A study on psychological aspects is made focusing on two different senses which fall upon a captain in navigation. One sense is called menace which originates in encumbrance for the freedom of the seas with not objects such as ships or islands but the mirror images of the captain's own ship in the boundaries formed by the objects. An analytical hypothesis is set forth to express the menace field by applying the image principle in hydrodynamics for an incompressible fluid. Some important examples of the menace fields are shown to assure the proposed hypothesis. Navigation surroundings are assessed in view of maneuverability of ships. Another sense is called danger how menace is coming or going away. The sense of danger is represented by the time differentiation of the menace field. It is discussed that the sense of danger relates strongly with fears for ship collision.