In this article, a novel ship domain model termed intelligent quaternion ship domain (IQSD) has been proposed by effectively considering ship maneuverability and human factors. Unlike previous ship domains, the proposed IQSD is much more dependable and flexible for navigators to make decisions. The key characteristics are as follows
the domain size is determined by the quaternion including 4 radii, i.e., fore, aft, starboard, and port, which sufficiently take factors affecting the domain (i.e., ship maneuverability, speeds and courses, etc.) into account;
the domain shape is modeled by another parameter k, which makes the IQSD more flexible because the ship boundary could be not only linear or nonlinear, but also thin or fat; 3) furthermore, the navigator states including skill ability and physical and mental states have been effectively modeled by a fuzzy system, which determines the shape parameter, k. To reasonably relate the IQSD to potential applications, i.e., collision risk assessment, collision avoidance and trajectory planning, etc., a generalized IQSD (GIQSD) with fuzzy boundaries has been developed by using fuzzy sets. As a consequence, the GIQSD would become more practical and convenient for applications because uncertainty and fuzzy information has been merged into the GIQSD. Furthermore, concepts of longitudinal and lateral risk based on the GIQSD have been defined to estimate the spatial collision risk (SCR) for ships encountered. Finally, comprehensive simulations have been conducted on various encounter situations and comparative studies with typical ship domains have been intensively analyzed. Simulation results demonstrate that the IQSD is more effective and flexible than previous ship domains, and the intelligent SCR based on the GIQSD is reasonable and dependable.