Sailor Speak of the Week – Dead Reckoning

Definition

Noun

• A navigation method using course, speed, and drift to determine one’s position.

Origin

Probably earlier than the 17th century. Believed to be a corruption of the term “deduced reckoning.”

Comments

Possibly one of the simplest methods of navigation, but not the most accurate and subject to compounding errors. It starts with a “fix” on a known location from which a vessel’s course and speed are plotted at certain intervals. For example, a vessel traveling on a course of 090 at 10 knots for 2 hours will have gone 20 nautical miles east after that period of time. (Distance = speed x time). Any currents or wind conditions affecting the vessel can also be taken into account. Of course, the problem with dead reckoning is that it’s dependent on your fixes being accurate and your speed being constant. You also need an accurate chronometer. It’s not so much an issue nowadays with electronic and satellite navigation, but it can still be used in a pinch.

References

King, D., Hattendorf, J.B, & Estes, J.W. (1997). A Sea of Words: A Lexicon and Companion for Patrick O’Brian’s Seafaring Tales (2nd Ed.). Owl Books.

Rogers, J.G. (1985). Origins of Sea Terms. Mystic Seaport.