The sandglass has been turned
Course WNW
A continuous course record supports dead reckoning.
Speed 6 kn
The knot comes from the historical use of a log line and timer.
Wind rising
Changing weather alters sail handling and watch decisions.
A floating chip first provided a rough speed estimate
Before the marked log line, a sailor could cast a floating object from the bow and time its passage along the known length of the ship. Distance divided by time yielded an estimate, but counting seconds on a moving deck, waves around the chip, and an uncertain ship length made the method crude. Dutchman's log is a name commonly associated with this simple practice.
Ocean navigation turned speed from an impression into a recurring data requirement. Out of sight of land, a navigator advanced the last reliable position by using compass course, speed and elapsed time. Looking at water beside the hull could suggest fast or slow, but it could not supply consistent logbook figures. The drag board, reel, marked line and sandglass made the observation repeatable.
The log-ship, reel and sandglass worked as one instrument
A typical log-ship was a sector-shaped board weighted along its curved edge. A bridle joined it to line on a freely turning reel. Once cast astern, the weight held the board nearly upright and its drag kept it moving more slowly than the vessel while the line ran out. A sharp pull released part of the bridle so the board lay flat for easier recovery.
The job required coordination: one person managed the reel, another timed and called the orders, and another counted marks. A Royal Museums Greenwich set has a 28-second glass, while Cutty Sark teaching material discusses roughly 30 seconds. Both can be historically meaningful because the time and line spacing formed a calibrated pair. Wet line stretched, glasses varied, and crews applied practical corrections; 28 seconds was not a timeless worldwide rule.
Marker spacing performed a proportional conversion
The principle was speed equals distance divided by time. A 28-second interval fits about 128.57 times into an hour, so a line interval near one nautical mile divided by 128.57 makes one interval correspond approximately to one nautical mile per hour. A 30-second glass requires a different spacing. Subdivisions between full knots supported fractional readings in the log.
The historical length of the sea mile also changed. Research on British logbooks describes sixteenth-century 42-foot spacing based on a roughly 5,000-foot mile. After improved geodetic work put a minute of latitude nearer 6,000 feet, Richard Norwood proposed about 50-foot spacing in 1637. The international nautical mile became exactly 1,852 metres in 1929; the United States adopted it in 1954 and the United Kingdom in 1970. Modern conversion must not be projected onto every early logbook.
A chip log measured speed through water, not GPS speed over ground
Because the board occupied the same local body of water as the vessel, the result primarily represented speed through water. If that whole body of water flowed east at two knots while a vessel made six knots north through it, the track over Earth also moved east. Multiplying the six-knot reading by time described relative motion through the water, before allowing for current, leeway and steering.
That limitation belongs to dead reckoning. Navigators recorded course, speed, wind and time, then compared the accumulated estimate with a landmark, latitude observation or later a longitude fix. A disagreement could contain current, compass error, leeway and log error; it did not automatically measure one of them. Modern GPS speed over ground and a ship's speed log still answer different questions.
Nineteenth-century mechanical logs changed the mechanism, not the knot
Royal Museums Greenwich notes that William Bourne described the log and line in 1574 and that it survived into the twentieth century, even after mechanical logs appeared in the nineteenth. A Smithsonian 1881 Thomas Walker patent model used a towed rotor connected to a counter mounted at the rail, allowing a continuous distance indication without hauling the entire mechanism aboard for every reading.
Pressure, electromagnetic and Doppler logs later made through-water measurements more continuous, while satellite navigation supplied over-ground motion. The knot endured because nautical miles coordinate conveniently with charts, navigation and marine weather. A vessel makes 12 knots, not 12 knots per hour: the latter divides an already time-based speed unit by time again. Nautical mile is distance; knot is speed.
Questions
Continue exploring this subject
How fast is one knot?
Under the modern international definition, one knot is one nautical mile per hour: 1.852 kilometres per hour, or about 1.151 statute miles per hour.
Why do sources mention both 28- and 30-second glasses?
The glass and marker spacing had to be calibrated together. Different instruments and practices used different paired values, so the time cannot be compared without the matching line interval.
Is a knot speed or distance?
It is speed. The nautical mile is distance: a five-knot vessel travels five nautical miles in one hour if that speed remains constant.
Could a chip log measure current directly?
No. It mainly measured speed through local water. Current was inferred by comparing dead reckoning with an independent position and accounting for leeway and other errors.
Sources
Continue the research
- Log and lineRoyal Museums Greenwich
- What is the difference between a nautical mile and a knot?NOAA National Ocean Service
- Inside Okeanos Explorer: Doppler Speed LogNOAA Ocean Exploration
- Log and Rotor, Patent ModelSmithsonian Institution
- British Logbooks in UK ArchivesInternational Comprehensive Ocean-Atmosphere Data Set