Abstract
The time required to sail between any two points is dependent on the distance, speed of the ship and the effects of external forces owing to the weather, sea conditions, etc. Accordingly, to select the most economic and efficient course, the navigator has to form his judgement on the basis of synthetical data founded on the efficiency and the condition of the ship, safety of the course, positions of the two terminals of the course, and season. In this paper, the writer, relying on assumptions, offered some considerations on the difference of distance between great-circle and rnumb-line, which is an element to be considered in the selection of the most economic course.
