An Estimating Method for Stability Qualities of Ships in Confused Seas, including the Effect of Shipped Seawater
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概要
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This paper is concerned with the estimation of stability qualities of ships in rough seas subjected to steady winds and gusts, and influenced by irregular waves and shipped seawater. Analysing results of experiments on the wind force, apparent centre of wind pressure and apparent centre of water pressure with several ship models, new formulae for wind moments acting on a ship in the upright and inclined conditions were obtained.The ship is assumed to be rolling among non-fully developed irregular seas generated by winds of given mean velocity, the period of component waves of most predominant energy being equal to the period of roll of ship, and the expected maximum amplitude among 100 swings is taken as the standard absolute amplitude. After many kinds of resonance curyes of ships were utilized with Neumann's spectrum, a simple relation was found between this standard amplitude and the angle of roll of ship on imagined synchronous regular waves with heights equal to the expected maximum height among 50 irregular waves.The ship is inclined to an angle by the action of steady wind and rolls about that angle. She is assumed to be struck by a gust that has the velocity √<1.5> times that of the steady wind, when she rolls up to the maximum angle corresponding to the standard amplitude on the windward side relative to the effective wave surface.The effect of seawater shipping on the deck is divided into two kinds. The first effect is due to shipped water stagnating on the deck when she is struck by the gust, and the second one is due to water shipping on the deck when she is rolling leeward after the striking of the gust.Formulae for these two effects were determined by investigating stability qualities of a cargo vessel with minimum stability in her critical conditions.The work ratio R is defined as the ratio of useful reserve dynamical stability to the maximum kinetic energy of ship after being struck by the gust. The critical value of work ratio R_c of 1.53 was obtained from the fact that the torpedo-boat "Tomozuru" of 744-ton displacement was capsized in a storm of mean wind velocity of about 20 m/sec when she was acting together with other two sister ships. A ship may be judged to be stable in given rough seas when the factor of safety for stability Co which is shown by R/R_c is greater than unity.
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