Speedometer

Accurate speedometers came into popular use in the latter half of the 19th century. Until this time, most sailors estimated their speed with a chip log, a float affixed to a rope knotted at intervals of 47.25 feet, 14.4 meters. Navigators threw the float off the boat and counted the knots as they hit the water. The navigators timed their count, using a 28-second sandglass to ensure consistency. The number of knots that ran out in 28 seconds equaled the boat’s speed in nautical miles.