A simplified, approximate method of power flow is proposed as a function of space averaged driving point mobilities and space averaged free vibration of machines. By using the calculated model, with beams which compose a machine and a ship hull, the characteristics of power flow are simulated. In paticular, it is shown that the vertical, horizontal and rotational component have nearly equal contributions, and the above mentioned simplified method is in agreement with the simulated results. In addition, power flow from a reduction gear to a machine room model ship is measured and the results are compared with the simplified method showing good accordance. As a convenient method, free vibration of the reduction gear has been estimated by the sound power level, thus demonstrating the effectiveness of this method.