• Journal of the Korean Society of Fisheries and Ocean Technology
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• v.28 no.2
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• pp.132-143
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• 1992

This paper presents a method in order to calculate the vortex distribution, the streak-line and the time-line around the flat and the cambered otter board in two dimensional flow using the discrete vortex method, and to calculate C sub(L) and C sub(D) of the otter boards varied with the passage of time by the numerical simulation using the Blasu's formula. The results obtained are summarized as follows: 1. Flow pattern around the otter boards calculated by the discrete vortex method was resembled closely that of the visualized photograph. 2. C sub(L) and C sub(D) calculated by the numerical simulation was very similar to the model test. 3. The circulation direction around the otter boards and the action direction of the shearing force can be recognized from the time-line around the otter boards. 4. Flow speed in the back side of the otter boards was faster than that in the front side, and the difference of the flow speed in both side of the cambered otter boards was about 1.3 times greater than that of the flat otter boards. 5. The clockwise vortex was generated in the trailing edge, and the counter-clockwise vortex was generated the leading edge of the otter boards. And they were shown the shape of Karman's vortex varied with the passage of time.

• Journal of the Korean Society of Fisheries and Ocean Technology
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• v.43 no.1
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• pp.49-61
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• 2007

Otter boards in the trawl are the one of essential equipments for the net mouth to be spread to the horizontal direction. Its performance should be considered in the light of the spreading force to the drag and the stability of towing in the water. Up to the present, studies of the otter boards have focused mainly on the drag and lift force, but not on the stability of otter boards movement in 3 dimensional space. In this study, the otter board is regarded as a rigid body, which has six degrees of freedom motion in three dimensional coordinate system. The forces acting on the otter boards are the underwater weight, the resistance of drag and spread forces and the tension on the warps and otter pendants. The equations of forces were derived and substituted into the governing equations of 6 degrees of freedom motion, then the second order of differential equations to the otter boards were established. For the stable numerical integration of this system, Backward Euler one of implicit methods was used. From the results of the numerical calculation, graphic simulation was carried out. The simulations were conducted for 3 types of otter boards having same area with different aspect ratio(${\lambda}=0.5,\;1.0,\;1.5$). The tested gear was mid-water trawl and the towing speed was 4k't. The length of warp was 350m and all conditions were same to each otter board. The results of this study are like this; First, the otter boards of ${\lambda}=1.0$ showed the longest spread distance, and the ${\lambda}=0.5$ showed the shorted spread distance. Second, the otter boards of ${\lambda}=1.0$ and 1.5 showed the upright at the towing speed of 4k't, but the one of ${\lambda}=0.5$ heeled outside. Third, the yawing angles of three otter boards were similar after 100 seconds with the small oscillation. Fourth, it was revealed that the net height and width are affected by the characteristics of otter boards such as the lift coefficient.

• Journal of the Korean Society of Fisheries and Ocean Technology
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• v.22 no.4
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• pp.49-55
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• 1986

The authors carried out an experiment to determine the horizontal opening of the mid water trawl gear, which is the same as was used in the foreward experiment concerned to the attitude and opening efficiency of otter board. The trawl net .used in the experiment was designed to have a square sectional four-seamed body with two three-sepenlted wings. Each wing tip was rigged with a net pendant of 70 m long and the fore ends of the pendants were directly jointed to the otter pendant without inserting hand rope, and a front weight of 200 kg weigh was rigged with in front of the lowest wing tip. To determine the opening between otter boards ani between wing tips, two 50 KHz fish finders were set sideways on the otter board and on the port middle wing tip so as to detect the distance froa the finders to the starboard otter board and to the wing tip respectively, and the informations were transmitted to the indicators onboard the ship through water-tight cables. The results obtained can be summarized as follows: 1. The openings between otter borads were 43-45 m in case of the warp 100 m, 53-55 m in the war;> 150 m and 54-59 m in the warp 200 m. It was increased in linear according to the increase of towin; speed in the range of 1. 1-1. 8 mlscc. 2. The determined values of the openings between otter boards were greater than the estimated values. which were calculated by the opening between warps at the gallows of the ship and the length of tt~ warp, assuming that the warps from the top rollers to the otter boards were straight. The rates of the determined values to the estimated values were 1. 23-11. 0 in case of the warp 100 m, 1. 23-1. 12 in the warp 150 m and 1. 23-1. 15 in the warp 200 m respectively. The rates were almost the same at the slow towing speed of 1. 3 mlscc or so, then the higher the speed, the less the rate, and the longer the warp, the greater the rate. 3. The rates of the openings between otter boards to the total length of net pendant and the net (excluded cod end) showed 43-43% in case of the warp 150-200 m. It was a little smaller than th~ standard. which is 50%. in the common bottom trawl. 4. The determined openings between wing tips were 18-19 m in case of the warp 100m, 21-22 m in the warp 150 rn, They showed that the higher the speed, the greater the openings even though its range was no more than 1 m or so. 5. The rates of openings between otter boards to th~ openings betwee:l wing tips were almost invariably 38%. 6. The rates of openigs b2tween otter boards to the lenth of h~ad rope were 60-65% in cas': of th~ warp lOa-150m. It were much larg2r than th~ standard. which is 50%. in the common botto:n trawl.

• Journal of the Korean Society of Fisheries and Ocean Technology
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• v.27 no.4
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• pp.286-292
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• 1991

The authors propose to use the vortex generators in order to improve of the efficiency for the cambered otter boards. The equipments and testing method of this model test was the same as the previous report. This study was tested for 6 models such as the single cambered, the V-shaped cambered and the slotted cambered otter board without and with vortex generators. The results obtained are as follows: \circled1 C sub(L) of the single cambered model otter board with vortex generators was increased about 10% in comparison with that of model without vortex generators, C sub(D) decreased 2%, and L/D increased 5~20%. \circled2 L/D of the V-shaped cambered model otter board with vortex generators was increased 10~20% in comparison with that of model without vortex generators. \circled3 C sub(L) of the two slotted cambered model otter board with vortex generators was increased about 20% within an angle of attack 25$^{\circ}$ in comparison with that of without vortex generators, C sub(D) increased 5~20%, and L/D was higher than prototype within an angle of attack 20$^{\circ}$. \circled4 The separation point of the model otter boards with vortex generators was removed back ward a little in comparison with that of the model without vortex generators. \circled5 Flow speed difference of the back side to the front side of model otter boards with vortex generators was increased a little in comparison with that of the models without vortex generators. \circled6 The size of separation zone in case of the model otter boards with vortex generators was decreased about 10% in comparison with that of the models without vortex generators.

• Journal of the Korean Society of Fisheries and Ocean Technology
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• v.27 no.4
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• pp.278-285
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• 1991

The authors propose to use the slot system in order to improve of the efficiency for the cambered otter boards. The experiment is divided into 2 parts, one is the efficiency model test, and the other is the visualization model test. The hydrodynamic characteristics of the model otter boards were tested by efficiency model test to measure the shearing, drag force of the models and visualization test using hydrogen bubble method to observe the streak-line and time-line of flow around the models, and milk spout method to observe the separation zone in the wake behind the models. This study tested for 5 models such ad without slot, slot position 0.2C, 0.4C, 0.6C and 0.8C. The results obtained are as follows: \circled1 The maximum C sub(L) of model otter board with slot position 0.6C in attack angle 27$^{\circ}$ was the highest of all models, it's value was 1.59. \circled2 In general, the L/D ratio of the one slot otter boards were 16~28% higher than otter board without slot. \circled3 The slot position 0.6C was better than any other slot position, and it's conformed by visiualization. \circled4 As to the model otter board with slot position 0.6C, flow speed of the back side was faster 1.3 to 1.7 times than in the front side. \circled5 The size of the separated zone in case of the model otter board with 0.6C was smaller than that of any other models.

• Journal of the Korean Society of Fisheries and Ocean Technology
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• v.22 no.4
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• pp.41-48
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• 1986

To investigate the midwater trawl gear available for the Korean near sea trawlers, the authors carried out a field experiment on the attitude and the opening efficiency of the otter board with the Pusan 404 (160GT, 750ps), a training ship of National Fisheries University of Pusan. The experimental trawl gear was designed to be operable by the ship and the otter board was made of single iron plate with 12% camber ratio. The special-prepared potentiometric angle detector was used for determining the attitude, and the 50 KHz fish finder for the opening between the otter boards. The results obtained can be summarized as follows: 1. The angle of attack varied by moving the towing point on the towing plate of the otter board. It showed 33 to 36 degrees when the point was set at the outmost position, 25 to 31 degrees at the middle position and 19 to 30 degrees at the inmost position, with a decreasing tendency according to the increase of towing speed in each cases. 2. The heel of the otter board always occured inwards, increasing slightly according to the increase of towing speed. 3. The tilt of the otter board always occured outwards, increasing slightly according to the move of the towing point inwards, and decreasing slightly according to the increase of towing speed. 4. The opening between otter boards showed the largest value when the towing point was set at the outmost position.

• Korean Journal of Fisheries and Aquatic Sciences
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• v.34 no.3
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• pp.238-244
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• 2001

A method of estimating the fluid drag of a fishing gear and otter board spread in a midwater trawl on full scale was described by implementing a three-dimensional semi-analytic treatment of the towing cable (warp) of a trawl system with the field experiments obtained with the SCANMAR system. The shape of hand rope, bridle and float(or ground) rope attached behind otter boards in a horizontal plane was assumed to be of form $y_r=Ax_r^B$. The distance between otter boards (otter board spread) obtained by the three dimensional analysis of a towing cable must be equal to that obtained by the functional equation of the shape of ropes behind otter boards, The angle of attack of ropes which can be obtained from the functional equation enables one to estimate the fluid drag of trawl net (net drag) by subtracting the fluid drag of the hand rope and bridles from the drag component of the tension of hand rope attached just behind the otter boards.

• Journal of the Korean Society of Fisheries and Ocean Technology
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• v.34 no.3
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• pp.294-301
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• 1998

Towing performance of a midwater trawl system was examined aboard the training ship KAYA(2900ps) at the East Sea using the midwater trawl gear that had been designed and manufactured in accordance with the vessel. In this experiment, the trawl system data, the towing speed, the length and tension of the warp, net mouth height, and the depth of otter boards and net were measured and analyzed. The results are as follows: 1. In case of heaving in the warp with constant towing speed, the tension was suddenly increased and then again was reduced and after returned to the original steady state tension. At this time, net height was reduced a bit by ascension of ground rope, but returned to it’s original value. In the case where the warp was paid out, the tension was suddenly decreased and after increased and then returned to the tension of the original state, and the net height was greatly increased instantly by the sinking of the ground rope and then returned to the steady state 2. In the case of increased towing speed mm constant warp length, the tension was increased, and reducing the net height, the gear depth was decreased. On the other hand, in the case where towing speed was reduced, the tension was reduced and the gear depth and net height was increased. 3. Otter boards show a swing motion in the scope of 5~ 10m continuously. Otter boards responded to the state change of the trawl system at first, and then the motion of the net appeared. 4. The depth of net center was about 20m deeper than that of the otter boards, it shows about 0.4 times the warp length at the 4knots towing speed.

• Korean Journal of Fisheries and Aquatic Sciences
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• v.20 no.6
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• pp.484-488
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• 1987

The model experiments were performed in tile circular water tank on the simple cambered and the super-V otter boards attached with the slotted fowler flap at the trailing edge in order to develop more efficient shearing characteristics. The dimension of the model otter boards was varied slightly in the flap chord ratio $0.20\~0.22$ and in the area $432\~426cm^2$ in accordance with the flap angle $30\~50^{\circ}$. The maximum shearing coefficient $C_L=1.78$ and hydrodynamic efficiency $C_L/C_D=4.0$ in the superV type were higher than their efficiencies $C_L=1.75$ and $C_L/C_D=3.7$ in the simple cambered type. As the shearing forces of the otter boards with flap were increased $20\~30\%$ mere than these without flap in spite of increasing the drag and the instability. The effect of flap should be fully investigated for the application.

• Journal of the Korean Society of Fisheries and Ocean Technology
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• v.24 no.2
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• pp.78-82
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• 1988

Near sea trawlers of Korea sometimes catch pelagic fishes like file fish by using midwater trawl gear even though usually catch bottom fish. It is reasonable to use the specific otter board as well as specific net in bottom trawling and in midwater trawling respectively. But, the trawlers are so small ranging 100 to 120GT, 700 to 100ps that it is very complicated to use different otter board for bottom trawling and for midwater trawling. The otter board for bottom trawling. is also used for the midwater trawling without any change even though the net is changed into the specific one. Although the otter board in the midwater trawling should be lighter than that for bottom trawling, to use otter board for bottom trawling directly for the midwater trawling without any change makes the net easily touch the sea bed and also make the horizontal opening of the otter boards be limited owing to the length of warp in the southern sea of Korea, main fishing ground of midwater trawling, which is 100m or so in depth. That is why the otter board for the midwater trawling should be made lighter than that in the bottom trawling, even if temporary. The authors carried out an experiment to achieve this purpose by attaching a large styropol float on the top of the otter board. In this experiment, underwater weight of the otter board was 630kg and buoyancy of the float was 510kg. To determine the depth and horizontal opening of the otter board, two fish finder was used. A transmitter of 50KHz fish finder was set downward through the shoe plate of otter board to determine the elevation of otter board from the sea bed, and a transmitter of 200KHz fish finder was set sideways on the starboard otter board to be able to detect the distance between otter boards. The obtained results can be summarized as follows: 1. The actual towing speed in the experiment varied 1.1 to 1.8 m/sec. 2. The depth of otter board was within 41 to 25m with float on the top and 45 to 26m without float in case of the warp length 100m, whereas the depth 68-44m with float and 74-46m without float in case of the warp length 150m. This fact means that the depth with float was 9-4% shallower than that without float. 3. The horizontal opening between otter boards was within 34-41m with float and 30-38m without float in case of the warp length 100m, whereas the opening was 44-50m with float and 37-46m without float in case of the warp length 150m. This fact means the opening with float was 10% greater than that without float in case of the warp length 100m, and 15% greater in case of the warp length 150m. 4. The horizontal opening between wing tips by using the otter board with float was 1m greater than by without float in case of the warp length 100m, whereas the opening by with float was 2m greater than by without float in case of warp length 150m. From this fact, it can be estimated that the effective opening area of the net mouth by using the otter board with float could be made 10% greater than by without float in case of warp length 100m, whereas the area with float 20% greater than by without float in case of warp length 150m.