Ship Domain in Restricted Waters : A study assessing Norwegian navigators´ perception of safe passing distance to a targeted ship in restricted waters
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Collisions at sea is a great threat to the navigational safety. To assess the navigational safety, navigators of ships make use of various criteria. The most commonly adopted ones are two proximity indicators called Closest Point of Approach and Time to Closest Point of Approach. Research suggests that these are insufficient for their intended purpose and generally require a lot of experience from any one applying them, especially in restricted and congested waters. A concept termed ship domain offers an intuitive alternative. The concept is concerned with defining a free space around a ship required for safe navigation and collision avoidance and thus it is a concept aiming at generalizing safe distance. The general problem is that the ship domain is heavily dependent on certain factors known to influence its shape and size, three of which are the type water area, relative bearing to an approaching targeted ship and the own ship´s size. This thesis have investigated the influence of these factors on the ship domain shape and size as perceived by Norwegian navigators in restricted waters in a quantitative, quasi-experimental, questionnaire-based study. It was found, with some caution due to a small sample size, that navigators perceived a ship domain in an increasing manner depending on ship size. The overall influence of ship size on the mean ship domain size could be approximated by a linear regression. The ship domain shape resembled a circle regardless of ship size. It was also found that the relative bearing of a targeted ship had an impact on a perceived safe passing distance. However, there was found no wider systematic differences corresponding to past research and hence some discussion follows this. Finally, future research should consider an empirical approach that considers actual ship navigation so that the results in this thesis can be confirmed. It is also recommended that future research considers a mixed method approach that allows probing for additional information, electing how navigators assess the ship domain.