The Log-Line Speed Scale
The measurement that gave the knot its name.
Origin
In use from the sixteenth century and refined through the eighteenth, remaining standard until mechanical logs replaced it.
The catch
The spacing of the knots was not arbitrary. At 47 feet 3 inches apart and timed against a 28-second glass, each knot that ran out equalled exactly one nautical mile per hour. The arithmetic was done once, by the ropemaker, so the sailor never had to.
Knots counted in 28 seconds = speed in nautical miles per hourThe line was knotted every 47 feet 3 inches, which is to a nautical mile what 28 seconds is to an hour. Enter a count to see the speed.
Worked examples
A weighted wooden quadrant, the chip log, was thrown over the stern and allowed to drag. As the line ran out a sailor counted the knots passing through his hands while another watched a sand glass. The method was crude, dependent on current and on the honesty of the count, but it was reproducible on any ship in any ocean, and it named the unit we still use.
On the evidence
The figure of 47 feet 3 inches is the standard given in eighteenth-century navigation manuals, and it is exact against a 28-second glass. Practice was looser. Recorded knot spacings vary from 42 to 51 feet, glasses ran anywhere from 14 to 30 seconds, and both drifted with wear and damp. Some masters knowingly used a short line to flatter the day’s run. The arithmetic below is correct; the readings it produced at sea were another matter.