中型船用ダンフォースアンカーの把駐性能及び把駐限界に関する研究

摘要

長崎大学水産学部の附属練習船「鶴洋丸」（155GT） （以下、「本船」とする）は空中重量125kgのダンフ ォース錨を搭載している。ダンプオース錨は日本で は昭和30年ごろから使われはじめたとされるが、扱 いやすく他の錨と比較して把駐係数が大きいという 特徴から主に小型船舶で使用されている。沖停泊中、 本船はこれまで気象・海象の影響により度々走錨す ることがあった。錨泊に当たり、常に錨による把駐 力が、風や潮流など自船に加わる外力よりも大きい 必要がある。%Kakuyo Mam (155GT, hereinafter referred to as "the Vessel"), a training ship of Nagasaki University Facility of Fisheries, which was completed in FY2004 and is equipped with two Danforth anchors, whose weight in air is 125 kilograms each. Due to the impact of weather and oceanographic phenomena, the Vessel has often dragged its anchor when anchoring. While anchoring, the anchor watchman has to make sure whether the holding power of the anchor of the Vessel surpasses external force by wind or tidal current. Another point is that the anchor watchman needs to know the holding limit of the anchor against wind or tidal current. In this study, the projected area above and below the water line of the Vessel was calculated from drawings, and also the external force by wind or tidal current was computed. Meanwhile, the tension acting on the anchor as well as wind and tidal current at the time of anchoring under rough weather was measured. Also, the maximum holding power of the anchor was measured by letting the Vessel drag its anchor, and a subsequent state of dragging anchor was reemerged. In addition, a dragging experiment using the soil of the seabed at the anchorage and a model anchor was conducted, and the variation of holding power due to the difference of the nature of the seabed was examined. As a result, certain findings were obtained and are reported here. (1) The holding coefficient of Danforth anchor varies depending on the nature of the seabed and is approximately proportional to the median particle diameter of the ground of the seabed. (2) In the model experiment, even after once the dragging anchor started, a half of the maximum momentary holding coefficient of the anchor can be expected as far as the anchor is biting the ground. (3) In performing a measurement under rough weather, relative force of wind and tidal current was estimated as approximately 2 tons at the moment when the largest tension (0.7 tons) acted on the anchor. The one-minute mean tension at that time was 0.25 tons, which was roughly in accordance with the calculated value.