船尾振動の発生原因に就いて

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In this paper, the author gives his view concerning the causes of the stern vibration of a ship, believing there are three, that is, (a) damping imposed on a ship in case of hull vibration (b) local vibration induced near the vibrating point at the stern of a ship, (c) the smallness of [mass×rigidity] of stern compared to that of amidship (a) It is theoretically proved in this paper that if there is resistance against vibration, the vibrating wave started from vibrating point is damped off on its way and that the amplitude decreases exponentially with the increase of the distance from the vibrating point. Since logarithmic decrement generally increases with the higher made of vibration, the stern vibration due to this cause occurs more easily, in the higher mades. with such a magnitude of damping as was obtained in former experiments, however, it is not good enough to cause the stern vibration such as we experience. If the stern vibration, therefore, be caused by this reason, it can possibly cause very large damping in higher mode of vibration in case of spiral cargo. (b) If vibrating waves sent forth from the vibrating point give rise to local vibration on the way of its propagation, the vibrating energy is adsorbed in the locally vibrating body and the vibration that is conveyed to farther part decreases. Especially, it is proved in this paper that if natural frequency of local vibration coincides with that of exciting force, there occurs no vibration in the distant part, total energy being absorbed in the local vibrating body. Loser's "Vibration neutraliser" corresponds to the case in which local vibrating body is at the vibration exciting point. The inherent frequency of the panel forming a ship is apt to be about 300〜500/m due to the effect of water in contact. Accordingly, the greater part of the stern vibration experienced by us seems to be due to this cause. Assuming, by the way, as frequency of local vibration=σ, frequency of the vibration of n th mode of the ship=ω_n frequency under the effect of local vibration =ω'_n, it follows that ω'_n<ω_n when ω_n≦σ ω'_n>ω_n when ω_n≦σ In case of ω_n≒σ, therefore, there appear two frequencies, and moreover, the curves representing the relation between ω'_n, and the degree of mode assumes two curves that jump discontinuously at n. And in case that a local vibration body is in the node of i th mode, there appears no effect of local vibration in the amplitude and frequency of the vibration of i th mode. The cause of the irregular form of the experimental curves that show the relation between frequency and number of nodes might be explained by local vibration. (c) At the stern, virtual mass in vibration as well as rigidity decreases rapidly. The author has theoretically solved in this paper the vibration in which two uniform beams of different mass and rigidity are connected each other and vibrating force acts at the free end. Consequently, it has been proved, as a matter of course, that the amplitude of the beam of small [mass×rigidity] becomes larger than that of large beam. This fact shows the possibility of stern vibration caused by the gradual decrease of [mass×rigidity] at the stern. with an actual ship, it has each of these three causes more or less, though any one of them is likely to play main part.
社団法人日本船舶海洋工学会の論文
1960-11-05

船尾振動の発生原因に就いて

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