Pages 149-156
The longitudinal dynamic instability in calm water, porpoising, of a personal water craft is experimentally investigated at up to Fn=6.0. The criteria of occurrence of porpoising are predicted by using a linear stability theory. The porpoising motion is also estimated by using a non-linear time domain simulation method. The measured restoring, added mass and damping coefficients are used in these methods. The predicted criteria and the simulated motion are in good agreement with the measured ones. In order to reveal the cause of porpoising instability, the measured forces are analyzed. The results show that the coupled heave and pitch restoring coefficients have different sign and in the same order as the other coefficients in high advance speed range. This means that the porpoising is caused by a self-excited oscillation due to the energy exchange between heave and pitch motions.
