Knots (boat speed) Explained

by admin on April 14, 2014

Most of today’s boats have knot meters installed to tell the operator the boat speed.  Now, really there are two speeds… through the water, and over the ground.  Our GPS systems can only calculate our speed ‘over the ground’, it does not take into account the river or tidal currents which will affect our speed over the ground.  A knot of speed is defined as one Nautical Mile distance traveled per hour of time.

Prior to under-water paddle-wheel speed transducers, used to send a signal to our electronic knot meters, our predecessors on the water used a “chip-log” or “log-line”, which was usually a wedge-shaped, weighted-on-one-end ( so it would float perpendicular to the water to increase drag) piece of wood.  It would have long, thin line attached to it with “knots” tied at evenly spaced intervals.  A 30-second sand-glass timer was turned as the log hit the water and the knots were counted for those 30 seconds.  I’ll let you do the math, but here are the basics:   A nautical mile is a unit of length which equates to one minute of arc measured along any meridian.  By international agreement it has ben set at 6076 feet.  So if we spaced the knots at 50 feet, and counted six knots departing our hands, what would our speed be? Answer: 50′ is appx 120th of a NM, and 30 seconds is 120th of an hour.  Presto!  (Now if I could just find my 30-second sand-glass.)

 

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