How to Knot a Rope to Gauge Its Speed on a Ship
In seafaring and air travel, the term knot refers to the ship's or aircraft's speed. Modern ships and aircraft use electronic equipment to measure knots, but this was not always the case. As recently as the early 20th century, sailors used an ingenious method for measuring the speed of ships and river currents, known as the log line. Measuring speed in knots required a weighted log-chip tied to the end of a log-line or rope with knots tied at equal intervals, a reel or spool to hold the rope and a sandglass for timing.
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Things You'll Need
- 600 feet of thin rope
- Twine
- Colored fabric scraps
- 30-second sandglass timer
- Rope spool with handles, or reel
- Boat or ship
- Log-chip, weighted on one end
Instructions
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Divide a 600-foot rope at intervals of 47.33 feet. Note that intervals must be exact in order to measure speed accurately. At each interval, interweave twine into the rope fibers and knot off the twine with three or four basic square knots. Create every tenth knot from colored fabric scraps, to make counting easier. Fasten one end of the rope to a handled spool or reel by tying it around the center rod between the handles, then wind up the rope by hand in a fashion similar to a spool of thread.
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Tie two pieces of twine through holes drilled in the corners of the weighted bottom of the log-chip. A log-chip is a flat piece of wood, shaped like a quarter-circle, with the curved edge weighted with pieces of iron -- which causes this edge to sink -- and two straight edges forming an angle that stands up out of the water to mark its location. Tie the free ends of the twine to a stray-line -- a single piece of twine -- and tie the stray-line to the end of the rope by incorporating it into a knot at the end of the rope. Knot a 3-inch by 6-inch piece of red fabric at this end of the rope to mark the point at which time measurement begins.
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Have one person hold the reel of rope by the handles, another person heave the log-chip, and a third person measure time with the 30-second sandglass. The person timing yells "heave the log" and turns the glass as soon as the red knot hits the water.
The second person heaves the log over the stern and the person holding the reel of rope allows the reel to spin freely in her hands until the timer yells "stop." At this point, the person holding the reel must firmly grasp the reel's handles to prevent the rope from unwinding any further.
The person who heaved the log-chip pulls the line back in as the person with the reel winds the rope onto the reel. The person timing the event counts the knots, then performs the speed calculations. Example: If 15 knots follow the log-chip into the water during the 30-second time frame, then the ship was traveling at a rate of 15 knots, or about 15 nautical miles per hour.
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Comments
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The design above would be appropriate to the old US Navy method (see Reference, US Navy: Naval Terms and Expressions) and note that this method uses a time period of 28 seconds. To get accurate results for a 30 second period, the knots must be spaced 50.63 feet apart for a standard nautical mile of 6076 feet.
