh escaping.
Trawls maale of PA-netting ya.rn !ril1 alo so and
(non-return
would thelefore
have less need for a flaplEr
valve).
-
7.
Resistance
4A-
agains!
blological
chenicals,
attack
etc.
in different
Different
slnthetic
fibres ate sohble
chemicals and this is used for identification.
It has, hcuever, never
been reported that trawls were darnagealby chefidcals bormally used on
vessels such as fue1, lubricants,
etc.
There is sotie suspicion that
yarns.
annonia used in aleep freezers could cause darnage to PA-[etting
yarns.
betting
!.ish sl-ime or other offal have no effect oh synthetic
The ccdnon aalvantag€ of all synthetics
ove! natural
fibres is total resistance
to rotting.
Eotrever, PA-fibres
are not as
rot,
PE
and
PvC fibres.
dependa.ble as was assumed because they nay
ullike
8.
Necessity
of dyeing and stiffenibg
gear.
coLour may
Trawls are active fishing
fheir
therefore,
be of litt1e
but brigbt
shiny nettinq may
siqnificance,
is wanted and sone
friqhten
fish.
For mid-{'ater tralr'ls low visibility
parC
trawl technologists
suggest that the upper
of trawls shouf,al have
qyed trawls are often
parts.
a different
colour to the side and lower
Dreferred bv snall vessels with 1ow towino sDeefls,
6.5
calculation
6.5.1
(Effective
net bv horse power
of Dimension of trawl
horse pow€r
Effective
T1Ie relation
between BIIP (Brake hor:se pcerer) and (EHP)
horse power) is e).pressed as:
EHP
=
BHP
Kl xK2
xK3
Kt
commonsafety coefficient
=
0.85
K,
Sea factor
=
0.85
Propeller
efficiency
snall
BHP to
EHP
be used
FV
75
coefficient
0.17 - 0.20
tra\
Mealiun size
o.20 - o.25
Big size
o.ag - o.29
in
this
fornula
is 60t of the real
Eere
t'
:
Cear tlraq
v
:
Towing speed (n/sec)
:
Effective
EHP
force
{kg)
1 H.P. = 75 kg/sec
holse power GE).
today
In important problen for grawl teohnologists
is holr to natch the sire of trawl g€ar to the Lowibg pc,wer or main engine
output.
The towing powe! of a trawler is influenceal by the output of
its malr engius, shape of hu1l, propell€r
d6sigb ahd trln of vessel
uDibr opelatlo!!.
rt is, therefore,
very dlffidult
to accurately
compute
tciaing pol'e!.
6.5.2
cear alrag force
6.5.2.1
ahrling b.a\rl
operation
(Gear lesistanco)
warp and hanal-lope r€sistahce
Hydraulic resistance of !,ra4) and hand-rope
is expressed ln the follolrlng:
&(ks)
R.
=
px1,x6xYz
!Cd
Eere d ls the alianeter of \darp oi hand-lope (d,
I is
warp or hand-rope (n) and V is the towlng speeil.
Cd ls
p
=
ttle ilrag coefficlent
=
O.08
tlensity
=
Io5
of sea water
6.5.2.2
-
the lenqth
of
(Fy'sec)
(kgs2dq)
9a.tet boaral resistance
The resiEtanoe of the otter boaral is
of the reBi-stance which th€ otter board
consLilEleal to be the lesult
reslstance bebteell t]re
lecel,ves frc,n the water flow and tfle ftlotlon
Ihe latt€t
fo?ce is not so marked,
otte! boartl antl the sea botton.
thqrefore.
onfy the reslstance of the ottor board btt water flo\t is
discugsed here.
The resistance
of otte!
board R" can be
er<preaEed aa follo!,ts:
Rb =
lcdxPxsxv2
Here, Ca ls the coefficient
of resistanc€
nunber and also by the angle of incidence
water (about o.3)
the alensity
of sea wate!
P
ls
s
is the area of the otter
(Vsec)
velocity
which valies
of tbe otter
by Retmolds I
boaral in the
= 105 (kSs2lmq)
board
(n2) and v is
the towlng
-50-
6.5.2.3
with
no fish
catch is
Net resistance
The resistance
of a conplele
expressed al4)roxinately
as follo\rs:
-
a
=
the maxinun circurnference
b
-
the naximurn Lehgth of the net
d
=
aliarneter of net twine
l,
=
length
ceneral
ner
abd
9"
of the net
(n)
(n)
of each mesh bar
(d/!, value nornatly
6,6
tratirl
method of designing
f,torn the average vaiuer.
the optimurn trawl
- First stage: the prototl?e
tr.awl is selected.
A point
is reached $hen the technical
studies on the resistance.
fishing
spxead
and fishing
height of the trawl are insufficient
to select t}1e tr.awl
proEocype.
- Seconal stage: the optinur trawting
speeil is deternined
while the prototype
trawl is used for catching the fish,
(For this
purpose tests.axe perfonrEd under actuat fishing
cohditions.
Further
calculations
of the optinurn ttawt anal its interaction
with ttle ship
are carIled out for this speed onty).
- third stage: the nost effective
utiLizatioo
of the
ship during trawling
is exainined.
The yielal is proportional
to tne
area of the trawl nouth, other conditiods beinq equal.
On the other
hand, the hydrodynanic trawt resisrance is arso pioportionaJ.
to the
area of the trawl nouth.
Ihe trawt resistanca
is the main part of
the total
resistance,
overcone by the propelter
of the tralr]er.
characteristics
Fourth stage: tJle probleh of irnproving
of t}le trawt used is solveil.
the hydronechanicat
If we now correspondiDgly
increase or reduce atl the
linear dinensions of, the nodernizeal trawt and the-rigging
forces, l,re
obtain the alesigin chalacteristics
of the optimum txawl, which disptays
naxinurn fishing
efficiency
with fuf1 utilization
of the shiprs Dower.
- 51 -
- Fifti
stage: the trawf is calculated with given
technical
chalacteristics
from the kno$m characteristics
of the plototype
trawl.
Ilavlng deterrnined tie sinilarity
scales, we can corll)letely
calculate
a1I the diipnsions
of the trawl tlet abd of its individual
parts,
warps, all rigqinq parts anal other chalacteristj-cs,
abd also prepare
the required drawings.
- Sixth stag€: tire check the resulCs obtaineal. afte! having
coq)leteal the calculation
and drawing.
Ehe easiest but least accurate
rpthod of checklnq is the calculation
of th€ spreaal and resistance
of
the tralrl.
A nore reliable
check is providetl by experinents.
To
inclease the efficiency
of these experiments, reduce the titE spent in
pelforldng
then and decrease the costs invol-ved, the techni.cal
charactelistlcs
of the project
are checkeal on noilels.
-
Seventh stage: at this Last stage, tests ate carried out
in tlte fishing
legion iD oialer to detertnine the fislring efficiency
of
the neu trawl in cqnparison with the prototype.
The tests are perfornpal
on ships by the methoal of paralle]
trawling
and corparison of the yields
obtained.
When the higher fishing
efficiency
of tbe ne\r trawl is
confimed
it is desirabLe to perform supplenentary tests in order Co
check the optimun trawLing speed caLculatetl at the seconil stage.
I! is
nost in{)ortant
to carry out such tests when tlEr€ is a !,rlale iliffelence
betwedn the alinensions of the design €nd the prototl.pe trawl.
thise seven funtlarEntat sitages of alesign conprise the
\thofe conplex of theoretical
calculatlons,
desigtling, processing and
npdef and fulL-sca1e tests, which must be perfomeal in orde! to create
an optimrnr tlawl.
7.
FISHING EFFTCIENCYOI' TRAWLNET
lhe flshing efficiency
of a trawl is usually expressed by a
coefficient
whlcrh, according to Baranov. is the ratio bet!,reen the catch
obtained and the entire amount of fish in the area of the fishing path,
c! : -!:!i
1-
where n is the nunber of fish in the alea of the _fishing path t
nr is the nurnber of fish t hich escaped froh the fishing
zone.
-52-
q can assune values f;on z€ro (when nr = n, i.e,,
The quantity
zone) to uJlity (\,rben nt = o, i.e.,
aU the fish escaped frofir tlle fishing
not a single fish escaped frorn'the fishtng
zondl . However, the numerical
value of q cannot be directty
ateternin€il slnce we do not know n, which
is the nunibei of fish in the fishlnd
areg.
The fishing
has been enpresseil by marking the catch
efficiency
(CPUE) as its indices,
gear
per unit effort
but in aspects of fishinq
by
the
catch
study, the inalices for fishinq
effici€ncl'
etapresseal
are
per unit working alea or r,rorking volulne of tlle fisbihg
net.
oonnection eith the
lhe fishing
efficiency
has a particula!
(height
cross-section
area of the net mouth
& spreading width) under
purpose
operation for the
of catching fish.
Otl til€ other hand shrinP
and sone flat fish,
are cauqht mor.e by nets designed with
for instance,
ll'tlerefore the
a wider net nouth opening regardless of 'let h€ight.
discussj.on of trawl net efficiency
for aliffelent
nust be different
species of fish.
t.
Hayashi nade a special alepressor device l,thich causeal the
net mouth to rise higher,
and becone squale shaped. He conpared the
net wlth a conventional
bro-boat t]'pe trawl net for fishing efficiency;
It
and found tiat
the forner net is 10-20 per cent higher in catch.
is sFicially
effective
for sea brear, coal and sharks.
-
fish and
2. sqrlE trawls have tickler
chaihs, for taking flat
shrinps fron the nud in the bottcin.
I'hese chains are rigged between
they scare the
the otter boards or fron the _gr:ound rope extensioBs.
shrirp off the bottcn and drj-ve then into the n€t which is a little
The tickler
chains may also ha\r€ the effect
ahead of the ground !ope.
above the bottom when the f,atter
of keepinq the qround xope a little
is rough or is litt€red
with rubbish.
efficaency
tt€re are many nethods of improving the fishitq
efficiency
the net
of a trawl net, in any event, to enhance the flshi4g
conditions:
is required to EEet the following
a)
the net rnust be of the correct
of the nain engine
b)
The net nust be well
c)
Ihe net nust be appropri
ate ly
d)
the net nust be suitable
for
e)
Itre net nust be well
size
balanced with
for
the horse power
the otter
shaped for
the botton
designed for
boarals
catching
topogxaphy
effectine
fishing.
fish
- 53 -
A.
EISII BETTAVIOURIN RE]-ATION TO TRAWL
From the point of vlew of the behaviour of flsh,
th€ entire
co[q)1ex of otter boards, cables and trawl net cah be arbitralily
divided
lnto four sections,
in each of lrhich the fish act in a chaEacteiistic
nranner.
1. The first
section extends fron the otter boards to the front
enals of the wing and incluales the cables anal l€gs.
llost characteristic
of this zone are the wi-ale turbulent
wakes behind the otter boarals
(Figure 29) . The voltex zone becomes filled
particles
wlth flne silt
up from the botton, and it extenals along the sweep-J.ines and
stirred
Itings towards the coal end.
Fron the alata analysed one general conclusion can be tliawn:
the tulbufent
rrake is an obstacle to alL kinds of fish.
h th€ir
attenpt
to avoid entering it, the fish nove towards the center of the
into the cours€ along which the net part
area boundeal by the sreep-1ines,
of the trawl rnoves. Botton fish such as skaee, flounder and others were
never 6een to lise fron the bottorn to escape flon the tlawled zone above
the wake. Pelagic fish (rnackelel, saraline) reacteal to the lrake in exactly
the sane marmer, forming scattereal aggregations ov6r a large area near
the botton.
Snall dispersed Shoals of these fish behaved sonpwhat
diffei'ent1y.
often abandoning the fishing
zone anal rising above the
upper boundary of the lrake. From the point of vi€w of concentrating
fish
in the path of the trawl, the sweep-lined obviously play a much smaller
lofe than the turbulent
wake because i-t was ofteb fo\rnd that the fish
'escaped lnto the space between the wing of the trawl anal the wake.
lthich senses of the fish are directly
involveal in percej.ving
the sake? Based on various data the foUovring conclusions can be
drawn.
If there is enough illunination
anal th€ water is sufficiently
transparent,
the fish car see the vrake from a consialerable distahce.
It should be borne in nind that the nolse ploduced by the otter board
and the bobbins mc'ving along the botton is clearly
audi.ble in the lrater
a long rray fron the trawl,
so that tshe fish can sense the approachinq
danger and become sti$ulated.
If illurdnation
and lrater transpalency
are poor, the fish
perceive the wa.ke with the aid of the lateral
1l$e organs/ but at a
nuch sholter distance.
They probably also perceiye the pressure
pulsations
lrhich originate
wakes behinal
in the zone of turbulent
the otter boarals.
-54-
2. The second section of the trawl is beEween the end of the
wings a]]d the reax par.t of the betly.
the secohd boundary of this
section cannot be precisely
defined because lt depebds on nany factor.s
(tle design of the betl.y, transparency of wate!, illunination,
etc.),
In this section fish of al1 kinds, when they are betvreen
the wings or inside the be1ly, do not nake any attenpt to escape fron
the trawl through the mesh although the size of this is larg€ enough
to enable then to do so easily.
vlhen the netting
approaches the fish,
they tuin so that their head points towards the trawl nouth anal begin
tq nove forrard.
Escape through the net in this zone was observed only
,,race for speed.
when the fish accialentally
cane close to tll€ net th their
and toucheal the net with the tail
or: fins.
T,he rhythtn of swirwlitrg
rnotions was interrupted,
the fish sudalenly lost speed, r.rere pressed
agalnst the net and, because of t-he targe nesh, foubd themsetves outside.
On the other hand, a certain nunfJer of fish in this zohe do not nake
any attempt to escape; they turn theix head against the direction
of
the trawl and thus swisr straight
into the cod ehil.
3. The third section of the trawl incluales the last leaf of
the betly anal the conical part of the cod end.
In this region the
construction
of the net envelope achieves a certain critical
value, and
the fish begin to fling
themselves convulsively
towaitls the nettirgl
attempting
to pass through the nesh.
Alt})ough most of the fish attempt to €scape fron the t!aw1
thxough the mesh, a few either drift
without stoppibg into the coal end,
their head turned against the novenent of th€ trawl, or attenpt to
escape through the nouth.
Ho;e\rer the fish wer€ never successful in
these attempts at the tr:awling speed of 3.0-3.5 knots according to
4. The fourth section of the trawl includes the entixe
part of the baq toqether \rith the coal end, vrhere the catch
cylindrical
accumulates,
Almost all fish that reach the coal end can no longei
(Fiqurc 30).
resist
actively
They arc engulfed by the powelful swirl
of water and form a chuming mass. The densily of this mass increases
towarals the rear of the cod enal, being greatest at the back; the fish
aie firmfy pressed against each other,
Car:f,ied along by the whirling
water, the fish are squeezed along the netting and, if the nesh slze
perTits,
some of them nay even be pushed througtr.
this process is
haphazald and does not depend on the fish themselves.
-55-
boalal under \tater.
Figure
29.
Vier{' of otter
!'iguxe
30-
The catch accunufates
a
in the cod end.
-56-
9.
REFERENGS
A.lekse€v, A.P. (1971).
Flsh t€havl.ou! and flshlng teclbiques
(Flanslated
flom RussiDt.
Islael
Plogran for Scientlfic
Translatlona,
ilerusalern.
Baranov, F.I. (1977). Selectetl works on fishhg g€ar
(Translateal flqn Russlan). Israel Proglan for Sclentlflc
Translatlona, ;ferusalem.
I'AO (1974).
Otter
R@.
board desigm antl perfornandr.
FAO (1959).
lloaleln flshtng
RotE.
(1964).
Uode4r fishing
RotrE.
FAO (197L).
Moalern flshlng
Rome.
llhe l,tdted
Nations,
gea! of t$€ world.l.
Eh€ Uniteal Natlons,
gear. of the worlal 2.
Ihe thited
Nations,
gear of the worlil
fthe thited
Nations,
3.
PriilDanr
A.L. (1973).
(Translat-eal
TranslatioiE,
Carner,
ilohn. (1967).
(Books) Ltd,
Ganer.
;Ioltn. (1967).
Itow to nalae anal aet nets.
(Books) Itd.,
I-drdon.
t|heory and deslgn of conmercial fishing
gear
frolr Russi.n' . Israel
Progran for Scientific
;I6!EsaIem.
Uoder.n deep 6ea tlalrlibg
London.
g€ar.
!'ishj.ng
Fishing
News
l.l€t s
(1973). lrodeung cc'dlercial ftshing qear by
Konalrat' Ev.v.P.,
tjre Eetlloil of analog mchani.s!|s.
Plshchevaya Proeyshlelmost
hrblishera, Moscon.
Kost!runh,
yu. N. (1971). T!€orI antl trawlLng
(Translated frqn Rrsslan) . Isra€I Program for Sclentlfic
Translatlons, \Terusalen,
-57-
Kutakov, 8., Kudryavtsev, N.r and Savtasov, V. (197t.). Mid-irater
trawling (Translated fron Russian).
Israet progran for
Scientific
Translations, Jerusalen.
Nikonorov, I.v. (1971). Methodsof continuous fishing
(Translated flom Russr.an). Israef proglarn for
Tranlsations, Jerusalen.
Scientific
Notnura, M., and Yanazaki, T. (1975) . Fishing techniques,
International
Cooperation Agency, Tokyo.
Japan