The Theoretical Prediction And Research Of Ship Motions

The vessel swing disturbed by the wave has been considered as an important physics phenomenon in that it directly influences the speed, deck wave, moment, sink and even other physics phenomena of the ship. It is the basic part of the seakeeping and most important to the study of the whole seakeeping. The method of theoretical research on the swing primarily relies on listing, analyzing and solving the differential equation of each different swing. It is quite difficult in math to estimate the accurate interaction between the ship and the fluid although some researchers have studied the swing of five or six degrees coupled in the inclined wave and have provided the homologous results. Due to the fact that some coupled items are relatively small on the quantity, in practice we tend to adopt a series of simplification hypotheses so that simple coupled swings could be handled.The rigid ship has six degrees when the navigating ship with constant speed U encounters the regular wave with the arbitrary angle μ. Each degree is coupled. And it is believed that pitch and heave motions are coupled, roll, sway and yaw motions are coupled, if the motion of surge is ignored. The former is called longitudinal motion while the latter is called lateral motion. This paper is intended to analyze and predict five degrees motions with the longitudinal coupled motion and lateral coupled motion. Firstly, I set up the coupled equation, and calculate the add-mass, damping coefficient, disturbed force and moment by linear theory in two dimensions, and then I can obtain the add- mass, damping coefficient, disturbed force and moment of the real ship in three dimensions by slice theory of Faltinsen, Salvensen and Tuck in two dimensions. The hydrodynamic coefficients are calculated by Lewis section method, and the wave disturbing load is obtained through Airy linear theory and scattered wave theory. The amplitude response and the acceleration response of several ships are calculated and analyzed. After that, I make a comparison between the results of the experiments and the theoretical calculation, and horizontally compare the calculated values of this article with the results of a few empirical methods, such as Kaunaly method, finite element method, Frank method and non-coupled experience formulas. Quite satisfying results are obtained. Finally the vertical and transverse accelerations and the statistic characteristics of the vessel swing are calculated. Besides, the paper makes a preliminary discussion of the nonlinear roll.