Navigation and Seamanship (1+1)

1.3.3. Dead Reckoning (DR)

Dead Reckoning (DR) is one of the four main divisions of navigation.

When the earliest mariners become sufficiently daring and skilled to venture beyond their known waters in which they could pilot their vessel, they developed dead reckoning as a means of keeping track of their position.

The term derived from ‘deduced’ or ded reckoning. The process by which a ship’s position was deduced or computed trigonometrically, in relation to a known point of departure.

Highly accurate modern charts permit solution by graphic methods rather than by laborious mathematics.

Dead Reckoning is the process of determining a ship’s approximate position by applying to its last well determined position a vector or a series of consecutive vectors 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 measurement without considering current.

The following are the key elements of DR

i. Only the true courses steered are used to determine a DR position.

ii. The distance used in determining a DR position

iii. A DR plot is always started from an established position, that is a fix or running fix.

iv. The effects of current are not considered in determining a DR position.

Though this is not an accurate method, a means of fixing the ship’s position is not always available, due to weather equipment failure, etc, under such conditions a navigator must rely on his dead reckoning for an indication of his position.