I. Introduction
THE increasing demands and complexity of subsea missions has called for the advancement of autonomous underwater vehicle (AUV) technology. The fine motion control of such vehicles necessitates a fault-tolerant system design. In particular, the thruster subsystem needs to be robust against unfavorable water conditions in which ventilation may occur. The lift and drag physics of the propellers may be used to derive a set of single or multiple state mathematical models for the thrusters [1]–[5]. The thruster servo motor controllers employed in the test vehicle can return shaft velocity and motor current draw information. These data were compiled with the measured thrust (via load cell) in order to develop some practical thruster models [6]. These procedures do not employ the measurement of the ambient water flow velocity [7]–[8], which would theoretically improve the accuracy of the model; however, the equipment to perform such experiments was not available, hence the practical approach.