The Beaufort scale for sailors, force by force
Thirteen numbers sailors have used for two hundred years. What each force looks like on the water, how it feels aboard a cruising yacht, and where most crews draw their lines.
In this piece
The Beaufort scale sorts the wind into thirteen forces by its mean speed, from force 0, a flat calm, to force 12, hurricane force. In knots, force 4 is 11 to 16, force 6 is 22 to 27 and force 8, a gale, is 34 to 40. Many official marine forecasts in Europe still give the wind as a Beaufort force, and most sailors think in forces whether the forecast does or not.
For a 10 to 12 m cruising yacht, the short version is this. Forces 3 to 5 are the sailing days. Force 6 is where most crews reef hard and start asking whether the passage can wait, especially if it is upwind. From force 7 up, the harbour is the better place to be unless you are already at sea.
The Beaufort scale in knots, force by force
The speeds are the World Meteorological Organization’s bands for the mean wind 10 m above the surface. The sea column describes open water once the wind has been blowing for a while. The last column is our view from the cockpit of a typical cruising boat of 10 to 12 m, with a competent crew; your boat and your crew may draw the lines a force either side.
| Force | Name | Knots | The sea, offshore | Aboard a 10 to 12 m yacht |
|---|---|---|---|---|
| 0 | Calm | under 1 | Flat, like a mirror | Engine on. Any leftover swell slats the sails and the boom. |
| 1 | Light air | 1 to 3 | Ripples like scales, no foam | Barely steerage under sail. Motor, or swim and wait. |
| 2 | Light breeze | 4 to 6 | Small wavelets, glassy crests that do not break | Full sail, 2 to 4 knots through the water. A gennaker or cruising chute earns its place. |
| 3 | Gentle breeze | 7 to 10 | Crests start to break, the odd white horse | Full sail, dry and easy. The best day there is for teaching someone to helm. |
| 4 | Moderate breeze | 11 to 16 | Small waves getting longer, white horses fairly common | Full sail and the boat at her best. Upwind, the first reef goes in near the top of the band. |
| 5 | Fresh breeze | 17 to 21 | Moderate, longer waves, many white horses, some spray | First reef, some genoa rolled away. Upwind is wet; a reach is fast and fun. |
| 6 | Strong breeze | 22 to 27 | Large waves forming, white foam crests everywhere, spray | Second reef and a small headsail. Upwind is hard, slamming work. Many crews wait. |
| 7 | Near gale | 28 to 33 | The sea heaps up, foam blows off in streaks | Deep reefs and a scrap of headsail. Most cruising crews stay in harbour. |
| 8 | Gale | 34 to 40 | Higher, longer waves, spindrift off the crests | Stay in. At sea: storm sails or bare poles, heave to or run. |
| 9 | Strong gale | 41 to 47 | High waves, crests tumble over, spray cuts visibility | Heavy weather tactics. The question is now the boat’s safety, not the passage. |
| 10 | Storm | 48 to 55 | Very high waves with overhanging crests, the sea looks white | Survival conditions. |
| 11 | Violent storm | 56 to 63 | Exceptionally high waves, the sea covered in foam | Survival conditions. |
| 12 | Hurricane force | 64 and over | Air full of foam and spray, the sea completely white | Survival conditions. |
Beaufort in m/s and km/h, with wave heights
For forecasts that use other units. The wave heights are the WMO’s “probable” significant heights for the open sea, far from land, with the wind blowing long enough to build a full sea.
| Force | Knots | m/s | km/h | Probable wave height |
|---|---|---|---|---|
| 0 | under 1 | 0 to 0.2 | under 1 | 0 |
| 1 | 1 to 3 | 0.3 to 1.5 | 1 to 5 | 0.1 m |
| 2 | 4 to 6 | 1.6 to 3.3 | 6 to 11 | 0.2 m |
| 3 | 7 to 10 | 3.4 to 5.4 | 12 to 19 | 0.6 m |
| 4 | 11 to 16 | 5.5 to 7.9 | 20 to 28 | 1 m |
| 5 | 17 to 21 | 8.0 to 10.7 | 29 to 38 | 2 m |
| 6 | 22 to 27 | 10.8 to 13.8 | 39 to 49 | 3 m |
| 7 | 28 to 33 | 13.9 to 17.1 | 50 to 61 | 4 m |
| 8 | 34 to 40 | 17.2 to 20.7 | 62 to 74 | 5.5 m |
| 9 | 41 to 47 | 20.8 to 24.4 | 75 to 88 | 7 m |
| 10 | 48 to 55 | 24.5 to 28.4 | 89 to 102 | 9 m |
| 11 | 56 to 63 | 28.5 to 32.6 | 103 to 117 | 11.5 m |
| 12 | 64 and over | 32.7 and over | 118 and over | 14 m and over |
The wave heights are for the open sea
Those heights assume an ocean: hundreds of miles of open water upwind and a wind that has blown for a day or more. Most of us sail somewhere smaller. In a bay with a mile of fetch, a force 6 raises a chop well under a metre. In the channels of an enclosed sea the waves stay lower than the table but come shorter and steeper, with breaking tops, and a 2 m sea at a short period can be harder on a boat and crew than the long 3 m sea the table describes. That is the normal state of the Cyclades in a strong meltemi.
The reverse happens too. Swell from a blow far away can roll into a bay on a day of force 2, and the Beaufort number tells you nothing about it. Wave heights in a forecast are significant heights, the average of the highest third, so some waves will be much bigger; our guide to reading a marine forecast explains how much bigger.
Gusts, and the force you actually sail in
A forecast force is the mean wind. The gusts inside it are often a force higher, and in the lee of high land or under a shower cloud they can be two or three forces higher. A forecast of force 5 near a mountainous coast can mean force 7 gusts coming off the hills. Plan the day on the mean, reef for the gusts.
Your own motion changes it again. Sailing upwind at 6 knots, the wind across the deck is roughly a force stronger than the true wind; running, it is a force or more lighter. A force 5 that felt gentle on the way out is a different animal when you turn round to come home.
SafeBoat colours each hour of its forecast by Beaufort force, green through amber to red, so the afternoon a day turns amber stands out before you read a single number.
Where sailors draw the lines
Reefing
Most cruising yachts of this size carry full sail comfortably upwind to somewhere around 15 to 18 knots of apparent wind, want the first reef by about 20 and the second somewhere in the mid to high 20s. Downwind, the apparent wind is lower, so you carry more sail for longer. That is the trap: the reef you did not need on the run is the one you badly need when you round up, and it is much easier to put in before you turn. The old rule still stands. Reef when you first think about it, and if the forecast says force 5 or more, put the first reef in before you leave the harbour.
Boats differ a great deal. A heavy, long-keeled boat stands up to a force 5 under full sail; a light modern cruiser with a big main may want a reef at the top of force 4. The boat’s handbook or the sailmaker’s notes sometimes give the wind speeds the designer had in mind.
Going out
There is no official line, only the boat, the crew and the direction. Our own habits for coastal day passages are roughly these: force 5 we sail in any direction, force 6 we take only off the wind, and in a forecast of force 7 we stay where we are. With a crew new to the boat, or anyone seasick-prone aboard, we knock a force off each of those.
The sea matters as much as the number. A force 5 against a current or over a leftover swell can be nastier than a force 6 on flat water downwind. Where the passage starts and ends matters too: leaving a sheltered harbour into a force 6 is one thing, arriving at a harbour entrance with the waves breaking across it is another.
At anchor
Force 6 and over, sustained, is where the choice of anchorage and the amount of chain start to decide how well you sleep. If the forecast has force 7 coming into an anchorage that is open to it, the time to move is in daylight, before it arrives. Our guide to how much chain to let out covers the anchoring side.
Where the Beaufort scale came from
Francis Beaufort was an Irish-born officer of the Royal Navy who later spent a quarter of a century as Hydrographer of the Navy. He did not invent the idea of a wind scale; others had tried before him. What he set down in his journal in 1805 was a scale a ship’s officer could use without instruments, and the instrument was the ship.
Beaufort’s forces were defined by what a full-rigged man-of-war could carry. At force 1, just enough wind to give her steerage. In the middle of the scale, the point at which she had to take in her lighter sails or reef her topsails. At force 12, a wind in which she could show no canvas at all. The Admiralty made the scale standard for logbook entries in 1838.
As steam replaced sail, describing the wind by a warship’s canvas stopped making sense to anyone. The descriptions were rewritten in terms of what the sea looks like, and in the twentieth century wind speeds were attached to each force. The knots in the tables above are the modern WMO bands, but the numbers 0 to 12 are still Beaufort’s.
Questions people ask
What is force 6 in knots?
22 to 27 knots of mean wind, which is about 39 to 49 km/h or 11 to 14 m/s. Its official name is strong breeze. British sailors have long called it the yachtsman’s gale, and beating into one in a cruising yacht you soon see why. Offshore, large waves start to form, white foam crests are everywhere, and the gusts are often force 7.
Is force 5 too much for a beginner?
Not with an experienced skipper aboard and the wind on or behind the beam: a force 5 reach is one of the best ways to learn what a boat can do. For a new skipper in charge for the first time, it is a lot. 17 to 21 knots means reefing, a heavy helm and spray upwind, and the gusts are often force 6. If that is you, pick a short leg with the wind aft of the beam, put the reef in before you leave, and turn back if the sea is building faster than you expected.
Can you tell the force by looking at the sea?
To within a force, yes, which is what the sea descriptions are for. Scattered white horses appear at force 3 and become common at 4. At force 5 there are many of them and the first spray; at 6, foam crests are everywhere; at 7, the foam starts to lie in streaks along the wind. Close under a weather shore the water can be flat while the wind is still blowing hard, so judge from the sea further out. Make a habit of guessing the force before you look at the instrument. After a season you will be right most of the time, and that is the skill the scale was built for.