This paper deals with real-time optimal motion planning for astable AUVs, and presents an approximate analytical solution for the optimal control problem of a symmetric astable AUV with symmetric thruster configuration. The results show that the motion along the target vector is optimal for such underwater vehicles. The numerical solutions are found to be in good agreement with the approximate analytical solution. A relationship between the number of thrusters and the energy consumed is derived analytically for forward motion. The effect of the number of thrusters on optimality is shown to be significant at higher speeds. In the case of omni-directional underwater vehicles, the number of thrusters plays major role in optimal motion planning. For monostable underwater vehicles, the approximate analytical solution can be used as a good starting point for numerical solution. © 2005 Elsevier Ltd. All rights reserved.