Ancient Navigation Knot or Knob of Wood Floated in Water Then Ship’s Rope Pulled to Straight Indicated Velocity of Vessel

If you tie a long rope to a chunk of wood, or tightly tangle the end of the rope into a big knot, either will float from where a vessel would sail away to see how long would take the given length of rope to go taught to begin dragging the knot (knob), this was the means anciently for the knot rate measure.