NAVIGATION · Lesson 1 of 5
Dead Reckoning
25 min · Beginner friendly
You’ll learn toPlot a DR position from course, speed and elapsed time · Use correct DR notation and update the plot regularly · Recognise the limitations and error growth of DR
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Your last certain position is behind you. Ahead: haze, and a compass, a and a watch. (DR) is the skill of turning those three instruments into an honest answer to where am I now? — the oldest navigation technique there is, and the foundation every other method builds on.
Take a guess
Decide before you read: your last known position was 40 minutes ago, and the log has shown a steady 6 knots. How far along the course line do you plot your position now?
Commit to an answer before reading on — a wrong guess here helps the right idea stick.
What Dead Reckoning Is
- Dead reckoning (DR)
- Estimating your present position by applying the and the distance run through the water to your last known position. It ignores tide and wind .
- Course steered
- The the helm has actually been steering, converted to true degrees for plotting on the chart.
- Log
- The instrument that measures speed and distance travelled through the water — not over the seabed.
- Knot (kn)
- One per hour. A nautical mile (M) is 1,852 metres — very close to one minute of on the chart.
Through the water is the phrase to hold on to. The log counts like a walker counting paces: movement relative to the water around the hull. If that water is itself moving — and in tidal seas it usually is — the log never notices. That is DR's power and its weakness in one sentence: always available, and blind to the tide.
Every DR is built from three recorded facts: course steered (in true), distance run (from the log, or from speed and time), and time elapsed. The mental shortcuts are worth learning early: 6 knots is a mile every 10 minutes, 3 knots a mile every 20.
Worked example
Worked example: distance run
You leave the fairway buoy off Westhaven at 1015, steering a steady course with the log showing 4.5 knots. How far have you run through the water by 1055?
- Time elapsed: 1055 − 1015 = 40 minutes.
- Apply the formula: distance = 4.5 × 40 ÷ 60.
- 4.5 × 40 = 180; 180 ÷ 60 = 3.
You have run 3.0 M through the water.
Knowledge check
Your log reads 5 knots and you hold a steady course for 24 minutes. How far have you travelled through the water?
Plotting a DR on the Chart
- Start at the last known position — ideally a proper fix — and note its time and log reading.
- Convert the course steered to true degrees.
- Draw a light pencil line from the position in that , marked with a single arrowhead — the symbol for a course steered through the water.
- Measure the distance run along the line, using the latitude scale at the side of the chart (1 minute of latitude = 1 M).
- Mark the DR with a short line across the course line and a dot, and write the time beside it.
Knowledge check
From the chartwork module — which symbol on the plot says this is a DR position?
Why DR Drifts From the Truth
Knowledge check
Which of the following does a dead reckoning position take into account?
A DR is an honest estimate, not a fact, and its errors accumulate — the longer you run on DR alone, the wider the circle of uncertainty around the pencil mark grows:
- Steering error — no helm holds a course perfectly; a few degrees of wander is normal, more in waves.
- Log error — logs commonly read a few percent fast or slow, and paddlewheel logs under-read at low speed or when fouled.
- — the whole body of water may be carrying you sideways, and DR ignores it completely.
- — the wind pushes the boat bodily downwind of her ; DR ignores this too.
As a planning figure, an unassisted DR can be a mile or more wrong after an hour in a strong stream. That is why DR is the skeleton of the plot: every other technique starts from it, improves it (the ) or resets it (a fix).
Keeping the Plot Alive
Good navigators keep the DR live: a deck log entry — time, log reading, course — every hour on passage, more often near dangers, and at every change of course or speed. Each entry is a point the whole plot can restart from.
Work it out
At 1200 you fix your position at Salt Island fairway buoy, log reading 31.4. You steer 045°T. At 1300 the log reads 36.4, and at 1345 it reads 40.1. How far along the course line from the buoy do you plot each DR?
Within 0.1 nm is fine.
Within 0.1 nm is fine.
Now do it with a pencil. On Chart SP1, run up a real DR from a real fix — graded the way an would grade it.
On the chart · Run up the DR
From the 0900 fix you steer 070°T at 5.0 kn through the water. At 0930 you want a DR.
Draw the course steered from the fix, mark the distance run along it, and plot the DR position.
| 0900 fix | as plotted (shown on the chart) |
| Course steered | 070°T |
| Boat speed | 5.0 kn |
| Time on this course | 30 minutes |
Draw on the chart, or add an object here and set its numbers:
Key points
- DR projects your position from course steered, distance through the water, and time — nothing else.
- Distance (M) = speed () × time (min) ÷ 60. Learn the shortcuts: 6 kn is 1 M per 10 minutes.
- Plot with standard symbols: fix = dot in circle; DR = line across the track with a dot; one arrowhead = . Label every position with the time.
- DR errors grow with time: steering wander, log error, tide and leeway all accumulate.
- Update the DR hourly (more often near dangers) and whenever course or speed changes, and record the numbers in the deck log.
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