Recently enormous amount of data can be obtained from running ships, so that the needs of the analysis of the data are increasing. For the analysis, the relationship among ship speed, engine revolution and engine power in calm sea is important. In order to estimate the relationship accurately, data in calm sea distributed widely in terms of ship speed are necessary, but ship speed does not change much in some voyages. In the paper, The method of correction for the displacement using Admiralty coefficient and weather correction method is explained to obtain more accurate relationship among ship speed, engine revolution and engine power in calm sea. Using the methods, the voyages of different displacements are investigated and the range of application of Admiralty coefficient is revealed.