Knots are the unit sailors use to measure speed – of the wind, of the boat through the water, of the tide. One knot is one nautical mile per hour, and because a nautical mile is one minute of latitude (1,852 metres, or about 1.15 statute miles), a knot ties speed directly to the distance scale printed up the side of every chart. That is the beginner’s essential: “15 knots of breeze” means the wind is travelling 15 nautical miles every hour. The experienced sailor adds the caveat that a wind-speed figure means nothing until you know whether it is true wind or apparent wind, and at what height it was measured – a masthead anemometer on a 50-footer reads noticeably higher than a hand-held at deck level in the same breeze.
Where the Word Comes From
The name is not a coincidence. Before instruments, a ship’s speed was found with a chip log: a weighted wooden quadrant on a long line, with knots tied into the line at regular intervals. The log was thrown over the stern, a small sandglass was turned, and a sailor counted how many knots ran through his fingers before the sand ran out. The spacing of the knots was matched to the glass so that the count gave nautical miles per hour directly. The knots in the line became the unit, and the unit outlived the line. It is a rare case of a nautical word whose etymology is exactly what it appears to be.
Why Sailors Never Switched to Kilometres
The knot survives because it is tied to the geometry of the planet rather than to an arbitrary standard. One minute of arc along a meridian is one nautical mile, so a boat making six knots covers six minutes of latitude in an hour, and a navigator can lay that off with dividers on the latitude scale without converting anything. Aviation kept the knot for the same reason. For rough mental conversion: multiply knots by two to get kilometres per hour (slightly over), or add 15 percent to get miles per hour. Half a metre per second is close to one knot, which is handy when reading European forecasts.
Wind Speed: True, Apparent and Gusts
On the water, knots most often describe wind. True wind is the wind over the sea surface. Apparent wind is what the boat and her instruments actually feel: true wind combined with the boat’s own motion. Beating at seven knots in 15 knots true, the apparent wind on deck may read close to 20; running in the same breeze it may drop to eight and the crew will complain about the heat. Forecasts and the Beaufort scale describe true wind, usually averaged over ten minutes at ten metres above the surface. Gusts in a forecast are typically a third stronger than the mean, and in unstable air or off high ground they can be half again as strong. A sailmaker sizing a sail wardrobe cares about all three numbers, because it is the apparent wind and the gusts, not the average, that load the cloth.
Why Knots Matter to a Sailmaker
Every sail has a wind range, and that range is written in knots of apparent wind. A light No. 1 genoa built from a soft laminate may be designed to be fastest from four to twelve knots and to be off the boat by 16; a heavy No. 3 may not come alive until the anemometer passes 18. Push a sail past its range and the cloth stretches, the draft moves aft, the leech hooks and the boat heels without going faster – and the damage to the fibres is permanent. UK Sailmakers crossover charts plot each sail against true wind speed and wind angle so a crew knows, before the race, which sail belongs up at 14 knots on a tight reach. For a cruising boat, the same thinking decides at what wind speed the first reef goes in and when the furling genoa should be rolled to its reinforced reef mark.
Frequently Asked Questions about Knots
Is it correct to say “knots per hour”?
No. A knot already means one nautical mile per hour, so “knots per hour” would be a rate of acceleration. Say “the wind is 20 knots” or “we were making seven knots.”
Does my masthead wind instrument read true or apparent wind?
The sensor itself measures apparent wind. The instrument system calculates true wind by combining that with boatspeed and heading, so the “true” figure is only as good as the log and compass feeding it, and it will be wrong if the log is fouled or badly calibrated.
Knots are the common language of the sea. Whether you are choosing between a No. 2 and a No. 3 before the start, deciding when to reef on a night passage or working out whether the tide will let you make the harbour before it turns, you are thinking in knots. Know your boat’s numbers, know your sails’ ranges, and trust the figure on the dial only when you know what it is actually measuring.