To analyze the effect of winds and waves upon a ship's speed is a very important problem which is concerned with the selection of the course, estimate of spare fuel and estimate of a ship's position. But each ship being different from any other in type, size, condition, ability, etc., it is not possible to formular any univrsal rules. In this paper, the authors has attempted, for an example, to analyze navigational records of the fishing training ship "Oshoro-maru" of Hokkaido University. As shown in Fig.1, all data are separated into 7 sections by the directions of the winds. The data are 520 in all ; 74 on the average in a section. The calculation of speed reduction is by the difference between the standard speed 9.45 Kt and the speed shown by the P. log under each condition ; the standard speed is that shown by the P. log when the standard rotation is 255 RPM and the influences of wind, wave and swell are nil. In order to know the relation between the force and direction of wind and speed reduction, as in Fig.2, the speed reduction is plotted on the ordinate ; wind scale on the abscissa. As the speed reduction (Δ)=aB+bB^2, if a smooth curve is drawn along the points plotted by data, the speed reduction due to wind force is shown by forms (1)〜(7) in each section, as shown in Fig.3. The mean direction angle of wind (class mark in each section) is plotted on the abscissa ; the speed reduction on the ordinate. The calculated values by forms (1)〜(7) are plotted and if the smooth curve is drawn along the plotted points, that curve approaches closely to the curve of 5th degree shown in (8)〜(14). In order to get the relation between wave height and speed reduction, the authors took the following method for deducing wave height from wind scale, that is, they got the average value of the grades of wind waves against each wind grade in navigational records, and replaced the wind grade by the average value of grades of wind wave scale. The intermediate value between the grades of the scale is shown by the proportional defference. When the relation between wind scale and wave height in this observation is plotted, it is as shown in Fig.5. The curve of which the points plotted in this connection are made smooth by the curve of a quadratic equation, is expressed by equations (15). As shown in Table 1, the relation between wind scale and wind velocity is epressed approximately by equation (16), and the relation between wave height and wind velocity is expressed by equation (17). Using the relation among equations (15), (16) and (17), and relation in Fig.3, as shown in Fig.6, the relationship between wave height and the speed reduction is expressed by a simple equation.