Module 8 - Drawing Instruments
Advanced Scenario Check: Stale EP and Reciprocal Bearing
What you will learn
Reconstruct a time-bounded EP and position line when a full-hour tide vector and a reciprocal bearing have created a plausible but unsupported clearing margin.
The advanced error is not untidy plotting. It is a tidy position assembled from incompatible time intervals and direction senses, then used as if the surrounding hazard margin had been proved.
A fictional yacht has a reliable fix at 0300 and is making 6.0 kn through the water. At 0320 the navigator advances an EP using a tidal stream of 1.2 kn but copies the full one-hour 1.2 M stream arrow from an earlier construction. A lighthouse bearing observed from the yacht is 038°T; the line drawn from the lighthouse is also sent along 038°T instead of the reciprocal 218°T. The resulting EP sits comfortably inside the intended clearing side.
The time basis fails first. Twenty minutes is one third of an hour, so the through-water run is 6.0 × 20/60 = 2.0 M and the tidal displacement is 1.2 × 20/60 = 0.4 M. A 1.2 M tide arrow applies one hour of stream to twenty minutes of vessel movement and displaces the EP by an extra 0.8 M before any other error is considered.
The bearing construction also fails. The observed 038°T describes the direction from vessel to lighthouse. To plot the line of position from the charted lighthouse towards the vessel, use its reciprocal: 038° + 180° = 218°T. The line is infinite, but the ray used to transfer the observation must carry the correct sense so the position is placed on the supporting geometry rather than a plausible parallel elsewhere.
Stop using the apparent clearing margin. Preserve the 0300 fix and original 0320 observation, rebuild the 20-minute DR and tide vectors, plot the reciprocal position line, label time and true reference, and compare the reconstructed position with depth, another visual or radar line and the electronic source's age and integrity. Do not erase the conflict before it can be explained.
The command response must separate plot repair from vessel control. One person reconstructs and reads back the chart evidence while helm, lookout and navigation maintain the already verified safe sector or execute the briefed fallback. If the required clearing side, distance or route cannot be supported in time, reduce exposure and abandon the intended continuation rather than steer from the attractive but invalid EP.
This scenario does not create a universal clearing-bearing inequality or a real-world helm order. Which side is safe depends on the lighthouse, hazard, route and vessel geometry shown on the current chart. Written revision can test the audit sequence; practical navigation requires the actual chart, conditions, equipment and competent command.
Worked example
Audit the two construction errors and state what evidence releases a corrected position.
- 1Elapsed time: 20 ÷ 60 = 1/3 hour.
- 2Through-water run: 6.0 kn × 1/3 h = 2.0 M.
- 3Tidal displacement: 1.2 kn × 1/3 h = 0.4 M, so the copied 1.2 M arrow overstates the interval by 0.8 M.
- 4Bearing reciprocal: 038°T + 180° = 218°T. Reversing 218° returns 038°, which independently checks the pair.
- 5Replot from the unchanged 0300 fix with the correct 0320 interval and line direction; keep the original marks identifiable until the discrepancy is resolved.
- 6Release only after a second chart read-back and independent position or hazard cue supports the reconstructed margin and the fallback remains available.
Sense check: At 6.0 kn the yacht cannot travel 6 M in twenty minutes, and a 1.2 kn stream cannot displace it 1.2 M in that interval. The simple thirds expose the time-base error before the chart is trusted.
| Evidence | Audit result | Release consequence |
|---|---|---|
| 20 minutes at 6.0 kn | 2.0 M through water | Reject any six-mile one-hour run used for the 0320 point |
| 20 minutes at 1.2 kn | 0.4 M tide | Reject the copied 1.2 M full-hour stream vector |
| Observed lighthouse bearing 038°T | Plot from the object on reciprocal 218°T | Reject the same-direction object-to-vessel ray |
| Tidy EP but unsupported margin | Geometry conflicts with the source interval and direction | Withdraw the point until reconstructed and cross-checked |
| Margin still unproved | No auditable release evidence | Use the already briefed fallback rather than continue |
Give the first defensible bridge command
The helmsman is waiting for a new course while the navigator insists that the tidy EP proves the yacht is clear.
1. Withdraw the EP
State that the 0320 position and clearing margin are not released because the tide interval and bearing sense are wrong.
2. Hold safe control
Maintain the last verified safe sector or execute the briefed fallback while lookout, depth and traffic monitoring continue; do not improvise a new heading from the invalid point.
3. Reconstruct and cross-check
Replot 2.0 M through water, 0.4 M tide and the 218°T reciprocal from the lighthouse, then compare another independent cue.
4. Release or abandon
Brief the corrected evidence, margin and next trigger only if they are supported; otherwise abandon the intended continuation before options close.
Sense check: The command is not 'plot faster.' It protects the yacht while the navigator removes two known construction errors and proves whether any usable margin remains.
Common mistake or limitation
- Copying a one-hour tidal arrow into a twenty-minute DR or EP because the speed label is unchanged.
- Drawing from the charted object along the observed vessel-to-object bearing instead of its reciprocal.
- Accepting a tidy point because it lies on the preferred side of a hazard before the inputs and geometry are audited.
- Erasing the original fix, observation or failed construction before the discrepancy and correction are traceable.
- Letting plot repair consume the remaining sea-room instead of maintaining a verified safe action or executing the fallback.
Recap
- Match every distance vector to the actual elapsed time: twenty minutes is one third of an hour.
- At 6.0 kn the twenty-minute run is 2.0 M; at 1.2 kn the tidal displacement is 0.4 M.
- An observed 038°T vessel-to-object bearing plots from the object on reciprocal 218°T.
- Withdraw an unsupported EP, protect vessel control, reconstruct and cross-check before releasing the route or margin.
Optional quick check
Section 7 of 7
At 0320, a 6.0 kn DR uses a 1.2 kn stream and an observed lighthouse bearing of 038°T. Which audit is correct?
Keep revising this topic
Sources and factual review
Reviewed 20 August 2026 - Compass Revision editorial review.
- RYA Yachtmaster Offshore examination scope
- RYA Coastal Skipper/Yachtmaster Offshore Theory course
- RYA Handy Plotter
- RYA Yachtmaster Handbook
- MCA MGN 610 Amendment 1 — navigation and SOLAS Chapter V
- MCA Navigation and Radar examination syllabus
- IHO S-4 chart specifications, edition 4.10.0
- USCG Boat Crew Handbook: Navigation and Piloting
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