# Factors Influencing the Action Point of the Collision Avoidance Manoeuvre
## 4 Discussion and Conclusions  
On the basis of the results of the statistical analysis, it can be stated that the navigator's decision-making process in encounter situations is influenced by such factors as:  
* Type of sea area
* Relative speed of the ships
* Size of the target ship  
As the presented results were obtained on the basis of data collected for one ship, in order to build a more comprehensive model of the navigator's behaviour in the encounter situations, it is recommended to repeat the research for other types and sizes of ships. One of the most interesting observations is that majority of the anti-collision manoeuvres start at a distance greater than the assumed in the literature [10,21,22] as the distance at which the give-way vessel should start the manoeuvre. This is mainly due to the development of navigational equipment such as radars and AIS that allows navigators to obtain information about the situation from long distances. The average distance between ships when the manoeuvres started was 9.4 NM (respectively, 10.3 NM and 7.6 NM for open and restricted sea areas). It was observed that over 70% of the manoeuvres started before the distance between the ships decreased to the assumed 6 NM limit (77% for the open sea areas and 58% for the restricted areas). The behaviour of the navigators in the pre-COLREG situations should be subjected to more detailed analysis in the future. The S-W test confirmed that if the situations are analysed separately for the open and restricted sea areas, it is possible to use the normal distribution to model the distances between ships at the start of the anti-collision manoeuvre. Further tests showed that the Weibull is the best fitted theoretical distribution. The results achieved for the analysis of the TCPA value for all situation at the start of the manoeuvre showed that most of them start between 15 and 25 min before CPA is reached. The average TCPA for all analysed situations is 26.7 min. The manoeuvres in open sea areas started earlier, the average TCPA was 30.1 min and for the restricted sea areas the average TCPA was 19.7 min. On the basis of statistical tests, it is not possible to definitely indicate the best fitted theoretical distribution. In further analysis, log-normal distribution should be considered. Figure 4: Relation between length of the target ship, TCPA and the distance between ships at the start of the manoeuvre. The second stage of analysis was focused on the assessment of the influence of relative speed and the size of the target ship on the decision-making of the navigator. The results show that the influence of relative speed is significant both for the distance and TCPA at the start of the manoeuvre. For the relative speed not higher than 10 knots, the average manoeuvre started at a distance of 4 NM and when the TCPA was ca. 28 min. For the relative speed between 10 and 25 knots, the average distance at the start of the manoeuvre was about 8 NM, and for the relative speed higher than 25 knots the average distance was about 12 NM. It can be stated that the higher the relative speed the lower the TCPA at the start of a manoeuvre. On the basis of the results, it can be assumed that for navigators the distance is a factor that has a greater influence on the decision of manoeuvre start than the TCPA. The results achieved for the size of the target ship shows the correlation between the length of the target ships and the distance between ships at the start of the manoeuvre. It is especially visible for ships smaller than 100 m, for which the manoeuvres start at a distance almost twice as little than for the bigger ships. A correlation between the size of the target ship and the TCPA at the start of the manoeuvre was not observed. It confirms the observation that decisions about the manoeuvre's start is made on the basis of the distance between the ships rather than the TCPA. For the open sea areas, where there is more space and the traffic intensity is lower, the manoeuvres were started earlier.