Abstract: According to the feature of the displacement ship S stern form，the mathematical equation of the dead wood was proposed，and the characteristics were defined，a series of dead wood forms were designed.Combining the Kijima’s method of calculating hydrodynamic forces with the effective aspect ratio，the effect of the dead wood on hydrodynamic forces of the hull was explored， and an evaluation method for the effect was provided.By using the separate maneuvering prediction method，the dead wood’s effect on the maneuverability was calculated.The results show that the adding of the straight-line stability is proportional to the area of the dead wood.