When Wing in Ground Vehicles （WIG Vehicles）, Seaplane take off on water, the law of motion may be very complicated, because of the effect of propeller thrust, hydrodynamic force and the aerodynamic force. The situation may be more complicated, if considering the gust, surf, and the skill of the pilot. The assumption that the wetted area of the fuselage is negative proportional to the velocity is introduced in the kinematical equations. Accordingly, the computation of the takeoff distance is simplified, and the relational graph between the propeller thrust and the takeoff distance can be draw. The simplified method can be used to help selecting the engine model or to evaluate the takeoff distance.