Chapter 7: Dead Reckoning
DR advances the last known position by course steered and speed through the water. Adding current and wind converts the DR into an estimated position and the ground track.
Plotting conventions
A fix is a circle with a dot and the time in four digits. A DR position is a semicircle with a dot; an estimated position is a square. Course is labelled above the track line as C 095, speed below as S 12. The DR track is always run from the last fix, never from an EP, and is maintained even when GNSS is available so that a sudden loss of electronics leaves a usable plot.
- Plot a DR position at every course change, speed change, fix and hourly
- Never carry DR forward from an EP
- Label everything — an unlabelled plot is unusable by a relief watch
Current sailing and the triangle
The current triangle relates three vectors: the vessel's course and speed through the water (heading/log), the current's set and drift, and the resulting course over ground and speed over ground. Set is the true direction toward which the current flows; drift is its speed in knots. To make good a desired track, lay off the desired track, the current vector, and solve for the heading to steer — the difference is the drift angle.
Leeway
Leeway is the downwind displacement caused by wind on the hull and superstructure, estimated from the wake angle and applied to the water track before current. High-freeboard vessels in strong beam winds may make 5°–10° of leeway; it is added on the downwind side of the heading.
Formula cards
Distance run
D = S × T
60 D = S × T when T is in minutes: the 60 D Street rule.
Set and drift from a fix
Set = direction DR → fix; Drift = distance DR→fix ÷ hours elapsed
The offset between DR and a subsequent fix is the integrated current effect.
Key takeaways
- • DR uses course and speed only — no current, no wind.
- • Set is a direction toward, drift is a speed; together they convert DR into EP.
- • Course made good is the ground track; course steered is the water track.
Knowledge check
Immediate feedback with rationales. Answer each question to see the explanation.
Chapter 7 quiz
0 of 3 answered
1.After 3 hours the fix lies 6 NM at 270° from the DR position. Set and drift are:
2.Which symbol denotes an estimated position?
3.Steering 090° at 10 kn with a set of 180° drift 2 kn, the vessel's ground track is: