Damping effects, the basic energy-loss mechanisms in practical structures, are seldom fully understood. Consequently it is not usually feasible to determine the damping coefficient by means of the corresponding generalized damping expression. For this reason the damping in most structural systems must be evaluated directly by experimental methods. Especially for a ship structure vibrating in the random ocean, the system damping becomes more complicated. In this report, by application of the stochastic modelling technique on the time series, of which the structural response signals were picked up aboard three different types of ship in sea trials, four statistical mean curves of damping ratios versus frequencies of the ship hull-girder are presented. The analyst may referto these curves to choose reasonable values of the frequency-dependent damping ratios in solving his problems of ship structure dynamics. Key words." ship hull-girder, vibrations, data acquisition and processing, time series, ARMA stochastic modelling, damping ratios. NOTATION a0, a~, a2 Coefficients of 2nd order polynomial a, Discrete white noise ci Modal damping coefficient, mode i 1