From several years ago, the free propeller (vane wheel), which is fitted at the rear of the main propeller, is going to be put to practical use for the purpose of the energy saving. The authors had a chance to measure the effect and the strength of the free propeller in both conditions before fitted and after fitted on the same full-scaled ship. From this data, it is found that ; a) the number of revolutions of the free propeller is 37.5% of that of the main propeller, b) the thrust force by the free propeller accounts for1417% of the total thrust force, c) on the blade stress of the free propeller, the mean value is 2.6 kg/mm2 and the peak-to-peak value is 8.8 kg/mm², and this value is permissible, d) the blade frequency components of the pressure fluctuations induced by the main propeller are reduced to 80% and the level of the hull vibrations is also reduced consequently.