This paper deals with the U-type hydrodynamic vibration damper (it is called 'U-type HVD' here) and the spring and hydrodynamic type dynamic damper (it is called 'SH-type DD') for reducing ship vibration. The former damper was developed by the NPL, and the latter one was devised by the authors. The damper effect of two kinds of the dynamic dampers was investigated experimentally and theoretically. The authors obtained the follwing conclusion. (1) The U-type HVD has the higher natural frequency, the lower is its air room height or free surface height. (2) The SH-type DD has two natural frequencies, in which the first order natural frequency originates from the coil spring vibration system, and the second one is from the air water vibration system. (3) The natural vibration of the SH-type DD can be analysed theoretically by using equivalent vibration model with two degree of freedom. (4) The authors obtained theoretical results of damper effect of hydro-dynamic dampers, which agrees with experimentl results, using the energy method.
