Module 6 - Position, Course & Speed
Advanced Scenario Check: Position
What you will learn
Reject an ETA built from incompatible speed and distance frames, recalculate progress over the ground and brief the resulting gate decision clearly.
A tidy speed-distance-time answer can be operationally wrong when speed through water is paired with a distance over the ground. This scenario rehearses the Yachtmaster habit of checking the frame before trusting the arithmetic.
At 2115 a fictional yacht has 6.6 M remaining along an intended 090°T ground track. A clear-by gate is 2240. The log shows 6.2 kn speed through water, while checked GNSS history over a stable interval shows COG 100°T and SOG 4.4 kn. A watch handover uses 6.2 kn against the 6.6 M ground distance and announces a 2219 ETA.
That 2219 answer mixes movement frames. The route distance is over the ground, while the 6.2 kn log value is through the water. Even raw SOG does not all advance the route when COG is ten degrees off the intended track. Project the ground vector onto the route or use a checked plot of distance made good.
The along-track speed is approximately 4.4 × cos 10°, or 4.33 kn. At that rate 6.6 M takes about 1 hour 31 minutes, giving an ETA near 2246—about six minutes after the clear-by gate before any additional allowance. The ten-degree track difference also creates danger-side displacement that must be monitored against the plan.
Do not preserve the earlier ETA by averaging log speed and SOG or by assuming one source is wrong. Check log calibration, GNSS integrity, heading and track references, tide, leeway, manoeuvring and the time interval; meanwhile close the failed gate and use the briefed hold, diversion or revised route while that option remains controllable.
This fictional calculation supports written revision only. The live decision depends on current position, vessel, weather, tide, traffic, charted dangers, sea room and the skipper's assessment.
Worked example
Compare the incompatible 2219 ETA with an ETA based on along-track ground progress.
- 1Incorrect frame: 6.6 M ÷ 6.2 kn STW = 1.065 hours, about 1 hour 04 minutes; 2115 + 1:04 gives about 2219.
- 2Track difference: 100°T COG − 090°T intended track = 10°.
- 3Along-track ground speed: 4.4 kn SOG × cos 10° = about 4.33 kn.
- 4Ground-route time remaining: 6.6 M ÷ 4.33 kn = about 1.52 hours, or 1 hour 31 minutes.
- 5Revised ETA: 2115 + about 1 hour 31 minutes = 2246, around six minutes after the 2240 gate. Close the gate before selecting the fallback or a fresh plan.
Sense check: An adverse ground result should not produce an earlier arrival merely because the through-water log is higher. The frame check explains why the 2219 answer is optimistic.
| Input | Frame | Valid use here |
|---|---|---|
| 6.2 kn log speed | Normally speed through water | DR or water-track work after calibration and reference checks |
| 4.4 kn SOG on 100°T COG | Ground-movement vector | Project onto the intended ground track over a checked interval |
| 6.6 M remaining | Distance along the planned ground route | Pair with along-track ground speed for this ETA |
| 2240 clear-by gate | Operational decision limit | Close when the monitored forecast no longer protects it |
Turn the recalculation into a watch handover
The relieving watch has already written 2219 beside the waypoint and asks whether to keep it until the instruments agree.
1. Correct the record
Strike through rather than silently overwrite 2219; record the 2115 time, 6.6 M ground distance, 100°T COG, 4.4 kn SOG and 6.2 kn STW with their sources.
2. Show the frame
State that 6.2 kn is through water, while the route and gate use progress over the ground; record about 4.33 kn along-track speed and a 2246 estimate.
3. State the decision
Close the 2240 gate and name the hold, diversion or replanning action that preserves sea room.
4. State the next evidence
Set the next accepted fix, along-track progress, cross-track, depth and time review and identify what would reopen a revised route decision.
Sense check: The handover should let another navigator reconstruct both ETAs, see why one is invalid and know which gate is now closed.
Common mistake or limitation
- Pairing speed through water with a remaining distance over the ground creates a neat but incompatible ETA.
- Using raw SOG as along-track speed ignores the component moving across the route when COG differs from intended track.
- Averaging STW and SOG does not reconcile their different physical meanings.
- Keeping a failed gate open until every discrepancy is diagnosed can consume the hold or diversion option needed to stay safe.
Recap
- Match speed, distance and time to the same movement frame.
- Use checked along-track ground progress for a ground-route ETA.
- Record why an old ETA was rejected rather than silently replacing it.
- Close a failed gate early and preserve a controllable fallback.
Optional quick check
Section 10 of 10
A 6.6 M ground-route distance is paired with 6.2 kn STW to give 2219, but along-track ground speed is about 4.33 kn and gives 2246 against a 2240 gate. Which conclusion is defensible?
Sources and factual review
Reviewed 20 August 2026 - Compass Revision editorial review.
Continue studying Position, Course & Speed
This public lesson is part of Revision Module 6. Full revision access adds the guided Learn, Practise and Test flow, flashcards and revision progress.