船舶が追波、斜め追波中を高速で航行する際、最大限の操 舵努力でも保針できないブローチング現象が起こる危険性 はかねてより指摘されてきた。ひとたびブローチングが生 じると、大きな旋回角速度による遠心力により転覆に至る危 険性もはらんでいる。そのためこのブローチング現象は海事 関係者にとって無視しえない重大な脅威であると認識され ている。%When a ship runs in following and quartering seas, she could occasionally face broaching-to, which is a major threat of ship capsizing, or difficulty to keep her desired course due to wave-induce forces. It is noteworthy that usually a PID auto-pilot is used in model experiments and numerical simulations for investigating broaching-to but a PID auto-pilot is not so suitable for identifying operational limits. Thus, in this paper, the optimal control problem for ship maneuvering motion in following and quartering seas was discussed. As a result, while a proportional auto-pilot fails to keep the desired course, optimal control obtained here successfully keeps the desired course as a Bang-Bang control. If wave steepness further increases, even this optimal control may face a difficulty to keep her desired course and then broaching-to could be more rigorously identified.
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