How Does Boat Speed Work?

Many wonder how does boat speed work. Luckily the answer isn't too terribly complicated. It merely requires some conversions and understanding some terms that might be new to you.

Boat speed is measured in knots. One knot is the equivalent of one nautical mile per hour. This measurement for boat speed can be abbreviated as kn, kt or kts.  Knots are also used in meteorology and air navigation. The a boat's speed is relative to the water on which it is travelling. For consistency, the speed of navigational waters is also measured in knots, such as currents and tidal streams. For practical purposes, the length of a nautical mile is the same as the length of one minute of latitude.

Knot equivalents. One nautical mile per hour is the approximate equivalent of 1.151 miles per hour, the exact equivalent of 1.852km per hour or 0.514m per second. 

The origin of the knot. The knot originated long ago, when the speed of boats and ships on the water was measured by knots tied onto a rope. A chip log was weighted on one end of the rope so it stood upright when tossed into the sea, thus causing good resistance against the water. This chip log was attached to a rope in which knots were tied every 47 feet and three inches. A sailor would use a sandglass to time 30 seconds. Another sailor would let the rope slip through his hands while standing at the stern of the boat and would count how many knots went through his hands. This knot count would then be recorded and used in the boat or ship's dead reckoning and navigation. In modern times, the accepted timing is 28 seconds instead of 30. The difference in this modern definition is less than 0.02%.

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