According to the COLREGs, every Officer On Watch (OOW) shall at all times maintain a proper look-out. Among all the available means of look-out, sight is the most basic and commonly used one. OOWs always need to make a full appraisal of the situation and of the risk of collision by sight, including estimating the range and bearing. Unlike the ranging equipment (such as Radar), OOWs, who are keeping a look-out by sight, cannot estimate the distance precisely according to their visual sense. In this paper, an experiment is carried out for finding the difference between the actual distance and the estimating one by OOWs with different onboard experience. The data are analyzed by using methods of statistics, Anomaly Detection, and Two-Step clustering for researching the human element of the visual sense during look-out. The factors, which can affect the visual sense, are revealed. Conclusions are drawn for improving the look-out and enhancing the safety of the navigation.