The added resistance in regular head waves at low forward speed acting on a huge bulk carrier is evaluated by using three dimensional source technique with forward speed. The numerical results are compared with measured data and are also compared with the calculated value of slender ship theory. The added resistance at low forward speed predicted by three dimensional source technique is much better agreement with measured ones than that by slender ship theory. Using added resistance data in regular waves ship speed in a seaway is also evaluated. The evaluated ship speed in regular moderate seas based on slender ship theory is in good agreement with ones based on a measured added resistance. In rough seas, however, the estimated speed loss obtained by using slender ship theory is too high to use. This is because of the error of added resistance at low forward speed. On the other hand, speed loss in rough seas estimated by based on three dimensional source technique is fairly good.