Definition
Pairs of shore beacons define two transits one nautical mile apart, and the vessel runs at steady revolutions on the course marked on the chart, which is set perpendicular to the transits. Timing between transits gives speed as 3600 divided by the elapsed seconds, so 6 minutes 40 seconds, that is 400 seconds, equals 9.0 knots. Reciprocal runs are averaged, traditionally by the mean of means, to remove the effect of tidal stream.
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