Theoretical And Experimental Research On Roll Damping Of The Displacement Type Vessel

The ship sailing in waves, it will produce a variety of movements because of the wind,waves and other disturbances. The roll motion is harmful to the safety of the ship. Rolldamping is an important influence factor to the roll amplitude, but the researchers have notfound a common method to estimate the values of roll damping of all ships yet, so the rolldamping characteristics and the forecasting methods research is a very significant work.This paper, based on two displacement type vessels model test under different speed,different appendages, studied the estimation methods of ship roll damping and roll dampingcharacteristics. There are two main methods to be applied to estimate roll damping, one isship model testing method, and the other one is empirical formula. At the same time, theaccuracy of the roll damping solution is verified and the characteristics of the roll dampingare analyzed under different speed and different appendages. Through the analysis and studiesof model test program data we find that: the contribution of the viscous damping, andsummarized the range of the values of the critical damping rate. For displacement type vesselwithout bilge keel the range is from2~4%, including the bilge keel the range takes7~9%,with the full appedanges the range is around10%; Investigating the contribution of differentappendages to the viscosity roll damping contribution, we find that the bilge keel contributionis the biggest, the second is the contribution of the rudder, and the contribution of shaft sheathis relatively small; The roll damping coefficient with speed is much larger than the situationwith zero velocity, the higher the speed, the greater the roll damping coefficient; The rolldamping coefficients of non-straight-walled hull are greater than straight-walled hull, and therate of the linear damping contribution is also greater than the straight-walled hull; Theinitial angle will be a factor to influence the damping values, the larger the initial angle is, thelarger the roll damping coefficient is; By the comparation of the regular waves roll motion wefound that the prediction results is in a good agreement with the experimental results, whichprove that the discussed methods to estimate roll damping in this article are available.The paper used the instantaneous roll angle to get the roll damping to simulate the rollmotion in time domain. For the free roll decay motion, the simulating results are comparedwith the experiment dates, it showed that in the zero-speed case, there are some differences,but there is a good fit in the speed cases. For roll motion in the regular waves, the simulation results are convergent; through the comparison of the results frequency domain and the resultsof the time domain we found that there are differences at the peak frequencies. So we need todo more researches on it.