Using a specially developed dynamometer, the hydrodynamic increase in inertia was determined for a variable-pitch propeller under load at various pitch settings and at various operating conditions. Tests also were carried out under "still-water" conditions. The results show the effect of amplitude of oscillation and propeller Reynolds number on the hydrodynamic inertia to be small. The influence of propeller load is also small over the normal range of operation. It would appear, therefore, that the hydrodynamic inertia can be determined to a reasonable degree of accuracy, by still-water tests with a nonrotating propeller.