For surface ships, course-keeping, speed changing and maneuvering involve primarily forces, moments and motions acting in all directions in the horizontal plane. For submarines, the third dimension also comes into play. Hydrodynamic motion forces and interactions acting on the vessel’s hull, rudder and other appendages are of first consideration and difficulty. However, it is important to recognize that the responses of a number of mechanical, electronic, environmental and, most importantly, human factors all influence controllability.