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NAOSITE: Nagasaki University's Academic Output SITE
Title
On the Comparison Between Two-seam Trawl Net and Four-seam Trawl
Net
Author(s)
Flores, Efren Ed.C.; Nakasai, Kei
Citation
長崎大学水産学部研究報告, v.32, pp.129-134; 1971
Issue Date
1971-12
URL
http://hdl.handle.net/10069/31092
Right
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129
On the Comparison
Between
Two-seam
and Four-seam
Efren
Ed.
Models of two-seam
be identical
The
trawl
FLORES and
in a circulating
Net
Net
Kei
net and four-seam
in size and tested
mechanical
elevation
C.
Trawl
Trawl
NAKASAI
trawl
net
type water
were
constructed
tank to compare
to
their
characteristics.
results
of the
of headline,
important
spread
net to be more efficient
working
performance
of wings and total
than the two-seam
of the
resistance,
nets,
such
as
showed the four-seam
net.
Introduction
In Japan,
construction
Europe
until
lately,
the
into two types—the
at
the
originated
beginning
trawl
nets
were
two-seam
trawl
of the century,
and
classified
net which
the
according
was
four-seam
to
introduced
trawl
net
the
from
which
in Japan.
The two-seam
in two-boat
trawl
net was employed
trawling.
Though
in the same fishing
ground,
in otter
the fishing
trawling
and
methods differed,
i. e. , the East China Sea.
the
four-seam
both were
operated
Thus the two types of
nets were to catch the same group of fishes,
however,
has been given as to why two different
of nets had to be used for operation
in the same area.
The mechanical
types
difference
between
no specific
explaination
the two types
of nets was
not clear and therefore
it could not be used to explain the above matter.
It is
then
they
local
considered
that
were
constructed
and
operated
following
conventions.
Up to the present,
overseas
fishing
in the seas adjacent
is otter
trawling,
trawling
to Japan.
that
the
of fishing
operations
have been
expanding
to
since the World War II due to the decrease of fish catch
Although
the net construction
It is considered
experiences
the Japanese
grounds
the method of operation
has shifted
employment
operations
of the
and not for
to the
gear
of these trawlers
four-seam
trawl
net.
is based on the long-term
its advantages
due to
mechanical
mechanical
character-
characteristics.
Presently,
there
istics of the trawl
the relationship
is not much study on the comparative
nets.
between
N. NOMURA and T. YASUI1) conducted
the resistance
and
trawl,
2) Danish seine commonly
used
waters,
3) two boat type trawl
generally
towing
in the
speed
of
north-eastern
operated
in the
1) ordinary
area
East
a study
on
otter
of Japanese
China
Sea,
4)
130
Bull． Fac． Fish．， Nagasaki Univ．， No． 32 （1971）
Japanese type small trawl operated manually and 5） Japanese type small otter
trawl， using their respective model nets． However， each net was treated indepen−
dently， i． e．， the models were of different size． lt is difficult to construct model
nets of different construction but of similar size if they are based on specific full
scale nets such as those mentioned above．
In this respect， the comparative study on the mechanical characteristics of
several fishing nets of different construction but of one type such as trawl nets，
is best approached by using theoretical analysis based only on the mode of
Qperation and the geometrical configuration of the net in operation． From this，
model nets of similar size but of different construction （i．e．， 2， 4， 6−seam nets）
could be made． This approach gives accurate comparison of various aspects of
the model net such as elevation of the net modth， spread of wings and resistance．
Furthermore， the construction of model net is not restricted by the size of a full
scale net since such need not be taken into consideration． On the contrary， an
efficient full scale net can be made using the results of the model net test．
T． TANIGUCHI2） investigated the resistance of four types of cod ends of
similar size， i． e．， 1） parabolic cylinder type， 2）i sphenic type， 3），．， pyramid
type and 4） box−type， on’the basis of theoretical＋analysis， a similar approach to
the above mentioned method but limited only to cod ends．
Therefore， to understand fully the mechanical differences between two types
of net construction， the authors attempted in this report to compare the mechanical
characteristics of two−seam and four−seam trawl nets statting with the theoretical
analysis and then experimentally investigating their models of identical size and
materials ．
Analytical studies on the mechanical characteristics of the four−seam・trawl
net were made by T． KAWAKAMI and K． NAKASAI3，4）． A study on the
same of the two−seam trawl net was done by the authors in a separate paper5）
using the same treatment as mentioned above．
Expenses for executing this study were partly defrayed by the Japanese
Ministry of Education．
Experiment
For accurate comparison of the mechanical characteristics between the two−
seam trawl net and the four−seam trawl net， their model nets were constructed
in identical size， i， e．， the mesh area being the same in all sections． The only
difference consists in the manner of construction as shown in Fig． 1． The
seaming line of the two−seam net starts from the end of the wing passing through
its center down to the edges of baiting and belly， then to the cod end． For the
four−seam net， the seaming line starts from the headline between the joints of
square and side wall， then down to the body and cod end． Another seaming line
131
E． FLoREs and K． NAKAsAI ： Comparison of Trawl Nets
ピ
2−Searn
一N
2
N2一
囲
N2
’／・、髄5
B創丁醜’ ・．
コ ロ し
玉
囲一
らし
N2’・、亀
2
1
KZ B しY
b
N2
1N
4−Seqm
N2
COD END
匡］国国
i・亀
N2
，二
N
ほ
巨］［ヨ国回国
由
SQUIIiRE
N2
s
．
VLDER
WING
匹］国
E灯．
WIN
Fig． 1． Experimental model nets．
runs from the groundline between the joints of side wall and belly， then down
to the cod end．
The dipaensions of the model nets and the experimental procedure are the
same as those in the previous paper5）． The two model nets were tested in the
same circulating type water tank maintaining the same conditions to get a good
accuracy of their comparison．
Comparisons were made on elevation of headline， spread of headline， sectional
horizontal spreads and resistance．
Result and Discussion
Since the important working performance of the net is characterized by the
horizontal spread， elevation of headline and resistance， the comparisons were
concentrated on these aspects．
Fig． 2−A shows the sectional horizontal spread of headline； 1） y． spread at
extension wing of Ni webbing， 2） y．． spread at wing of N 2 webbing and 3）
yh spread at square， for beam length of 20cm at different water velocities．
At y ev， the four−seam net has a wider spread than the two−seam net both
maintaining a constant spread throughout． Spread orww shows the same relation−
ship except for the 10−20 cm／sec velocity range where the four−seam net shows a
decreasing change． However， yh gives a change in relationship with the four−
seam net having a lesser sptead than the two−seam and is maintained throughout．
132
Bull． Fac． Fish．， Nagasaki Univ．， No． 32 （1971）
㎝ε
yh
5
一 2 Seam
少 ＿一4Seom
3
、 、
、
、 、 、 、 一 一
@ 噛、㌔ 一＿＿＿ ＿＿＿ ＿＿＿＿一一一一一
y”
2
yw
＿＿＿＿＿＿ ＿＿一一 一一一 一一一一 一一 一一一一 一 一一一 一
一．
Io
50
20 30 40
@ V｛Gm18GC，
Fig．2・A． Sectional horizontal spread of headline at extension wing（yw）， wing
（yww）and square（yh）for two・seam and four・seam trawl nets in
relation to water velocity（V）at 20cm beam length．
The total spread of the headline shows that both nets have the same value．
However， at this beam length， the elevation of net month， Z， of the four−seam
net is higher than that of the two−seam net from about 25 cm／sec water velocity
as shown in Fig． 3． This may be attributed to the spread relationship at pth．
Fig． 2−B shows the same sectional’ horizontal spread of the headline at beam
length of 35cm． Both y． and y．w fluctuate more than those of Fig． 2−A with
the change of water velocity， and yh presents the same result as in the previous
beam， length except for a slight increase in difference as water velocity increases．
乙．，
＼ ＼＼
＼ ＼＼
3．0
＼ 、
＼ ＼
＼ ＼
＼ ＼
＼ ＼
＼ ＼
＼ 、、
＼ ＼
N ＼
N ＼
＼ ＼
＼； ＼ 、
ヘミ ＼ ＼、
2．0
＼ 、
＼ 、
＼ 、
＼ 、、
、 、、
＼
、 ＼
＼ ＼
、、 、
、 、
、 、
、 、
、 、
1．O
s 一．
一一一一一一一一20
一’
@．一一N．
、
、
＼
、
、
、
、
、
、
、
．一 35
20
N 35
50 、
50
Vl㎝ノ8ec）
IO 20 50 40
Fig． 3． Observed values of elevation of headline （Z） for two−seam and four−
seam nets in relation to water velocity （V） at 20， 35， 50cm beam lengths．
133
E． FLoREs and K． NAKAsAI ： Comparison of Trawl Nets
m．．
C8叩
”一一一一 一『一、』一一一一一＿＿一 yh
一．
@g
一．． 一一
一一 N
一一一
7
5
一一一一一一一 ’Yww
t t ．一一
一一 一一 pt
一一一・一 2 Seam
一一一 4 Seqrn
4
〈一一 一一一一一．一一． Yw
s
tO 2b v（cmJsec） 50 40 50．
Fig． 2−B． Sectional horizontal spread of headline at extention wing （yw）， wing
（yww） and square （yh） for two−seam and four−seam trawl nets in
relation to water velocity （V） at 35 cm beam length．
Referring back to Fig． 3， Z of the four−seam net is still higher than that of the
two−seam net．
In Fig． 2−C at 50 cm beam length， the change in horizontal spread is noti−
ceable， For yw and yww， the four−seam net has lesser spread than the two−seam
net． However， at 30cm／sec water velocity， the wings of the two−seam net
collapsed showing the instability of the net at higher velocities．
Even with the same yh for both nets， the Z value is higher with the four−
seam net than with the two−seam net． Since the number of floats and the size
C揃
9
＝一 Yh
8
一 一 一 一 一 一 一 一一 一 一 一 ．一一一 一 一 一 一 一 一m一 一 一“ N
N N−
N し
SN YWW
7
一 2 Seam
6
一一一 @4 Seam
、 、
、 、 ！！
y
W
、 、 ∼
ヘ ノ
、 、 ＿ ＿ ＿ ＿ ＿ 一 ’
IO 20 50 40 50
V（cm／sec）
Fig． 2−C， Sectional horizontal spread of headline at extension wing （yw）， wing
（yww） and square （yh） for two−seam and four−seam trawl nets in
relation to water velocity （V） at 50 cm beam length．
134
Bull． Fac． Fish．， Nagasaki Univ．， No． 32 （1971）
and kind of webbing are the same in both nets， these can not be the cause of
the difference in headline elevation． The only one aspect left out 一to affect the
elevation of the headline is the construction of the net at square， body and sides．
The force acting on the side wall of the four−seam net is transmitted to the
ribline where the seaming line is stapled． For the two−seam net， the upper
section of the side wall is a part of the baiting and one half of its resistance is
transmitted to the latter． Since the forward section of the baiting （square） is，
stapled to the headline， its resistance affects the headline elevation as explained
in the previous paper5）．
The observed values of the towing forces （f−fW， F−fWr） or total resistance of the
nets do not show any difference， although
15
in the theoretical values， the two−seam
e 一2 Seam
net shows a greater resistance than the
●一一｝一4Seoゆ
●
／
／
Z
●
for the different types of cod ends
o ●
remains the same． This was also true
eo●●・●
to
O ㌔f
irrespective of construction， the resistance
OO
下
range as shown in Fig． 4． Therefore，
謹 four−seam net within the ekperimental
investigated by T． TANIGUCHI 2） ．
Based on the above results， both
一
5
卸／
／
！
／
calculated and observed values proved
that the four−seam net is mechanically
more efficient than the two−seam net・
IO 20 30 40 50
ew．β獅
Fig． 4． Observed （O， e） and calculated
（．， ・・・…） values of towing forces
of two−seam net（fw／f）and four−seam
net （Fw／F） in relation ’to respective
angle of force （Bw） and （ew）．
Reference
1） NoMuRA， M． arid T． YAsul： Model experiments on trawl nets of various types，
Bull． Jap． Soc． Sci． Rish．， 18， （12） ， 727−733 （1953）
2） TANIGucHI， T． ： On the resistance Of various cod ends fixed in a stream 一1， Bntt．
Jap． Soc． Sci． Fish．， 21， （5）， 29！−296 （1955）
3） KAwAKAMI， T． and K． NAKASAI ： On the mechanical chara’cter of the drag n．et−li，
Bugl． Jap． Soc． Sci． Fish．， 28， （7）， 644−670 （1962）
4） KAWAKAMI， T．’ and K． NAKAsAI ： Theoretical derivation of characteristic curve of
trawl nets， Mem． Colt． Agr． Kyoto Llniv．， No． 94， （Fish． Ser． No． 9）， 1−16 （1968）
5） NAKAsAI， K．，and E． FLoREs ： Theoretical studies on the mechanical characteristics
of two−seam trawl net， This Bul．t．， 32， 113−128 （1971）