Dead Reckoning and Estimated Position
Projecting a vessel from a known position using course, speed and time, then allowing for tide, leeway and uncertainty.
How to prepare for Day Skipper Theory
Learn the navigation foundations first, combine tide and weather into berth-to-berth plans, then rehearse collision avoidance, electronic cross-checks, seamanship and emergency decisions.
Core concepts
Concept 1
A dead-reckoning position advances a known position by the vessel's steered course and distance through the water.
Exam cue: Start from a reliable position and record the time.
Concept 2
An estimated position adds known or estimated external effects such as tidal stream and leeway and should be labelled with time and method.
Exam cue: Convert speed and elapsed time to distance before applying the correct directional vectors.
Concept 3
DR and EP remain valuable gross-error checks and degraded-mode records even when GNSS continuously displays a position.
Exam cue: Show uncertainty and update the estimate when new observations or sensor evidence arrive.
Risk pitfalls and guardrails
Calling a GNSS fix an EP or treating an EP as an observed fix.
Guardrail: Do not present the 601-item authoring target as an official RYA paper and do not treat theory study as a substitute for recognised assessment or practical skippering competence.
Applying tidal stream in the wrong direction or for the wrong duration.
Guardrail: Do not present the 601-item authoring target as an official RYA paper and do not treat theory study as a substitute for recognised assessment or practical skippering competence.
Carrying an old DR forward without widening uncertainty or obtaining a new check.
Guardrail: Do not present the 601-item authoring target as an official RYA paper and do not treat theory study as a substitute for recognised assessment or practical skippering competence.
Memory anchors
DR
Known position advanced by course steered and distance through the water.
EP
A DR adjusted for estimated effects such as tide and leeway.
Distance
Speed in knots multiplied by time in hours gives nautical miles.
Time Label
Every DR, EP and fix needs the time to which it applies.
Independent History
A navigation record helps detect or recover from GNSS error or failure.
Checkpoint rule
Do the check-up only after you can summarize each concept in one sentence and identify one dangerous pitfall from memory.
Knowledge Check (after reading)
Short check-up to confirm understanding of this module.
Check-up Questions
What inputs define a basic dead-reckoning position?
What turns a DR into an estimated position?
Answer all questions to submit.
Next step personalized recommendations
Continue learning
Move forward only after this module is stable.
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