Measuring Boat Speed in Knots with Knots

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Measuring Boat Speed in Knots with Knots

Postby jimh » Wed Sep 23, 2020 1:12 pm

Traditionally boat speed was measured by trailing astern a small line attached to a board. The board would tend to remain stationary while the boat sailed away. The line was marked with knots at particular intervals. The seaman measuring the boat speed would count the number of knots spooled out in a particular time period. The number of knots would be equal to the boat speed. The duration of the timed interval and the distance between knot markers would need to be coordinated. We derive the spacing between knots by assuming a reasonable time interval and length of the trailed line.

Timing was historically done with an hour glass designed for a short duration. This usage predates the modern stopwatch. The hour glass might be constructed for a duration of one-minute.

To determine spacing of the knots there must be a correlation between the length of a nautical mile and the distance between knots. One nautical mile is considered to be the equivalent of 6,076-feet. A speed of one nautical mile in one hour is then a rate of 6076-feet/60-minutes, or 101.26-feet in 1-minute. The spacing of the knot markers on the line would then be at intervals of 101.26-feet of 101-feet 3.2-inches or in proportion to that distance, depending on the time of observation to be used.

If the trailing line is to be observed for one minute and if the boat speed could be as much as 10-knots, then the length of the line must be 10 x 101.26-feet or 1,012.6-feet. This is a substantial amount of line to deploy and reel in.

If the counting time and the knot spacing are both reduced by a factor of 0.5, then the line length proportional to 1-knot will be reduced to 50.63-feet, the marker spacing will be reduced to 50.63-feet, and the counting time reduced to 30-seconds. For a boat making 10-knots, ten markers or 506.63 feet are deployed in 0.5-minutes, or a rate of 1012.6-feet-per minute. In 60-minutes the distance would be 60,756-feet or 10-nautical miles. Such a line would have to be over 500-feet long. An hour glass with an accurate 30-second interval is also needed.