Damping Resistances in Rolling, Pitching, and Vibration of a Ship in her Motion-ahead

Abstract

In his previous paper, the author obtained the coefficient of vibration damping of a plate that is parallel to a water stream, and movable normal to the same stream, with a theory regarding the dynamic damping of the vibrations of a ship in motion-ahead. Although the vibration damping of any ship increases almost linearly with speed, since the vibrational force usually varies as the square of the same speed, the amplitudes of the ship vibrations are apparently augmented with speed increase. Since, however, no data on vibration test of an actual ship was given in the previous paper, there has remained some doubt as to whether or not the damping condition in a full-sized ship would obey the law that the author formulated.In the present paper, with a view to confirming the results given in the previous paper, the author directs his attention to the results of the rolling experiments of the “Revenge” that was conducted about fifty years ago. Based on the data of his model, experiments with a plate vibrating in a circulation water tank that were published in his previous paper and also reexamined in this one, he predicted the damping coefficient of the rolling of H.M.S. “Revenge” in her motion-ahead (free of the actual coefficient of the vessel itself), from which he finds that, notwithstanding the large differences between the conditions of the vibrating model plate and the conditions that attended the rolling experiments of the “Revenge, ” in oscillatory frequency as well as in displacement amplitude, the damping coefficient of rolling of the “Revenge” predicted from the data of model experiments fairly agrees with the same coefficient that had been actually observed. The author thus concluded that the data of his model experiments can straightway be applied to the vibrations of a full-sized ship-a ship with the frequencies and amplitudes within the range of those model plates in the author's experiments. In extending his investigation to the case of the pitching motion of a ship, the reason for the pitching of a ship in her motion-ahead in a calm sea being damped more quickly than in her stand-still condition was ascertained. Since, as a matter of fact, the problem is merely qualitative, it was impossible for the author to give accurate values of the dynamic damping coefficients under consideration.