Judging the distance between one's own ship and other ships is more difficult at sea than on land, because landmarks or reference objects are scarce at sea. Although we can now measure the precise distance to an encountered ship using nautical instruments, such as a radar distance indicator or the automatic identification system, ship crews occasionally judge distances by visual observation, particularly in the case of obstacle avoidance. These estimated distances are typically used for making decisions regarding collision risk, and errors might be influenced by experience. In this study, we evaluate errors in the distance to encountered ships estimated by visual observation. Estimated distances were more precise for experienced crew than were those for trainees. There were tendencies to overestimate the distance to encountered ships with increasing ship size, and to underestimate the distance with decreasing actual range. The colour of the ship did not affect the estimated distance. These results may contribute to ensuring safe ship operation.
