the approximate value of the current resistance of any fixed-net at the given current velocity is to be counted easily.
(3) Floating-and-sinking-phenomenon of the Sinker
Now, Q is the vertical component of the force to the sand-bag rope, and B the force got by reducing the weight of the sinker and the net in water from the buoyancy of
the floater under water.
As to the relationship between Q and B there are two cases, viz. the one when Q is smaller than B, and the other when Q is larger than B. As to the former, at the section b, in the 12th line of Table III, it is denoted with*; (Q—B) in the 15th line is negative.
The cause of such a phenomenon as this is assumed to be due to the deformation
10-10 .
2-I 0
O "On.
-O-'
5 12 1025 1535 205 Vcm/sec
Fig. 10. Average value of C, Cr and Cq.
178 Mem. Fac. Fish. Kagoshima Univ. Vol. S
of the net-construction under the running water, and the following considerations are made from the above experimental results: first, concerning the resistance of one
flat-net held by the sand-bag rope in the running water, in case of the flat-net whose flat-net-foot detatches itself completely from the water-bottom, the relationship between the various kinds of forces is shown in Fig. 11-a. The case in which the net-foot remains attached to the bottom is shown in Fig. 11-b.In Figures, O ; floater,
Forces acting on the Net.
R ; force to the sand-bag rope,
0 ; angle which the sand-bag rope makes with the water-bottom, b ; buoyancy of the under-water floaters,
a ; resultant of R and b, a ; force balancing to a,
d ; sum of the under-water weight of the sinkers and the under-water weight of the net,
e ; hydrodynamic force acting on the net and the sinkers, / ; horizontal component of the hydrodynamic force, h ; vertical component of the hydrodynamic force, (j> ; angle between / and e,
O'; point at which the action line of the sinker-and-net gravity intersects with a', O"; sinker,
R'; force acting on the sinker, r' ; force balancing to R', N, OO"; net.
Out of the experimental value shown in Table II and III,, the values of b, d, h, a, e, 0,/
are computed and shown in the same table, line 12, 13, 15, 16, 17, 18, 19 respectively.
According to the table, in the case when the net-foot detatches itself completely from the water-bottom the following may be described as to the hydrodynamic force :
a) i) When the current velocity increases, the magnitude of the hydrodynamic force e increases also, and its direction </> gradually approaches to the horizontal line. This phenomenon is observed in both flat-net and square-net.
//) The horizontal component / of the hydrodynamic force is, of course, equal to the horizontal component of the force acting on the sand-bag rope. Therefore, the
Kanamori : On the Physical Analysis of the Fixed-fishing-net
Resistance 179
coefficient of horizontal resistance CP is equal to the coefficient of horizontal component of the hydrodynamic force.
Hi) The vertical component h of the hydrodynamic force, may be considered as the lift of the hydrodynamic force. This magnitude decreases when the current velocity increases. By the way, as Cq is counted, basing on the vertical force acting on the sand-bag rope, so, it is not the so called the coefficient of the lift. But in the fixed-net the horizontal resistance is dominant, so, there may be not so much difference between CQ and C&, as shown in the 22nd line, Table III.
b) On B*jW and the length of the sand-bag rope
Whether the net-foot detatches itself completely from the water-bottom or not is generally determined by whether (Q—B) = h is positive or negative. In the net where the ratio between floating power B* and sinking power W, viz, B*jW is equal to 2, h is positive. This may be described in anothor expression, namely the result of ,x2-test on the flat-net C (Table III) is highly significant, and that of ;t2-test on the square-net
C is significant. When, h of the net, where the value of B*/ W is equal to 1 or 0.5,
is negative, the net-foot remains attached to the water-bottom.In short, from the above experimental results, the following may be described. When
B*/W is large, the net-foot detatches itself from the water-bottom easily. When B*/W becomes small, the net-foot does not easily detatch itself from the bottom, and the
smaller becomes B*/ W the harder becomes the net-foot-detachment from the water-bottom.Besides the value of B*/W, the fact whether the net-foot detatches itself from the
water-bottom or not has relationship with the length of the sand-bag rope, viz, the longer is the sand-bag rope the easier becomes the net-foot detatchment from the water-bottom, as shown Fig. 4 and 5. In this experiment, when B*/W=l, this is to be numerically
observed as shown in the 13th line, Table III.c) As shown in Table IV, while all the sinkers of the net-foot remain attatched to
the water-bottom, as to the resistance, in addition to the force to the sand-bag rope,
there is a force to the net-foot, and so, the occasion when the net-foot remains attatchedto the water-bottom is now considered. In Fig. 11-b, the force R' to the sinkers,
increases as the current velocity increases. (See Table IV). And the sum of the horizontalcomponent of a' and that of r' is the horizontal component of the force acting on the net. When the net-foot remains attatched to the water-bottom, the sinker is pressed
downward. Then, it may be easily assumed that the vertical component of the resistanceto the net-foot does not work to blow up the net-foot. And moreover, as stated above, whether the net-foot detatches itself from the water bottom or not has a relationship with the value of B*/W. But in this experiment it is difficult to measure the angle which the net-foot makes with the horizontal line when the net-foot remains attatched to the bottom in the running water. This difficulty may be due to the fact that the used
tension pickup is larger than the sinker.Then, in order to make any further research on the floating and sinking phenomenon of the net-foot, it seems to be necessary to make the experiment with the tension pickup reduced into minor type, and under the conditions in which the value of B*/W is let
to change variously.
Acknowledgement