Dead Reckoning (DR): Dead Reckoning is a method used to estimate the ship's position by projecting its course and speed from a known starting point. It involves calculating the vessel's position based on its course and speed over a given time, considering only the effects of the wind and current set and drift. DR positions are plotted on the chart at regular intervals to track the ship's progress.
Wind Effects: The wind can significantly impact a ship's course and speed. To account for wind effects, seafarers consider the vessel's heading and the true wind direction and speed. By plotting these factors on the chart, they can determine the ship's course made good (CMG), which is the direction in which the vessel is actually moving over the ground.
Current Set and Drift Effects: Current set refers to the direction of the tidal or oceanic current, while current drift is the speed at which the current is flowing. These factors can cause the ship to deviate from its intended course. To calculate the effects of current set and drift, seafarers use tidal stream atlases, tide tables, and tidal stream charts. By comparing the vessel's speed, course made good, and the current set and drift, they can estimate the leeway (sideways drift) and apply appropriate corrections to the DR position.
Fix Position: A fix position is a more accurate determination of the ship's location by combining various sources of information. To obtain a fix, seafarers use visual observations, radar fixes, celestial navigation, or electronic position fixes from GPS. By plotting these fixes on the chart, seafarers can obtain a precise representation of the ship's position at a specific moment.
Throughout the voyage, seafarers continuously update the vessel's position by combining DR positions, wind effects, and current set and drift effects, as well as fixes. This allows for a more accurate understanding of the ship's location and assists in maintaining the desired course.
