A knot is the unit of speed used at sea and in the air: one nautical mile per hour, or 1852 metres per hour, which is very nearly 1.15 statute miles per hour or 0.514 metres per second. Boat speed, wind speed, current and speed over the ground are all quoted in knots. The beginner needs to know that a knot is a speed and that the phrase “knots per hour” is a landsman’s error, since the hour is already inside the unit. The experienced navigator adds why the knot survives: a nautical mile is one minute of latitude, so a boat making six knots covers six minutes of latitude in an hour, and distance, speed and time can be read straight off the chart with a pair of dividers and no conversion.
Where the Word Comes From
The name is literal. Before there were logs on transducers, a ship’s speed was measured with a chip log: a weighted wooden board on a line knotted at fixed intervals, streamed astern while a sandglass ran. The board stayed roughly still in the water, the line ran out, and the sailor counted how many knots passed through his fingers before the glass emptied. With the knots spaced at 47 feet 3 inches and a 28-second glass, the number of knots counted equalled the ship’s speed in nautical miles per hour. The instrument is long gone; the word stayed. This is also why the modern speed instrument is still called a log, and why the distance it records is the log reading.
Which Speed Are You Reading?
The boat has several speeds, all in knots, and mixing them up causes trouble. Boat speed, from the paddlewheel or ultrasonic log, is speed through the water. Speed over the ground, from GPS, is speed relative to the seabed and includes any tidal stream or current. A boat logging six knots through the water against a two-knot tide is making four over the ground, and her tacking angles and her time to the next mark are set by the second figure, not the first. Wind speed adds a third and fourth: true wind is the wind over the water, apparent wind is what the instruments and the sails feel, which is true wind combined with the boat’s own motion.
Why Knots Matter to the Sailmaker
Every sail is designed around a range of wind speeds in knots, and every sail inventory is built on crossovers – the true wind speed at which one headsail hands over to the next. A light genoa may be built for zero to twelve knots true, a medium jib for ten to twenty, a heavy jib for eighteen and above, with the ranges overlapping so the crew has a choice. Cloth weight, laminate construction and the amount of shape the designer builds in are all chosen for the apparent wind loads those numbers imply; a sail carried a few knots above its range is a sail being stretched out of shape. Boat speed matters as well, because the faster the boat, the more the apparent wind comes forward and increases, which is why fast boats carry flatter sails than the true wind alone would suggest. When you tell a sailmaker your usual conditions, give them in knots, and be honest about the top of the range.
Cruising and Racing Perspectives
For the cruiser, knots are passage planning. Hull speed for a displacement boat is roughly 1.34 times the square root of the waterline length in feet, so a 30-foot waterline gives about seven and a third knots as a practical maximum, and a realistic passage average is well below that. Knowing your boat makes five knots on average, not seven, is what gets you to the tidal gate on time and into harbour before dark. Cruisers also live by wind speed in knots: the Beaufort scale is still spoken, but the forecast and the instruments give knots, and reefing decisions are made on them.
For the racer, knots are the currency of every decision. Target boat speed for a given true wind speed and angle, read from the polar diagram, tells the helm whether to point higher or foot off. A tenth of a knot over a two-hour beat is several boat lengths at the top mark. Wind speed calls the sail changes; the difference between an eighteen-knot and a twenty-two-knot forecast is a different jib and a reef in the main.
Frequently Asked Questions about Knots
Is it correct to say “knots per hour”?
No. A knot is already one nautical mile per hour, so “knots per hour” would be a rate of acceleration. Say “six knots”, never “six knots per hour”.
How fast is a knot in miles per hour and kilometres per hour?
One knot is about 1.15 statute miles per hour and about 1.85 kilometres per hour. Ten knots is a brisk 11.5 mph or 18.5 km/h.
The knot is the oldest unit on the boat still in daily use, and the one everything else is measured against: how fast you go, how hard it blows and how far it is to the mark. Speak it correctly, know which of your speeds you are reading, and the numbers will always make sense.