The Great Ouse

looks calm but that is only skin deep, the water is flowing at about 7 miles per hour (just over 8 knots) as the tide ebbs, but there is no wind to agitate the surface.Grandson, as usual, was with me so I explained about the knot and its derivation.

He had no idea that sailors would dangle a weighted knotted rope from the boat and as the boat increased speed the rope would no longer hang straight down and more knots would become visible.