Module 1 - Nautical Terms
Coastal Skipper Terminology
What you will learn
Use DR, EP, set, drift, leeway, transit, clearing bearing, tidal gate, fetch, lee shore and making way as bounded evidence in a coastal decision.
Advanced terms are not labels to memorise in isolation. They distinguish estimates from observations, intended limits from achieved motion and environmental factors from guaranteed outcomes.
Set is the direction towards which a tidal stream or current flows; drift is its rate, normally in knots. Leeway is a vessel's sideways displacement through the water caused principally by wind. Each value is an estimate or observation tied to a time, place, vessel and condition—not a permanent correction.
A dead-reckoning position is advanced from a known position using the vessel's movement through the water over elapsed time under the chosen plotting convention. An estimated position adds estimated external effects such as tidal stream or current and leeway. A DR and EP are both constructed estimates with growing uncertainty; neither is a measured fix and an EP is not automatically more true.
A transit or range exists when two identified objects are observed in line. It can provide a useful line of position or directional check without a compass, but it is not an exact fix and one line cannot show movement along itself. A clearing bearing is a pre-planned bearing boundary used on a stated side of an identified object with a protected margin. Its usefulness depends on correct chart work, object identification, reference conversion, observation and continued monitoring.
A tidal gate is a timing constraint or opportunity where stream, height, sea state or another tidal effect materially changes feasibility or margin. It is not always an instant when passage changes from impossible to safe. A lee shore has wind blowing towards it and can erode sea room; a weather shore has wind blowing away from it and may reduce local fetch, but shoreline shape, topography, refraction, swell and changing wind can limit any shelter.
Fetch is the distance over water for which wind blows in a broadly consistent direction; wave development also depends on wind strength, duration, depth and other conditions. Met Office sea-state words such as slight, moderate and rough correspond to stated wave-height bands. They describe sea state, not the height of every individual wave or a guarantee of comfort or safety.
Under the COLREGs, underway means not at anchor, made fast to the shore or aground. Making way through the water means moving relative to the surrounding water. Rule 35 uses that distinction for sound signals in or near restricted visibility; it should not be treated as a shortcut for every lights or collision-avoidance rule.
Worked example
A passage plan uses a transit to mark a turn and a clearing bearing to keep a shoal on the safe side while an estimated cross-stream acts.
- 1Plot the desired ground track, danger and safe-side clearing boundary using one stated direction reference.
- 2Advance a DR from the last reliable position using recorded course steered, speed through the water and elapsed time under the chosen convention.
- 3Apply the estimated stream and leeway to construct an EP, recording the source and time of those assumptions.
- 4At the transit, treat the alignment as one observed line of position or turn cue—not as an exact two-dimensional fix.
- 5Compare the observed bearing, depth and electronic or visual fixes with the plan; revise before the clearing margin is threatened.
Sense check: If the transit is held but the yacht could still move along that line, another independent observation is needed to establish position or motion in that direction.
| Term | What it provides | What it does not prove |
|---|---|---|
| DR | A position advanced from recorded movement through the water | A measured fix or external-effect correction |
| EP | A DR adjusted using estimated external effects | A guaranteed improvement or true position |
| Transit | An observed alignment and line of position | An exact two-dimensional fix |
| Clearing bearing | A monitored safe-side bearing boundary | Safety without correct plotting, identification and margin |
| Tidal gate | A time-dependent constraint or opportunity | A universal safe/unsafe switch |
Use the terms as evidence in a pilotage update
A yacht approaches a headland with a lee shore to leeward. The plan includes a tidal-gate window, a clearing bearing and a transit. The skipper compares the DR and EP with fixes and observed progress, then revises the plan if the stream, sea state or vessel performance differs from the assumptions.
1. State the estimate
Record the DR and EP with time and the course, speed, stream and leeway assumptions used.
2. Observe independently
Use identified transits, bearings, depth, electronic fixes or other suitable evidence rather than allowing one estimate to confirm itself.
3. Protect limits
Monitor the clearing boundary, sea room and tidal-gate margin continuously; act before the margin is consumed.
4. Revise
If observed track, sea state or timing differs, update the assumptions and choose a safer course, speed, delay or alternative.
Sense check: A well-labelled EP is still an estimate. The plan is stronger when independent observations agree and the skipper retains room to respond when they do not.
Common mistake or limitation
- An EP is not a corrected truth. It is a DR modified by estimated effects and can be worse if the stream, leeway, speed or starting position is wrong.
- A transit gives a line of position or alignment, not an exact fix. One transit can miss motion along its own line.
- A clearing bearing is not a magic heading to maintain. It is a charted safe-side boundary with a stated reference, margin and observation plan.
- A weather shore does not guarantee shelter, and a tidal gate is not universally passable or impassable at one named state of tide.
- Longer fetch alone does not determine wave size; wind strength and duration, depth, swell and local effects also matter.
Recap
- DR and EP are constructed estimates; independent fixes test them.
- Transits and clearing bearings are useful only with correct identification, reference, geometry and margin.
- Tidal gates, lee or weather shores and fetch describe conditional planning factors, not guaranteed outcomes.
- Underway and making way through the water are distinct COLREG terms; Rule 35 uses the distinction for restricted-visibility sound signals.
Optional quick check
Section 9 of 9
Two identified charted objects are exactly in line. What can you conclude from that observation alone?
Sources and factual review
Reviewed 20 August 2026 - Compass Revision editorial review.
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