In order to maintain safe navigation, mariners are required to deal with given navigational situations. These are necessary tasks to be carried out, which constitute the workload for mariners. Accordingly, it is crucial for mariners not to get into situations where the workload is deemed to exceed the mariners' own competency. In particular, the lookout procedures for avoiding collisions with encountering vessels are affected by the number and movements of encountering vessels, which often increase the workload on mariners. Consequently, regarding the lookout procedures, deriving alternative measures for effective monitoring of encountering vessels in order to decrease the workload, could improve effectiveness of lookout by mariners with a view to achieving safe navigation. Measures for lookout consist of multiple actions such as visual observation and instrumental monitoring. However, experienced mariners judge the risk of collisions primarily based on visual observation, because they are well aware of the limitations and characteristics of information obtained from nautical instruments such as radar/ARPA. Therefore, in this paper, the authors focus on lookout especially by visual observation to derive such effective measures. As a result, by using the relation of the crossing angle of the courses and the ratio of velocity between one's own vessel and each encountering vessel, the authors propose that there is a specified area for lookout with a higher risk of collision. It is hoped that the proposed measure will provide a rational criterion for so-called “appropriate lookout” as a guide for mariners who need to improve their lookout competency.
