The ground ropes function is to keep the net in contact with the sea bottom, and herd fishes to
the entrance of the bag net. In the case of thinking about selective fishing with a drag net, it is
necessary to clarify the shape and herding function of the ground rope. But there are very few
reports about it. In this paper, the author considered forces acting on the ground rope under towing conditions,
and decided the shape of it analytically. The author designed a fish herding model
the ground rope based on the theoretical probability of fiches entering the funnel of a net. Based
on these results, the author investigated the shape and gear efficiency of the ground rope. Finally
the author intended to obtain rational instructions for designing of ground rope.
(1) The construction and weight distribution of three types of ground rope: Danish Seine, Bull
Trawl and Trawl, (typical drag nets operated in Japan), were compared. Weight distribution of
both Danich Seine and Trawl are uniform. But the mean weight of the later is much heavier than
that of the former. It the case of comparison of Trawl and Bull Trawl, the former is long and increases its weight towards the middle.
The latter is short and very heavy and shows little difference in its weight distribution.
The difference in fishing methods are clearly influenced in the construction of the two types ground rope.
(2) Three forces: hydraulic resistance of netting concentrated at the lacign line, hydraulic resistance of ground rope,
and sliding frictional force between sea bottom are acting on the ground rope
under towing conditions. At equilibrium those forces made a fixed ground rope shape. The author assumed the ground rope as a connection of segments,
and considered an equlibrium among
three components acting on them: horizontal, vertical and tangential components. In general it is
impossible to express the vertical and horizontal component acting on each segment of the ground
rope as a simple function. Assuming that the horizontal component acting on each segment of the
ground rope is constant at any part, the length and end distances are known ahead of time, the
shape of the ground rope was expressed graphically based on the distribution of vertical components of it which were obtained experimentaly.
(3) The vertical component acting on each segment of the ground rope under towing conditions
was expressed as the sum of hydraulic resistance, sliding frictional force and sand drag. Drag of
clyinder type of bobbins at different lengths and that of sphere type of bobbins in various directions
were measured both on wet concrete and in a sand tank with water. The drag of two types bobbin on a rocky bottom (F_D) had no directional qualities, and may be expressed as the product of
weight of bobbin (W_B) and drag coefficient (μ_90); F_D=μ_90W_B.
The drag coefficient of cylinder type of bobbins and sphere type of bobbins were 0.75 and 0.68
respectively. The drag of cylinder type of bobbins in towed on a sandy bottom is dependent upon
the on attack angle (90°-θ), and may be expressed the product of the weight of bobbin (W_B) and
drag coefficient (μθ) as follows; F_D=μθW_B.
The drag coefficient is a function of θ and is expressed as follows; μθ=0.68 (0.72 sin^[1.5]θ+cos^[1.5]θ). In the case of a sphere, the drag coefficient was denoted a constant value of 0.68.
Hydraulic resistance of model ground rope composed of clyinder type of bobbins was measured
in a circulating tank. Hydraulic drag coefficient (C_D) showed a constant value of about 0.45.
(4) Three types of model ground rope differing in weight distribution were made. These were
examined both on wet concrete and sandy sea bottom under various conditions for the purpose of
measuring shape and resistance. Close agreement was shown between experimental values of the
shape and resistance of them, and those of calculated values obtained by analytically using the drag
coeffieients mentioned above. When the distribution of forces acting on ground rope uder towing
condition are known, the shape of it can be decided using the method proposed in this study. On
the other hand the weight distribution of ground rope for making a certain ground rope shape can
be decided using the method proposed in this study.
(5) The author paid attention to fish reaction to the ground rope and considered a fish herding
model for the ground rope. Theoretical probability of fishes entering the funnel of the bag net was
calculated, and six theoretical types of ground rope: triangle, trapezoid, half ellipse, arc, parabola
and catenary were compared. The fishing efficiency of trapezoid, parabola, half ellipse and catenary showed almost same values under towing condition based on the fish herding model for flat
fish. The wing tips distance at maximum fishing efficiency was wider than usual towing conditions.
Higher fishing efficiency would be expected under that towing condition.
Fishing efficiency of two types of ground rope, middle-weighted Bull Trawl type and heavy-weighted Trawl type,
were compared by towing on a sandy sea bottom using the fish herding model proposed in this study. Fishing efficiency of the former was a little higher than that of the latter. This corelates well with the results of field experiments.
(6) The ratio of resistance of ground rope to the total resistance of drag net was about 30-35%
under towing condition. It is imposible to change a ground rope shape under towing condition by
means of variation on the weight distribution of the ground rope. In the case of thinking on the
ground rope shape, it is necessary to consider the hydraulic resistance of netting which were concentrated to the lacing lines and acting on the ground rope. The trapezoid shape glound rope was
showed the fishing efficiency using the fish herding model proposed in this study. About 30-35%
of resistance of drag net should act on the points both edge of bosom of ground rope to make a
trapezoid-shape ground rope. It is one method to concentrate whole drag acting on ground rope
and hydraullic resistance of netting there as a results of many considerations on weight distribution
of ground rope and the design of lacing lines.

雑誌名

鹿児島大学水産学部紀要=Memoirs of Faculty of Fisheries Kagoshima University

巻

38

号

2

ページ

103 - 156

作成日

1989-12-25

ISSN

0453087X

NII書誌ID

AN00040498

著者版フラグ

publisher

NDC

664

別言語のタイトル

Fundamental Studies on the Function of Ground Rope of Drag Net