In the preceding report, the authors treated of the operational limitations during scouting work, on the basis of the characteristics of ship motion, by using an actual 135 GT purse seiner working in the fishing ground. In the present paper, the main objective was shifted to the same problem during a cycle of shooting and hauling work of the net. In order to examine this problem, the process of a cycle of fishing work was classified into the six steps, and the ship motions and accelerations were measured under the four sea conditions. From the results of spectrum analysis and statistical analysis of double amplitudes relating to rolling, the probability of deck-edge immersion was estimated, and the discussion was concentrated to the change in safety with progress of fishing work under respective sea conditions. And the discussion was extended to the safety of the crew on deck during fishing work.

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In the previous report, a method of estimating in-water image contrast using its black-and-white photograpf was revealed. The present study proposes a new method of estimation by using televised image from video tape. The televised image of a test board was captured by video-capture program and saved as file into memory unit of a computer. Using the Image 1.45 software, gray value of televised image was measured to estimate its contrast value Ci. From the relationship between contrast Ci and Cg. calculated from target and background gray values, a high correlation (r:0.95) was found. A relative equation between the Ci and the Cg was presented by the following equation : Ci=0.134 ln Cg+0.933. Using this formula, the application of the technique for small targets such as net set in a large sized water tank are discussed.

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浅海の砂底に設置した人工魚礁の埋没現象には底質の局所洗堀が深く関わっていると考えられる。模型実験により得られる結果が実際の海に設置した魚礁周辺で生じる洗掘現象に適用できるかどうかを確認するために,タイ国東部海岸のペイ湾に実験と同じ形状の魚礁を設置して比較検討した。用いた魚礁はこの地方で一般に用いられる3タイプと,新たに考案した四面体形状のものである。模型と実物との縮尺は20分の1とした。両者の結果を比較すると,洗堀の発達の過程はよく似ており,模型で生じる洗掘穴の深さと実物の埋没深さの魚礁形状による変化傾向も同じであった。実際の海における魚礁の埋没現象には向きと速さが日変動・季節変動する流れが深く関わっていると考えられた。

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岬先端で剥離する渦の発生周期と渦の吸い込み現象について検討した。剥離渦は自らスピンダウンしながら余剰エネルギーを生じ,その余剰エネルギーは底層水の吸い上げに消費される。剥離後の渦は非粘性のRankinn Combined Vortexとし,剥離時の岬の前面に発達する境界層厚さを1/7乗速度分布則およびプラントルの混合距離理論を用いて定式化し,吸い込み速度,発生周期について実験と比較して理論の有効性を確認した。

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The relationship between distribution of juvenile japanese littleneck Ruditapes phillippinarum and physical environment, especially bottom condition and water current, was investigated in west Mikawa-bay. Sediment samples were collected by Smith-McIntyre grab. These samples were sieved with lmm mesh screen and bivalves were picked up and measured shell length. The oxydised layer and the reduced layer of substratum were observed throughout the transparent corer, and then upper 10 cm of cored sediments were used for particle size analysis. The water current-of the bay was analyzed by the three dimensional hydrodynamic model. The juveniles always distributed more abundantly in the mouth of Yahagi river than in other stations. The thickness of oxidized layer in bottom sediment was more than 20 cm in the river mouth, whereas most of other station showed less than 10 cm. The landward water current in bottom and lower strata of the river was observed according to the numerical analysis. Thus, it was considered that the bottom condition and the water movement contributed to the survival of juveniles and the entrainment of planktonic larvae to the habitat, respectively.

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