Advanced Scenario Check: Position

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.