A nonlinear mathematical model capable of predicting loads and motions of high speed planing craft in the longitudinal plane has been developed. The development of the model is based on the mathematical model presented by Zarnick (1978). Following the principle of 2D strip theory and wedge water entry problem, a high speed planing hull is divided into a number of transverse sections. Sectional forces and motions are evaluated, and then, by integrating along the ship length, total force and moment are obtained which leads to corresponding instantaneous acceleration. By integration using a time marching scheme, velocity and displacement are obtained. The influence of the controlling parameters, such as number of sections and time step, on the accuracy and stability of the simulation in calm water, regular and irregular waves is investigated. The accuracy of the underlying mathematical model is investigated and the deficiencies identified. The optimum model is finally validated against the original model of Zarnick(1978) and the experiments of Fridsma (1969). An extension of the model to be capable of simulating roll motion is proposed and implemented. This extension may be useful when the prediction of high speed planing motions in oblique seas is proposed. The initial validation process has been carried out but subjected to the full validity of application. Moreover, as the original approach of the mathematical model was used in prediction of seaplane landing (Wagner, 1931), an additional aim and objective to the present PhD project is to find a novel technique to predict the loads on fuselage of an aircraft emergently landing (ditching) into the water. Experimental tests related to these simulations are planned and carried out in order to use their results as validation references to the modified mathematical model. The deliverable of the project is an analysis of optimization of the mathematical model capable of predicting loads and motions of high speed planing craft. As well as the implementation of capability of predicting impact loads and initial post-impact motions of aircraft ditching into the water.