A singly-nonorthogonal coordinate system for Reynolds equations is derived in order to perform the calculation of the flows around ship hulls precisely and economically. In this coordinate system, i) the angle between X^1 and X^2 axes on the body surface or ii) the angle between X^1 axis on the body surface and X^3 axis taken as the distance from the surface, need not to be orthogonal, but the others are perpendicular to each other. Using this coordinate system, boundary layer calculations are performed around a ship model which hull form is represented mathematically, and these results are compared with experimental data.