A Study on Coupled Pitch and Heave Porpoising Instability of Planing Craft : 1st Report

Abstract

An experimental investigation was done on the calm water longitudinal dynamic instability at high advance speeds of a planing craft. Free attitude tests were used to detect the conditions causing porpoising and the motions involved. Experiments were done by systematically changing ship's weight, position of center of gravity and towing speed. It was found that porpoising is caused by heave and pitch coupling when their periods were the same and the porpoising limits varies with longitudinal center of gravity and ship's weight. By captive model tests, the influence of hydrodynamic forces on porpoising was studied. Experiments were done systematically changing angle of trim, draft and towing speed. From measured hydrodynamic forces, restoring forces of heave and pitch were calculated. It was found that the coefficients of heave induced pitch restoring forces are negative at high advance speeds. As a result, there is a possibility for parametric excitation due to transfer of energy between heave and pitch motions. Stability analysis was done, using Routh-Hurwitz stability criterion, to predict the porpoising limits. But, the results did not agree with the experiment.