In order to evaluated the response of motion of an arbitrary shaped body with a constant forward speed in waves, the hydrodynamic forces and moments acting on it have to be predicted accurately. The strip method has been usually applied to obtain these hydrodynamic forces acting on a ship shaped body, where the two dimensional hydrodynamical characteristics of each cross section are used. However, the strip method has some deficiencies arising from the fact that the effect of the forward speed can not be considered rationally, and that some three dimensional effects are not taken into consideration. The author developed the method to calculate the three dimensional hydrodynamic forces and moments acting on an arbitrary shaped body with a constant forward speed, in which the sigularities are distributed on its surface, with an intention of investigating the effect of the forward speed as well as some three dimensional effects on the hydrodynamic characteristics. For a half submerged elliptical model with a constant forward speed, the sigularity distributions of diffraction and radiation problems were determined by this calculus. Wave exciting forces, wave pressure distributions on its surface, added mass and damping coefficients were calculated by this method and compared with both the experimental values and ones calculated by the strip method.