A computer model was constructed that allowed two vessels involved in a possible collision situation to take collision avoidance action following the "International Regulations for Preventing Collisions at Sea". The mariners’ actions were modelled by the concepts of the domain and the RDRR (Range to Domain/Range-rate). The domain was used to determine if a vessel was threatening and the RDRR to determine the time at which a vessel should give-way to a threatening target. Each vessel in the simulation had four domains corresponding to the type of encounter in which the vessel was involved. Values for the time at which a vessel manoeuvres and the domain radii were determined from an analysis of high quality cine films of the radar at H.M. Coastguard at St. Margaret's Bay, Dover. Information was also taken from simulator exercises set up on the Polytechnic radar simulator. The two ship encounter was then developed to become the multi-ship encounter and eventually was able to model over 400 vessels over a two day period through a computer representation of the Dover Strait. A further development included a computer graphical representation of a radar simulator running in real-time, and which allowed a mariner to navigate one of the vessels using computer control. A validation of the computer model was undertaken by comparing the simulated results with those observed from the cine films. Following the validation several examples of the computer model being used as a decision support system were included.