The determination of sailing ship’s behavior accounting for the Collision Regulations plays an essential part in the choice of a decision procedure and the verification of its effect in navigational decision support systems. One of the basic aims of such systems is to provide procedures defining the direction of course alterations and/or speed accounting for the regulations in force. There are a number of publications on the subject. Based on the performed analysis of the collision regulations and on navigators’ and sailors’ knowledge the authors have developed an anti-collision algorithm for close quarters situations in the open sea in good visibility