Dead reckoning vs estimated position

Offshore Theory, written by a superyacht bosun with a commercial Yachtmaster Offshore · 30 October 2026

Dead reckoning and the estimated position both answer the same question when there is nothing to take a fix on. The difference is what they leave out. A dead reckoning position uses only what the boat did through the water; an estimated position adds what the wind and the water did to the boat.

Dead reckoning (DR)

From the last known position, lay off the course steered for the distance on the log. That is all. The DR position is marked with a short line across the track and the time.

It needs nothing but the compass, the log and a clock, and in water without a stream and in light winds it is close to the truth. Its weakness is that the log only measures water going past the hull: it cannot feel the water moving with the boat.

Estimated position (EP)

Start the same way, then add two things. First leeway: the water track is the heading turned downwind by the leeway angle. Then the tidal stream for the same time, laid off from the end of the water track. The end of the stream is the EP, marked with a small triangle and the time.

How far apart they get

A worked example, with round figures. From a fix at 1000 you steer 045°T and log 8.0 miles in the hour. Leeway is 5° with the wind on the port beam, and the stream sets 120°T at 1.5 knots.

• DR at 1100: 8.0 miles on 045°. That is 5.66 miles east and 5.66 north of the fix.
• EP at 1100: 8.0 miles on 050° through the water, then 1.5 miles on 120°. That is 7.43 miles east and 4.39 north.
• The gap: 1.77 miles east and 1.27 south, which is 2.2 miles, with the EP bearing about 126° from the DR.

Two miles after one hour, from a modest stream and a few degrees of leeway. Over a night passage the gap between the two can be the gap between clear water and a headland.

Which one to use

• Use the EP for navigation whenever you have the stream and a sensible estimate of leeway. It is the best estimate you have.
• Keep the DR as a check. It shows how much of the EP depends on the stream figure. If DR and EP are miles apart, the stream figure is doing a lot of the work, and a mistake in it moves you just as much.
• Replace both with a fix whenever you can. A fix restarts the reckoning from a known point; the difference between the EP and the fix tells you how good your stream and leeway figures really were.

Keeping the reckoning

The reckoning is only as good as what you write down: the time and log reading at every alteration of course, the course steered (not the course ordered), and the stream for each hour. In the course, a whole afternoon is kept this way, leg by leg, and then shown to be wrong in exactly the way the stream predicts.

Where this is in the course

Dead reckoning and running a position line on →

Tidal streams and the estimated position →

Free reading notes: dead reckoning and the running fix →

Questions

What is dead reckoning?

A position worked out from the last known position using only the course steered and the distance run through the water, from the log. It leaves out the tidal stream, and as used here, leeway.

What does an estimated position add?

The effects the log cannot feel: leeway, which moves the boat through the water away from her heading, and the tidal stream, which moves the water itself.

How are they marked on the chart?

A dead reckoning position is a short line across the track with the time. An estimated position is a small triangle with the time. A fix is a circle.

Is an EP always better than a DR?

It is better when the stream and leeway figures are good. If the stream figure is a guess, the EP inherits the guess. Both are estimates; neither replaces a fix.

How often should I update the reckoning?

At every alteration of course, at each hour or log reading you plot, and whenever you get a fix, which restarts it from a known position.

Related guides

• Course to steer and the tidal triangleThe one-hour vector triangle drawn step by step, with leeway, and down to a compass course.
• Set, drift and the estimated positionFrom the dead reckoning to the EP: the stream for the time run, leeway, and the triangle drawn the right way.
• The cocked hat and crossing bearingsChoosing marks, the angle of cut, the order you take them in, and what the cocked hat tells you.
• The running fixOne mark, two bearings and the run between them: how to transfer the line, with and without a stream.