Working out your position from a known starting point, your course, your speed and the time elapsed.
Dead reckoning (DR) means calculating where you are from where you were: take the last known position, the course you've steered, your speed and the time since then, and plot the result on the chart. It needs no satellites, only a compass, a log, a clock and careful notes.
How it works
distance run = speed × time
At 6 knots for 1 hour 30 minutes you've covered 9 nautical miles. Plot 9 miles along your course from the last fix, and that's your DR position.
Strictly, a DR position uses only course and speed through the water. Corrected for leeway and current, it becomes an estimated position (EP), usually much closer to the truth.
Why it still matters
- When the electronics fail. A flat battery or a GPS fault shouldn't leave you without a position.
- As a cross-check. If the GPS position and your reckoning disagree badly, something is wrong with the GPS, the chart or your assumptions.
- It trains the eye. Knowing roughly where you should be makes it easier to notice when you're not.
Errors add up with time, so every reliable fix (from bearings, a buoy passed close by or GPS) starts the reckoning afresh. That's why a traditional logbook records time, course, log reading and position at regular intervals.