Broaching is a phenomenon whereby a ship cannot keep a constant course in severe following and quartering seas with relatively high speed. Once this dangerous phenomenon happens, even a ship complying with the current Intact Stability Code (IS Code) of the International Maritime Organization(IMO) could capsize. This paper focuses on broaching of slender vessels with twin screw propellers and twin rudders because they are often operated in high speed range which make susceptible to broach. First, existing experimental results at the Haslar basin were explained by a mathematical modelling proposed by the authors and extended here to predict broaching of high-speed slender ships in severe following and quartering seas. Second, we discussed the effect of the fin-stabilizer (anti-rolling fin) with which high speed vessels are often equipped, on broaching. Then, we identified an optimal control gains to reduce danger of capsizing due to broaching. Effects of the fin size and its aspect ratio as well as the area of rudder were also investigated. The above results are expected to be used for designing high-speed slender ships with a fin-stabilizer set for avoiding dangerous broaching at seas
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