At 7 knots, however, the records of the depth of the depressor varied a little while it was trawled

105

Field Test of Gyoken Depressor*

Keishi SHIBATA

Hamuro and Ishii1) desingned a new type depressor for use of mid water Trolling. Effectiveness of this depressor was proven in the test by attaching to the tuna trolling off Samoa Islands in the equa- torial Pacific, in the summer of 1965 (Hamuro 1966). The author had an opportunity to carry out a field test of this type of depressor on the occasion of the cruise of the research vessel, the Tansei Maru, in Sagami Bay on April 26,1966.

The depressor was shackled with a swivel at the end of towing warp, 9 mm in diameter, and a Netsonde to record the depth and temper- ature of water on the deck by means of acoustic telemeter, was at- tached to the warp just above the depressor (Fig.1).

One series of the test was made by attaching a plankton net, 56cm in mouth diameter, conical, 1 mm mesh Pylen cloth, to the warp at a point of 2 meters above the depressor, and another series of the test was made by towing the depressor, without attaching plankton net.

Each series of the test consisted of towing at seven different speeds, i, e, 2, 3, 4, 5, 6 and 7 knots. In every case the warp was paid out until the depressor pointed out the depth of about 50 meters which was monitored by the acoustic telemeter.

The results of the test were as follows : Within the range of 2 to 6 knots of towing speed, the depressor was very stable in both cases, i, e, with and without plankton net. The angle of the warp measured on deck was almost constant at each speed. At 7 knots, however, the records of the depth of the depressor varied a little while it was trawled.

The depth of the depressor calculated from the angle of warp on deck and the length of warp paid out was compared with the depth recorded by acoustic telemeter. Whereas there was a remarkable difference between the two, the latter was considered to indicate more reliable value (Fig. 2).

The above two estimatimations of the depth of depressor would have been derived from hydrauric elements of followings : Diving force of the depressor, weight of warp and attaching gears, and hy- drauric resistance of warp and plankton net. The towing warp draws various catenary curves in accordance with the resultant force of the above hydrauric elements, and the diving force of the depressor is in proportion with the twoing speed. Accordingly, the measured depth

of the depressor indicates a greater value than that estimated from the angle and length of warp paid out.

* Plankton gear test cruise of the Tansei Maru, Ocean Research Insitute , University of Tokyo, in Sagami Bay, April 22-27, 1966

ユ06 SHrBATA：Field Test of Gyoken Depressor

夢辮蓑

Fig． 1 Gyoken depressor， transducer of acoustic depth and        temperature telemeter， Netsonde．

       The detail of the depressor is shown in Table 1．

ミO 勿ミ 0

漣ヒΦ心︒㍉    砂 0   0

@ 

@ 

T0

So≧︑︒§亀

．一一一一a      ・わ

c

Fig． 2

2 3 4 5 6

  siotped r la，oV

7

Length of warp requiered for towing depressor at 50 meters depth．

a ； depth of depressor measured by acoustic telemeter b；depth of depressor calculated from angle and length of    warp， towing with plankton net

c ； towing without plankton net

Bu11． Fac． Fish． Nagasaki Univ．，22（196ワ） 107

   It is concluded that this type of depressor is effective for the towing of a small plankton net with the 9 ne−m warp at a speed beilow

6 Knots． Fig． 5 i・11ustrates the depressor and plankton net towed at a speed of 6 knots．

50m _ケ

Fig． 5

      N，ss；orr

Towing plankton net with depressor at 6 knots．

P ： plankton net， 56cm in mouth diameter N ： Netsonde

D ： Gyoken depressor W：Warp， 9 moz in diameter

   The author expresses his sincere thanks to the Ocean Research Institute， University of Tokyo for giving an opportunity to do this test． Thanks are also due to Mr． Kato of the Furuno Electric Co．

Ltd．， for his kindness in supplying a depressor and a Netsonde for this test． The author is indebted to Prof． S． Motoda， Hokkaido  Jni−

versity， who organized the cruise of testing plankton gear． The help given in this field work by author s colleague， Capt． S． Abe， Nagasaki University， is greatly appreciated．

Table 1

 0ver−all length     ii breadth     i7 height

 Outer diameter of body  Area of main planes

   ！i stabilizing horizontal plane

   u i7 vertical plane

 Incidence angle of main plane  Total weight （in air）

   ij （in water）

 Material

Dimension of Gyoken−depressor（Hamuro 1966）．

］OOcm ワ4㎝

55cm

1 20 cm O． 2］ m2

0．］lm2

0． 1 28 m2

8．so 2ユ．6k牙

9k牙 plastic

ZO8 SHiBATA：Field Test of Gyoken Depressor

Table 2 Depth of the position of the Gyoken−depressor measurted by       means of Netsonde and the depth of the position calculated       from angle and length of the warp paid out．

Depressor without plankton net Towing Warp length

speed paid out

2 Knots 5 4 5 6 7

 L67m

80 88 ユ08 125 155

Warp Depth calculated angle from length and

      angle of warp

 0 480

5ワ

65 66 ワ2 ワ6

L cos e＝d    44．8    45．6    40．0    42．ワ    58．6    5ワ．5

Ratio of calculated depth to the depth measured by Netsonde

    dt／d＝＝50     0．896     0．8ワ2     0．80     0．854     0．ワワ2     0．ワ5

Ratio of warp length to depresser depth

  L／50   1．54   1．6   1．ワ6   2．16   2．50   5．ユ0

Depressor w！th plankton net

∩∠スリ4rO！0ロ﹂ 92 102 112 ユ29 145 170

！0∩∠只︶りム70rO＝U！O∠Uワロ〜ワ 51．4 47．8 4ワ．5 59．9 42．5 44

1．028 0．956 0．946 0．ワ98 0．84ワ O．88

1．84 2．04 2．24 2．58 2． 90 5．40

Reference

1） Hamuro， T；；Ocea勿FisheryσS／1， Apri1，1ワ（1966）