Conclusion
This paper developed and verified the collision ratio as an index to determine the time at which to begin a collision avoidance maneuver, and the following four conclusions can be made.
First, the risk of collision is evaluated using the collision ratio with three VIDs. Each of the domains, such as 1.0×Lpp, 2.4×Lpp and 3.6×Lpp, are related to the collision, the safety of the privileged own ship and the safety of the burdened own ship, respectively. The nonlinear tendency of the collision ratio according to the approach of the own ship to the target ship is consistent with another researcher’s opinion on the risk of collision at sea. It is therefore possible to infer the time at which to begin the collision avoidance maneuver according to status of the own ship using the collision ratio.
Second, the criteria for the collision avoidance maneuver are determined according to the characteristics of the collision ratio. If the own ship has the duty to avoid the target ship, the own ship begins the collision avoidance maneuver when the collision ratio becomes 0.33. On the other hand, if the target ship has the duty to avoid the own ship, the own ship begins the collision avoidance maneuver when the collision ratio has reached 0.66. The applicability of these criteria is verified by comparing the minimum relative distance between the own ship and the target ship.
Third, the maneuverability of the own ship can be considered in order to infer the time at which to begin the collision avoidance maneuver when the collision ratio has been used. The deduced times match well with the initial track reaches for the assumed vessels with different maneuverabilities. If the own ship can easily deviate from the original course, the time at which to begin the collision avoidance maneuver becomes delayed.
Fourth, the critical collision condition, including the slower ship’s dilemma, can be considered for the time inferred using the collision ratio. Basically, the encounter situations that have been designed represent critical collision conditions, and there is no problem in predicting the risk of collision for all conditions. The unusual risk of collision for the slower ship’s dilemma was proven to be predicted using the collision ratio.