The webbing of the model net was woven with cotton thread. The strip plane of the model net is shown in Fig. 5. 3. The frame lines and purse line o{ the model net were of cotton and a Saran (polyvinyliden chloride) twine, respectively.
The density of the cotton twine was 1. 39 and that of the Saran twine was 1. 70.
Fine quality cork, of density O. 239, was chosen as the material to make the floats.
Copper wire (density, 8.93) of adequate length was used {or the leads in this model.
Purse rings, O. 4 mm. in diameter, were made of copper wire, O. 36 mm. in diameter.
41'"
~
~A
-B
r - D E F c:.
C
-127 ".,..,
o , 10 ! 20 I 30 , "'"
Scab.
Fig. 5.3 Strip plane of the 11250 model seine.
Y. IrT.1KA: Studies on the Mechanical Characters of Purse Seine 35 Forty of these rings were hung to the lead line at regular intervals. The dimensions, arrangements of webbing and accessories of this model net are tabulated in Tables 5. 5 - 5. 7 and Figs. 5. 4 and 5. 5.
TABLE 5.5 PARTICULARS OF THE WEBBINGS USED FOR THE 1/250 MODEL SEINE.
Thread Portion of
the Net Mark Knot Count (Cotton)
20'S
Mesh Size (Bar- ) measure
Vertical No. of Meshes
Length Webbing of
Horizontal No. of Meshes
Bag 1 A Reef 10 0.891cm 10 7. 27em 4
Bag 2 B II 8 0.842 6 /I /I
Bag 3 C /I 6 0.891 18
"
/IBase Wing D /I II
"
31 6.06 3Intermediate
Part E II 4
"
32 36.36 20Tapered Wing F II
" "
25 30.30 17Tip Wing G
"
6 II 14 3.03 2TABLE 5.6 PARTICULARSOF THE ROPES USED FOR THE 1/250 MODEL SEINE.
Name of Rope Mark Material Diameter Length Note
Cark Line A' Cotton 0..96mm. 115.1em. Doubled
Lead Line 1 B' II
"
86. 7 II'I 2 C' II II Hl.2 Tripled
Selvage Line D' II 'I 7.9
Purse Line 1 E' Saran* 2.06 3.6 "Sekiyama"
/I 2 F' II
"
140.0Bridle G' Cotton
*
polyvinyliden chlorideE' ~
I
Fig. 5.4 Schematic diagram to show the arrangement of the ropes.
36 Memoirs of the FaGulty of Agriculture of Kinki Univ. 2 (1964) TABLE 5.7 ARRANGEMENT OF THE FLOATS AND SINKERS ON
THE 1/250 MODEL SEINE.
Cork Line Lead Line
Portion of the Net Mark Weight of Corks Length Mark Weightof Leads Length al 0.32g· wt . 4. 120m .
Bag a 0.82t:·wt . 10.91om .
0.23 1/
I
blb 0.52 9.09 c' 0.44 9. 70
d' 0.27 6.06
Intermediate Part c 1/ /I el 0.18 3.64
/' 0.26 4.85
d 0.46 1/ gl 0.27 1/
hi O. 15 2.40 •
0.43 1/
il 0,19 1/
j' 0.20 1/
kl 0.23 1/
/ 0.50 1/
{I 0.36 3.64
Tapered Wing ml 0.55 6.06
nl O. 16 1.82
g 0.21 4.85
0' O. 14 1/
P' O. 10 1/
ql 0.07 1/
h 0 1. 54
r l 0 1. 58
Cl ? c d. e f JI.
--l-~,----!"'--"t~-,~,,,,-,,,,,,,,,
!J'l»'~
Fig. 5.5 Schematic diagram to show the arrangement of the floats and sinkers.
5. 4. 2 EXPERIMENTAL APP ARA TUS
In actual operation the pursing begins as soon as the set is finished. The speed of pursing is rapid at the beginning and becomes slower towards the end, whereas the speed of setting is almost constant. It .takes about 3 minutes to set the seine and
- - - - - - - -
-Y. IrT.~KA: Studies on the Mechanical Characters of Purse Seine 37 about 13 minutes to haul in the purse line completely. Therefore, in order to satisfy Eq. (5. 9) the model seine must be set in about 0.72 seconds and the pursing of the model must be done in about
3.
1 seconds. However, model operations in such short lapses of time are difficult to realize on technical grounds. For this reason, the setting and the pursing apparatus were specially designed.The setting apparatus consisted of a circular frame which has 8 small rings at regular intervals, holder lines, and a system of pulleys. The model net was hung from the circular frame by the hold~r lines through the small rings. The circular frame with net was suspended by the holder lines so that the bottom margin of the net was about 1 em. over the water surface. When the holder lines were released the model net was "set". The schematic diagram of the setting a.pparatus is shown in Fig. 5.6.
- - - - - _._--- - _. - -
-Fig, 5.6 Schematic diagram of the setting apparatus with a stroboscopic illuminator.
The pursing apparatus consisted of a 1/8 H. P. electric motor, a "pursing"
drum, and a system of gears and pulleys. The schematic diagram of this apparatus is shown in Fig. 5. 7. The pursing was done mechanically ata constant speed of 16.5 em. Isec., which was the mean speed of the pursing in the actual operation.
Moreover, a stroboscopic illuminator and a tension J;ecorder of the purse line were specially designed for this experiment. The stroboscopic illuminator consisted of a covering box with a rotating shutter disc. A 500 Watt spot-light was mounted in the box. The schematic diagram of this apparatus is shown in Fig. 5. 6. When the rotating shutter was rotated' at a constant angular velocity, the model net was illuminated at regular intervals. With this apparatus the sinking behaviour of the net in successive stages was photographed on a single dry-plate by multiple exposures.
38 Memoirs of the Faculty of Agriculture of Kinki Univ. 2 (1964)
Fig. 5.7 Schematic diagram of the pursing apparatus with purse 1ine tension recorder.
Key: --A: Motor,Bl and B2: Worm-gearing, Cl-4: Pulleys, D: Clutch, E: Pursing drum, F: Recording Drum,
G: Carriage, H: Spring, I: Pen, ]: Purse line.
The tension recorder consisted of a recording drum, and a carriage with pen attached to a steel spring. The schematic diagram of the apparatus is shown in Fig.
5.7. The drum was rotated at a constant angular velocity by a motor. When the pursing was begun the tension on the purse lines, pulled the carriage along on the rails recording the change in tension on the purse line during the whole pursing oper-ation on the drum.
5. 4. 3 EXPERIMENTAL METHODS AND RESULTS
1. To investigate the sinking movement of the bottom margin of the entire net the sinking of the lead line was photographed from both the wing side and the bag side. An example of the photographs obtained is shown in Plate 5. 1. In this plate, the 10 horizontal lines shows the lead line in successive stages of descent at regular intervals of time. Therefore, the depth
t,
to which the lead line reaches In time t was easily calculated. If L is the maximum depth to which the lead line reached in time T, then the relation between the non-dimensional quantities tiL and tiT ISfound to be linear for both the wing and bag parts of the seine, as shown in Fig.
5. 8. It may be considered that the sinking movement of the bottom margin of a purse seine is at a uniform rate.
/
Y. IrTAKA: Studies on the Mechanical Characters of Purse Seine 39
Plate 5.1 Photograph showing the sinking movement of the bottom margin of the model seine.
1.0
09
/
08 07
/ /
06
W~~/:;?
05
all-j{ o1:--
Ba.;r04
t
03
J/
02 01
l~
~;f
0 0] 0.2 03 04 05 06 07 08 09 10
?T
Fig. 5.8 Showing the relation between the depth of lead line and the time required for lead line to attain that depth.
40 Memoirs of the Faculty of Agriculture of Kinki Univ. 2 (1964)
To investigate the sinking behaviour of the bottom margin of the seine during pursing oprations, the model was pursed mechanically at a speed of 16.5 em. /sec. , one half second (I) and (me second (II) after the seine was dropped. For the full-scale gear the corresponding times are about 2. 1 minutes and 4.2 minutes, respective-ly, after the set was started. During the pursing operation the transformation of the net was filmed with a movie camera at a speed of 16 frames per second. An example of the results obtained is shown in Fig. 5. 9.
.
40't~-]
60 . 80 '(0.5)2-' - - - 2-' 2-' - 2-'
(13.0) 50 (140) 54 (15.1) 58 <.) 6·') 62 (J2,7) 4~ (138) 53 (148) 57 L-(_15_,9_l--':...J
Fig. 5.9 Transformations of the net. A: wing, B: fish bag, parenthesized figures: corresponding elapsed time for full scale gear (in Minutes), figures: film number since the net was set.
Fig. 5. 10 shows the relation of the values of I and t in the whole course of the operation. These values are converted to the corresponding st:;'lge of operations of the full-scale seine. It can be seen from this figure that the lead line sinks very rapidly to about 6 meters and then sinks more slowly at a uniform rate. This initial rapid sinking of the bottom edge is thought to be due to dropping of the model net from above the water surface to make the "set". If the linear part of the curve is extended backwa~d, this curve intersects the origin, which is to be expected. From the figure the following assumptions for the full-scale net may be considered.
a) For about 4 minutes after pursing is started, the bottom margin of the fish bag may still continue to sink at a uniform rate.
b) The bottom margin of the fish bag may sink to its limit of about 60 meters (40% of the stretched depth L of webing), in about 8 minutes after the set was started.
c) If pursing is started 2 minutes after the set IS finished, the maximum depth to
41 Y. IrTAKA: Studies on the Mechanical Characters of Purse Seine
which the bottom margin reaches may only be about 10 meters (7% of L) more than if the pursing is started as soon as the set is finished.
M
70 60 50
..e
4030 20
tJ..,.~c1(I) 10
0
Fig. 5.10 Relation between the depth I of the bottom margin of fish bag and the time t required to attain that stage. Solid line:
maximum depth to which the bottom margin reaches, broken line: depth to which lead line reaches, P (I) and P (II): times when the pursing was begun in the case of (I) and (II), respectively.
2. To investigate the change of tension in the purse line during pursing opera
tions, t"";o series of pursing experiments were made: the pursing was begun(A) when the bottom margin of the net had sunk completely, and (B) when it was still in the course of sinking. The results are summarized in Fig. 5.11. It can be seen from this figure that in the case of (A) the tension in the purse line begins to increase.
sharply as soon as the pursing begins, but in the case of (B) the tension does not increase until the first 1/6 of the pursing process is completed. After about 1/2 of the pursing operation is completed the changes of tension in both cases coincide with each other.
42 Memoirs of the Faculty of Agriculture of Kinki Univ. 2 (1964)