One of the important goals of designing of planing boats is to achieve higher speeds, one of the major obstacles to this issue is longitudinal instability.
In this research, a new method is proposed to solve the longitudinal instability of steped planing boats. In this method, using the transverse curvature or variable deadrise angle of the body, the instability of boat has been corrected. Two bodies, called Nimala1 and Nimala2, were tested in National Iranian Marine Laboratory. The main difference between the two transverse curvature bodies is that Nimala1 has a constant deadrise angle and nimala2 has a variable deadrise angle in a section. To determine the effect of the variable deadrise angle on the longitudinal instability, each of the models has been tested up to longitudinal instability. Due to the equipment installed during the test of the model in the towing tank, it was not possible to measure the movement and therefore numerical analysis is used as a complement to the exprimental test. The results show that the variation in cross-curvature has a significant effect on longitudinal instability. By shifting the center of pressure to Aft, the delay of the porpoising was changed from 5.89 volumetric Fn in the fixed deadrise angle model, to 8.34 volumetric Fn in Variable deadrise angle model. Also, the resistance of this body is not significantly different from that of a body with constant transverse curvature, especially at high speeds.