In a nutshell, this method used a plank of wood (usually wedge shaped and weighted on one end so it would float perpendicular to the water to increase drag) tied to a long thin line that had knots tied at evenly spaced intervals.
The wood would be tossed into the water and the line let out while a sailor used a sand-glass to time the number of knots let out in the given timespan.
As for the interval and the time-span, this varied somewhat in the beginning, but for reference, one mid-eighteenth century version (attested in A Voyage to South America by Jorge Juan and Antonio de Ulloa) had the knots at 1/120th of a mile with a 30-second timer.
This has all since been standardized based on the nautical mile (today equaling 1.852 kilometers). One knot then equals one nautical mile per hour. In landlubber terms, this is about 1.15 miles per hour or 1.852 kilometers per hour.
More pertinently, this is equal to 1/60th a degree of latitude or longitude or one minute of arc (assuming the Earth is a perfect sphere, which it’s not -being squashed at the poles and bulging in the middle -but this is a good enough approximation). Thus, if you were traveling at one knot, it would take you approximately 60 hours to go 1 degree of longitude or latitude.
So, today, if you’re using a 28-second timer, to get your accurate speed in knots, you need to have the interval of knots at 14.4018 meters (47 feet, 3 inches). The number of knots that are unrolled during that span is your speed in knots.
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- Before “knots” a common way to measure a ship’s speed was simply to drop a log or other floating object into the water at the front of a ship, then time how long it took for it to reach the back of the ship. Your speed could then be calculated using this time and the known length of the ship.