Definition
Six minutes is one tenth of an hour, so distance run in six minutes equals speed in knots divided by ten. At 12 knots that is 1.2 nautical miles, and in three minutes half of that, 0.6 nautical miles. The rule is invaluable for rapid pilotage checks, timing between marks and estimating closing ranges without instruments, and it inverts equally well: covering 0.8 nautical miles in six minutes indicates 8 knots.
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