Determination of an evasive combined z-maneuver with a known beginning

The method of determining of the combined Z-maneuver with a known start to avoid collisions with several ships is covered in the article. Three classes and 11 types of approaching of own vessel with a dangerous target are used when choosing maneuvers. These types are determined by the relative bearing and the aspect angle of the target. The domains of danger are formed at the targets when assessing the situation threat and maneuver searching. These domains have the shape of a circle. The center of domain is shifted of the target mass centre to bow on one third part of the domain radius. A two-dimensional spatial criterion is used to identify dangerous targets. Three phases (advance measures, permissible actions after the first phase, urgent actions for both vessels) are highlighted in the process of approaching of own ship with the dangerous target. The requirements of the COLREG are taken into account by increasing the domain radius of the most dangerous target for maneuver variants, which according to these rules should be avoided. The dynamics of the own vessel in the forecasts of course or speed alterations are taken into account by using approximate formulas. These formulas are used for prediction of the “weak” and “average” joint course and speed changes also, on the assumption of their independence. In complex situations the course and speed maneuvers are determined on the diagram of safe velocity vectors. A method for calculating diagram taken into account the dynamics of own ship, as well as variants of this diagram used to choose evasive actions, are proposed in paper. It is possible for a given value of a speed/course change to define realms of zones of safe course/speed values and deviation of the route before returning to the previous elements of the movement. The proposed method allows to simplify the selection of an effective variant of the combined Z-maneuver with obtaining information about the distance of deviation from the route in difficult situations. It is important in confined water areas. This method provides the ability to display a realm of zones of permissible maneuver variants and can be used in ECDIS in solving the tasks of ships collision prevention. The reliability of the method was tested on examples of collision avoidance in various situations.