Advanced Scenario Check: Stale EP and Reciprocal Bearing

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.