Dead reckoning is the process of determining a ship's approximate position by applying to its last well-determined position representing the run that has since been made, using only the true courses steered, and the distance run as determined by log, engine revolutions, or calculations from speed measurements, without considering current. By using the course and speed vectors ahead of the present position, the ship's predicted DR position for any desired time can be determined.
Dead reckoning is carried out as a vessel advances along its passage. It can be done in advance as the planned or projected plot of the movements of the vessel at a later time.
The elements of dead reckoning are, only the true courses steered are used to determine a DR position.
The distance used in determining a DR position is obtained by multiplying the engine speed by the time involved in the run.
A DR plot is always started from an established position, a fix or running fix. The effects of current are not considered in determining a DR position. It is very important to maintain a accurate DR.If a ship made good the exact course and speed ordered, and there was no wind or current, dead reckoning would provide a accurate position. However, these conditions rarely exist, a DR position is only an approximation of the true position, and the need for maintaining a constant and accurate dead reckoning plan should be obvious. A navigator must know his position, or approximate position, to determine when to make turns, to predict the time of sighting lights or other aids to navigation, and to identify landmarks.
Dead reckoning is done on a chart or plotting sheet for the area the ship is steaming.
Some of the terms used in dead reckoning must be defined,not all books on or myself use the same navigation terms and definitions.
Heading (Hdg.). The horizontal direction in which a ship points or heads at any instant, in angular units, clockwisse from 000° through 360°, The heading of a ship is also called ship's head.
Course (C). As applied to marine navigation, the direction in which a vessel is to be steered, or is being steered; the direction of travel through the water. The course is measured from 000° clockwise from the reference direction to 360°. Course may be designated as true, magnetic, compass.
Course Line. In marine navigation, the graphic representation of a ship's course, normally used in the construction of a dead reckoning plot.
Speed (S). The ordered rate of travel of a ship through the water; in knots.
Fix. A position established at a specific time to a high degree of accuracy. A running fix is a position of lesser accuracy based in part on present information and in part on information transferred from a prior time.
DR Position. A position determined by plotting a vector or series of consecutive vectors using only the true course, and distance determined by speed through the water, without current.
Estimated Position (EP). The more probable position of a ship, determined from incomplete data or data of questionable accuracy. In practical usage it is often the DR position modified by the best additional information available.
Dead Reckoning Plot, called DR plot. In marine navigation it is the graphical representation on the nautical chart of the line or series of lines which are the vectors of the ordered true courses, and distance run on these courses at the ordered speeds, while going from a fixed point. The DR plot starts at a fix or running fix; it is labeled as to courses, speeds, and times of various dead reckoning positions, usually at hourly intervals or at times of change of course or speed. A DR plot properly represents courses and speeds that have been used; a plot may be made in advance for courses and speeds that are expected to be used.
Estimated Time of Departure (ETD). The estimate of the time of departure from a specified location in accordance with a scheduled move to a new location.
