The paper deals with the results of a research carried out on propeller models at the Water Tunnel of the Italian Navy in Rome to the purpose of studying the damping behaviour of the propeller in torsional vibrations. The reason of this research is due to the fact that, up to now, the methods for calculating the propeller damping coefficient are either empirical or theoretical and, as it is known, there were never made systematic experiments to check them. On the other hand, calculating such damping coefficient is very useful and important to the engine manufacturer, who usually deals with the torsional vibration calculation. The research was planned by Registro Italiano Navale in co-operation with Grandi Motori Trieste, with the Italian Navy and with the financial contribution of the Italian National Research Council. The research was carried out in three stages with a special apparatus having a cardan joint as exciting system and a set of interchangeable quill shafts as system for varying natural vibration frequencies. Propeller models of Wageningen B-series were employed; they were all 4-blade propeller models, except for two 3-blade ones. The tests were carried out by changing the amount of excitation, the frequency values, the advance coefficient and therefore the mean torque, the rotational speed and the geometrical characteristics of the propellers. The obtained results showed the parameters on which the propeller damping coefficient depends and the relating approximating functions for calculating such a coefficient for four-blade propellers. From a few calculations made with this method on existing propulsion plants there was found a good correspondence with the actual results. Nevertheless, this calculation method is to be considered, for the time being, as a rough one, because it needs a further investigation and checking by means of torsional vibration records.